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Why are Merkel cells innervated by an axon, and not a dendrite?

Why are Merkel cells innervated by an axon, and not a dendrite?


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Here are two images from Google. Afferent neurons receive input and send it to the central nervous system. Input is received by the neuron's dendritic end and send away centrally via axon terminals that are sheathed with Schwann cells. Axon terminals synapse to motor neurons (muscles and glands).

How is it possible that Merkel cells, which supposed to be a sensory cell, synapse with an axon, and not with a dendritic end?


There is nothing out of the ordinary with the Merkel cell, as it functions in the same way as photoreceptors and hair cells (see Further Reading #1).


Fig. 1. Merkel cell. Source: Gallery for Share.

The Merkel cell depicted in Fig. 1 is a specialized receptor neuron. It has no axon. It relies on a secondary sensory neuron (a somatosensory afferent) onto which it synapses to direct the touch signal to the brain.

Note that neural information flow goes from dendrite to axon. An axon terminates into a synapse. The Merkel cell, which does not have an axon, features a synapse directly onto the dendrite of the secondary neuron (See Further Reading #2).

Further Readings
1. Can axons act as receptors?
2. What are the functions and differences between axons and dendrites?
.


Why are Merkel cells innervated by an axon, and not a dendrite? - Biology

Merkel cells are touch-sensitive cells that transduce touch via Piezo2 channels.

The Merkel cell–neurite complex contains two sensory receptor cell types.

Merkel cells and neurons together mediate different aspects of touch responses.

The Merkel cell–neurite complex is a unique vertebrate touch receptor comprising two distinct cell types in the skin. Its presence in touch-sensitive skin areas was recognized more than a century ago, but the functions of each cell type in sensory transduction have been unclear. Three recent studies demonstrate that Merkel cells are mechanosensitive cells that function in touch transduction via Piezo2. One study concludes that Merkel cells, rather than sensory neurons, are principal sites of mechanotransduction, whereas two other studies report that both Merkel cells and neurons encode mechanical inputs. Together, these studies settle a long-standing debate on whether or not Merkel cells are mechanosensory cells, and enable future investigations of how these skin cells communicate with neurons.


INTRODUCTION

Many differentiated cells have highly polarized arrays of microtubules that likely play a large role in establishing their specialized architecture and function. Neurons are strikingly polarized and initially seemed that they would be the clearest example of cells in which microtubule orientation formed the basis of directional transport and cell polarity (Black and Baas, 1989). Most neurons have a cell body in which the bulk of proteins are synthesized, dendrites that are specialized to receive signals, and axons that are specialized to send them. Where examined, microtubules in vertebrate dendrites have mixed orientation, and in axons they have uniform orientation with all plus ends distal to the cell body. Thus, the simplest model for selective transport from the cell body to dendrites is use of a minus end-directed motor. However, current models of transport into dendrites rely on plus end-directed motors (Setou et al., 2004 Hirokawa and Takemura, 2005 Kennedy and Ehlers, 2006 Levy and Holzbaur, 2006). These models raise the question: are minus-end-out microtubules important for directional transport or neuronal polarity?

Axonal microtubule orientation has been examined in a variety of neurons, all with the same result: >95% of plus ends are oriented away from the cell body (plus-end-out). Original studies on axonal microtubule orientation relied on decoration of microtubules with exogenous tubulin, which forms curved hooks on the sides of existing microtubules, and analysis by electron microscopy. The direction of hook curvature indicates microtubule polarity. This method was used to determine axonal microtubule orientation in many different types of vertebrate neurons (Burton and Paige, 1981 Heidemann et al., 1981 Baas et al., 1987, 1988 Troutt and Burnside, 1988). More recently, the direction of movement of proteins that bind to growing microtubule plus ends was used to analyze axonal microtubule orientation in cultured mouse hippocampal and Purkinje neurons (Stepanova et al., 2003) and cultured Aplysia neurons (Erez et al., 2007). Using both assays, in sensory and central neurons, in organisms ranging from the invertebrate Aplysia to mammals, >95% of axonal microtubules have been found to be plus-end-out. Additionally, second harmonic generation microscopy has confirmed axonal microtubules in vivo and in vitro have uniform microtubule orientation (Dombeck et al., 2003). Uniform plus-end-out microtubule orientation thus seems to be a universal and evolutionarily conserved signature of axons.

Similarly, mixed orientation of microtubules has been considered a signature of dendrites (Alberts et al., 2002). However, dendrites are generally much more difficult to study, and their microtubule organization has been examined much less than that of axons. The hook method has been used to analyze dendritic microtubule orientation in one type of neuron with branched dendrites in vivo: frog mitral cells, which are interneurons. In these dendrites, approximately equal numbers of microtubules had plus and minus ends distal to the cell body throughout the length of the dendrite (Burton, 1988). Both hook labeling and microtubule plus end-binding protein dynamics have been used to analyze microtubule orientation in dendrites in cultured rodent interneurons. In proximal dendrites, both methods showed mixed microtubule orientation, with roughly equal numbers pointing in each direction. Close to dendrite growth cones, most microtubules had plus ends out (Baas et al., 1988 Stepanova et al., 2003). Thus, the prevailing model of microtubule orientation in vertebrate neurons is mixed in proximal dendrites, and plus-end-out in distal dendrites (Figure 1).

Figure 1. Known microtubule orientation in vertebrate and Drosophila da neurons, and possible scenarios for the arrangement of microtubules in fly dendrites. (A) In frog mitral cells and cultured rodent interneurons, microtubules in dendrites have mixed orientation, whereas in Drosophila da neurons ∼95% have minus ends distal to the cell body based on EB1-GFP dynamics. In all neurons examined, plus-end-out microtubules predominate in axons. (B) The failure to find a significant population of plus-end-out microtubules in da neuron dendrites can be accounted for by several explanations. 1) The arrangement of microtubules in sensory da dendrites could be different from the arrangement in interneuron dendrites. 2) Analysis of microtubule orientation by EB1-GFP dynamics could have missed a significant population of stable plus-end-out microtubules. 3) Minus-end-out microtubules could predominate in all Drosophila neurons.

