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Anatomy of nervous system's sensory pathways

Anatomy of nervous system's sensory pathways


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When I touch my hand on a hot stove, I feel pain. I'm interested in knowing all the main "endpoints" (components/parts of the body) that are involved in relaying this pain signal. As I understand it so far, they are:

  1. Nerve (thermoreceptor)
  2. Fibers
  3. Spinal cord
  4. Brain stem (medula => pons => midbrain)
  5. ???
  6. Somatosensory Cortex

A special sensory nerve in my hand, called a thermoreceptor, receives a thermal input (the hot stove) and fires a pain signal to my spinal cord.

My spinal cord then sends the signal up into the brain stem, where the signal travels through the medula, the pons and the midbrain.

At this point the signal is somehow wired to my somatosensory cortex; the part of the brain that ultimately processes the signal and interprets it as "pain".

Have I identified all the key players here? What connects the midbrain/brainstem to the somatosensory cortex?


The key player you are lacking is the thalamus, which integrates a lot of stimuli, and pre-processes them before sending them to the cortex


Anatomy of nervous system's sensory pathways - Biology

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Anatomy and Neural Pathways Modulating Distinct Locomotor Behaviors in Drosophila Larva

The control of movements is a fundamental feature shared by all animals. At the most basic level, simple movements are generated by coordinated neural activity and muscle contraction patterns that are controlled by the central nervous system. How behavioral responses to various sensory inputs are processed and integrated by the downstream neural network to produce flexible and adaptive behaviors remains an intense area of investigation in many laboratories. Due to recent advances in experimental techniques, many fundamental neural pathways underlying animal movements have now been elucidated. For example, while the role of motor neurons in locomotion has been studied in great detail, the roles of interneurons in animal movements in both basic and noxious environments have only recently been realized. However, the genetic and transmitter identities of many of these interneurons remains unclear. In this review, we provide an overview of the underlying circuitry and neural pathways required by Drosophila larvae to produce successful movements. By improving our understanding of locomotor circuitry in model systems such as Drosophila, we will have a better understanding of how neural circuits in organisms with different bodies and brains lead to distinct locomotion types at the organism level. The understanding of genetic and physiological components of these movements types also provides directions to understand movements in higher organisms.

Keywords: Drosophila larvae brain circuits information processing locomotion neural communication sensory systems.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Schematics of topological organization, myotopic…

Schematics of topological organization, myotopic map and CNS regions involved in locomotion. (…

Interneuronal circuits regulating intrasegmental muscle…

Interneuronal circuits regulating intrasegmental muscle contractions. ( A ) Schematic of Drosophila larva…

Neuronal pathways controlling forward and…

Neuronal pathways controlling forward and backward locomotion. ( A ) Schematic of Drosophila…

Neural circuits are involved in…

Neural circuits are involved in bilateral motor coordination. ( A ) A schematic…

Neural circuit involved in mechanosensory-nociception.…

Neural circuit involved in mechanosensory-nociception. ( A ) The neural circuitry involved in…

The neural circuits involved in…

The neural circuits involved in thermo-nociception-mediated bending and rolling. Neural circuits depicting multiple…

Neural circuit involved in chemotaxis.…

Neural circuit involved in chemotaxis. ( A ) Schematic depicting larval turning towards…

Schematics of neural circuit involved…

Schematics of neural circuit involved in phototaxis behavior. ( A ) Larval schematic…


Chapter Review

Sensation starts with the activation of a sensory ending, such as the thermoreceptor in the skin sensing the temperature of the water. The sensory endings in the skin initiate an electrical signal that travels along the sensory axon within a nerve into the spinal cord, where it synapses with a neuron in the gray matter of the spinal cord. The temperature information represented in that electrical signal is passed to the next neuron by a chemical signal that diffuses across the small gap of the synapse and initiates a new electrical signal in the target cell. That signal travels through the sensory pathway to the brain, passing through the thalamus, where conscious perception of the water temperature is made possible by the cerebral cortex. Following integration of that information with other cognitive processes and sensory information, the brain sends a command back down to the spinal cord to initiate a motor response by controlling a skeletal muscle. The motor pathway is composed of two cells, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the cerebral cortex and synapses on a cell in the gray matter of the spinal cord. The lower motor neuron is that cell in the gray matter of the spinal cord and its axon extends into the periphery where it synapses with a skeletal muscle in a neuromuscular junction.


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About this course

In this anatomy course, part of the Anatomy XSeries, you will be introduced to the central and peripheral nervous systems. You will learn about basic neuroanatomy, sensory pathways, motor pathways and the autonomic nervous system.

The course includes illustrated lecture videos and quizzes to help you expand and test your knowledge of the nervous system.

By the end of this course, you will have a better understanding of how the entire body influences, and is influenced, by the nervous system.


Anatomy of nervous system's sensory pathways - Biology

absolute refractory period: time during an action period when another action potential cannot be generated because the voltage-gated Na + channel is inactivated

action potential: change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal unique to neurons and muscle fibers

activation gate: part of the voltage-gated Na + channel that opens when the membrane voltage reaches threshold

astrocyte: glial cell type of the CNS that provides support for neurons and maintains the blood-brain barrier

autonomic nervous system (ANS): functional division of the nervous system that is responsible for homeostatic reflexes that coordinate control of cardiac and smooth muscle, as well as glandular tissue

axon hillock: tapering of the neuron cell body that gives rise to the axon

axon segment: single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the initial segment, and the last, which is followed by the axon terminal)

axon terminal: end of the axon, where there are usually several branches extending toward the target cell

axon: single process of the neuron that carries an electrical signal (action potential) away from the cell body toward a target cell

axoplasm: cytoplasm of an axon, which is different in composition than the cytoplasm of the neuronal cell body

