Degeneration and regeneration of peripheral nerve in the rat trigeminal system. I. Identification and characterization of the multiple afferent innervation of the mystacial vibrissae

Sensory Adaptation in the Whisker-Mediated Tactile System: Physiology, Theory, and Function

In the natural environment, organisms are constantly exposed to a continuous stream of sensory input. The dynamics of sensory input changes with organism's behaviour and environmental context. The contextual variations may induce >100-fold change in the parameters of the stimulation that an animal experiences. Thus, it is vital for the organism to adapt to the new diet of stimulation. The response properties of neurons, in turn, dynamically adjust to the prevailing properties of sensory stimulation, a process known as "neuronal adaptation." Neuronal adaptation is a ubiquitous phenomenon across all sensory modalities and occurs at different stages of processing from periphery to cortex. In spite of the wealth of research on contextual modulation and neuronal adaptation in visual and auditory systems, the neuronal and computational basis of sensory adaptation in somatosensory system is less understood. Here, we summarise the recent finding and views about the neuronal adaptation in the rodent whisker-mediated tactile system and further summarise the functional effect of neuronal adaptation on the response dynamics and encoding efficiency of neurons at single cell and population levels along the whisker-mediated touch system in rodents. Based on direct and indirect pieces of evidence presented here, we suggest sensory adaptation provides context-dependent functional mechanisms for noise reduction in sensory processing, salience processing and deviant stimulus detection, shift between integration and coincidence detection, band-pass frequency filtering, adjusting neuronal receptive fields, enhancing neural coding and improving discriminability around adapting stimuli, energy conservation, and disambiguating encoding of principal features of tactile stimuli.

Whisker-Mediated Touch System in Rodents: From Neuron to Behavior

A key question in systems neuroscience is to identify how sensory stimuli are represented in neuronal activity, and how the activity of sensory neurons in turn is “read out” by downstream neurons and give rise to behavior. The choice of a proper model system to address these questions, is therefore a crucial step. Over the past decade, the increasingly powerful array of experimental approaches that has become available in non-primate models (e.g., optogenetics and two-photon imaging) has spurred a renewed interest for the use of rodent models in systems neuroscience research. Here, I introduce the rodent whisker-mediated touch system as a structurally well-established and well-organized model system which, despite its simplicity, gives rise to complex behaviors. This system serves as a behaviorally efficient model system; known as nocturnal animals, along with their olfaction, rodents rely on their whisker-mediated touch system to collect information about their surrounding environment. Moreover, this system represents a well-studied circuitry with a somatotopic organization. At every stage of processing, one can identify anatomical and functional topographic maps of whiskers; “barrelettes” in the brainstem nuclei, “barreloids” in the sensory thalamus, and “barrels” in the cortex. This article provides a brief review on the basic anatomy and function of the whisker system in rodents.

Fetal extracellular matrix nerve wraps locally improve peripheral nerve remodeling after complete transection and direct repair in rat

In peripheral nerve (PN) injuries requiring surgical repair, as in PN transection, cellular and ECM remodeling at PN epineurial repair sites is hypothesized to reduce PN functional outcomes by slowing, misdirecting, or preventing axons from regrowing appropriately across the repair site. Herein this study reports on deriving and analyzing fetal porcine urinary bladder extracellular matrix (fUB-ECM) by vacuum assisted decellularization, fabricating fUBM-ECM nerve wraps, and testing fUB-ECM nerve wrap biocompatibility and bioactivity in a trigeminal, infraorbital nerve (ION) branch transection and direct end-to-end repair model in rat. FUB-ECM nerve wraps significantly improved epi- and endoneurial organization and increased both neovascularization and growth associated protein-43 (GAP-43) expression at PN repair sites, 28-days post surgery. However, the number of neurofilament positive axons, remyelination, and whisker-evoked response properties of ION axons were unaltered, indicating improved tissue remodeling per se does not predict axon regrowth, remyelination, and the return of mechanoreceptor cortical signaling. This study shows fUB-ECM nerve wraps are biocompatible, bioactive, and good experimental and potentially clinical devices for treating epineurial repairs. Moreover, this study highlights the value provided by precise, analytic models, like the ION repair model, in understanding how PN tissue remodeling relates to axonal regrowth, remyelination, and axonal response properties.

Diffuse traumatic brain injury and the sensory brain

In this review we discuss the consequences to the brain's cortex, specifically to the sensory cortex, of traumatic brain injury. The thesis underlying this approach is that long-term deficits in cognition seen after brain damage in humans are likely underpinned by an impaired cortical processing of the sensory information needed to drive cognition or to be used by cognitive processes to produce a response. We take it here that the impairment to sensory processing does not arise from damage to peripheral sensory systems, but from disordered brain processing of sensory input.

The morphology of the rat vibrissal follicle-sinus complex revealed by three-dimensional computer-aided reconstruction

The vibrissal follicle-sinus complex (FSC) is a sensory receptor of the mammalian integumentary system that is located around the mouth. The purpose of the present study was to identify the actual 3-dimensional structure of the rat vibrissal FSC. Rat skin tissue was serially sectioned at a thickness of 10 μm and then stained with Masson's trichrome. The serial sections were reconstructed 3-dimensionally using Reconstruct software. The rat vibrissal follicle is a spindle-shaped structure that is embedded within a blood sinus and enveloped within a thick collagenous capsule. The vibrissal FSC is innervated by the deep vibrissal and superficial vibrissal nerves. The deep vibrissal nerve, travelling in the basal-to-apical direction, penetrates the thick collagenous capsule of the vibrissal FSC. The sinus system can be divided into a superior portion, known as the ring sinus, and an inferior portion, known as the cavernous sinus. The ring sinus contains a C-shaped structure, the ringwulst, which is suspended from the mesenchymal sheath of the follicle. Collagenous trabeculae can be seen in the cavernous sinus but not in the ring sinus. The ring sinus encircles the follicle obliquely and asymmetrically. The ringwulst encircles the follicle incompletely, in a C-shaped fashion. This study has demonstrated the previously underappreciated 3-dimensional structure of the vibrissal FSC, which differs from previously reported descriptions, and provides data that will enhance the understanding of vibrissal function.

A comparative analysis of vibrissa count and infraorbital foramen area in primates and other mammals

Vibrissae are specialized sensory "hairs" that respond to mechanical stimuli. Sensory information from vibrissae is transmitted to the brain via the infraorbital nerve, which passes through the infraorbital foramen (IOF). Several analyses have documented that primates have smaller IOFs than non-primate mammals, and that haplorhines have smaller IOFs than strepsirrhines. These grade shifts in IOF area were attributed to differences in "vibrissa development." Following earlier analyses, IOF area has been used to derive a general estimate of "whiskeredness" in extinct primates, and consequently, IOF area has been used in phylogenetic and paleoecological interpretations. Yet, the relationship between IOF area and vibrissa count has not been tested, and little is known about how IOF area and vibrissa counts vary among mammals. This study explores how relative IOF area and vibrissa count differ among 25 mammalian orders, and tests for a correlation between IOF area and vibrissa count. Results indicate that primates and dermopterans (Primatomorpha) have smaller IOFs than most non-primate mammals, but they do not have fewer vibrissae. In addition, strepsirrhines and haplorhines do not differ from one another in relative IOF area or vibrissa counts. Despite different patterns documented for IOF area and vibrissa count variation across mammals, results from this study do confirm that vibrissa count and IOF area are significantly and positively correlated (p < 0.0001). However, there is considerable scatter in the data, suggesting that vibrissa counts cannot be predicted from IOF area. There are three implications of these finding. First, IOF area reflects all mechanoreceptors in the maxillary region, not just vibrissae. Second, IOF area may be an informative feature in interpretations of the fossil record. Third, paleoecological interpretations based on vibrissae are not recommended.