Dendrites contain organelles and proteins, including rough endoplasmic reticulum, the Golgi complex, and neurotransmitter receptors, that are rare in axons (Craig and Banker, 1994). One simple way to generate this asymmetry would be to selectively transport proteins and organelles into dendrites with a minus end-directed motor, because minus-end-out microtubules are only present in dendrites (Black and Baas, 1989). Selective transport of several types of vesicles from the cell body into dendrites has been observed previously (Burack et al., 2000 Rosales et al., 2005). Dendrites also lose dendritic shape and organelles when minus-end-out microtubules are reduced (Yu et al., 2000). However, the role of minus-end-out microtubules in selective transport into dendrites has been called into question for several reasons. First, most organelles can move bidirectionally and so seem to have both plus- and minus-end–directed motors associated with them (Welte, 2004). Second, cultured neurons have a region at their tip in which minus-end-out microtubules are rare (Baas et al., 1988). Third, plus-end–directed kinesins have been found in association with neurotransmitter receptors (Setou et al., 2000, 2002). Thus, current models of directional transport into dendrites rely on specific targeting of kinesins or kinesin-cargo pairs to plus-end-out dendritic microtubules (Setou et al., 2004 Hirokawa and Takemura, 2005 Kennedy and Ehlers, 2006 Levy and Holzbaur, 2006), and the role of minus-end-out microtubules in polarized transport and neuronal polarity is not clear.

However, recent analysis of microtubule orientation in branched Drosophila sensory neurons raised the possibility that minus-end-out microtubules may actually be the most important component of the dendritic microtubule cytoskeleton. Using EB1-green fluorescent protein (GFP) dynamics to infer microtubule orientation, >95% of microtubules in dendritic arborization (da) dendrites were found to have minus ends distal to the cell body (Rolls et al., 2007). Several possible explanations exist for the difference between these results and those in vertebrate neurons: 1) dendrites of sensory neurons (examined in flies) have different arrangements of microtubules than interneurons (examined in vertebrates) 2) EB1-GFP dynamics revealed the orientation of a special subset of microtubules, and stable plus-end-out microtubules are present in Drosophila dendrites or 3) all Drosophila dendrites have mostly minus-end-out microtubules (Figure 1).

To distinguish between these possibilities, we used EB1-GFP dynamics to generate a complete map of microtubule orientation in all major classes of Drosophila neurons: sensory neurons, interneurons, and motor neurons. We found that minus-end-out microtubules predominate in dendrites from all three types of neurons, and we propose that minus-end-out microtubules are a conserved signature of dendrites. Our map of microtubule orientation makes very specific predictions about the layout of microtubule tracks at dendrite branch points: that microtubules run between the cell body and dendrites, but not from one dendrite to the other. To test the completeness of our map, we analyzed the layout of stable microtubules and the paths taken by endosomes at dendrite branch points. Results from both methods agreed with predictions of the map based on EB1-GFP dynamics, and they were inconsistent with a set of stable plus-end-out microtubules in dendrites.

An even more striking test of our microtubule map was offered by unipolar neurons. Our microtubule map predicts that there are no continuous tracks for cargo transport between the cell body and dendrites of unipolar neurons. We confirmed this prediction by tracking endosomes in unipolar neurons. The absence of a direct route between the cell body and dendrites makes sense only in the context of our microtubule map.


What is Efferent?

Efferent neurons (also known as motor neurons) can be found inside the central nervous system (in the grey matter of spinal cord and medulla oblongata), and they are responsible for receiving information from the central nervous system and transmitting nerve impulse to the periphery of the body such as muscles, glands etc.

Figure 02: Efferent Neuron

The cell body of the motor neuron has a satellite shape. Also, it has a long axon and several shorter dendrites. Moreover, the axon forms a neuromuscular junction with the effectors. Hence, the impulse enters through dendrites and leaves it through the single axon to the other end.


Neural Control and Coordination MCQ

  • a. Hypothalamus
  • b. Inner layer of the cerebrum
  • c. Medulla oblongata
  • d. Amygdalin and hippocampus

The part of brain that connects with spinal cord is:

  • a. Cerebrum
  • b. Midbrain
  • c. Cerebellum
  • d. None

The part of brain related with processing of knowledge and thinking is:

  • a. Cerebrum
  • b. Midbrain
  • c. Cerebellum
  • d. None

Which part of the brain control rate of respiration and cardiovascular rexflex?

  • a. Midbrain
  • b. Pons
  • c. Cerebellum
  • d. Cerebrum

Part of the brain that control memory is:

  • a. Amygdalin
  • b. Hippocampus
  • c. Neo-cortex
  • d. All three

Which part of the brain control the chemical signals in the body?

  • a. Midbrain
  • b. Hypothalamus
  • c. Cerebrum
  • d. Cerebellum

Which part of the brain among these options is not associated with reflex action?

  • a. Spinal cord
  • b. Afferent neuron
  • c. Efferent neuron
  • d. Medulla oblongata

Which of the following is not a sensory receptor?

  • a. Gustatory receptor
  • b. Olfactory bulb
  • c. Pacinian corpuscles
  • d. Dorsal root ganglion

The recptor which detect the colour is

  • a. Rods
  • b. Cones
  • c. Both
  • d. None

How many types of cone cells are there in the eye?

  • a. One
  • b. Two
  • c. Three.
  • d. Four

Place of highest visual acuity is:

  • a. Blindspot
  • b. Macula Lutea
  • c. Fovea centralis
  • d. None

Vitreous humor is present between which two part of the eye?

  • a. Retina and the lens
  • b. Lens and the Retina
  • c. Both
  • d. None

Defficiency of which vitamin cause night blindness?

  • a. A
  • b. B
  • c. E
  • d. K

Failure of iris muscle to adjust the lens lead to

  • a. Short-sightedness
  • b. Farsightedness
  • c. Both
  • d. None

Corpora quadrigemina of the brain process?