biogenic amine: class of neurotransmitters that are enzymatically derived from amino acids but no longer contain a carboxyl group

bipolar: shape of a neuron with two processes extending from the neuron cell body—the axon and one dendrite

blood-brain barrier (BBB): physiological barrier between the circulatory system and the central nervous system that establishes a privileged blood supply, restricting the flow of substances into the CNS

brain: the large organ of the central nervous system composed of white and gray matter, contained within the cranium and continuous with the spinal cord

central nervous system (CNS): anatomical division of the nervous system located within the cranial and vertebral cavities, namely the brain and spinal cord

cerebral cortex: outermost layer of gray matter in the brain, where conscious perception takes place

cerebrospinal fluid (CSF): circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood

chemical synapse: connection between two neurons, or between a neuron and its target, where a neurotransmitter diffuses across a very short distance

cholinergic system: neurotransmitter system of acetylcholine, which includes its receptors and the enzyme acetylcholinesterase

choroid plexus: specialized structure containing ependymal cells that line blood capillaries and filter blood to produce CSF in the four ventricles of the brain

continuous conduction: slow propagation of an action potential along an unmyelinated axon owing to voltage-gated Na + channels located along the entire length of the cell membrane

dendrite: one of many branchlike processes that extends from the neuron cell body and functions as a contact for incoming signals (synapses) from other neurons or sensory cells

depolarization: change in a cell membrane potential from rest toward zero

effector protein: enzyme that catalyzes the generation of a new molecule, which acts as the intracellular mediator of the signal that binds to the receptor

electrical synapse: connection between two neurons, or any two electrically active cells, where ions flow directly through channels spanning their adjacent cell membranes

electrochemical exclusion: principle of selectively allowing ions through a channel on the basis of their charge

enteric nervous system (ENS): neural tissue associated with the digestive system that is responsible for nervous control through autonomic connections

ependymal cell: glial cell type in the CNS responsible for producing cerebrospinal fluid

excitable membrane: cell membrane that regulates the movement of ions so that an electrical signal can be generated

excitatory postsynaptic potential (EPSP): graded potential in the postsynaptic membrane that is the result of depolarization and makes an action potential more likely to occur

G protein: guanosine triphosphate (GTP) hydrolase that physically moves from the receptor protein to the effector protein to activate the latter

ganglion: localized collection of neuron cell bodies in the peripheral nervous system

gated: property of a channel that determines how it opens under specific conditions, such as voltage change or physical deformation

generator potential: graded potential from dendrites of a unipolar cell which generates the action potential in the initial segment of that cell’s axon

glial cell: one of the various types of neural tissue cells responsible for maintenance of the tissue, and largely responsible for supporting neurons

graded potential: change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it

gray matter: regions of the nervous system containing cell bodies of neurons with few or no myelinated axons actually may be more pink or tan in color, but called gray in contrast to white matter

inactivation gate: part of a voltage-gated Na + channel that closes when the membrane potential reaches +30 mV

inhibitory postsynaptic potential (IPSP): graded potential in the postsynaptic membrane that is the result of hyperpolarization and makes an action potential less likely to occur

initial segment: first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated

integration: nervous system function that combines sensory perceptions and higher cognitive functions (memories, learning, emotion, etc.) to produce a response

ionotropic receptor: neurotransmitter receptor that acts as an ion channel gate, and opens by the binding of the neurotransmitter

leakage channel: ion channel that opens randomly and is not gated to a specific event, also known as a non-gated channel

ligand-gated channels: another name for an ionotropic receptor for which a neurotransmitter is the ligand

lower motor neuron: second neuron in the motor command pathway that is directly connected to the skeletal muscle

mechanically gated channel: ion channel that opens when a physical event directly affects the structure of the protein

membrane potential: distribution of charge across the cell membrane, based on the charges of ions

metabotropic receptor: neurotransmitter receptor that involves a complex of proteins that cause metabolic changes in a cell

microglia: glial cell type in the CNS that serves as the resident component of the immune system

multipolar: shape of a neuron that has multiple processes—the axon and two or more dendrites

muscarinic receptor: type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor

myelin sheath: lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS facilitates the transmission of electrical signals

myelin: lipid-rich insulating substance surrounding the axons of many neurons, allowing for faster transmission of electrical signals

nerve: cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system

neuron: neural tissue cell that is primarily responsible for generating and propagating electrical signals into, within, and out of the nervous system

neuropeptide: neurotransmitter type that includes protein molecules and shorter chains of amino acids

neurotransmitter: chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell

nicotinic receptor: type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor

node of Ranvier: gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon

nonspecific channel: channel that is not specific to one ion over another, such as a nonspecific cation channel that allows any positively charged ion across the membrane

nucleus: in the nervous system, a localized collection of neuron cell bodies that are functionally related a “center” of neural function

oligodendrocyte: glial cell type in the CNS that provides the myelin insulation for axons in tracts

peripheral nervous system (PNS): anatomical division of the nervous system that is largely outside the cranial and vertebral cavities, namely all parts except the brain and spinal cord

postsynaptic potential (PSP): graded potential in the postsynaptic membrane caused by the binding of neurotransmitter to protein receptors

precentral gyrus of the frontal cortex: region of the cerebral cortex responsible for generating motor commands, where the upper motor neuron cell body is located

process: in cells, an extension of a cell body in the case of neurons, this includes the axon and dendrites

propagation: movement of an action potential along the length of an axon

receptor potential: graded potential in a specialized sensory cell that directly causes the release of neurotransmitter without an intervening action potential

refractory period: time after the initiation of an action potential when another action potential cannot be generated

relative refractory period: time during the refractory period when a new action potential can only be initiated by a stronger stimulus than the current action potential because voltage-gated K + channels are not closed

repolarization: return of the membrane potential to its normally negative voltage at the end of the action potential

resistance: property of an axon that relates to the ability of particles to diffuse through the cytoplasm this is inversely proportional to the fiber diameter

response: nervous system function that causes a target tissue (muscle or gland) to produce an event as a consequence to stimuli

resting membrane potential: the difference in voltage measured across a cell membrane under steady-state conditions, typically -70 mV

