The role of sensory innervation in the development of mechanoreceptors

Demise of nociceptive Schwann cells causes nerve retraction and pain hyperalgesia

ABSTRACT

Recent findings indicate that nociceptive nerves are not "free", but similar to touch and pressure sensitive nerves, terminate in an end-organ in mice. This sensory structure consists of the nociceptive nerves and specialized nociceptive Schwann cells forming a mesh-like organ in subepidermis with pain transduction initiated at both these cellular constituents. The intimate relation of nociceptive nerves with nociceptive Schwann cells in mice raises the question whether defects in nociceptive Schwann cells can by itself contribute to pain hyperalgesia, nerve retraction, and peripheral neuropathy. We therefore examined the existence of nociceptive Schwann cells in human skin and their possible contribution to neuropathy and pain hyperalgesia in mouse models. Similar to mouse, human skin contains SOX10+/S100B+/AQP1+ Schwann cells in the subepidermal border that have extensive processes, which are intimately associated with nociceptive nerves projecting into epidermis. The ablation of nociceptive Schwann cells in mice resulted in nerve retraction and mechanical, cold, and heat hyperalgesia. Conversely, ablating the nociceptive nerves led to a retraction of epidermal Schwann cell processes, changes in nociceptive Schwann cell soma morphology, heat analgesia, and mechanical hyperalgesia. Our results provide evidence for a nociceptive sensory end-organ in the human skin and using animal models highlight the interdependence of the nerve and the nociceptive Schwann cell. Finally, we show that demise of nociceptive Schwann cells is sufficient to cause neuropathic-like pain in the mouse.

Muscle spindle function in healthy and diseased muscle

Almost every muscle contains muscle spindles. These delicate sensory receptors inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for motor control, for maintaining posture and for a stable gait. Many neuromuscular diseases affect muscle spindle function contributing, among others, to an unstable gait, frequent falls and ataxic behavior in the affected patients. Nevertheless, muscle spindles are usually ignored during examination and analysis of muscle function and when designing therapeutic strategies for neuromuscular diseases. This review summarizes the development and function of muscle spindles and the changes observed under pathological conditions, in particular in the various forms of muscular dystrophies.

Feature-selective encoding of substrate vibrations in the forelimb somatosensory cortex

The spectral content of skin vibrations, produced by either displacing the finger across a surface texture¹ or passively sensing external movements through the solid substrate^2,3, provides fundamental information about our environment. Low-frequency flutter (below 50 Hz) applied locally to the primate fingertip evokes cyclically entrained spiking in neurons of the primary somatosensory cortex (S1), and thus spike rates in these neurons increase linearly with frequency^4,5. However, the same local vibrations at high frequencies (over 100 Hz) cannot be discriminated on the basis of differences in discharge rates of S1 neurons^4,6, because spiking is only partially entrained at these frequencies⁶. Here we investigated whether high-frequency substrate vibrations applied broadly to the mouse forelimb rely on a different cortical coding scheme. We found that forelimb S1 neurons encode vibration frequency similarly to sound pitch representation in the auditory cortex^7,8: their spike rates are selectively tuned to a preferred value of a low-level stimulus feature without any temporal entrainment. This feature, identified as the product of frequency and a power function of amplitude, was also found to be perceptually relevant as it predicted behaviour in a frequency discrimination task. Using histology, peripheral deafferentation and optogenetic receptor tagging, we show that these selective responses are inherited from deep Pacinian corpuscles located adjacent to bones, most densely around the ulna and radius and only sparsely along phalanges. This mechanoreceptor arrangement and the tuned cortical rate code suggest that the mouse forelimb constitutes a sensory channel best adapted for passive 'listening' to substrate vibrations, rather than for active texture exploration.

Afferent Innervation, Muscle Spindles, and Contractures Following Neonatal Brachial Plexus Injury in a Mouse Model

PURPOSE

We used an established mouse model of elbow flexion contracture after neonatal brachial plexus injury (NBPI) to test the hypothesis that preservation of afferent innervation protects against contractures and is associated with preservation of muscle spindles and ErbB signaling.