Hypoglossal nucleus projections to the rat masseter muscle

We investigated in the rat whether hypoglossal innervation extended to facial muscles other than the extrinsic musculature of the mystacial pad. Results showed that hypoglossal neurons also innervate the masseter muscle. Dil injected into the XII nucleus showed hypoglossal axons in the ipsilateral main trunk of the trigeminal nerve. After Gasser's ganglion crossing, the axons entered into the infraorbital division of the trigeminal nerve and targeted the extrinsic muscles of the mystacial pad. They also spread into the masseter branch of the trigeminal nerve to target the polar portions of the masseter muscle spindles. Retrograde double labelling, performed by injecting Dil into the pad and True Blue into the ipsilateral masseter muscle, showed labelled hypoglossal neurons in the medio-dorsal portion of the XII nucleus. The majority of these neurons were small (15-20 microm diameter), showed fluorescence for Dil and projected to the mystacial pad. Other medium-size neurons (25 microm diameter) were instead labelled with True Blue and projected to the masseter muscle. Finally, in the same area, other small hypoglossal neurons showed double labelling and projected to both structures. Functional hypotheses on the role of these hypoglossal projections have been discussed.

Hypoglossal nuclei participation in rat mystacial pad control

Recently, we showed that extra-trigeminal axons, originating from the hypoglossal nucleus, travel with the infraorbital division of the trigeminal nerve (ION), which is known to innervate the rat mystacial pad. Dil was monolaterally injected into the rat XII nucleus to analyse the peripheral distribution of hypoglossal axons to the mystacial pad, to evaluate their involvement in facial sensory-motor control. Electromyographic responses of mystacial pad motor units to electrical stimulation of the ION were recorded, along with the evoked responses to electrical stimulation of the ipsilateral XII nucleus. The results showed that hypoglossal axon terminals target the ipsilateral extrinsic musculature of the mystacial pad, but they do not have any contact with the intrinsic muscles. ION electrical stimulation increased electromyographic activity in the ipsilateral pad extrinsic muscles, even following VII nerve transection. Hypoglossal nucleus electrical stimulation induced field potentials and monosynaptic responses in the same motor units that persisted even following VII nerve transection, these disappearing after cooling the ION. We suggest that the small hypoglossal neurons projecting to the extrinsic musculature of the mystacial pad are part of a hypoglossal-trigeminal loop that participates in the sensory-motor control of the rat vibrissae system.

Relationship between physiological response type (RA and SA) and vibrissal receptive field of neurons within the rat trigeminal ganglion

Cells within the trigeminal ganglion (Vg) encode all the information necessary for the rat to differentiate tactile stimuli, yet it is the least-studied component in the rodent trigeminal somatosensory system. For example, extensive anatomical and electrophysiological investigations have shown clear somatotopic organization in the higher levels of this system, including VPM thalamus and SI cortex, yet whether this conserved schemata exists in the Vg is unknown. Moreover although there is recent interest in recording from vibrissae-responsive cells in the Vg, it is surprising to note that the locations of these cells have not even been clearly demarcated. To address this, we recorded extracellularly from 350 sensory-responsive Vg neurons in 35 Long-Evans rats. First, we determined three-dimensional locations of these cells and found a finer detail of somatotopy than previously reported. Cells innervating dorsal facial features, even within the whisker region, were more dorsal than midline and ventral features. We also show more cells with caudal than rostral whisker receptive fields (RF), similar to that found in VPM and SI. Next, for each vibrissal cell we determined its response type classified as either rapidly (RA) or slowly (SA) adapting. We examined the relationship between vibrissal RF and response type and demonstrate similar proportions of RA and SA cells responding to any whisker. These results suggest that if RA and SA cells encode distinct features of stimuli, as previously suggested, then at the basic physiological level each whisker has similar abilities to encode for such features.

Oscillatory calcium responses mediated by P2Y2 purinergic receptors in terminal Schwann cells of longitudinal lanceolate endings isolated from rat vibrissae

The longitudinal lanceolate endings are mechanoreceptors that detect hair movement. We have previously shown that terminal Schwann cells, glial elements of the sensory devices, respond to an application of the sensory modulator adenosine 5'-triphosphate (ATP) by an elevation in the intracellular Ca2+ concentration ([Ca2+]i), suggesting a regulatory role for these cells in the cutaneous sensation. To define the spatiotemporal dynamics of the cell signaling and the pharmacological properties of the receptors responsible, arrays of the lanceolates were enzymatically isolated from the rat vibrissal follicle and subjected to [Ca2+]i image recording by time-lapse confocal microscopy during bath application of ATP analogues. The terminal Schwann cells formed extensive networks, connecting with one another by their lamellar processes associated with lanceolate axon endings. Stimulation of the cells with 100 microM ATP evoked [Ca2+]i waves propagating along the cell processes. In each Schwann lamella, the initial wave evoked by a given trial of the stimulant arose from a specific locus within the cell process, whereas subsequent waves were sometimes observed to travel from its proximal portion. This implies a subcellular compartmentalization that may enable each Schwann lamella to modulate the activity of its accompanying lanceolate terminal through its own Ca2+ signal as well as to regulate neighboring lanceolates through interlamellar signal propagation. Pharmacological experiments have shown that the Schwann cell responses are mediated by the P2Y2 receptor, which has recently been reported to couple to multiple effector molecules in addition to stimulating the phosphoinositide signaling pathway involved in various glia-neuron interactions.

Calcium-binding protein-immunoreactive innervation of the rat vibrissa

Immunohistochemistry detected calcium-binding proteins (CaBPs) in corpuscular and Merkel nerve endings of the rat vibrissa. CaBP-immunoreactive (ir) corpuscular endings were divided into two types: ramified and unramified endings. Ramified endings were subdivided into reticular and Ruffini endings. Unramified endings were identical to longitudinal lanceolate endings which have been described previously. Reticular and unramified endings as well as Merkel endings co-expressed neurocalcin (NC)- and parvalbumin (PV)-immunoreactivity (ir). However, such endings were devoid of peptide 19 (PEP19)-ir. PV-ir Ruffini endings were immunoreactive for PEP19 but not NC. The retrograde tracing method revealed that 34, 21 and 18% of trigeminal neurons which project to the infraorbital nerve exhibited NC-, PEP19- and PV-ir, respectively. In addition, 73 and 36% of the PV-ir neurons showed NC- and PEP19-ir, respectively. The content and co-expression of CaBPs in vibrissal low-threshold mechanoreceptors may depend on their terminal morphology.

Expression of neuropeptides and growth-associated protein 43 (GAP-43) in cutaneous and mucosal nerve structures of the adult rat lower lip after mental nerve section

The reinnervation of the adult rat lower lip has been investigated after unilateral section of the mental nerve. Rats were sacrificed at 4, 7, 9, 14, 30, and 90 days after the operation. A further group of animals with section of the mental nerve and block of the alveolar nerve regeneration, was sacrificed at 14 days. Specimens were processed for immunocytochemistry with antibodies against PGP 9.5, GAP-43 or neuropeptides (CGRP, SP and VIP). Four days after nerve section, axonal degeneration seems evident in the mental nerve branches and inside skin and mucosa. GAP-43 immunoreactivity is intense in the mental nerve 7 days after nerve section and it reaches its maximal expression and distribution in peripheral nerve fibres at 14 days. At 30 days, the decline in its expression is associated with the increase of PGP9.5-, SP-, and CGRP immunopositivity. VIP is observed only in perivascular fibres at all times observed. Present results suggest that, after sensory denervation of the rat lip, nerve fibres in skin and mucosa remain at lower density than normal. The different time courses in the expression of neuropeptides and GAP-43 suggest a possible early involvement of GAP-43 in peripheral nerve regeneration.

Divergent movement of adjacent whiskers

The current view of whisker movement is that approximately 25 whiskers on each side of the face move in synchrony. To determine whether whiskers are constrained to move together, we trained rats to use two whiskers on the same side of the face in simple behavioral tasks and videotaped the whiskers during the task. Here we report that the movement of adjacent whiskers is usually synchronous but can diverge: 1) the distance between whiskers can vary dramatically during movement; 2) one whisker can move while the second one remains stationary; 3) two whiskers can simultaneously move in opposite directions; and 4) one whisker can be maintained in contact with an object while the other is retracted and protracted. The frequency of whisker movement during the task falls within the previously reported range for rats whisking freely into air or performing roughness discrimination with their whiskers. Our data also suggest that whisker movement can be divided into three distinct phases: protraction, retraction, and a measurable delay between these movements. We conclude that, although whiskers often move in concert, adjacent caudal whiskers can be moved independently of each other.