Schwann cell: glial cell type in the PNS that provides the myelin insulation for axons in nerves

saltatory conduction: quick propagation of the action potential along a myelinated axon owing to voltage-gated Na + channels being present only at the nodes of Ranvier

satellite cell: glial cell type in the PNS that provides support for neurons in the ganglia

sensation: nervous system function that receives information from the environment and translates it into the electrical signals of nervous tissue

size exclusion: principle of selectively allowing ions through a channel on the basis of their relative size

soma: in neurons, that portion of the cell that contains the nucleus the cell body, as opposed to the cell processes (axons and dendrites)

somatic nervous system (SNS): functional division of the nervous system that is concerned with conscious perception, voluntary movement, and skeletal muscle reflexes

spatial summation: combination of graded potentials across the neuronal cell membrane caused by signals from separate presynaptic elements that add up to initiate an action potential

spinal cord: organ of the central nervous system found within the vertebral cavity and connected with the periphery through spinal nerves mediates reflex behaviors

stimulus: an event in the external or internal environment that registers as activity in a sensory neuron

summate: to add together, as in the cumulative change in postsynaptic potentials toward reaching threshold in the membrane, either across a span of the membrane or over a certain amount of time

synapse: narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell

synaptic cleft: small gap between cells in a chemical synapse where neurotransmitter diffuses from the presynaptic element to the postsynaptic element

synaptic end bulb: swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse

temporal summation: combination of graded potentials at the same location on a neuron resulting in a strong signal from one input

thalamus: region of the central nervous system that acts as a relay for sensory pathways

thermoreceptor: type of sensory receptor capable of transducing temperature stimuli into neural action potentials

threshold: membrane voltage at which an action potential is initiated

tract: bundle of axons in the central nervous system having the same function and point of origin

unipolar: shape of a neuron which has only one process that includes both the axon and dendrite

upper motor neuron: first neuron in the motor command pathway with its cell body in the cerebral cortex that synapses on the lower motor neuron in the spinal cord

ventricle: central cavity within the brain where CSF is produced and circulates

voltage-gated channel: ion channel that opens because of a change in the charge distributed across the membrane where it is located

white matter: regions of the nervous system containing mostly myelinated axons, making the tissue appear white because of the high lipid content of myelin


Nervous System Study Guide

➢ Describe the functions of the nervous system – how information flows in this system The nervous system is hardwired. It is in charge of things that have to happen rapidly. The three basic functions of the nervous system as a whole are communication, control, coordination.

➢ Know the anatomical &amp functional organization of the nervous system Cells = neurons + neuroglia (glia = glue) → Nervous Tissue = Grey matter + white matter in Central Nervous system → Organs = Brain + spinal cord in Central Nervous System also nerves

  • ganglia in Peripheral Nervous System → Systems = Central Nervous System &amp Peripheral Nervous System

Nervous System Hierarchy ↓ Central Nervous System + Peripheral nervous system ↓ Sensory Division + Motor Division ↓ ↓ General + Special Somatic + Autonomic -Touch -Smell -Pain -Taste -Temp -Vision -Stretch -Hearing -Pressure -Equilibrium -Proprioception

➢ Differentiate all pairs of vocabulary words as given on handout Nervous System = Uses electrical impulses through neurons Endocrine System = Uses hormones Central Nervous System = It is so important, it is covered by bone and performs integration Peripheral Nervous System = Carries out commands Sensory = Gathers information and carries signals from outside towards the central nervous system Motor = Carries signals from central nervous system out to the body Afferent = Sensory neurons that carry nerve impulses from stimuli towards the CNS + brain Efferent = Motor neurons that carry impulses away from CNS → muscle for movement Somatic = Voluntary: effector is skeletal muscle tissue Autonomic = Involuntary: effector is smooth muscle tissue, cardiac muscle tissue, and glands Sympathetic = Responds to emergencies Parasympathetic = Maintains homeostasis General Senses = Body wide – touch, pain, temperature, stretch, pressure, proprioception Special Senses = Smell, taste, vision, hearing, equilibrium Grey matter = Made of unmyelinated axons organized into nuclei (clusters of cell bodies) located in the cortex or grey horns of the Central Nervous System White matter = Myelinated axons organized into tracts (bundles of axons)

Horn = Have cell bodies of some kind of motor neuron Column = Nuclei = Grey matter in the CNS Cortex = Outer layer (Grey matter) in the brain Nuclei = Grey matter in CNS Tracts = Bundles of myelinated axons in white matter in the CNS Nuclei = Grey matter in the CNS Ganglia = A structure containing a number of nerve cell bodies, typically linked by synapses Tract = Bundles of myelinated axons in white matter in the CNS Nerve = Bundle of axons Nerve = Bundle of axons Ganglion = Collection of cell bodies Nerve = Bundle of axons Neuron = A specialized cell transmitting nerve impulses a nerve cell. Neuron = A specialized cell transmitting nerve impulses a nerve cell. Neuroglia = Glue which function to support neurons Cranial Nerves = Either sensory or motor or mixed neurons Spinal Nerves = All have both sensory and motor neurons Axon = Take information away from the cell body and splits to become the neuromuscular junction Dendrite (Nerve Fibers) = Take information towards cell body Reflex = Sensory division of the PNS (the receptors) take info which is carried by sensory neurons in a nerve to the CNS and integration is performed by the interneurons and then info is carried by motor neurons in a nerve to the effectors which are the motor division of the PNS. Reflex Arc = The entire process from RECEPTOR to EFFECTOR depending on whether or not the reflex will be positive or negative Withdrawal Reflex Arc = pathway of information for performing a reflex Autonomic Reflex Arc = Neurons talk to neurons talk to neurons until they talk to an effector Synapse = Point of communication between synaptic knob and some other cell (could be another neuron or effector) General Sensory neurons are always unipolar