  • a. Auditory signals
  • b. Visual signals
  • c. Smell
  • d. None

Smallest bone of the body is:

  • a. Malleus
  • b. incus
  • c. Stapes
  • d. Radius

Main function of inner ear is to:

  • a. Hear
  • b. Balance
  • c. Chemical detection
  • d. None

Which receptor is sensitive to auditory signals?

  • a. Reissner’s membrane
  • b. Organ of Corti
  • c. Crista and macula
  • d. None

Which gland is innervated with nerve signals from hypothalamus?

  • a. Pituitary
  • b. Thyroid
  • c. Parathyroid
  • d. Pancreas

Which among these is not a ear bone (ossicle)?

  • a. Stapes
  • b. Incus
  • c. Malleus
  • d. Otolith

Which among these is not a part of vestibular apparatus

  • a. Semicircular canals
  • b. Macula
  • c. Cochlea
  • d. b and c

Neural Control and Coordination MCQ/Objective Question Chapter 21 Biology


Discussion

Our results provided a concrete model for the development of the PVD dendrites. While the 3° branches are instructed by the sublateral stripes of SAX-7, the 4° branches were precisely guided by circumferential stripes of SAX-7 on the hypodermal membrane underneath the muscle cells. The precise co-localization between the SAX-7 stripes and the PVD 4° branches and the complete lack of 4° branches in the sax-7, mnr-1 or dma-1 mutants demonstrate that the pre-patterned SAX-7 stripes instructed the development of PVD dendrites. Consistent with our previous findings, the SAX-7 signal is detected by the PVD receptor DMA-1. This recognition also requires co-ligand MNR-1 on the hypodermal cells. Since DMA-1 is specifically expressed in the PVD neurons, but not in many other neurons that extend neurites along the hypodermal cells, only PVD develops exuberant branches.

Here, we provided a satisfying explanation to how the SAX-7 stripes are localized to specific parts of the hypodermal cells with a distinct pattern. Our genetic analysis demonstrates that UNC-52/Perlecan is important for patterning the SAX-7 stripes and the PVD dendrite. In unc-52 mutants, both the SAX-7 stripes and the PVD 4° branches were disorganized. Interestingly, the disorganized PVD 4° branches still followed the disrupted SAX-7 stripes, strongly suggesting that SAX-7 patterns dictate the PVD dendrite morphology. UNC-52 is only detected in the basement membrane associated with muscle cells and it is concentrated beneath the muscle dense bodies (DB) and M-lines under body wall muscles (Francis and Waterston, 1991 Mullen et al., 1999 Rogalski et al., 1993). This explains why the PVD 4° branches only develop underneath the muscle cells but not at more medial locations.

UNC-52 plays essential functions to link the sarcomere structure in the muscle to specific subcellular hemi-desmosomes in the hypodermal cells. At the muscle plasma membrane, UNC-52 binds to the dense bodies and M lines through integrin complexes. At the hypodermal cell membrane, UNC-52 is important for patterning LET-805 and hemi-desmosome, fibrous organelles (FOs) (Cox and Hardin, 2004 Francis and Waterston, 1991). Our genetic epistasis analyses demonstrated a developmental hierarchy. UNC-52 is essential for the subcellular localization of the hemi-desmosome, including the intermediate filament MUA-6. MUA-6 in turn patterns SAX-7, likely through the interaction with the cytosolic domain of SAX-7. SAX-7 uses its extracellular domain, together with MNR-1 to attract PVD dendrite growth and stabilization by binding to DMA-1. This model highlights the importance of cell-cell interactions in the morphogenesis of neurons. In Drosophila class IV dendritic arborization (da) neurons integrin-laminin interaction attaches dendrites to the ECM (Han et al., 2012 Kim et al., 2012). Disruption of such interactions causes the dendrite to be embedded in the epidermis. In another related study, Yasunaga et al., showed that the degradation of basement membrane components by Mmp2 is required to reshape dendrite during development of Drosophila (Yasunaga et al., 2010). Similarly, HSPGs in zebrafish play important roles in guiding the peripheral axons to innervate the skin (Wang et al., 2012), suggestive of an evolutionarily conserved mechanism of peripheral neurite patterning by ECM molecules.

PVD is postulated to function as a proprioceptor, sensing and controlling body posture, based on the abnormal posture caused by ablating PVD (Albeg et al., 2011). Our hypothesis not only demonstrates the mechanism of PVD 4° dendrites development through muscle derived cues, but also illustrates the close association between the sarcomere and the PVD dendrites. In addition, it is worth noting that the hemi-desmosome like fibrous organelle is thought to act as tendon like structures that transmit tension of muscle contraction to cuticle. Thus, this developmental program utilizes the UNC-52/Perlecan and SAX-7 to bundle the tension transmitting cell junctions with a proprioceptive sensory dendrite.

The best-understood mammalian proprioceptive receptor is the muscle spindle. Similar to the hypodermal-muscle-dendrite complex in PVD, the muscle spindle contains the sheath cells, specialized muscle fibers and nerve terminals. The sensory nerve terminals wrap around the muscle cells several rounds with evenly spaced turns. Remarkably, in both worms and mammals, the sensory nerves are oriented perpendicular to longitudinal axis of the sarcomeres, a configuration that might maximize the stretching effects from the muscle onto the nerve (Cheret et al., 2013). Future studies are needed to test if the cellular and molecular interactions that we described here play a similar role in patterning the muscle spindles.