METHODS

A model of preganglionic C5 through C7 NBPI was first tested in mice with fluorescent axons using confocal imaging to confirm preserved afferent innervation of spindles despite motor end plate denervation. Preganglionic and postganglionic injuries were then created in wild-type mice. Four weeks later, we assessed total and afferent denervation of the elbow flexors by musculocutaneous nerve immunohistochemistry. Biceps muscle volume and cross-sectional area were measured by micro computed tomography. An observer who was blinded to the study protocol measured elbow flexion contractures. Biceps spindle and muscle fiber morphology and ErbB signaling pathway activity were assessed histologically and immunohistochemically.

RESULTS

Preganglionic and postganglionic injuries caused similar total denervation and biceps muscle atrophy. However, after preganglionic injuries, afferent innervation was partially preserved and elbow flexion contractures were significantly less severe. Spindles degenerated after postganglionic injury but were preserved after preganglionic injury. ErbB signaling was inactivated in denervated spindles after postganglionic injury but ErbB signaling activity was preserved in spindles after preganglionic injury with retained afferent innervation. Preganglionic and postganglionic injuries were associated with upregulation of ErbB signaling in extrafusal muscle fibers.

CONCLUSIONS

Contractures after NBPI are associated with muscle spindle degeneration and loss of spindle ErbB signaling activity. Preservation of afferent innervation maintained spindle development and ErbB signaling activity, and protected against contractures.

CLINICAL RELEVANCE

Pharmacologic modulation of ErbB signaling, which is being investigated as a therapy for congestive heart failure, may be able to recapitulate the protective effects of afferent innervation in spindle development and contracture prevention. Muscle spindle preservation may also have implications in proprioception and motor learning, both of which are impaired in NBPI.

Dependence on the transcription factor Shox2 for specification of sensory neurons conveying discriminative touch

Touch sensation is mediated by specific subtypes of sensory neurons which develop in a hierarchical process from common early progenitor neurons, but the molecular mechanism that underlies diversification of touch-sensitive mechanoreceptive neurons is not fully known. Here, we use genetically manipulated mice to examine whether the transcription factor short stature homeobox 2 (Shox2) participates in the acquisition of neuronal subtypes conveying touch sensation. We show that Shox2 encodes the development of category I low-threshold mechanoreceptive neurons in glabrous skin, i.e. discriminative touch-sensitive neurons which form innervations of epidermal Merkel cell and Meissner corpuscles. In contrast, other sensory fiber endings, including those innervating Pacinian corpuscles, are not dependent on Shox2. Shox2 is expressed in neurons of most or all classes of sensory neurons at early embryonic stages and is later confined to touch-sensitive neurons expressing Ret and/or TrkB. Conditional deletion of Shox2 and analysis of Runx3(-/-);Bax(-/-) mutant mice reveals that Runx3 is suppressing Shox2 while Shox2 is necessary for TrkB expression, and that these interactions are necessary for diversification of TrkB(+) and TrkC(+) mechanoreceptive neurons. In particular, development of TrkB(+)/Ret(+) and TrkB(+)/Ret(-) touch-sensitive neurons is critically dependent on Shox2. Consistently, Shox2 conditional mutant mice demonstrate a dramatic impairment of light touch responses. These results show that Shox2 is required for specification of a subclass of TrkB(+) sensory neurons which convey the sensation of discriminative touch arising from stimuli of the skin.

The transcription factor c-Maf controls touch receptor development and function

The sense of touch relies on detection of mechanical stimuli by specialized mechanosensory neurons. The scarcity of molecular data has made it difficult to analyze development of mechanoreceptors and to define the basis of their diversity and function. We show that the transcription factor c-Maf/c-MAF is crucial for mechanosensory function in mice and humans. The development and function of several rapidly adapting mechanoreceptor types are disrupted in c-Maf mutant mice. In particular, Pacinian corpuscles, a type of mechanoreceptor specialized to detect high-frequency vibrations, are severely atrophied. In line with this, sensitivity to high-frequency vibration is reduced in humans carrying a dominant mutation in the c-MAF gene. Thus, our work identifies a key transcription factor specifying development and function of mechanoreceptors and their end organs.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Molecular identification of rapidly adapting mechanoreceptors and their developmental dependence on ret signaling

In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the "early Ret(+)" DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles, and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret(+) neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.