Three-dimensional microanatomy of longitudinal lanceolate endings in rat vibrissae

The longitudinal lanceolate endings are ubiquitous sensory terminals in the sinus and nonsinus hairs of mammals that form a palisade around the hair follicle. To analyze how the nerve endings detect hair movements, the present study re-examined their fine structure and relationships with surrounding connective tissue in rat vibrissae by using a combination of three methods: immunohistochemistry for S-100 protein, scanning electron microscopy of NaOH-macerated specimens, and transmission electron microscopy of serial sections. Observations showed the lanceolate endings to be represented by triplet units with a flattened axon terminal flanked on each side by a Schwann cell lamella, as reported previously. Two distinct parts were discriminated in the lanceolate ending: a principal portion in which the axon terminal protruded numerous fine fingers from between the Schwann cell coverings, and an apical cone that enclosed a large axon finger in an attenuated Schwann sheath. Long foot processes of Schwann cells fanned out distally from each apical cone. The principal portions of the lanceolate endings were firmly linked to the surrounding connective tissue by the narrow edges equipped with axon fingers, suggesting their continuous deformation by sustained hair deflections. In contrast, the apical cones were freely suspended in an amorphous matrix with only the end feet of the Schwann cell projections attached to rigid tissue elements. This part of the ending was proposed as a possible transducer site to generate rapidly adapting receptor potentials, both retreating and overshooting during the acceleration and deceleration phases of a given vibrissal movement.

Innervation of a single vibrissa in the whisker-pad of rats

Innervation of the gamma straddler vibrissa and distribution of individual gamma vibrissal sensory nerve fibers within infraorbital nerve branches were investigated in adult rats. In the close vicinity (2 mm) of the whisker-pad, one-fifth of the nerve fibers innervating the gamma vibrissa were still located in nerves to neighboring (beta, delta, C1 and D1) whiskers, while one-tenth of the innervating fibers were placed in nerves of more distant sinus hairs. This convergence of gamma vibrissal sensory nerve fibers to single vibrissa from neighboring nerves is suggested to be a consequence of overlapping vibrissal representation in the Gasserian ganglion.

Structure and innervation of the vibrissal follicle-sinus complex in the Australian water rat, Hydromys chrysogaster

Light and electron microscopic techniques were used to examine the structure and innervation of the mystacial vibrissal follicle-sinus complex (F-SC) in the Australian water rat. The F-SCs of this semiaquatic rodent show the same morphologic elements described in terrestrial rats but differ in size, structure, and innervation. Most striking is the size of the water rat's caudal F-SCs, measuring 6.3 mm in length and 2.4 mm in diameter. The sinus system is divisible into a ring sinus and a cavernous sinus and shows a distinct asymmetry. At the highest level of the cavernous sinus, the outer root sheath forms a ridge in the direction of the trabeculae, which bind the ridge to the capsule. A ringwulst is present only in small and medium-sized F-SCs. The mean number of myelinated axons counted in the deep vibrissal nerve (DVN) of most caudal F-SCs was 537, indicating an innervation density of the water rat's vibrissal system at least 2.5 times as high as that of terrestrial rats. The total number of nerve fibers of the small superficial nerves was less than 10% of that of the DVN. These fibers innervate almost exclusively the area of the inner conical body. Structural specializations of the water rat F-SC are discussed as an analogous development in mammals adapted to the aquatic environment, primarily in terms of thermoregulation, whereas its high degree of innervation is assessed to lend support to the hypothesis that the vibrissal system is of special significance in aquatic mammals.

Mesencephalic innervation of the vibrissal follicle-sinus complex in the mouse embryo

Peripheral projections of neurones whose cell bodies lie in the mesencephalic nucleus of the fifth cranial nerve, situated between the central grey and mesencephalic reticular formation, were studied in mouse embryos aged between day 9 and 15 and in postnatal day 1 mice. Nonspecific neural antibody staining allowed visualisation of the developing cranial nerves, in particular the descending mesencephalic tract. This facilitated successful dissection of the descending mesencephalic tract and trigeminal ganglion in the heads of fresh mouse embryos and postnatal mice. The fluorescent dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil), was injected into the descending mesencephalic tract in mouse embryos aged 12.5, 13.5 and 15 days of gestation and also into postnatal day 1 mice. Following a period of incubation, 100 microm sections were viewed under visible light and episcopic fluorescence. Mesencephalic neurones were observed to pass superiorly over the trigeminal ganglion and enter the maxillary division to innervate vibrissal follicle-sinus complexes, whilst none was observed innervating mandibular and maxillary intraoral structures. There was no fluorescent labelling in non-Dil injected control specimens. Using a highly specific neuronal tracer, this study shows that mesencephalic neurones in the periphery project exclusively to follicle sinus complexes in the developing mouse embryo and remain at least until postnatal day 1. These observations, contrary to those made in other animals, indicate a species specificity of mesencephalic peripheral projections.

Ultrastructural observations of synaptic connections of vibrissa afferent terminals in cat principal sensory nucleus and morphometry of related synaptic elements

Previous work suggests that slowly adapting (SA) periodontal afferents have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with transmitter release may be related directly to bouton size. The present study examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra-axonal labeling with horseradish peroxidase and a morphometric analysis. All SA and FA afferent boutons contained clear, round, synaptic vesicles. All the FA and most SA boutons were presynaptic to dendrites, but a few SA boutons were axosomatic. Both types of bouton were frequently postsynaptic to unlabeled axonal ending(s) containing pleomorphic, synaptic vesicles (P-ending). The size of labeled boutons was larger in FA than SA afferents, but the size of dendrites postsynaptic to labeled boutons was larger for SA than FA afferents. Large-sized FA and SA boutons made synaptic contacts with small-diameter dendrites. The size of FA and SA boutons was larger than that of their associated P-endings. A morphometric analysis made on the pooled data of SA and FA boutons indicated that apposed surface area, active zone number, total active zone area, vesicle number, and mitochondrial volume were highly correlated in a positive linear manner with labeled bouton volume. These relationships were also applicable to unlabeled P-endings, but the range of each parameter was smaller than that of the labeled boutons. These observations provide evidence that the two functionally distinct types of vibrissa afferent manifest unique differences but share certain structural features in the synaptic organization and that the ultrastructural "size principle" proposed by Pierce and Mendell ([1993] J. Neurosci. 13:4748-4763) for Ia-motoneuron synapses is applicable to the somatosensory system.

Effect of neonatal capsaicin and infraorbital nerve section on whisker-related patterns in the rat trigeminal nucleus

In the present study, we investigated the effect of neonatally administered capsaicin on whisker-related pattern formation in the rat trigeminal complex. Both normal whisker-related patterns of barrelettes and the modified patterns seen after neonatal section of the infraorbital nerve were assessed. Capsaicin caused no change in the pattern or size of cytochrome oxidase (CO) barrelettes in the principal trigeminal nucleus (Vp) or trigeminal nucleus interpolaris (Vi) or caudalis (Vc). Injections of horseradish peroxidase (HRP) or wheatgerm agglutinin conjugated to HRP (WGA-HRP) into the posteroorbital (PO) whisker follicle in vehicle-treated animals showed that WGA labelled a larger number of trigeminal ganglion cells than HRP (203 +/- 23; cf. 158 +/- 19), with an increased labelling of small-diameter neurons (HRP: 25.9 +/- 7.7 microm; WGA: 23.2 +/- 7.2 pm). Capsaicin caused a loss of smaller diameter cells but had no effect on the location, cross-sectional area, or rostrocaudal extent of the transganglionically labelled HRP terminations in Vp, Vi, Vc, and cervical dorsal horn. WGA-HRP labelling revealed similar, but less dense, central terminal areas as HRP and an additional area of superficial terminals in the caudal medulla; these were also unaffected by capsaicin treatment. After infraorbital nerve section, CO patches and transganglionically labelled afferent terminations, corresponding to innervated nonmystacial whiskers, were approximately doubled in size. Capsaicin had no effect on the increased size of these spared whisker patches or their afferent terminal areas. These results suggest that barrelette formation is not dependent on unmyelinated afferents and that the changes in response properties seen after capsaicin, such as increased receptive fields, reflect functional changes rather than anatomical expansion of afferent terminal areas.