DISCUSSION

Alligatorids have a dense array of sensory receptors (ISOs) extending around the mouth and cranial regions (4200±94 ISOs in A.mississippiensis) whereas crocodylids have ISOs distributed across almost every scale of the body surface (6200±389 ISOs) as well as on the head (2900±134 ISOs in C. niloticus). Since the earliest reports of ISOs (Maurer, 1895 von Wettstein, 1937) and their use in the dichotomous identification of crocodilian skins (King and Brazaitis, 1971), their function has remained a topic of speculation. Although detailed morphological studies undertaken in Caiman receptors (von Düring, 1973 von Düring, 1974 von Düring and Miller, 1979) strongly suggested a mechanosensory role for ISOs, physiological characterization of their function has been limited to the trigeminal receptors of a single species (A. mississippiensis) (Soares, 2002). Anatomical studies of crocodylid post-cranial ISOs from Crocodylus porosus focused on a potential role of the organs as osmoreceptors (Jackson and Brooks, 2007 Jackson et al., 1996). This hypothesis is based in part on models of how ISOs mechanically flatten under osmotic pressure in a saltwater environment and on experiments measuring the mass of water consumed by the estuarine crocodiles. This led to the hypothesis that ISOs are the first identified vertebrate integumentary osmoreceptors (Jackson and Brooks, 2007). Other investigators have proposed that ISOs could function as magnetoreceptors (Rodda, 1984) or electroreceptors (Bullock, 1999).

Electrophysiological recording preparation as used in body recordings. In this case, the single-unit responses from the hindlimb were recorded from the saphenous nerve. Once units were identified mechanically, the receptive field was submerged in hyperosmotic solutions to monitor for activity. No activity related to immersion in the saltwater solution was detected.

Electrophysiological recording preparation as used in body recordings. In this case, the single-unit responses from the hindlimb were recorded from the saphenous nerve. Once units were identified mechanically, the receptive field was submerged in hyperosmotic solutions to monitor for activity. No activity related to immersion in the saltwater solution was detected.

Structure of ISOs

The ISOs appear to share many structural similarities with known mechanoreceptors. These include the push-rod receptor organs distributed across the snouts of monotremes (Andres and von Düring, 1984 Andres et al., 1991) and the Eimer's organs found on the glabrous skin on the rhinarium of moles (Catania, 1995). Numerous ‘Tastflecken’ (‘touch spots’) found on the small warts of bufonid toads and ranid species of frogs have also been identified (Lindblom, 1963 Ogawa et al., 1981). Cutaneous cephalic corpuscles with protruding centers appear in some colubrid snakes (Jackson, 1971 Jackson and Doetsch, 1977). Herbst and Grandry corpuscles comprise the tactile bill tip organs found in ducks (Berkhoudt, 1979 Gottschaldt and Lausmann, 1974). In all these cases, the receptor appears as a smooth, domed structure with an apex suitable for transducing deflection to a series of specialized afferents.

In juvenile crocodilian ISOs, the external, keratinized dome typically had a diameter of 0.5 mm or less for those distributed across the jaws whereas larger ISOs (1.2 mm) were found on crocodylid body scales. Despite this size difference, both populations of ISOs appeared remarkably similar in internal composition. The stratum corneum is thin over the organ (5 μm), presumably allowing a range of motions to compress the structure. This layer of three to five β-keratin cells (Alibardi, 2011) functions both in structural integrity of the ISO and acts as scaffolding for the most apical of the fine nerve terminals. In transverse sections, highly branched melanocytes can be seen throughout the keratinized layers and underlying collagenous layers and impart the distinctive pigmentation seen in most of the ISO bodies. A number of mechanoreceptors are apparent in sectioned ISOs. These mechanoreceptors can be broadly categorized based on their morphology and distribution, as described by von Düring and Miller (von Düring and Miller, 1979). These distinctions are as follows: (1) receptors of the epidermis, (2) receptors of the connective tissue with Schwann cell elaborations or myelination, (3) receptors of the connective tissue lacking Schwann cells and (4) Merkel cell neurite complexes. Among tactile specializations of the first group, crocodilians, as well as reptiles more generally (Landmann and Villiger, 1975 von Düring, 1973), are notable for having expansions of the receptor terminals, compared with the finer, tapered free nerve terminals found in most other vertebrates (Fig. 4B). The dermal Merkel column, similar to the ubiquitous epidermal Merkel neurite complex, traditionally has been interpreted as slowly adapting in other species. These columns were isolated to regions under each ISO whereas similar Merkel cells are found ubiquitously across the epidermal body surface in fishes (Lane and Whitear, 1977), amphibians (Nafstad and Baker, 1973), birds (Nafstad, 1971) and mammals (Halata, 1970 Munger, 1965). Lamellated corpuscles, comparable to the paciniform structures of mammals (Pease and Quilliam, 1957), have been characterized as rapidly adapting (Andres and von Düring, 1973 Iggo and Muir, 1969 von Düring and Miller, 1979). Indeed, both rapidly adapting and slowly adapting afferents were observed in our physiological data.

Crocodilian behavioral responses following water surface disturbance. (A) Individual images from a film sequence recorded under infrared lighting and white noise presentation to block audition as a juvenile A. mississippiensis orients towards a surface wave generated by a small food pellet (white arrows). (B) Schematic of the orienting movements presented in A. From the animal's initial location, a lateral, sweeping head movement is repeated until the head makes tactile contact with the floating pellet and it is rapidly captured. Scale bar, 10 cm.

Crocodilian behavioral responses following water surface disturbance. (A) Individual images from a film sequence recorded under infrared lighting and white noise presentation to block audition as a juvenile A. mississippiensis orients towards a surface wave generated by a small food pellet (white arrows). (B) Schematic of the orienting movements presented in A. From the animal's initial location, a lateral, sweeping head movement is repeated until the head makes tactile contact with the floating pellet and it is rapidly captured. Scale bar, 10 cm.

The close association between the discoid terminals and the supporting epidermal cells of the stratum corneum and lucidum has been observed before in reptile scales (von Düring and Miller, 1979) and in mammalian glabrous skin (Munger and Ide, 1988), and this relationship also holds for the crocodilian ISOs both from the cephalic and body regions. Highlighting the intimate association with the free nerve terminals, fluorescent lipophilic label (DiI) applied to bundles of myelinated fibers of the maxillary nerve often labeled the keratinized epidermal layers directly over the ISO while remaining absent from adjacent scaled regions.