➢ Describe the components of gray matter vs. white matter Gray matter is located in the central nervous system. Specifically, it is located in the cerebral cortex of the brain and the posterior, lateral, and anterior grey horns of the spinal cord. Gray matter is made of primarily of unmyelinated axons, cell bodies, and dendrites.

White matter is located in the central nervous system. Specifically, it is located in the corona radiata of the brain and in the posterior, lateral, and anterior funiculi of the spinal cord. White matter is also organized into tracts (bundles of axons) and is bundles of myelinated axons

➢ Classify neurons by structure and function know location of each part (Cell body, dendrites, axon Cell body is the main part of the neuron. Dendrites come off of the cell body. Axons take the information away from the cell body towards other neurons, muscles, or glands. There are three main types of axons sensory neuron, interneuron, motor neuron. There are three structures that correspond with the types of axons unipolar (only one process emerging from the cell), bipolar (have two processes which extend from each end of the cell body, and multipolar (have one axon or two or more dendrites).

➢ Compare the different ways that axons may be found: naked, with neurolemma, myelinated, etc. There are 4 types of possibilities axons can have: 1. Myelinated axons in the CNS (by oligodendrocytes) 2. Myelinated axons in the PNS (by Schwann cells) 3. Unmyelinated axons in the CNS (NAKED) 4. Unmyelinated axons in the CNS (a little protection from Schwann cells but mainly protected by neurolemma)

➢ What structure is essential for regeneration of peripheral nervous tissue Schwann Cells secrete nerve growth factor

PART III – SPINAL CORD AND SPINAL NERVES

➢ Describe the anatomical and cross-sectional structures of the spinal cord and structures that protect it The spinal cord is made up of two types of nervous tissue grey matter and white matter. The grey matter is made up of unmyelinated axons, a bunch of cell bodies, and dendrites. The white matter is made up of myelinated axons which are organized into bundles of axons called tracts. In the spinal cord the white matter tracts are called the posterior, anterior, and lateral funiculus. Each individual funiculus (white matter) has different types of neurons posterior has general sensory neurons ONLY, anterior has general sensory and somatic motor neurons, lateral has a ton of general sensory and somatic motor but also autonomic motor tracts. Grey matter in the spinal cord is made up of horns posterior horns have cell bodies of interneurons, anterior horns have cell bodies of somatic motor neurons, and lateral horns have cell bodies of preganglionic autonomic motor neurons.

Aside from the tissue that makes up the spinal cord itself, it also consists of several components exteriorly. Specifically, it consists of meninges which are CT coverings which also cover the brain. The meninges consist of epidural space, dura mater, subdural space, arachnoid layer, subarachnoid space, pia mater. There are also denticulate ligaments that are extensions of the pia that attach to the dura.

➢ Meningeal Layers in order, associated spaces, what fills each space Starts with the epidural space which is around the spinal cord only, next is the dura mater which is a white fibrous CT layer which is continuous with epineurium, then subdural space, then arachnoid layer, then subarachnoid space which is filled with CSF, finally pia mater which is stuck onto the spinal cord

➢ Describe the composition (hierarchical structure) and CT coverings of a nerve how are they named Nerve composition – Axon (myelin or not) → Endoneurium (layer of areolar CT) → Fascicles → Perineurium (dense irregular CT) → Nerve (organ) → Epineurium (continuous with dura mater WFCT)

➢ Describe the relationship between vertebrae, spinal nerves, spinal segments, dermat omes and myotomes Spinal nerves are all mixed and there is no such thing as a purely sensory or purely motor spinal nerve! Spinal nerves always exit at the intervertebral foramen. 31 pairs of spinal nerves names for their spinal cord segments 8 cervical spinal nerves, 12 thoracic spinal nerves, 5 lumbar spinal nerves, 5 sacral spinal nerves, 1 coccygeal spinal nerve

Each set of spinal nerves are in between vertebrae. The first spinal nerve emerges between the skull and the atlas. Each spinal nerve innervates one derma tome (skin region sensory) and 1 myotome (motor, muscle group).

➢ Differentiate: dorsal and ventral rootlets, roots, and rami Dorsal ramus is small and is in charge of everything posterior to the intervertebral formina. The ventral ramus is large and it is in charge of everything anterior (it branches).