Neonatal whisker removal reduces the discrimination of tactile stimuli by thalamic ensembles in adult rats

Simultaneous recordings of up to 48 single neurons per animal were used to characterize the long-term functional effects of sensory plastic modifications in the ventral posterior medial nucleus (VPM) of the thalamus following unilateral removal of facial whiskers in newborn rats. One year after this neonatal whisker deprivation, neurons in the contralateral VPM responded to cutaneous stimulation of the face at much longer minimal latencies (15.2 +/- 8.2 ms, mean +/- SD) than did normal cells (8.8 +/- 5.3 ms) in the same subregion of the VPM. In 69% of these neurons, the initial sensory responses to stimulus offset were followed for up to 700 ms by reverberant trains of bursting discharge, alternating in 100-ms cycles with inhibition. Receptive fields in the deafferented VPM were also atypical in that they extended over the entire face, shoulder, forepaw, hindpaw, and even ipsilateral whiskers. Discriminant analysis (DA) was then used to statistically evaluate how this abnormal receptive field organization might affect the ability of thalamocortical neuronal populations to "discriminate" somatosensory stimulus location. To standardize this analysis, three stimulus targets ("groups") were chosen in all animals such that they triangulated the central region of the "receptive field" of the recorded multineuronal ensemble. In the normal animals these stimulus targets were whiskers or perioral hairs; in the deprived animals the targets typically included hairy skin of the body as well as face. The measured variables consisted of each neuron's spiking response to each stimulus differentiated into three poststimulus response epochs (0-15, 15-30, and 30-45 ms). DA quantified the statistical contribution of each of these variables to its overall discrimination between the three stimulus sites. In the normal animals, the stimulus locations were correctly classified in 88.2 +/- 3.7% of trials on the basis of the spatiotemporal patterns of ensemble activity derived from up to 18 single neurons. In the deprived animals, the stimulus locations were much less consistently discriminated (reduced to 73.5 +/- 12.6%; difference from controls significant at P < 0.01) despite the fact that much more widely spaced stimulus targets were used and even when up to 20 neurons were included in the ensemble. Overall, these results suggest that neonatal damage to peripheral sense organs may produce marked changes in the physiology of individual neurons in the somatosensory thalamus. Moreover, the present demonstration that these changes can profoundly alter sensory discrimination at the level of neural populations in the thalamus provides important evidence that the well-known perceptual effects of chronic peripheral deprivation may be partially attributable to plastic reorganization at subcortical levels.

Comprehensive immunofluorescence and lectin binding analysis of intervibrissal fur innervation in the mystacial pad of the rat

The innervation of the intervibrissal fur in the mystacial pad of the rat and mouse was examined by immunofluorescence with a wide variety of antibodies for neuronal related structural proteins, enzymes, and peptides as well as for lectin binding histofluorescence with Griffonia simplicifolia (GSA). Anti-protein gene product 9.5 (PGP) immunofluorescence labeled all sets of axons and endings. The innervation in the upper dermis and epidermis was distributed through a four tiered dermal plexus. From deep to superficial, the second tier was the source of all apparent myelinated mechanoreceptors, the third tier of nearly all the peptidergic and GSA binding innervation, and the fourth tier of nonpeptidergic GSA negative innervation (peptide-/GSA-). Three types of mechanoreceptors-Merkel, transverse lanceolate, and longitudinal lanceolate endings-innervated guard hair follicles. All had similar labeling characteristics for 160 kDa and 200 kDa neurofilament subunits, peripherin, carbonic anhydrase, synaptophysin, and S100. Palisades of longitudinal lanceolate endings were part of piloneural complexes along circumferentially oriented sets of transverse lanceolate endings, peptidergic free nerve endings (FNEs), and peptide-/GSA- FNEs. The longitudinal lanceolate endings were the only mechanoreceptors in the mystacial pad that had detectable calcitonin gene-related peptide. The epidermis contained four types of unmyelinated endings: simple free nerve endings (FNEs), penicillate endings, cluster endings and bush endings. Only the simple FNEs were clearly peptidergic. Virtually all others were peptide-/ GSA-. Each bush ending was actually an intermingled cluster of endings formed by several unmyelinated axons and occasionally an Adelta axon. In contrast to the other unmyelinated innervation to the epidermis, bush endings labeled with an antibody against the Schwann cell protein S100. The necks and mouths of follicles, as well as superficial vasculature, were innervated by a mixture of unmyelinated peptidergic and/or GSA labeled sensory and sympathetic axons. Small presumptive sweat glands were innervated by three sets of peptidergic axons of which one was immunoreactive for somatostatin. Potential functions of the various sets of innervation are discussed.

The sensorineural specializations of the trunk tip (finger) of the Asian elephant, Elephas maximus

BACKGROUND

The dorsal extension of the tip of the trunk of Asian elephants (Elephas maximus), often referred to as "the finger," possesses remarkable mechanical dexterity and is used for a variety of special behaviors including grasping food and tactile and ultimately chemosensory recognition via the vomeronasal organ. The present study describes a unique sensory innervation of this specialized region of the trunk.

METHODS

The tip of the dorsal aspect of the trunk is referred to as the trunk tip finger and has been studied grossly in 13 living elephants. One tip from a male Asian elephant was obtained for histologic study when it was accidentally severed. The tissue was fixed in 10% neutral buffered formalin, and portions were either sectioned frozen or embedded in paraffin and serial sectioned. Sections were stained with silver in both cases.

RESULTS

The skin of the trunk tip finger differs from that of the surrounding areas; it contains a high density of free nerve endings, numerous convoluted branched small corpuscles, and vellus vibrissae that resemble vellus hairs, which do not protrude beyond the skin surface. The finger is thus densely innervated with three distinctive types of sensory terminals. Corpuscular receptors consist of small Pacinian corpuscles and convoluted branched simple corpuscles. Both are present in the superficial dermis. Abundant regular vibrissae are present in the skin surrounding the trunk tip finger. Short vibrissae that do not protrude from the skin surface, referred to as vellus vibrissae, are abundant in the finger tip. Both types of vibrissae are innervated by hundreds of axons resembling the mystacial vibrissae of rodents. Free nerve endings are numerous in the superficial dermis, often making intimate contact with the basal cells of rete pegs.

CONCLUSIONS

The dorsal finger of the trunk tip of Asian elephants has a unique sensory innervation that resembles aspects of sensory innervation of mystacial skin of rodents or lip tissue of monkeys. This dense sensory innervation can be correlated with the tactile ability of these animals to use the trunk finger to grasp small objects for feeding and to insert chemically active samples into the ductal orifices of the vomeronasal organ for subsequent chemosensory processing.

Experimental trigeminal nerve injury

The successful reinnervation of peripheral targets after injury varies with the axonal population of the nerve that is injured and the extent of the dislocation of its central component from the peripheral endoneurial tube. Larger-diameter axons such as those supplying mechanoreceptors recover more readily than narrower axons such as those supplying taste. A complex, bi-directional interaction between lingual epithelium and sprouting nerve results in the redifferentiation of taste buds after denervation. Dentin and the dental pulp provide a strong attraction to sprouting nerves and will become reinnervated from collateral sources if recovery of the original innervation is blocked. The most effective repair technique for transected lingual nerves is one which brings the cut ends together rather than one that provides a temporary bridge. Injuries can result in cell death in the trigeminal ganglion but only if the injury is severe and recovery is prevented. Lesser damage results in chromatolysis and the increased expression of neuropeptides. All nerve injuries bring about changes in the trigeminal nucleus. These occur as changes in receptive field and the incidence of spontaneously active neurons, effects which are consistent with the unmasking of existing afferents. These functional changes are short-lived and reversible. Morphologically, nerve injury results in terminal degeneration in the nuclei and an increased expression of the c-Fos gene and some neuropeptides. Only a chronic constriction injury induces behavioral changes. The adult trigeminal system retains considerable plasticity that permits it to respond successfully to nerve injury. Much remains to be learned about this response, particularly of the trophic factors that control peripheral recovery and the central response to more severe injuries.