Neuron/target plasticity in the peripheral gustatory system

Taste bud volume on the anterior tongue in adult rats is matched by an appropriate number of innervating geniculate ganglion cells. The larger the taste bud, the more geniculate ganglion cells that innervate it. To determine if such a match is perturbed in the regenerated gustatory system under different dietary conditions, taste bud volumes and numbers of innervating neurons were quantified in adult rats after unilateral axotomy of the chorda tympani nerve and/or maintenance on a sodium-restricted diet. The relationship between taste bud size and innervation was eliminated in rats merely fed a sodium-restricted diet; individual taste bud volumes were smaller than predicted by the corresponding number of innervating neurons. Surprisingly, the relationship was disrupted in a similar way on the intact side of the tongue in unilaterally sectioned rats, with no diet-related differences. The mismatch in these groups was due to a decrease in average taste bud volumes and not to a change in numbers of innervating ganglion cells. In contrast, individual taste bud volumes were larger than predicted by the corresponding number of innervating neurons on the regenerated side of the tongue; again, with no diet-related differences. However, the primary variable responsible for disrupting the function on the regenerated side was an approximate 20% decrease in geniculate ganglion cells available to innervate taste buds. Therefore, the neuron/target match in the peripheral gustatory system is susceptible to surgical and/or dietary manipulations that act through multiple mechanisms. This system is ideally suited to model sensory plasticity in adults.

The transcription factor Egr3 modulates sensory axon-myotube interactions during muscle spindle morphogenesis

The Egr family of zinc-finger transcription factors, consisting of Egr1, Egr2, Egr3, and Egr4, are involved in cellular growth and differentiation. Adult Egr3-deficient mice are ataxic and lack muscle spindle proprioceptors that normally develop at the sites of Ia afferent-myotube contacts during embryogenesis. To resolve whether spindles form and then degenerate, or whether they never form in the absence of Egr3, we examined the spatiotemporal expression of Egr3 relative to spindle development. In wild type mice, Egr3 was expressed in developing myotubes shortly after they were innervated by Ia afferents and its expression was controlled by innervation because it dissipated following nerve transection. In Egr3-deficient mice, myotubes received Ia afferent innervation and assembled normally into spindles during embryogenesis. However, newborn Egr3-deficient spindles had few internal myonuclei in intrafusal fibers and thin capsules. Moreover, slow-developmental myosin heavy chain was not induced in embryonic Egr3-deficient spindles suggesting that impairments in differentiation were present before they could be detected morphologically. After birth, sensory and motor innervation withdrew from the Egr3-deficient spindles, and the spindles disassembled. In spite of the spindle disassembly and retraction of afferents from muscles, the cell bodies of proprioceptive neurons within dorsal root ganglia were retained. We conclude that Egr3 has an essential role in regulating genes required for the transformation of undifferentiated myotubes into intrafusal fibers, and hence for the phenotypic differentiation of spindles.

Expression of myosin heavy chain isoforms in regenerated muscle spindle fibres after muscle grafting in young and adult rats--plasticity of intrafusal satellite cells

Satellite cells are the myogenic precursor cells of postnatal skeletal muscles. After muscle injury they can proliferate, differentiate, fuse and form myofibres. We have analysed regeneration of distinctly different types of intrafusal fibres in rat muscle spindles. We have introduced the new technique of heterochronous allotransplantation and compared it with the previously used standard autografting method. The allotransplantation method enables one to graft muscles from very young animals; we have used the extensor digitorum longus (EDL) muscles from 2- to 28-day-old rats, which were grafted into EDL muscles of adult inbred recipients. The regenerated "intrafusal" fibres did not express the spindle-specific slow tonic and alpha cardiac-like myosin heavy chain (MyHC) isoforms and they did not exhibit the dual mATPase reaction typical of the nuclear bag2 fibres and the characteristic regional differences in MyHC expression and in the mATPase reaction of nuclear bag1 and nuclear bag2 fibres. On the other hand, they expressed either fast twitch or slow twitch/beta cardiac MyHC isoforms and exhibited an alkali or acid stable mATPase reaction along their whole length, like extrafusal fast type 2 and slow type 1 muscle fibres, respectively. In all regenerated muscle spindles only motor, but no sensory axons were found. More than 85% of muscle spindles in our sample contained regenerated spindle fibres of the same extrafusal fibre type (either type 2 or type 1), in contrast to control muscle spindles, which always contained intrafusal fibres of three different intrafusal fibre types (nuclear bag1, nuclear bag2 and nuclear chain fibres). There were no differences in MyHC expression and mATPase activity between spindle fibres regenerated in grafts taken from young rats of various ages or between allotransplanted and autotransplanted EDL muscles. The present results demonstrate that regenerated "intrafusal" fibres resemble, according to MyHC expression, extrafusal fast or slow muscle fibres. It can thus be concluded that intrafusal satellite cells derived from distinctly different nuclear bag1, nuclear bag2 and nuclear chain fibres show great plasticity, as their MyHC expression can be respecified towards the extrafusal muscle fibre phenotype by foreign alpha-motor innervation.