Flow of info – Dorsal root → spinal cord → dorsal ramus, ventral ramus, spinal nerve

➢ Define the term plexus and name the nerve plexuses associated with given named nerves Plexus is a network of nerves. Cervical plexus → phrenic nerve Brachial plexus → radian, median, ulnar nerves Lumbar plexus → Femoral nerve Sacral plexus → tibial, common fibular, and sciatic nerve

➢ List the anatomical structures that contribute to speed of nerve impulse conduction Myelin sheaths (Schwann cells) + nodes of Ranvier

➢ Compare and contrast the sympathetic and parasympathetic divisions of the autonomic nervous system (both structure and function) CONTRAST SYMPATHETIC PARASYMPATHETIC

Function Respond to an emergency (always on excitatory or inhibitory)

Maintain homeostasis (conserve energy) always on excitatory or inhibitory Location of preganglionic neuron cell body

Thoracolumbar T1 → L Lateral Grey horn

Craniosacral -CNN (III,VII, IX, X) -Lateral grey horn (S2 → S4) Neuron Length Preganglionic neurons are short

Preganglionic neurons are long Ganglia location + names Closer to spinal cord sympathetic trunk ganglia (paired) collateral ganglia

In or near the wall of effector terminal ganglia

Output Divergent! (ex. 1 preganglionic: 30 postganglionic)

Focused (ex. 1 preganglionic: 4 postganglionic)

Neurotransmitters Preganglionic → postganglionic = Acetylcholine

Postganglionic → effector = norepinephrine

Preganglionic → postganglionic = Acetylcholine

Postganglionic → effector = Acetylcholine

➢ Explain the relationship of the hypothalamus to the Autonomic Nervous System The hypothalamus is apart of the Diencephalon. It is a collection of 12 paired nuclei. It is made up of mammillary bodies which act as the sensory relay station for smell (very anterior). They also link neurons and the endocrine systems so they can both be in charge at the same time of controlling the pituitary gland (master endocrine gland) which secretes hormones that control other endocrine glands. The hypothalamus controls the ENTIRE autonomic nervous system!! It controls the pineal gland and contains homeostatic control centers for temperature, hunger, thirst, and satiety. On top of that, it is the center of the limbic system [emotional brain] which has to do with rage, pleasure, and sex drive.

➢ Explain the relationship of the adrenal medulla to the sympathetic response Cannot find information about this in notes.

➢ Describe how the wiring of the sympathetic division facilitates rapid, mass activation of the effectors Sympathetic wiring is very complex. It has to do with connection for pupil dilation (to let more light in), arrector pili muscle, and eccrine glands, smooth muscle tissue in blood vessel walls, airway dilation, heart rate, beatforce increase, adrenal gland = medulla releases epinephrine (adrenaline = norepinephrine), ejaculation.

➢ Differentiate: sensory and autonomic ganglia, sympathetic and parasympathetic ganglia

CONTRAST SENSORY [DORSAL

ROOT] GANGLION

AUTONOMIC GANGLION

Information General sensory autonomic motor

Shape of cell body Unipolar Multipolar

Synapses No Yes [preganglionic → postganglionic]

Linkage between ganglia No Some are, sympathetic trunk ganglia/chain ganglia

➢ For each of the major parts of the brain know: o 2 ̇ degree embryonic vesicle, associated ventricle, location, and basic gross anatomy/functions a.) Dorsal hollow nerve cord b.) Anterior end enlarges and forces 1 ̊ embryonic vesicles to split c.) Further divide to form 2 ̊ embryonic vesicles → 5 pair

➢ List the central nervous system structures that grows like a rams horn during expansion of the telencephalon Lateral ventricles

➢ Explain the formation and circulation of cerebrospinal fluid (through ventricles, subarachnoid space, venous circulation) CSF is produced at the choroid plexus (network of capillaries) then goes to the lateral

ventricles → Third ventricle → Fourth ventricle → interventricular foramen → third ventricle

→ cerebral aqueduct → 4th ventricle → R/L lateral aperture, central canal, or median aperture

→ subarachnoid space → through arachnoid villi → dural venous sinuses (joins O2 poor

blood) → superior sagittal sinus → inferior sagittal sinus → straight sinus → occipital sinus →

confluence of sinuses → L/R Transverse sinus → Sigmoid sinus → Jugular foramen → internal

jugular veins → R/L brachiocephalic veins → superior vena cava → right atrium

Sensory and motor pathways ➢ Explain the clinical importance of knowing the major pathways of the nervous system No explanation of this given

➢ Name some common somatic (cutaneous and proprioceptive) receptors Motor pathways always begin with the cortex or nucleus. The most common one we need to know is the pyramidal or corticospinal pathway. Information in the pathway is traveling down from the brain which means that it is conscious control of skeletal muscle. It starts with pyramidal cell in precentral gyrus. The pyramidal cell axons create pyramids (inferior to pons in the medulla oblongata)

The other one is the extrapyramidal pathway. Its information is subconscious control of skeletal muscle tissue and originates in the basal nuclei, and midbrain.

➢ Compare the location and function of the major sensory and motor pathways: o Dorsal funiculus, spinothalamic, spinocerebellar pyramidal/corticospinal &amp extrapyramidal Sensory pathways always begin with a sensory receptor. Motor pathways always being with cortex or nuclei

➢ Identify name, location and organization of the 1 ̊ somatosensory cortex and the 1 ̊ motor cortex ??


Ascending Tracts

Ascending tracts carry sensory signals up the spinal cord. Sensory signals typically travel across three neurons from their origin in the receptors to their destination in the brain: a first-order neuron that detects a stimulus and transmits a signal to the spinal cord or brainstem a second-order neuron that continues as far as a “gateway” called the thalamus at the upper end of the brainstem and a third-order neuron that carries the signal the rest of the way to the cerebral cortex. The axons of these neurons are called the first- through third-order nerve fibers.

Figure 3. Locations of major sensory system shown in cross section of the spinal cord

Figure 4. Spinal cord ascending tracts to the brain

The major ascending tracts are as follows. The names of most of them consist of the prefix spino- followed by a root denoting the destination of its fibers in the brain, although this naming system does not apply to the first two.

Gracile fasciculus

The gracile fasciculus carries signals from the midthoracic and lower parts of the body. Below vertebra T6, it composes the entire posterior column. At T6, it is joined by the cuneate fasciculus, discussed next. It consists of first-order nerve fibers that travel up the ipsilateral side of the spinal cord and terminate at the gracile nucleus in the medulla oblongata of the brainstem. These fibers carry signals for vibration, visceral pain, deep and discriminative touch (touch whose location one can precisely identify), and especially proprioception from the lower limbs and lower trunk. Proprioception is the nonvisual sense of the position and movements of the body.