Merkel cells do not require trophic maintenance from the nerves in adult human skin

A 34-year-old Japanese man with hereditary sensory neuropathy was examined to evaluate the distribution, density and inter-relationship between Merkel cells and peripheral nerves in the skin. An epidermal sheet of affected plantar skin showed numerous CAM 5.2-reactive Merkel cells, whereas PGP 9.5-reactive peripheral nerves were completely absent in the epidermis and dermis. These findings strongly suggest that Merkel cells do not require trophic maintenance from nerves in adult human skin.

Innervation of nonmystacial vibrissae in the adult rat

Vibrissal follicle-sinus complexes (F-SCs) in the mystacial pad of rodents are heavily innervated by different types of sensory nerve endings. One site in mystacial F-SCs, the inner conical body (ICB), is uniquely well innervated only in those species, such as the rat, that rhythmically whisk their mystacial vibrissae. In this study, we examined the innervation of rat nonmystacial F-SCs, which are not whisked. Supraorbital, posteroorbital, lateral cervical, median cervical, submental, and carpal forelimb F-SCs were cut on a cryostat and were either prepared for anti-human protein gene product (PGP 9.5) immunofluorescence or stained using the Winkelmann silver technique. Much of the innervation of the nonmystacial F-SCs is similar to that of mystacial F-SCs. All are innervated by a large deep vibrissal nerve (DVN) and several smaller superficial vibrissal nerves (SVNs). As in the mystacial pad, the SVNs show a distribution of Merkel and free nerve endings qualitatively similar to the rete ridge collar of all the nonmystacial F-SCs as well as provide circumferentially oriented endings to the ICBs to all but median-cervical and carpal F-SCs. Not only was the ICB innervation relatively sparse in median-cervical and carpal F-SCs, but a large portion of the carpal ICB innervation also ascended from the DVNs, which make only a small ICB contribution in other locations. Similar to mystacial pad F-SCs, the DVNs provided Merkel and lanceolate endings to the level of the ring sinus as well as reticular and irregular lanceolate-like endings to the level of the cavernous sinus. However, all but the posteroorbital F-SCs have relatively few lanceolate endings. Carpal F-SCs also have relatively few ring-sinus Merkel endings, which are diffusely distributed, are limited to the superficial portion of the outer root sheath. They also lack reticular and irregular lanceolate-like endings in the cavernous sinus. However, carpal F-SCs have a unique set of corpuscular endings in the ICB, ring sinus, and cavernous sinus that are rarely seen in other F-SCs. PGP 9.5 immunofluorescence also revealed two sets of fine-caliber profiles at the level of the ICB and ring sinus that were not previously seen in mystacial F-SCs. Although there was no correlation between ICB innervation and whisking, the regional variations in F-SC innervation suggest that functional differences may exist between vibrissae at different locations in the body.

Effect of vibrissae deprivation on follicle innervation, neuropeptide synthesis in the trigeminal ganglion, and S1 barrel cortex plasticity

Deprivation of vibrissae from an early age causes plasticity in S1 barrel cortex. This method of deprivation is most likely to induce plasticity by altering the balance of primary afferent activity from the deprived and spared vibrissae. To study whether or not induction or expression of this type of plasticity might be affected by follicle nerve injury caused by the deprivation technique, three different methods of detecting nerve injury were used: counting axon numbers in the distal follicle nerve, quantifying morphological changes in axons, and measuring neuropeptide expression in the trigeminal ganglion cells. First, nerves innervating follicles chronically deprived of vibrissae from birth had the same number of myelinated and unmyelinated axons as nerves from normally reared animals. Second, axons innervating deprived follicles showed no morphological changes in myelination or mitochondria characteristic of damaged nerves. Third, the corresponding nerve cell bodies in the trigeminal ganglion did not show upregulation of galanin or neuropeptide Y expression. In contrast, animals receiving mild injury of the follicle nerve endings (by cauterization of the follicle) showed profound changes in axonal myelination and mitochondria and increases in neuropeptide expression. These results imply that vibrissae deprivation does not act by inducing injury of the follicular nerve, suggesting that changes in the balance of follicle nerve activity are the cause of cortical plasticity. Consistent with this notion, a fourth experiment demonstrated that trimming the vibrissae induces cortical plasticity comparable to that induced by complete vibrissae removal.

Innervation of putative rapidly adapting mechanoreceptors by calbindin- and calretinin-immunoreactive primary sensory neurons in the rat

Calbindin and calretinin are two homologous calcium-binding proteins that are expressed by subpopulations of primary sensory neurons. In the present work, we have studied the distribution of the neurons expressing calbindin and calretinin in dorsal root ganglia of the rat and their peripheral projections. Calbindin and calretinin immunoreactivities were expressed by subpopulations of large- and small-sized primary sensory neurons and colocalized in a majority of large-sized ones. The axons emerging from calbindin- or calretinin-immunoreactive neurons innervated muscle spindles, Pacini corpuscles and subepidermal lamellar corpuscles in the glabrous skin, formed palisades of lanceolate endings around hairs and vibrissae, and gave rise to intraepidermal nerve endings in the digital skin. Since most of these afferents are considered as rapidly adapting mechanoreceptors, it is concluded that calbindin- or calretinin-expressing neurons innervate particular mechanoreceptors that display physiological characteristics of rapid adaptation to stimuli.

The innervation of the mystacial pad of the rat as revealed by PGP 9.5 immunofluorescence

The innervation of the mystacial pad in the rat was investigated with the aid of antihuman protein gene product (PGP) 9.5 immunofluorescence. PGP 9.5 is ubiquitin carboxyl-terminal hydrolase, which is distributed throughout neuronal cytoplasm. This technique revealed all previously known innervation as well as a wide variety of small-caliber axons and some endings of large-caliber afferents that had not been observed before. Newly revealed innervation affiliated with vibrissal-follicle sinus complexes included 1) fine-caliber, radially oriented processes in the epidermal rete ridge collar; 2) a loose network of fine-caliber, circumferentially arrayed processes in the centrifugal part of the mesenchymal sheath at the level of the ring sinus; 3) a loose haphazard network of fine-caliber and medium-caliber processes in the mesenchymal sheath and among the trabeculae of the cavernous sinus; 4) a loose network of circumferentially arrayed processes within the mesenchymal sheath of the cavernous sinus and in close proximity to the basement membrane; 5) a dense network of reticular-like endings provided by large-caliber afferents to the mesenchymal sheath in the upper part of the cavernous sinus; and 6) fine-caliber innervation to the dermal papilla at the base of all vibrissal shafts. In the intervibrissal skin, a dense distribution of fine-caliber individual and clustered profiles was detected in the epidermis. In addition to previously known innervation, Merkel endings were consistently observed in the epidermis at the mouths of guard hairs, loose networks of fine-caliber axons were found around the necks of occasional guard hairs, and fine-caliber profiles were frequently affiliated with vellus hairs. Vascular profiles were heavily innervated throughout the dermis. Axons and motor end plates of the facial nerve innervation to papillary muscles also were labeled. Transection of the infraorbital nerve eliminated all but the facial nerve innervation. Unilateral removal of the superior cervical ganglion eliminated the innervation to the dermal papillae but caused no other noticeable reduction. PGP 9.5-like immunofluorescence was also moderately expressed in apparent Schwann cells, in Merkel cells only in the external root sheath of vibrissal follicles, and in apparent dendritic and/or Langerhans cells usually located in the epidermis and occasionally in the follicles. PGP 9.5-like immunofluorescence persisted in highly vacuolated profiles along the usual courses of medium to large-caliber axons 2 weeks after nerve transection. The possible functional role of the newly discovered innervation is considered along with that of previously identified afferents.