Glial growth factor rescues Schwann cells of mechanoreceptors from denervation-induced apoptosis

Golgi tendon organs and Pacinian corpuscles are peripheral mechanoreceptors that disappear after denervation during a critical period in early postnatal development. Even if regeneration is allowed to occur, Golgi tendon organs do not reform, and the reformation of Pacinian corpuscles is greatly impaired. The sensory nerve terminals of both types of mechanoreceptors are closely associated with Schwann cells. Here we investigate the changes in the Schwann cells found in Golgi tendon organs and Pacinian corpuscles after nerve resection in the early neonatal period. We report that denervation induces the apoptotic death of these Schwann cells and that this apoptosis can be prevented by administration of a soluble form of neuregulin, glial growth factor 2. Schwann cells associated with these mechanoreceptors are immunoreactive for the neuregulin receptors erbB2, erbB3, and erbB4, and the sensory nerve terminals are immunoreactive for neuregulin. Our results suggest that Schwann cells in developing sensory end organs are trophically dependent on sensory axon terminals and that an axon-derived neuregulin mediates this trophic interaction. The denervation-induced death of mechanoreceptor Schwann cells is correlated with deficiencies in the re-establishment of these sensory end organs by regenerating axons.

Degeneration and regeneration of cutaneous sensory nerve formations

Auxiliary structures of the cutaneous sensory nerve formations (SNF) are dependent on sensory innervation during their critical period of development. Denervation of mature cutaneous corpuscles results in survival of the terminal Schwann cells and the capsular structures which are probably responsible for successful reinnervation of the cutaneous SNF. In addition, the basal lamina tubes of Schwann cells are connected with the terminal Schwann cells and play an important role in the guidance of regrowing axons to their original targets. Long-lasting denervation causes atrophic changes of the terminal Schwann cells and alterations of their molecular equipment. These atrophic changes in the terminal Schwann cells may be responsible for erroneous reinnervation of cutaneous SNF. A population of the cutaneous Merkel cells surviving denervation may also serve as targets for regrowing sensory axons. The basal laminae of terminal Schwann cells are produced under control of the sensory terminals during maturation of cutaneous SNF. In adult animals, the basal laminae are capable of stimulating differentiation of migrated Schwann cells to the terminal Schwann cells without the presence of the sensory terminals. Nonspecific cholinesterase (nChE) is secreted by the terminal Schwann cells and is attached to their extracellular matrix. The synthesis of these molecules in adult animals is not influenced by the sensory terminals. However, the presence of nChE molecules is associated with living terminal Schwann cells. Fetal orthotopically grafted dorsal root ganglion (DRG) neurons have the ability to reinnervate cutaneous SNF of adult hosts. When cutaneous areas are denervated, axons from adjacent sensory nerves may extend collateral branches into this area. The capacity for such extension is dependent on: (1) type of sensory nerve ending, C and A delta fibers having significantly greater capacity than sensory axons of larger caliber; (2) age of the animal, immature animals generally showing a greater capacity for collateral sprouting; (3) the state of the adjacent axons, those already in a growth mode being more capable than "resting" ones; and (4) the regional and mechanical conditions at the site of denervation, hindpaw skin being much less extensively reinnervated by collateral fibers than that of the trunk.