Several features of the trigeminal system of crocodilians stood out when examining the innervation of the cranium. First, there was an exceptional density of nerve fibers supplying the skin of the face and a vast network of branching nerve bundles just below the epidermis. Throughout the dermis, ensheathed groups of myelinated afferents projected across the rostro-caudal length and outwards towards the epidermis, as seen in the cleared Sudan Black B specimens. The bundles emerged through small foramina of the maxilla and dentary. This organization is reminiscent of mechanosensory end organs found in the foramina of anterior margins of the beaks of water-foraging birds with bill tip organs (Cunningham et al., 2010) and highlights the shared archosaurian phylogeny between crocodilians and birds (Hedges and Poling, 1999). It seems likely that by having the majority of the maxillary and mandibular nerves shielded in bone, crocodilians are armored against many potential injuries that might be encountered when feeding communally, while simultaneously maintaining an acutely sensitive skin surface via the fibers running through the foramina.

Trigeminal afferents and their organization

A large proportion of the neurons in the trigeminal ganglion responded to stimulation of the areas most densely covered in ISOs near rostral points of the pre-maxilla and mandible and surrounding the teeth. In addition, many afferents responded to very light contact to the teeth, underscoring previous ultrastructural investigations of sensory nerve endings within the dental ligament and attachment tissues in Caiman crocodilus (Berkovitz and Sloan, 1979 Tadokoro et al., 1998). In general, the smallest receptive fields were found rostrally on the upper and lower jaws and near the teeth. This overall pattern of small receptive field size and corresponding ‘overrepresentation’ in the ganglion is reminiscent of cortical magnification of behaviorally important skin surfaces observed in mammals (Krubitzer, 2007 Sur et al., 1980). For many species, the most important skin surfaces used for exploring objects are densely innervated by afferents with the smallest receptive fields, and the skin surfaces have correspondingly large representations in the central nervous system. Examples of functionally significant skin surfaces with consequently large nervous system representations include the forelimb of the raccoon (Welker and Seidenstein, 1959), the bill surface in the platypus (Pettigrew, 1999) and the lips and tongue of humans (Penfield and Boldrey, 1937). The overall pattern found for crocodilians, which have the highest density of ISOs and smallest receptive fields around the teeth, provides an important clue to ISO function. We suggest that ISOs play a key role not only in capturing prey based on water movements (Soares, 2002) and contact, but also in discriminating objects that have been grasped in the jaws and guiding the manipulation of prey once it has been secured. This interpretation is consistent with other recent findings in vertebrates that have revealed very large cortical representations of the dentition and oral structures that had been previously unappreciated (Jain et al., 2001 Kaas et al., 2006 Remple et al., 2003).

Within the ganglion, neurons that responded to the rostral head were typically located ventrolaterally whereas neurons responding to stimulation of the caudal regions of the jaws were positioned dorsomedially. As would be expected, responses to stimulation of the pre-maxilla, maxilla and quadratojugal of the upper jaw were recorded from the anterior regions of the ganglion, in proximity to the entrance of the maxillary nerve into the ganglion, and areas responsive to stimulation of the dentary were recorded from the posterior regions, near the mandibular nerve's division from the ganglion. The electrophysiologically derived topography of the crocodilian trigeminal ganglion was consistent with maxillary representations in the maxillo-mandibular lobe as documented in horseradish peroxidase tracer studies from hatchling chicks (Noden, 1980).

In both the Nile crocodiles and alligators, receptive fields, some as small as the area of a single ISO, were sensitive to indentation thresholds produced by the finest von Frey filaments corresponding to a force of 0.078 mN. These measurements represent sensitivities more acute than those of primate fingertips (Johansson et al., 1980) – skin surfaces that are widely appreciated for their sensitivity (Darrian-Smith, 1984 Kaas, 2004). Similarly, tactile responses were elicited by mechanical displacements as small as 3.9 μm – an indentation threshold lower than found for the human hand (Johansson, 1978). These findings are evidence of the extreme and surprising sensitivity of the crocodilian face and may represent a requirement for the detection of subtle water disturbances (Soares, 2002) in addition to the discrimination of different objects and prey.

RA, SA I and SA II type responses were identified in recordings from the surface of single trigeminal ISOs of alligators as well, in keeping with the diverse array of mechanoreceptors and afferent end organs found in each receptor. A prior electrophysiological study from the plantar nerve of alligators and caiman, which lack ISOs on the body, have also found RA, SA I and SA II afferents on the hindlimbs (Kenton et al., 1971). However, in this preceding study, the finest indentation forces found for these cutaneous regions lacking ISOs (as is the case in alligatorid limbs) were more than six times greater than the median indentation forces for responses from the ISO covered areas (0.08 mN) in this report, suggesting that the organs provide a considerable increase in sensitivity. ISOs therefore seem to be structures that impart great sensitivity to an otherwise armored and shielded body surface.

Analysis of 14 RA responses collected for stimulus frequencies up to 350 Hz indicated that the lowest indentation thresholds were found at 20–30 Hz within the 5–150 Hz range. Larger displacement distances were necessary to elicit 1:1 entrained responses of the afferent to frequencies both below and above the 20–30 Hz window. The 20 Hz vibration stimulus has been noted as one of the optimal frequencies to induce orientation behaviors towards water surface disturbance in Notonecta glauca – a predatory aquatic insect that localizes and orients towards prey-borne surface waves transmitted via mechanoreceptive tarsal scolopoidal organs and abdominal sensory hairs (Lang, 1980 Weise, 1974). Thus the tuning of afferents to this frequency in crocodilians is consistent with prior behavioral observations of juvenile alligators orienting towards water surface ripples (Soares, 2002). In addition, responses from SA (both types I and II) and RA units often extended beyond 200 and 300 Hz and were elicited by 40–80 μm displacements, suggesting that relatively higher-frequency vibrations can also be readily transduced by ISOs.