SmI cortical barrels in an Australian marsupial, Trichosurus vulpecula (brush-tailed possum): structural organization, patterned distribution, and somatotopic relationships

This study reports on the cerebral cortex of an Australian marsupial, Trichosurus vulpecula (brush-tailed possum). It consists of an analysis of layer IV of somatosensory cortex in tangential sections of flattened specimens and in oblique radial sections stained to show Nissl substance or myelin, or tested for succinic dehydrogenase. It includes results of electrophysiological mapping experiments that ascertained the somatotopic significance of the cytoarchitecture of this cortical region. Layer IV has two interlocking cytoarchitectural fields: one granular (the barrelfield, comprising cell-dense barrels 150 to 500 microns in diameter) and one dysgranular. Only neurons within the granular field responded to light cutaneous stimulation. In the barrelfield cell-sparse septa (about 100 microns wide), low in succinic dehydrogenase activity and containing many radial myelinated axons, separate adjoining barrels. Possum barrels are "solid," lacking the prominent hollows characteristic of most rodent barrels. In some specimens three to five small neuronal "lobules" may constitute each large barrel. In tangential sections the size, shape, and arrangement of barrels combine to form a histological caricature of the possum's body, especially of the face and forepaw. Six rows of "mystacial barrels" are homeomorphic to the six rows of large mystacial vibrissae, and "forepaw barrels" are homeomorphic to the glabrous palmar and apical digital pads. Correlating cortical recording sites and receptive fields confirmed that individual barrels represent specific cutaneous regions. These results show that the cortical barrels of brush-tailed possums are remarkably similar to those of rodents, in structure, arrangement, and functional significance.

Structure, vascularization, and innervation of the mystacial pad of the rat as revealed by the lectin Griffonia simplicifolia

The mystacial pad of the rat is endowed with rows of vibrissal follicle-sinus complexes (F-SCs) that receive a dense and rich variety of innervation, much of which is C fibers. Each F-SC consists of a follicle at the core of a spindle-shaped, encapsulated vascular sinus. Previous studies have shown that the B subunit of the lectin Griffonia simplicifolia (GSA I-B4) binds selectively to a subset of small neurons in the trigeminal ganglion and to a subset of C fibers preferentially distributed to inner lamina II and outer lamina III of nucleus caudalis in the brainstem trigeminal complex in the rat. These laminae are also a major site of termination for afferents in superficial vibrissal nerves (SVNs) that innervate the upper portion of F-SCs. To determine the peripheral distribution of the afferents that bind GSA I-B4, mystacial pads from rats were prepared for fluorescence microscopy with GSA I-B4 conjugated to rhodamine. At the neck of each F-SC, numerous circumferentially oriented bundles of fine-caliber axonal profiles were labeled in the inner conical body, which receives nearly all of its innervation from the SVNs. A sparse, random distribution of fine-caliber profiles from deep vibrissal nerves was labeled at the level of the cavernous sinus in the deep half of the F-SCs. GSA I-B4 also labeled a variety of nonneural structures. By binding to vascular linings, GSA I-B4 revealed a dense, highly organized capillary system within the mesenchymal sheath that forms the inner lining of the vascular sinuses. Thus each F-SC appears to have a closed capillary system within the open vascular sinus. Trabeculae within the lumen of the cavernous sinus were also revealed to span between the sinus capsule and the mesenchymal sheath only about midway along the length of the follicle instead of the entire deeper half, as was previously believed. in addition, GSA I-B4 bound to the surface of follicular cells preferentially in the superficial half of the F-SCs. Sweat glands within the intervibrissal fur and some cells within sebaceous glands in F-SCs were also labeled as well as their ducts. The potential functional implications of these various features are discussed.

Unmyelinated innervation of sinus hair follicles in rats

Unmyelinated nerve fibres comprise approximately one third of the innervation of rodent sinus hair follicles but their function is unknown. They may play a role as high-threshold sensory fibres, or may be autonomic efferents controlling the vascular sinus. In the present experiments capsaicin and surgical sympathectomy were used to establish whether these unmyelinated fibres are afferent fibres or autonomic efferents. The deep vibrissal nerves of mystacial follicles (C1 and C4) and a non-mystacial follicle (the postero-orbital, PO) were assessed in normal adult animals (n = 6) and compared with those treated with neonatal capsaicin (n = 6) or bilateral superior cervical ganglionectomy (n = 7). In capsaicin-treated animals, counts of fibres in the deep vibrissal nerves from all follicles showed normal numbers of myelinated axons, but approximately 80% reduction in unmyelinated fibres (normal mean +/- SD: C1 94 +/- 10, C4 89 +/- 9, PO 85 +/- 6; after neonatal capsaicin: C1 17 +/- 8, C4 16 +/- 6, PO 18 +/- 6; n = 6, P < 0.001 for all follicles). After sympathectomy there was no significant reduction in myelinated or unmyelinated fibre numbers. Labelling of PO follicles with WGA-HRP showed minimal numbers of labelled cells (0-10) within the superior cervical ganglion, also suggesting minimal sympathetic innervation. This sparse sympathetic supply to the follicle was further demonstrated by a lack of tyrosine hydroxylase reactivity within the follicle complex; tissues outside the dermal capsule showed reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential differentiation of sensory innervation in the mystacial pad of the ferret

The mystacial pad of the ferret has an elaborate sensory innervation provided by three types of terminal nerves that arise from the infraorbital branch of the trigeminal nerve. Deep and superficial vibrissal nerves innervate nearly exclusive targets in the large follicle-sinus complexes (F-SCs) at the base of each tactile vibrissa. Dermal plexus nerves innervate the fur between the vibrissae. Each type of nerve provides a similar variety of sensory endings, albeit to different targets. In this study, Winkelmann and Sevier-Munger reduced silver techniques revealed that most of the endings differentiate postnatally in an overlapping sequence like that observed previously in the rat. Afferents from the deep vibrissal nerves begin to differentiate first, followed successively by those from superficial vibrissal nerves and the dermal plexus. Within each type of nerve, Merkel endings begin to differentiate first, followed successively by lanceolate endings and circumferential endings. In the ferret, the differentiation of the intervibrissal fur and its innervation is slightly delayed but substantially overlaps the development of the vibrissal innervation, whereas in the rat it occurs almost entirely later. There was no evidence of a transient exuberant or misplaced innervation or other secondary remodeling. Differentiating afferents and endings are located only in the sites normally seen in the adult, suggesting a high degree of afferent-target specificity. In the ferret, innervation is virtually lacking in one target--the inner conical body of the F-SCs, which is densely innervated in the rat. This lack was due to a failure of innervation to develop rather than to a secondary elimination of a transient innervation.

Sensory innervation in the inner conical body of the vibrissal follicle-sinus complex of the rat

The innervation of the inner conical body of the vibrissal follicle-sinus complex of the rat was examined by high-voltage and conventional transmission electron microscopy of serial and semi-serial sections. The inner conical body is innervated by axons supplied almost exclusively by several superficial vibrissal nerves that arise from the infraorbital branch of the trigeminal nerve and converge upon the neck of the follicle-sinus complex. Each superficial vibrissal nerve contains a few A delta myelinated axons and several bundles of 20-30 unmyelinated axons. These axons enter the inner conical body and distribute circumferentially within 7-10 ring-like arrays that encircle the vibrissal follicle and are stacked through the superficial-to-deep extent of the inner conical body. Each ring consists of 1 or 2 myelinated axons and several small bundles of 2-15 unmyelinated axons enclosed in sheaves of parallel collagen fibrils. Myelinated axons provide exclusively lanceolate endings that may arise at the termination of the axon or at nodes of Ranvier. Within the small bundles, unmyelinated axons individually terminate in succession as abrupt cytoplasmic swellings referred to as cytoplasmic blebs, which contain mitochondria or clusters of clear or dense-core vesicles. Because of their affiliation with collagen fibrils and the proximity of myelinated axons, the blebbed endings may have been misinterpreted as Ruffini endings in previous studies. Their structure, distribution, and origin from unmyelinated axons suggest that the blebbed endings may constitute a unique array of low-threshold C-mechanoreceptors.