Cuneate fasciculus

The cuneate fasciculus joins the gracile fasciculus at the T6 level. It occupies the lateral portion of the posterior column and forces the gracile fasciculus medially. It carries the same type of sensory signals, originating from T6 and up (from the upper limbs and chest). Its fibers end in the cuneate nucleus on the ipsilateral side of the medulla oblongata. In the medulla, second-order fibers of the gracile and cuneate systems decussate and form the medial lemniscus, a tract of nerve fibers that leads the rest of the way up the brainstem to the thalamus. Third-order fibers go from the thalamus to the cerebral cortex. Because of decussation, the signals carried by the gracile and cuneate fasciculi ultimately go to the contralateral cerebral hemisphere.

Spinothalamic tract

The spinothalamic tract and some smaller tracts form the anterolateral system, which passes up the anterior and lateral columns of the spinal cord. The spinothalamic tract carries signals for pain, temperature, pressure, tickle, itch, and light or crude touch. Light touch is the sensation produced by stroking hairless skin with a feather or cotton wisp, without indenting the skin crude touch is touch whose location one can only vaguely identify.

In this pathway, first-order neurons end in the posterior horn of the spinal cord near the point of entry. Here they synapse with second-order neurons, which decussate and form the contralateral ascending spinothalamic tract. These fibers lead all the way to the thalamus. Third-order neurons continue from there to the cerebral cortex. Because of decussation, sensory signals in this tract arrive in the cerebral hemisphere contralateral to their point of origin.

Spinoreticular tract

The spinoreticular tract also travels up the anterolateral system. It carries pain signals resulting from tissue injury. The first-order sensory neurons enter the posterior horn and immediately synapse with second-order neurons. These decussate to the opposite anterolateral system, ascend the cord, and end in a loosely organized core of gray matter called the reticular formation in the medulla and pons. Third-order neurons continue from the pons to the thalamus, and fourth-order neurons complete the path from there to the cerebral cortex.

Posterior and anterior spinocerebellar tracts

The posterior and anterior spinocerebellar tracts travel through the lateral column and carry proprioceptive signals from the limbs and trunk to the cerebellum at the rear of the brain. Their first-order neurons originate in muscles and tendons and end in the posterior horn of the spinal cord. Second-order neurons send their fibers up the spinocerebellar tracts and end in the cerebellum.

Fibers of the posterior tract travel up the ipsilateral side of the spinal cord. Those of the anterior tract cross over and travel up the contralateral side but then cross back in the brainstem to enter the ipsilateral side of the cerebellum. Both tracts provide the cerebellum with feedback needed to coordinate muscle action.

Figure 5. Rami of the spinal nerve

Figure 6. Spinal nerve fiber anatomy


Sensory Perception

A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.

Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.

Sensory Receptors

Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.

Structural Receptor Types

The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending , with dendrites embedded in tissue that would receive a sensation (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity or (3) a specialized receptor cell , which has distinct structural components that interpret a specific type of stimulus ((Figure)). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor .

Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move.

Functional Receptor Types

A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. Some stimuli are physical variations in the environment that affect receptor cell membrane potentials. Other stimuli include the electromagnetic radiation from visible light. For humans, the only electromagnetic energy that is perceived by our eyes is visible light. Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.

Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor . Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor . Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.

Sensory Modalities

Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.

Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense , which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.

Each of the senses is referred to as a sensory modality . Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors.

Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities , of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation , can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.

Gustation (Taste)

Only a few recognized submodalities exist within the sense of taste, or gustation . Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Research at the turn of the 20th century led to recognition of the fifth taste, umami, during the mid-1980s. Umami is a Japanese word that means “delicious taste,” and is often translated to mean savory. Very recent research has suggested that there may also be a sixth taste for fats, or lipids.

Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance ((Figure)): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.

Salty taste is simply the perception of sodium ions (Na + ) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na + and Cl – , which dissolve into the saliva in your mouth. The Na + concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na + into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.

Sour taste is the perception of H + concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked.

The first two tastes (salty and sour) are triggered by the cations Na + and H + . The other tastes result from food molecules binding to a G protein–coupled receptor. A G protein signal transduction system ultimately leads to depolarization of the gustatory cell. The sweet taste is the sensitivity of gustatory cells to the presence of glucose dissolved in the saliva. Other monosaccharides such as fructose, or artificial sweeteners such as aspartame (NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet receptors. The affinity for each of these molecules varies, and some will taste sweeter than glucose because they bind to the G protein–coupled receptor differently.

Bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. However, there are a number of different ways in which this can happen because there are a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some bitter molecules increase G protein activation within the gustatory cells, whereas other bitter molecules decrease G protein activation. The specific response depends on which molecule is binding to the receptor.

One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less attractive to herbivores.

Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food.

The taste known as umami is often referred to as the savory taste. Like sweet and bitter, it is based on the activation of G protein–coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory.

Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.

Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two groups known as “tasters” and “non-tasters” based on the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family?

Olfaction (Smell)

Like taste, the sense of smell, or olfaction , is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity ((Figure)). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded membrane potential in the olfactory neurons.

The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.

The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.

Olfactory System: Anosmia Blunt force trauma to the face, such as that common in many car accidents, can lead to the loss of the olfactory nerve, and subsequently, loss of the sense of smell. This condition is known as anosmia . When the frontal lobe of the brain moves relative to the ethmoid bone, the olfactory tract axons may be sheared apart. Professional fighters often experience anosmia because of repeated trauma to face and head. In addition, certain pharmaceuticals, such as antibiotics, can cause anosmia by killing all the olfactory neurons at once. If no axons are in place within the olfactory nerve, then the axons from newly formed olfactory neurons have no guide to lead them to their connections within the olfactory bulb. There are temporary causes of anosmia, as well, such as those caused by inflammatory responses related to respiratory infections or allergies.