Spinal nerve afferents

As one of the goals of this project was to collect physiological data regarding sensory function of post-cranial ISOs in crocodiles, it was necessary to record from spinal nerves innervating the integumentary surface. Although still responding to forces of 13.725 mN and finer, afferents from the body were not as sensitive as those distributed across cephalic regions in either crocodiles or alligators. Discrete single units across the limbs were typically large except for those found in distal regions of certain digits (IV and V on the forelimb and IV on the hindlimb). There also appeared to be sensitive regions, responding to indentation forces of 0.392 and 0.686 mN, on the webbing present between digits III and IV on the forelimb. These results are consistent with the concept of the ISOs being discrete tactile receptor units as they are present on some of the smallest scales of the body perhaps the increased receptor density per unit area imparts a greater degree of acuity. This idea is supported by the notion that the digits IV and V of the forelimb, which are notably more slender and do not have the claws found on the other digits, might be adapted to detecting somatosensory cues when the animal is floating in the water (Vliet and Groves, 2010). When foraging for fish, Caiman yacare partially open their mouths and fully extend their forelimbs, adopting a ‘cross posture’, and indeed, fish have been observed nipping at the caiman's digits (Olmos and Sazima, 1990), suggesting that tactile information from the digits could mediate predatory behaviors.

Another motivation for physiological investigation of the integument comes from Jackson's intriguing series of experiments into the potential osmoreceptive capabilities of post-cranial ISOs of crocodiles (Jackson and Brooks, 2007 Jackson et al., 1996). However, in recording directly from the afferents innervating ISO-covered body surfaces in Nile crocodiles, no single- or multi-unit responses attributable to exposure to hyperosmotic solutions were observed. The results were similar to those from alligators that were used as an experimental control without body ISOs. Finally, no responses were detected in response to electrical stimuli (Scheich et al., 1986), suggesting that ISOs play no role in electric field detection, and by extension, that crocodilians do not have electroreception.

We suggest that the crocodilian ISOs function as part of an elaborate mechanosensory system and are adaptive to a number of aquatic behaviors. When filmed under 940 nm IR illumination, both crocodiles and alligators readily struck at and captured fish (supplementary material Movie 1, clip 1) and occasionally oriented towards minute water surface disturbances, similar to the results reported by Soares (Soares, 2002). Beyond providing positional cues to the source of the stimuli, the ISOs are densely distributed throughout the upper palate and areas adjacent to the teeth within the oral cavity – a location unlikely to receive and transduce the pressure from expanding surface waves. Disjunct regions of greater receptor density were observed near the eye and nares, similar to supraorbital and rhinal microvibrissae areas found in mammals relying on trigeminally mediated tactile discrimination (Brecht, 2007 Ling, 1966 Lyne, 1959). When actively foraging, crocodilians open their mouths and move so as to sweep the arrays of cranial ISOs across the surface and underwater, rapidly capturing and securing objects that make contact with their heads, and releasing any non-edible matter, indicating that it is likely that they can discriminate between multiple different materials using tactile cues alone. As a testament to these discriminatory abilities, mother crocodilians often manipulate their eggs as they begin hatching, gently cracking away the shell with their teeth (part of a feeding apparatus capable of inflicting crushing bites and dismembering large prey) and allowing the hatchlings to seek protection in her mouth (Hunt, 1987 Pooley and Gans, 1976) – a situation in which blunted tactile acuity would be maladaptive. Although the question remains as to why ISO distribution differs between the alligatorid and crocodylid species, results from recording from the spinal nerves suggest that both species tested are sensitive to low thresholds of force. Some have speculated that ISOs homologous to the post-cranial populations of crocodylids are present far deeper within the integument in alligatorids (Richardson et al., 2002). While crocodilians are certainly capable of accurately ambushing and capturing prey by relying on their acute visual systems (Heric and Kruger, 1966 Pritz, 1975) in lighted conditions, even on the darkest nights, prey still face a formidable mechanosensory system if they unexpectedly come into contact with these reptiles.


1 Answer 1

Short answer
Axonal outputs are coupled to dendrites or other effector tissues, such as muscular fibers or glandular tissue. There are no redundant axonal outputs, or redundant dendritic inputs. There is a tight coupling between the two both during developmental formation of synapses and during the maintenance of the two.

Background
Neurons can have extensive dendritic trees (e.g., Purkinje cells in the cerebellar cortex, Fig. 1) and also elaborate arborated axon terminals (*e.g., in the neuromuscular junction, Fig. 2).

In the cases shown below, the functions can be defined as simple logical operators, namely integration (Purkinje cell) and amplification (neuromuscular junction). The Purkinje cells receive and integrate input from the brainstem. The motoneurons innervating muscle use multiple axon terminals to target a larger area of the muscle so that stronger, synchronous muscle contractions can be achieved.

The development and maintenance of dendrite-axon connections are tightly coupled (e.g., Shen & Lowan, 2010). Generally, when either input to the dendrite is diminished (axonal regression), or output of the axon is cancelled (dendritic regression), the plasticity of the neural tissue will lead to degeneration or re-directing of the axon, or regression or re-innervation of the dendrite, respectively (e.g. Marc et al., 2003).

As regarding to your math be aware axons can branch, axon arborations can target a single dendritic tree, axons can terminate on other tissues, such as glands and muscles (no dendrites there!). In other words, your mathematical example is an oversimplification. Further cells with elaborate dendritic trees as shown in Fig. 1 are common in cortical layers, but are not the norm. The ratios of the cell types have to be taken into account as well.

Lastly, there can be the textbook connections between neurons, namely axo-dendritic connections, but also axo-axonal (Schmitz et al, 2001), dendrodendritic (Shepherd, 2009), and even axosomatic synapses between neurons (Fig. 3).