Dual innervation of the rat vibrissa: responses of trigeminal ganglion cells projecting through deep or superficial nerves

The rat vibrissal follicle-sinus complex is innervated by a deep vibrissal nerve (DVN) and several smaller fascicles traveling in the dermis [conus or superficial vibrissal nerves, (SVNs)]. The function of the SVNs is unknown, although it has been suggested in a comparative study that they form part of a diffuse, multivibrissal system. Anatomical and electrophysiological methods were used to test this hypothesis and to determine if DVN and SVN fibers have differing response profiles. No ganglion cells were double-labeled after retrograde tracer injections in the DVN and SVNs of single follicles. Electron microscopy showed that selective transection of the DVN caused no SVN degeneration or vice versa. Thus, the dual innervation of the vibrissa arises from separate ganglion cells that project through separate nerves. Ganglion cells with A-row vibrissa receptive fields were studied before and after cutting the DVN and/or SVNs to the responsive vibrissa in order to identify their peripheral trajectories. In this sample, 83% projected through a DVN and 17% via a SVN. SVN or DVN cells were not spontaneously active. All cells responded to single vibrissae only; none were responsive to intervibrissal hairs or skin. Latencies to electrical stimulation were similar for DVN and SVN cells. Adaptation rates and threshold measurements were also similar in the two groups: 60% of the DVN cells and 80% of the SVN cells gave slowly adapting responses to sustained vibrissal displacement; threshold displacements ranged from less than 1 degrees to greater than 15 degrees for both SVN and DVN cells. Direction sensitivity was found in all DVN and SVN slowly adapting cells, with most cells responding to movements in one or two quadrants. For SVN cells, sequential circumferential nerve sections indicated that the fiber's directional sensitivity matched the direction of the fiber's entry into the follicle. The two groups differed in their responses to pushing in or pulling on the hair shaft. All the DVN cells were responsive to both of these stimuli, while for SVN cells pushing activated only 40% and none were responsive to pulling the hair. Another difference in the two groups was that no injury discharges occurred after cutting SVNs, but were present in 44% of DVN cells. These data suggest that DVN and SVNs are similar in the majority of response properties. There is also no evidence to support the hypothesis that SVNs provide diffuse, multivibrissal inputs.

The rat's postero-orbital sinus hair: II. Normal morphology and the increase in peripheral innervation with adjacent nerve section

The morphology and innervation of the postero-orbital (PO) sinus hair has been studied in normal rats and in adult animals in which an adjacent nerve, the infraorbital nerve, was sectioned on postnatal day 0 or day 7. The normal morphology of the follicle was similar to that of mystacial sinus hairs. However, the normal innervation differed from mystacial follicles in three respects: (1) instead of a separate innervation, the deep vibrissal nerve (DVN) and dermal plexus were supplied by a common follicle and skin nerve, named here the postero-orbital cutaneous nerve, a branch of the zygomaticofacial nerve; (2) the entry of the DVN through the capsule was highly variable; in some cases fascicles entered in close proximity, but in others they were widely distributed around the capsule; and (3) two or three small nerves, called here anastomosing nerves, were found to leave the PO follicle. These arose from the DVN after it had passed through the capsule to the cavernous sinus. The anastomosing nerves passed back through the capsule and ascended on the outer surface of the follicle to join the dermal plexus. Each nerve contained 1-4 myelinated fibres and 11-35 unmyelinated fibres. Infraorbital (IO) nerve section on day 0 caused a 19% (P less than 0.001, n = 8) increase in numbers of fibers to the DVN on the lesioned side. Most of the increase was due to unmyelinated fibres with no significant change in myelinated axons. No change in axon numbers in the DVN occurred after day 7 lesions. Labelling of the mystacial pad and the PO follicle did not result in any double labelling of cells in the trigeminal ganglion, in either normal or lesioned animals, making it improbable that the increased numbers of unmyelinated axons arose from rerouting of infraorbital fibres. It is suggested that the increased innervation of the PO follicle may arise by the rescue of ganglion cells from developmentally programmed cell death.

Non-innervated Merkel cells and Merkel-neurite complexes in human oral mucosa revealed using antiserum to protein gene product 9.5

Merkel cells are non-keratinized cells present in many different epithelia, but whose origin and functional role are still controversial. They were here investigated by means of antisera to the neural and neuroendocrine markers protein gene product 9.5 (PGP 9.5), and neurone-specific enolase. The expression of both markers in Merkel cells of human gingival and palatal mucosa was confirmed. Merkel cell-neurite complexes and isolated non-innervated Merkel cells had a similar morphology when stained by either antiserum. Merkel-neurite complexes were clustered in relatively large numbers in the lingual gingiva, thus constituting structures closely similar to the 'touch domes' in the skin. Clusters of non-innervated cells showing the same immunohistochemical features as Merkel cells were also demonstrated. In other areas of the oral mucosa, the innervated and non-innervated elements were only occasionally seen but there were many encapsulated Meissner-like receptors. When comparing the two different antisera, anti-PGP 9.5 appeared to provide a more consistent labelling of small fibres inside the epithelium and of bulb-like terminals on Merkel cells.

Expression of ALZ-50 immunoreactivity in the developing principal sensory nucleus of the trigeminal nerve: effect of transecting the infraorbital nerve

Many neurons in the CNS die as a consequence of normal development. As these neurons die, they may be programmed to produce 'death proteins'. We explored the possibility that an antigen recognized by ALZ-50, a protein expressed in Alzheimer's-type neurofibrillary tangles, is generated during the process of neuronal death. The effects of transecting the infraorbital nerve on the expression of ALZ-50 immunoreactivity and neuronal death in the principal sensory nucleus of the trigeminal nerve (PSN) was examined. In normal rats, a small number of PSN neurons was ALZ-50-positive on postnatal day (P) 3. Transections on the day of birth (i.e. during the period of naturally occurring neuronal death) led to a 5-fold increase in the number of immunoreactive neurons expressing a 56-kDa protein on P3. In contrast, lesions on P25 (i.e. after the period of naturally occurring neuronal death) did not induce any neurons to exhibit ALZ-50 immunoreactivity. Thus, the 56-kDa protein recognized by ALZ-50 appears to be a death protein which is transiently expressed during the period of naturally occurring neuronal death. It is appealing to speculate that the pathological degeneration described in Alzheimer's brains results from the up-regulation of a quiescent developmental program.

Central projections of primary sensory neurons innervating different parts of the vibrissae follicles and intervibrissal skin on the mystacial pad of the rat

The cell bodies and central projections of neurons innervating the vibrissae follicles and adjacent skin in the rat were investigated by retrograde and transganglionic transport of HRP. The cell bodies of neurons innervating the vibrissa follicle via the deep vibrissa nerve (DVN) were the largest, followed by those innervating the follicle via the superficial vibrissa nerve (SVN). The smallest cell bodies were those innervating the intervibrissal skin. The DVN neurons terminated centrally as an almost uninterrupted column through the trigeminal sensory nuclear complex. The DVN projections to nucleus caudalis and C1 dorsal horn were entirely restricted to laminae III, IV, and V. Besides the projections to lamina V, the DVN projections were strictly localized somatotopically at all levels replicating the peripheral organization of the vibrissae. The SVNs projected sparsely to midlevels of the main sensory nucleus but not to nuclei oralis and interpolaris. The main SVN projections appeared in laminae I-III of nucleus caudalis. In addition, a small projection to lamina V was observed. The projections to laminae II and III were organized mediolaterally in a similar way as the DVN projections; those to laminae I and V were less restricted. The intervibrissal skin neurons projected sparsely to the caudal main sensory nucleus and to the border between nuclei oralis and interpolaris. The projections to nucleus caudalis were restricted to laminae I-III and V and were organized in a similar way as the SVN projections.

Merkel cells in vitro: production of nerve growth factor and selective interactions with sensory neurons