Loss of the sense of smell can result in food tasting bland. A person with an impaired sense of smell may require additional spice and seasoning levels for food to be tasted. Anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair.

The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. This explains why some elderly people salt their food more than younger people do. However, this increased sodium intake can increase blood volume and blood pressure, increasing the risk of cardiovascular diseases in the elderly.

Audition (Hearing)

Hearing, or audition , is the transduction of sound waves into a neural signal that is made possible by the structures of the ear ((Figure)). The large, fleshy structure on the lateral aspect of the head is known as the auricle . Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane , or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear . The middle ear consists of a space spanned by three small bones called the ossicles . The three ossicles are the malleus , incus , and stapes , which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear , where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.

The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule , which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia . These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window .

The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli . The scala vestibuli extends from the oval window, travelling above the cochlear duct , which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani , returns to the base of the cochlea, this time travelling under the cochlear duct. The scala tympani ends at the round window , which is covered by a membrane that contains the fluid within the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves ((Figure)). The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani.

A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct ((Figure)). The cochlear duct contains several organs of Corti , which tranduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane , which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.

The organs of Corti contain hair cells , which are named for the hair-like stereocilia extending from the cell’s apical surfaces ((Figure)). The stereocilia are an array of microvilli-like structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the hair cells to the overlying tectorial membrane , which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. When no sound is present, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized.

View the University of Michigan WebScope to explore the tissue sample in greater detail. The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea?

As stated above, a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. The unit of Hertz measures the frequency of sound waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows ((Figure)). Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave). The hair cells along the length of the cochlear duct, which are each sensitive to a particular frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible light into its component colors.

Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?

Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct?

Equilibrium (Balance)

Along with audition, the inner ear is responsible for encoding information about equilibrium , the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibule of the inner ear. Head position is sensed by the utricle and saccule , whereas head movement is sensed by the semicircular canals . The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.

The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane ((Figure)). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.

The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane ((Figure)). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla . The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula , a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.

Somatosensation (Touch)

Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.

Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves capsaicin , the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C. The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.

If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low frequency vibrations are sensed by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure and vibration is transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors.

Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent over-contraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in (Figure).

*No corresponding eponymous name.
Mechanoreceptors of Somatosensation
Name Historical (eponymous) name Location(s) Stimuli
Free nerve endings * Dermis, cornea, tongue, joint capsules, visceral organs Pain, temperature, mechanical deformation
Mechanoreceptors Merkel’s discs Epidermal–dermal junction, mucosal membranes Low frequency vibration (5–15 Hz)
Bulbous corpuscle Ruffini’s corpuscle Dermis, joint capsules Stretch
Tactile corpuscle Meissner’s corpuscle Papillary dermis, especially in the fingertips and lips Light touch, vibrations below 50 Hz
Lamellated corpuscle Pacinian corpuscle Deep dermis, subcutaneous tissue Deep pressure, high-frequency vibration (around 250 Hz)
Hair follicle plexus * Wrapped around hair follicles in the dermis Movement of hair
Muscle spindle * In line with skeletal muscle fibers Muscle contraction and stretch
Tendon stretch organ Golgi tendon organ In line with tendons Stretch of tendons

Vision

Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye ((Figure)). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva . The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland , located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball ((Figure)). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus , medial rectus , inferior rectus , and lateral rectus . When each of these muscles contract, the eye to moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea . The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris , which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see (Figure)).

The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.

The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic , which includes the white sclera and clear cornea . The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible ((Figure)). The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic , which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body , a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibers . These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris —the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil , which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic , or retina , which contains the nervous tissue responsible for photoreception.

The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor . The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor .

The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer . It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer . There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve (see (Figure)). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.

Note that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea . At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity , or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see (Figure)). As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea.

Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment ((Figure)). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin . The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins , which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.

At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon , which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiation longer than 720 nm fall into the infrared range, whereas wavelengths shorter than 380 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.

Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal . Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization . Before interacting with a photon, retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans– conformation, forming all-trans-retinal, which has a straight hydrocarbon chain ((Figure)).

The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the 11-cis-retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy.

The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue ((Figure)). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.

The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.

Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.

Sensory Nerves

Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.

Spinal Nerves

Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help to underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral , in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.

Cranial Nerves

Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral , meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system.

Chapter Review

The senses are olfaction (smell), gustation (taste), somatosensation (sensations associated with the skin and body), audition (hearing), equilibrium (balance), and vision. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.

The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells. The structural classifications are either based on the anatomy of the cell that is interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cell), or where the cell is located relative to the stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy.

The nerves that convey sensory information from the periphery to the CNS are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain. Spinal nerves have mixed populations of fibers some are motor fibers and some are sensory. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex. The cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are connected to the same side of the brain from which the sensory information originates.

Interactive Link Questions

Watch this video to learn about Dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, PA, who became interested in science at an early age because of her sensory experiences. She recognized that her sense of taste was unique compared with other people she knew. Now, she studies the genetic differences between people and their sensitivities to taste stimuli. In the video, there is a brief image of a person sticking out their tongue, which has been covered with a colored dye. This is how Dr. Reed is able to visualize and count papillae on the surface of the tongue. People fall into two large groups known as “tasters” and “non-tasters” on the basis of the density of papillae on their tongue, which also indicates the number of taste buds. Non-tasters can taste food, but they are not as sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she is a non-taster, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to tastes? Can you see any similarities among the members of your family?