Neomyogenesis in neonatally de-efferented and postnatally denervated rat muscle spindles

The ultrastructure of muscle spindles de-efferented by the extirpation of the lumbosacral spinal cord at the age of 2 days and subsequently deprived of their sensory innervation by the section of the sciatic nerve at 3-4 weeks of age was studied in serial sections of 2-month-old rat hindlimb muscles. De-efferentation leaves the primary sensory neurons and their peripheral axons intact and capable of inducing the muscle spindle morphogenesis during the critical period of their development. In de-efferented and subsequently denervated muscle spindles, new supernumerary intrafusal muscle profiles (SIPs) appeared in the muscle spindle A region. They were formed in intimate spatial relation with the original intrafusal muscle fibres (IMFs) predominantly from activated satellite cells derived from both nuclear bag (larger diameter) and nuclear chain fibres. SIPs, however, lacked the typical nuclear accumulations, as well as other ultrastructural distinctions present in control IMFs. The majority of differentiated SIPs separated from original IMFs, whereas the less differentiated SIPs were usually closely apposed to the surface of the parent IMFs and both were covered by the common basal lamina. In some spindles, the original IMFs and/or new SIPs at different stages of their differentiation were found together and they formed clusters of variable shape and composition. In the majority of clusters, all profiles seemed to be isolated along their entire length, although in few clusters, occasional cytoplasmic connections of variable length between intrafusal profiles were found. This result is important for the interpretation of the forthcoming study of expression of muscle spindle-specific myosin heavy chain isoforms in denervated SIPs in rat muscle spindles gradually deprived of their motor and sensory innervation.

Reinnervation of Pacinian corpuscles by CNS axons after transplantation to the dorsal column: incidence and ultrastructure

We have investigated the capacity of injured axons in the spinal dorsal columns of young adult rats to reinnervate grafted Pacinian corpuscles. A branch of the hindlimb interosseous nerve with a group of crural Pacinian corpuscles attached to it was autotransplanted to the surface of the spinal cord and the nerve stump was implanted into the dorsal column. Two to three months later 16 grafts were removed for examination by light and electron microscopy. By 3 months after transplantation almost all Schwann cell columns of the grafted nerve branch were occupied by regenerated myelinated and unmyelinated axons. Of 41 corpuscles examined by electron microscopy 24 were reinnervated by 1-3 myelinated fibres which gave rise to multiple terminals in the inner core. The remaining corpuscles appeared to be denervated. Only two of the reinnervated corpuscles contained regenerated endings which reiterated the distinct ultrastructure of normal presynaptic terminals of CNS axons, characterized by clusters of lucent vesicles and paramembranous densities. All other corpuscles were reinnervated by terminals which resembled peripheral mechanosensory endings as they contained mitochondria and very few vesicles. One such corpuscle was coinnervated by small terminals filled with large dense cored vesicles. We assume that the majority of grafted Pacinian corpuscles have been reinnervated by dorsal column axons and that the regenerated terminals with the ultrastructure of peripheral mechanosensory endings derive from central axons of primary sensory neurons, which are apparently capable of constructing mechanosensory-like terminals in response to signals from the Pacinian corpuscles. The vesicle-filled endings are probably formed by second order sensory neurons, corticospinal neurons and small peptidergic neurons unable to adjust their terminals to the new target.

The structural and functional development of muscle spindles and their connections in fetal sheep

In this paper we have studied the structural and functional development of hindlimb muscle receptors and the connections of their afferent fibres in fetal sheep (n = 26) from 67-143 days of gestation (term = 146 days). By recording extracellular discharges in dorsal root ganglia (L7, S1) we have shown that muscle spindle afferents first respond to a ramp-and-hold stretch at mid-gestation (approximately 75 days). Silver-stained preparations of muscle spindles revealed that afferent fibres are just beginning to form annulospiral windings at this age. It therefore appears that the annulospiral formation is not a necessary requirement for the generation of the response. By 87-92 days some receptors had developed a discharge at resting muscle length. Discharges were generally more robust and easier to elicit and static and dynamic components could be identified in the response to stretch. Although static sensitivity was generally low it was more evident than dynamic sensitivity. By 107-115 days it was possible to clearly distinguish between muscle and tendon afferents and to tentatively classify muscle responses as originating from primary or secondary afferent spindle endings. With increasing gestational age there was a progressive increase in the length and complexity of the spindle innervation in parallel with the maturation of functional activity. Biocytin injections into the dorsal root ganglia revealed afferent projections to the motoneuron pools by 67 days. Silver-staining of muscles showed that innervation of extrafusal fibres was also present by this age. We therefore conclude that the neural pathways necessary for reflex activity involving muscle spindles are present and functional from early in gestation and could contribute to early fetal movements.