A method has been developed for obtaining mixed primary cultures of dissociated epidermis enriched in Merkel cells. Merkel cells obtained from embryonic rat buccal pads were grown in serum-free medium and identified in vitro using a variety of histological and immunohistochemical markers. Quinacrine, a fluorescent amine, which has been used to identify Merkel cells in situ, labeled a morphologically distinct population of cells in vitro. Cells labeled with quinacrine had a large, phase bright nucleus with prominent nucleoli, surrounded by a phase dark perinuclear ring. Antibodies directed against neuron-specific enolase, another marker for Merkel cells in situ, and antibodies against a well-characterized neuroendocrine vesicle antigen also labeled this population of quinacrine fluorescent cells. Electron microscopic examination of our cultures indicated that cells containing characteristic features of Merkel cells including cytoplasmic dense-cored granules were present. A small but significant increase in the number of Merkel cells was observed over time in culture. Merkel cells supported the survival and outgrowth of both trigeminal ganglion sensory neurons and sympathetic neurons from the superior cervical ganglion in serum-free medium in the absence of exogenous nerve growth factor (NGF). Immunoblots probed with antibodies directed against NGF demonstrated that NGF was present in the medium taken from these cultures. NGF-like immunoreactivity colocalized to cells containing quinacrine fluorescence in situ and in vitro. Addition of antibodies directed against NGF to cocultures of Merkel cells and neurons decreased survival of sympathetic neurons by 90% and decreased survival of sensory neurons by 60%. These results suggest that Merkel cells are capable of providing trophic support for their normal complement of sensory neurons by producing NGF. Selective recognition of these targets was studied in vitro by characterizing the interactions between Merkel cells and growth cones from sensory or sympathetic neurons using both time-lapse videomicroscopy and standard morphometry of fixed cocultures. The majority of trigeminal ganglion sensory neurons (approximately 60%) extended growth cones onto clusters of Merkel cells. Neurites which contacted clusters of Merkel cells were significantly more highly branched than those growing on collagen. In contrast, the majority of sympathetic neurons (greater than 90%) failed to grow onto Merkel cells. Growth cones of sympathetic neurons often "collapsed" and retracted when contact was made with a cluster of Merkel cells. Fixation of Merkel cells with paraformaldehyde prior to coculture did not affect this difference between sensory and sympathetic neurite extension onto the Merkel cells. However, prior fixation of Merkel cells eradicated the apparent Merkel ce-induced branching of sensory neurites.(ABSTRACT TRUNCATED AT 400 WORDS)

Degeneration and regeneration of peripheral nerve in the rat trigeminal system: III. Abnormal sensory reinnervation of rat guard hairs following nerve transection and crush

The present study was undertaken in an attempt to better understand the abnormalities of cutaneous sensibility that are present in patients following nerve injury with concomitant cutaneous denervation and subsequent reinnervation. Reinnervated intervibrissal pelage of the rat mystacial pad was studied in silver-impregnated sections 3 and 5 months after transecting and 2 and 5 months after crushing the infraorbital nerve. The sensory terminals on guard and vellus hairs were analyzed in serial paraffin sections and in thick frozen sections. In normal rat mystacial skin, approximately nine/ten of innervated guard hairs have a typical piloneural complex consisting of a palisade of highly regular lanceolate terminals surrounded by circularly arranged Ruffini terminals and free nerve endings (FNEs). The remaining one of ten innervated guard hairs has only circularly arranged presumptive FNEs and Ruffini terminals. Vellus hairs, either singly or in clusters, typically have only circularly arranged terminals that in many cases are simple FNEs. We first recognized abnormalities in innervation of hairs following nerve transection and fully expected nerve terminals to be completely normal following nerve crush. Almost all reinnervated sensory nerve terminals associated with guard hairs were markedly abnormal following nerve transection and quantitatively abnormal following nerve crush. Following nerve transection, lanceolate terminals were almost completely absent, and they were remarkably reduced in number following nerve crush. Vellus hairs when reinnervated typically lacked the complex circular presumptive Ruffini terminals. These findings may be in part the basis for the abnormal cutaneous sensory perceptions (dysasthesias and paresthesias) noted in human subjects following damage to nerves with subsequent sensory reinnervation of the skin.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of Merkel cells and mechanosensory axons of the rat by styryl pyridinium dyes

The epidermal Merkel cells and their sensory innervation serve tactile sensation in vertebrates. In this study the fluorescent cationic mitochondrial dye, 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (4-Di-2-ASP), which has recently been used as a vital stain for motor and autonomic nerve terminals, was tested for its ability to stain Merkel cells and sensory fibers in the snout of the rat. Brightly-fluorescent structures resembling Merkel cells as well as nerve fibers and their terminations were evident in whole mounts of the vibrissal follicle. Unilateral denervation of the vibrissal follicles soon after birth resulted in a staining pattern remarkably similar to that obtained after labelling of the Merkel cells selectively with the fluorescent marker quinacrine, but all fiber staining was abolished. Likewise, in the separated epidermis of other skin regions, including the hairy and glabrous skin of the nose, the staining pattern revealed by 4-Di-2-ASP was indistinguishable from that obtained by quinacrine fluorescence. These results indicate that certain styryl pyridinium dyes may be used as vital stains for epidermal Merkel cells as well as cutaneous mechanosensory axons.

Anatomical consequences of neonatal infraorbital nerve transection upon the trigeminal ganglion and vibrissa follicle nerves in the adult rat

A large body of experimental literature has demonstrated that neonatal infraorbital nerve damage in rodents produces anatomical and/or functional alterations of the normal whisker representation in central trigeminal structures. Less is known about the organization of primary afferent components of the trigeminal system following this manipulation. Such information provides an important basis for interpreting the central changes observed following damage of infraorbital nerve fibers at birth. We have therefore examined the composition and order of peripheral innervation in the pathway from the trigeminal ganglion to the vibrissa follicles in adult rats subjected to unilateral neonatal infraorbital nerve transection. Electron microscopy was used to determine the number and diameter of myelinated and unmyelinated fibers in vibrissa follicle nerves of these animals. Wheat germ agglutinin-horseradish peroxidase and fluorescent retrograde tracers were employed to examine the number and diameter, as well as the topographic organization and branching, of ganglion cells innervating the vibrissae in these rats. The data presented below indicate that neonatal infraorbital nerve transection has the following consequences within the adult trigeminal nerve and ganglion: 1) an alteration of the gross morphology of vibrissal nerves, 2) a significant reduction in the average number (85.4%) and diameter (32.6%) of myelinated, but not unmyelinated, follicle nerve axons, 3) a significant decrease in the average number (36.8%) of trigeminal ganglion cells innervating vibrissa follicles, 4) no significant change in the distribution of ganglion cell diameters, 5) an increase in peripheral branching (1.8-fold) of these ganglion cell axons, and 6) an alteration of somatotopic order within the trigeminal ganglion. Taken together, these data indicate that neonatal infraorbital nerve transection produces a profound reorganization of the primary afferent component of the trigeminal neuraxis.

Structure-function relationships in the rat brainstem subnucleus interpolaris: II. Low and high threshold trigeminal primary afferents

Prior studies indicate that vibrissa, guard hair, hairy skin, mucosa, and nociceptive trigeminal primary afferents give rise to morphologically distinct terminal arbors in the medullary dorsal horn. The present study describes the extent to which similar structure-function relationships exist in the rostrally adjacent subnucleus interpolaris (SpVi). Seventy-three axons were physiologically characterized and visualized by standard intra-axonal HRP labeling techniques. They responded to guard hair (GH) or vibrissa (VIB) deflection; gentle pressure applied to hairy skin (HS), glabrous skin (GS), lingual mucosa (LM), or an incisor (PER); or a noxious pinch of the face (NOX). Response latencies to trigeminal ganglion shocks were equivalent for all categories with low threshold receptive fields (mean = 0.44 ms), and these were significantly shorter than those of fibers with high threshold NOX receptive fields (mean = 0.88 ms). All axons gave off transversely oriented collaterals into SpVi with rostrocaudal discontinuities in their arbors. Collaterals were topographically organized. Axons innervating the rostral mouth and face terminated medially, and those that supplied the caudal face innervated successively more lateral SpVi. The dorsal face was represented in the ventral SpVi, whereas the ventral face and mouth were represented more dorsally. This transverse topography extended largely throughout the rostrocaudal extent of SpVi. VIB, GH, GS, and LM collaterals had similar configurations with circumscribed arbors. HS, PER, and NOX arbors had a "stringy" shape without a clear terminal focus, save for the fact that PER and NOX collaterals often terminated in rostrally displaced substantia gelatinosa at the level of the caudal SpVi. Analysis of variance, considering only those data from mystacial VIB, GH, and HS fibers, indicated significant differences for all of the following measures: number of collaterals, number of boutons per collateral, arbor area, arbor circumference, and arbor circularity (form factor). A similar analysis, considering all fiber types, indicated significant differences for only the following measures: number of collaterals, arbor area, and arbor circumference. Individual group comparisons between the more heavily sampled functional categories indicated that GH afferents had significantly fewer collaterals, fewer boutons per collateral, smaller arbor area, shorter arbor circumference, and more circular arbors than those of HS axons. VIB fibers tended to fall between GH and HS afferents with respect to number of collaterals, arbor area, circumference, and circularity. The remaining functional groups were not as orderly.(ABSTRACT TRUNCATED AT 400 WORDS)