Fig. 2. Axonal terminals in a neuromuscular junction. source: Farrel (2017)


Fig. 3. Different kinds of synapses. source: Modesto Junior College


DISCUSSION

In this study, we produced electron microscopy image volumes of mouse cerebellum at P3 and P7 and reconstructed climbing fiber branches and their Purkinje cell targets in order to learn more about the changing organization of climbing fiber input to Purkinje cells in the first postnatal week. Our results reveal several features of climbing fiber-Purkinje cell synapse rearrangement. First, single climbing fibers (with branches working in concert) form many additional synapses to focus their innervation onto a subset of their Purkinje targets. Second, single-synapse strengths (measured by PSD area) become more uniform and do not correlate with the overall strength of a climbing fiber-Purkinje cell connection. These two points, combined with our confirmation that synaptic pruning does not occur, lead us to conclude that synapse addition is the primary mode of functional differentiation in the first postnatal week. This structural information is more consistent with electrophysiological evidence of synaptic strengthening in the first postnatal week (Hashimoto and Kano, 2003 Bosman et al., 2008) than results suggesting changes begin in the second week (Scelfo and Strata, 2005). Third, we infer that synapse addition between P3 and P7 involves positive feedback between climbing fibers and Purkinje cells. This feedback appears to be nearly linear in nature and predicts that climbing fibers establish preferences by adding one synapse every 1 to few hours.

We also addressed a challenge that beset our analysis and is present in many other connectomic image volumes, i.e., that practically all axons are incomplete because connectomic volumes are small relative to neuron sizes. We combined information about large-scale climbing fiber morphology (Sugihara, 2005) with our own reconstructions of terminal arbors and their synaptic connectivity to quantify the total number of climbing fibers that innervated the 30 Purkinje cells in our P7 image volume. This information provides important context about climbing fiber-Purkinje cell synapse rearrangement: there appears to be parity between young climbing fibers and Purkinje cells. If detailed synaptic connectivity were not determined for our P7 dataset, a range of other scenarios could seem appropriate. In particular, because each Purkinje cell is innervated by 4 climbing fibers on average (Results), the maximum number of climbing fibers that could have innervated 30 Purkinje cells was

120 (4 distinct axons per cell) and the minimum number is 4 (if the same 4 axons innervated every cell). The actual number of inputs, 30, is exactly what one would expect to find innervating a volume of 30 Purkinje cells in adult cerebellum, because adult climbing fibers innervate Purkinje cells sparsely, rarely innervating more than one cell in a field of 30. More specifically, an adult climbing fiber forms synapses with

10 Purkinje cells out of thousands in a few cerebellar lobules (Sugihara et al., 2001 Fujita and Sugihara, 2013). Climbing fiber branching in development thus appears to be economical: the number of climbing fibers that innervate Purkinje cells in a local region in the first postnatal week is just enough to assure that each axon ends up with a postsynaptic target and that none branched there in vain.

This information, taken together, provides a glimpse at how development translates into structural changes in brain circuitry. It also provides useful constraints on the mechanisms that underlie developmental rewiring in this region of cerebellum.

Finally, although we studied synaptic connectivity from climbing fiber branches onto Purkinje cells, these electron microscopy volumes also contain ample information about all other cell types found in cerebellar cortex, their organelles, and their synaptic connectivity. These image volumes are thus useful resources for investigations of normal cerebellar development in mouse.

Branches of the Same Climbing Fiber Exhibit Similar Synaptic Preferences via a Contact-Mediated Mechanism

Our analysis indicates that groups of climbing fiber branches in the P7 volume had statistically significant similarities in their synaptic connectivity ( Figure 7C ). These similarities were not explained by close fasciculation of those branches. Indeed, each climbing fiber branch appeared to have a completely individual branching pattern (http://bit.ly/bbandrossP7cfgroups). Based on a number of arguments (Results), we think it likely that each such group originates from one climbing fiber. This finding leads to an important implication for the way that preferred Purkinje cells are chosen. Namely, climbing fibers do not establish synaptic preferences with a particular Purkinje cell through specific, directed arborization during the first postnatal week. Rather, they establish large numbers of synapses at sites where they happen to be in contact. In this sense preferences are contact mediated rather than axon growth mediated. This idea is strengthened by the observation that climbing fiber branching is not pronounced in the vicinity of preferred cells compared with other Purkinje cells. Rather, the only evidence of the site of the preferred Purkinje cell is the greater density of synapses per axon length (http://bit.ly/bbandrossP3NGL, P3 http://bit.ly/bbandrossP7NGL, P7). One interpretation of this situation is that between P3 and P7 climbing fibers add synapses along existing branches that are already juxtaposed with a preferred Purkinje cell target.

This finding also illustrates an important point for connectomics datasets in general: namely, it may be possible to regroup broken axon pieces by leveraging their synaptic connectivity, as we have done for the P7 cerebellum dataset. This strategy should allow for more complete axonal reconstructions and therefore more accurate connectivity analysis. This type of connectivity-based inference is only possible with analysis in high-resolution connectomics datasets, in which all synapses formed by branches of an axon can be identified and their distributions across targets can be directly measured.

Functional Differentiation Differs between the Cerebellum and the NMJ

Our observations indicate that in the first postnatal week, climbing fibers develop preferences for certain Purkinje cells as they add synapses ( Figures 7B and ​ and7C). 7C ). Based on the idea that multiple branches originate from the same climbing fiber (Results), there may be hundreds of synapses added by one climbing fiber onto one Purkinje cell between P3 and P7, while other climbing fibers innervating the same Purkinje cell change their synaptic input much more modestly. For example, the branches in group 1 of our preference analysis collectively form 105 synapses onto Purkinje cell 17, their preferred target, but only 4 synapses onto Purkinje cell 1, a non-preferred target. Given the loss of all but one input over the next few weeks and previous electrophysiology work (Hashimoto et al., 2011), it seems likely that the climbing fiber with the large increment in synapses will be the one that remains after synaptic rewiring is complete. Importantly, there was no evidence in our studies of synapse loss in the first postnatal week as strengthening occurred ( Figures 3 and ​ and5), 5 ), although in later weeks massive elimination takes place (Kano et al., 2018).