Calbindin D-28k-immunoreactivity in rat muscle spindles during postnatal maturation and after denervation

Calbindin D-28k-immunoreactivity has been demonstrated in some of the intrafusal muscle fibres and in the capsule of adult rat muscle spindles. In this study, the immunocytochemical localization of calbindin D-28k in the muscle spindles of triceps surae muscle was studied during postnatal maturation and after denervation. In young rats calbindin D-28k-immunoreactivity was seen in a few intrafusal fibres, first at the age of 4 days. At the 7th day, three calbindin D-28k-immunoreactive fibres and one unlabelled fibre were seen in most muscle spindles, as in adult rats. The spindle capsule and perineurial sheath of nerves were first seen to exhibit calbindin D-28k immunoreactivity at the age of 14 days, and thereafter the localization of calbinding D-28k-like immunoreactivity was similar to that in adult rats. After denervation, calbindin D-28k-immunoreactivity remained in intrafusal muscle fibres and the spindle capsule for a long period. After two months of denervation, calbindin D-28k immunoreactivity could still be seen in the spindle capsule, but the intrafusal fibres were not labelled. The innervation is known to have trophic effects on the intrafusal fibres. The present findings suggest that the expression of calbindin D-28k-immunoreactivity in maturating muscle spindles may be induced by the developing innervation. The decrease of calbindin D-28k-immunoreactivity in intrafusal fibres after denervation may be due to the loss of trophic factors released by the nerves.

Sensory nerve endings in the puborectalis and anal region of the fetus and newborn

The study was carried out in seven fetuses (ovulation ages were 16, 18, 20, 22, 26, 28, and 31 weeks, respectively) and in five full-term newborn babies. The modified Bielschowsky method and Barker's block silver impregnation were used. It was found that the muscle spindle was arranged in the puborectalis and the external sphincter muscles from the 20th week onward; the tendon organ was located in association with the musculo-tendinous junction of the puborectalis from the 20th week onward; the Pacinian corpuscle lay in the plane of cleavage between the internal anal sphincter and the external anal sphincter from the 22nd week onward, and in the presacral space from the 28th week onward, and the later lagged behind the former in development; the globular endings lay in the presacral space from the 28th week onward; and the free nerve ending was distributed in the epithelium of the anal mucosa and the epidermis around the anus from the 31st week onward. These sensory nerve endings tend to mature gradually with increasing age. It was concluded that these endings may be responsible for the reflex of the external anal sphincter and the anal sensation.

Development of the sensory innervation of muscle spindles in the kitten

This is a report of the changes observed in the pattern of sensory innervation of muscle spindles in hindlimb muscles of kittens during the first four weeks of life. The structural analysis, made on teased, silver-stained preparations, was complemented by a series of recordings of afferent responses of kitten spindles during ramp-and-hold stretches of the muscle. The primary endings of spindles from newborn animals showed a large degree of variability in their branching pattern and branches formed a network across the intrafusal fibres. In older animals there was less variability and lateral branches of stem axons began to encircle the intrafusal fibres. The process of maturation was characterized by a more uniform shape of the endings and more complete, evenly spaced, annulospiral terminals. Recordings of the responses of primary endings of spindles during muscle stretch showed that several features of the adult response were already present in the newborn, although the overall rate of discharge was very much lower. It was concluded that the changes observed in the structure of the sensory endings of kitten spindles did not have clearly identifiable physiological correlates. It appears that an annulospiral shape of the sensory terminals is not a necessary prerequisite for the generation of stretch responses. The predominant factor which appears to determine the responses of spindles to stretch is the maturity of the intrafusal fibres, in particular, the bag2 fibre.