Answers will vary, but a typical answer might be: I can eat most anything (except mushrooms!), so I don’t think that I’m that sensitive to tastes. My whole family likes eating a variety of foods, so it seems that we all have the same level of sensitivity.

(Figure) The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. What is anchored to this membrane so that they can be activated by movement of the fluids within the cochlea?

(Figure) The hair cells are located in the organ of Corti, which is located on the basilar membrane. The stereocilia of those cells would normally be attached to the tectorial membrane (though they are detached in the micrograph because of processing of the tissue).

Watch this video to learn more about how the structures of the ear convert sound waves into a neural signal by moving the “hairs,” or stereocilia, of the cochlear duct. Specific locations along the length of the duct encode specific frequencies, or pitches. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for the amplification and transfer of sound from the external ear to the inner ear?

The small bones in the middle ear, the ossicles, amplify and transfer sound between the tympanic membrane of the external ear and the oval window of the inner ear.

Watch this animation to learn more about the inner ear and to see the cochlea unroll, with the base at the back of the image and the apex at the front. Specific wavelengths of sound cause specific regions of the basilar membrane to vibrate, much like the keys of a piano produce sound at different frequencies. Based on the animation, where do frequencies—from high to low pitches—cause activity in the hair cells within the cochlear duct?

High frequencies activate hair cells toward the base of the cochlea, and low frequencies activate hair cells toward the apex of the cochlea.

Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals.” What aspect of retinal processing is simplified by that statement? Explain your answer.

Photoreceptors convert light energy, or photons, into an electrochemical signal. The retina contains bipolar cells and the RGCs that finally convert it into action potentials that are sent from the retina to the CNS. It is important to recognize when popular media and online sources oversimplify complex physiological processes so that misunderstandings are not generated. This video was created by a medical device manufacturer who might be trying to highlight other aspects of the visual system than retinal processing. The statement they make is not incorrect, it just bundles together several steps, which makes it sound like RGCs are the transducers, rather than photoreceptors.


Pathways

Ascending Pathways

Sensory information enters the spinal cord on the same side of the body as the stimulus. Ascending tracts cross over the midline of the body to the contralateral side of the thalamus. The thalamus directs the signal to the cerebral cortex for conscious perception. The pathway is direct with very few neurones involved.

1. Dorsal Columns

Dorsal columns transmit information from touch and kinaesthesia these are both classified as low threshold information. There are two major dorsal columns the gracile fasiculus situated medially which conveys information from the hindlimbs and caudal trunk and the cuneate fasciculus which is situated more laterally and conveys information from the forelimbs and cranial trunk.

2. Spinothalamic Tracts

Spinothalamic tracts transmit information from temperature and "pin prick" pain these senses are classified as fast, initial pain sensations. These tracts compare with the ascending reticular formation.

3. Spinocervicothalamic Tracts

The spinocervicothalamic tracts transmit information from touch and kinaesthesia, although these are absent in man.

Spinocerebellar Tracts

These tracts transmit information from proprioception receptors, including information from muscle receptors, joint receptors and golgi tendon organs. Most sensory information enters the spinal cord on the ipsilateral side to the stimulus but some do cross to the contralateral side of the body. Contralateral signals pass back to the ipsilateral side of the body in the brain. Information is processed in the cerebellum and is therefore processed unconsciously.

Dorsal Spinocerebellar Tract

The dorsal spinocerebellar tract relays muscle spindle and golgi tendon organ information from the hindlimbs to the cerebellum.

Cuneo-cerebellar Tract

The cuneo-cerebellar tract serves the same purpose for the forelimbs as the dorsal spinocerebellar tract does for the hind limbs, but is much smaller.

Ventral Spinocerebellar Tract

The ventral spinocerebellar tract is similar to the dorsal spinocerebellar tract but it takes a less direct route to the cerebellum. The forelimb equivalent is called the rostral spinocerebellar tract.

Ascending Reticular Formation (Spinoreticular Tract)

The ascending reticular formation is thought of as the true pain sensation as the pain lasts longer. Sensory information enters the spinal cord on the ipsilateral side of the stimulus. Some signals cross to the contralateral side of the body. The tract consists of several short neurones. Therefore the ascending reticular formation is bilateral and multineuronal, although this pain pathway is thought to be more primitive than the spinothalmic tract. In humans, the ascending reticular formation is superceded by the spinothalamic tract. In animals, the ascending reticular formation is the main pathway for pain to reach the cerebral cortex.

Pain is not a sensory modality. The sensory modality that is checked in a neurologic exam is Nociception which is the patients response to a noxious stimuli. Pain is a subjective cerebral response. Noxious stimuli can result in responses ranging from itches, to nausea, to simply being in agony. Noxious stimuli may be transmitted to the brain by one of two pathways: the Spinothalamic Tract or the Ascending Reticular Formation.

Spinothalamic Tract

This fast, initial pinprick is detected by free nerve endings and causes an impulse along large, myelinated fibres. The pain sensation is localised, and ends quickly.

Ascending Reticular Formation

This pain sensation is detected by free nerve endings which causes an impulse along small, unmyelinated fibres. This results in a delayed perception of the sensation of pain, and that pain is often less localised but more persistent.

Clinical Relevance During trauma the small unmyelinated fibres are the last fibres to fail as they are close to the spinal cord. If deep pain sensation is lost in a case of trauma, then the prognosis is poor.
Hyperalgesia is an increased pain sensation. This occurs when tissue is damaged because chemicals are released which increase the sensitivity of nociceptors, so that even light pressure can cause pain. Hyperalgesia may have evolved to aid the healing of injuries.


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