This sequence of events for climbing fibers contrasts with developmental synapse rearrangement of motor axons at the NMJ, although in both cases postsynaptic targets are initially innervated by multiple axons and end up with a single axonal input. At the NMJ, multiple axons that converge undergo a process of synapse exchange: an axon adds synaptic territory by taking over space that was vacated by a different axon (Walsh and Lichtman, 2003 Gan and Lichtman, 1998 Turney and Lichtman, 2012). Synapse addition and loss are thus inextricably linked, with synapse loss appearing to be required for strengthening of the ultimate surviving input (a pattern also observed elsewhere: Chen and Regehr, 2000 Lichtman, 1980). Our results, however, provide evidence that early synaptic strengthening of climbing fiber inputs is unrelated to synapse removal by other fibers. A classical Hebbian mechanism may underlie the establishment of a dominant input in the cerebellum (Hebb, 1949 Kawamura et al., 2013 Lichtman and Balice-Gordon, 1990).

Persistence of Weak Climbing Fiber Inputs

The persistence of weak climbing fiber inputs despite the emergence of a dominant input during the first postnatal week raises the question of why these neurons would maintain weak connectivity. Physiologically effective climbing fibers in an adult establish many hundreds of synapses on a target cell, so it is unlikely that climbing fibers forming only a few synapses have functional significance. Axons may maintain weak connections to provide footholds on other target cells should the dominant input be damaged during development (Carrillo et al., 2013 Turney and Lichtman, 2012) or in case interactions between climbing fibers and Purkinje cells require a dominant input to refocus its resources elsewhere as has been proposed at the developing NMJ (Walsh and Lichtman, 2003). In this sense, Purkinje cells may be hedging their bets by remaining connected to multiple climbing fibers before the competition is resolved. From a climbing fiber perspective, the same may be true: an axon may remain connected to many targets to assure that it still innervates a few after most synaptic pruning has occurred.

Comparative Connectomics

Connectomics per se is a descriptive approach. It relies on inductive reasoning so that hypotheses are generated more easily than tested. One way, however, to generate and test hypotheses in connectomics datasets is to compare samples that differ in some way. In this study we have compared connectomics data from two developmental stages to learn how neural circuits become modified in early postnatal life. The power of this strategy is that a fundamentally static technique (looking at stained postmortem tissue) can be used to infer information about a highly dynamic phenomenon (the maturation of neural circuits). One challenge is that comparing connectomes is still a nascent approach. Although we possess potentially vast amounts of structural data from two time points, we do not yet know the ideal ways to make statistically rigorous comparisons. We contend that these two datasets contain material for an essentially unlimited number of hypotheses about how the cerebellum changes over the first postnatal week. Determining the best way to extract the things that are different and the things that are not from multiple samples will require turning these 𠇍igital tissues” into systematized databases that can be scrutinized automatically. This will likely be one of the central thrusts of connectomics going forward.


Conclusion

I have tried to summarize a bottom-up effort, undertaken with my collaborators during two decades, aimed at elucidating the mechanics in a simple cortical network that represents electrically a whisker deflection. Activity in this network can drive a behavioural response, such as gap-crossing. Conclusions that follow from this effort are as follows.

The biophysical properties in vitro must first be extablished for individual groups of cell types and their synaptic connections constituting the network, meaning that a bottom-up approach must be followed by synthesizing the network from defined elements, that is neurons with electrically active dendrites (Fig. S2).

The function of the different elements must be described when they are embedded in the in vivo network (Figs. 19 and 22). This requires recordings of synaptic potentials and unit recordings in vivo followed by an in silico anatomical reconstruction of individual pathways at the subcellular level. Such reconstructions are necessary to be able to make educated guesses, that is simulations, on what features of an AP pattern emitted by a projection cell ensemble are ‘read’ by the ensembles of target cells.

Average 3D anatomical reconstructions of complete (ensemble) pathways must be established by calculating axodendritic overlaps for entire ensembles of projecting and targeted cell types in a quantitative way by establishing a basic ‘digital neuroanatomy’ of the cortex.

The approach we took, in order to discover structure–function relationships that help to unravel simple design principles of cortical networks, was first to determine functions and then reconstruct the underlying morphology assuming that ‘form follows function’, a dictum of the architect Louis Sullivan (Sullivan, 1896 ) and a Bauhaus design principle, keeping in mind that the Max Planck Institute for Medical Research in Heidelberg, where most of the work described here began, was the first laboratory building designed by a Bauhaus architect. Sullivan also suggested: ‘Whether it be the sweeping eagle in his flight, or the open apple-blossom, the toiling work-horse, the blithe swan, the branching oak, the winding stream at its base, the drifting clouds, over all the coursing sun, form ever follows function, and this is the law. Where function does not change, form does not change.’

How this applies to structure–function relationships of the cortex remains to be seen. At present, however, it seems that ‘what we cannot reconstruct in silico and model, we have not understood’.

Disclaimer: Supporting information has been peer-reviewed but not copyedited.

Figure S1. Rodent somatosensory cortex and brain slice preparations.

Figure S2. Subcellular structure elements of L5tt pyramids that are functionally relevant shown in a simple cortical network consisting of two pyramidal cells and one (multipolar) non-pyramidal cell.

Figure S3. Whisker evoked subthreshold responses in granular layer 4. From Brecht and Sakmann (2002b).

Figure S4. Average postsynaptic potential that a single thalamic AP evokes in a cortical L4 cell. From Bruno & Sakmann (2006).

Figure S5. Response adaptation of whisker evoked PSPs in infragranular and granular layers during repetitive (10Hz) deflections of a principal whisker. From Brecht and Sakmann (2002b) & Manns et al. (2004).

Figure S6. Density of cell bodies along a columns vertical axis. From Meyer et al. (2010b).

Figure S7. Thalamocortical innervation of vS1. From Wimmer et al. (2010) & Meyer et al. (2010a).

Figure S8. Thalamo-cortical (VPM) axon overlap with L5tt dendrites. From Narayanan et al. (2015).

Figure S9. Unit AP-activity of L5st and L5tt cells during free whisking in awake animals. From DeKock & Sakmann (2009).

Figure S10. Principal whisker and surround whisker evoked PSP inputs (upper panels) and AP outputs (lower panels) in three (VPM, deep cortical layers of columns and POm) sequentially stacked modules of the vS1 network.

Figure S11. Whisker specific projection fields of fluorescent boutons of bulk labeled L5tt cells located in two separated columns.

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