Specificities of afferents reinnervating cat muscle spindles after nerve section

1. We have made quantitative assessments of the sensory reinnervation and recovery of peroneus brevis muscle spindles following section and epineurial repair of the common peroneal nerve. After 6-50 weeks recovery, single-unit, dorsal-root recordings were made of the responses to ramp-and-hold or sinusoidal stretch of the reinnervated spindles, which were subsequently examined in teased, silver preparations. 2. Assessments of recovery used data obtained from cross-union experiments in which foreign afferents (including Ib) were given the opportunity of reinnervating spindles in the absence of their native (Ia, spindle II) afferents; and from an examination of tenuissimus spindles reinnervated by Ia and spindle II afferents in the absence of Ib afferents. These studies revealed: (i) that regenerating Ib afferents can terminate in sites originally occupied by the endings of Ia or spindle II afferents, and respond to stretch like normal Ia and spindle II afferents; (ii) that Ib and spindle II afferents reinnervating spindles are histologically identical apart from diameter range; and (iii) that some cutaneous afferents can reinnervate spindles and give highly abnormal, phasic stretch responses. 3. Recovery of afferents reinnervating spindles was marked by increases in conduction velocity and proportions firing tonically, but their firing rates at the three phases of ramp-and-hold stretch were considerably lower than normal and showed no tendency to increase. 4. Some relatively fast afferents that gave spindle II-type responses were identified as Ib afferents reinnervating secondary-ending sites; conversely, some relatively slow afferents that gave Ia-type responses were identified as spindle II afferents reinnervating primary-ending sites. 5. The estimated loss of spindle afferents from tenuissimus after nerve section (52% Ia, 49% spindle II) was considerably less than the estimated loss of these afferents from peroneus brevis after section of the common peroneal nerve (79% Ia, 86% spindle II). The proportion of spindles in tenuissimus reinnervated by free-ending afferents was also much lower (22%) than in peroneus brevis (73%). These differences are partly attributed to the greater size and degree of afferent complexity of the common peroneal nerve. 6. Similar proportions of spindles in peroneus brevis were reinnervated by Ia and Ib afferents after both partial (27% Ia, 20% Ib) and complete (21% Ia, 20% Ib) section of the common peroneal nerve.(ABSTRACT TRUNCATED AT 400 WORDS)

Clustering of primary afferent fibers in peripheral nerve fascicles by sensory modality

The spatial organization of cutaneous afferent fibers in the cat saphenous nerve was studied by recording from functionally identified units in split filaments. It was found that within each fascicle, fibers tend to be clustered together with others of like modality; they are not randomly distributed. These results suggest that the sensory modality of primary afferent neurons is determined prior to their innervation of the skin.

Retrograde and transganglionic degeneration of sensory neurons after a peripheral nerve lesion at birth

The sciatic nerve of newborn rats (less than or equal to 16 h old) was crushed with a watchmaker forceps. During the first 4 weeks after the injury, examination of ipsilateral L4 through L6 dorsal root ganglia, their dorsal nerve roots, and the dorsal funiculus revealed the presence of degenerating myelin and axons. Chromatolysis was not observed. In the spinal cord, the degenerating argyrophilia was restricted to the medial part of the dorsal funiculus (fasciculus gracilis). This is interpreted as transganglionic degeneration of the central processes of the pseudounipolar cells. Twelve weeks after nerve crush, there was a noticeable reduction in the size of the leg, foot, and muscles innervated by the sciatic nerve as well as a substantial loss (P less than 0.001) of neurons and myelinated axons in ipsilateral spinal ganglia and their dorsal nerve roots. The reduction was most prominent among the larger sensory neurons (greater than 40 microns) and the larger myelinated axons. A total loss of about 60% of sensory neurons was found in the L4 through L6 spinal ganglia. About 58 and 64% of the myelinated axons were lost in L4 and L5 dorsal roots, respectively. The remaining perikarya and dorsal root axons were hypoplastic.

Quantitative ultrastructural stereology of synapses in nucleus dorsalis after a peripheral nerve injury at birth

Utilizing recent techniques in quantitative stereology, this investigation studied the synaptology of nucleus dorsalis (Clarke's column) in 12-week-old rats whose sciatic nerves were crushed in the 1st postnatal day. Four morphometric variables were analyzed at the levels of L1 and L3 spinal cord segments: total surface area of synaptic contact zones per unit volume (SV), total length of synaptic contact zones per unit area (LA), average length of synaptic membrane (L), and numerical density of synapses per unit volume (NV). The original raw data were corrected for Holmes's effect. The results indicated that peripheral nerve crush at birth induced a transganglionic change in central sensory terminals with a loss of numerous synapses. A significant loss (P less than 0.001) of about 32% in the SV and LA and a significant loss (P less than 0.001) of about 36% in the NV were observed on the experimental side. There was no preferential loss of synapses in either segment. The mean synaptic membrane length showed no significant difference between the control and experimental sides. The control values of the four morphometric variables calculated for L3 were lower than those calculated for L1. The loss of synapses after a peripheral nerve lesion was probably due to the loss of sensory neurons and their central processes, but there were other possibilities.

Development of immunohistochemical characteristics of intrafusal fibres in normal and de-efferented rat muscle spindles

Intrafusal muscle fibres in adult muscle spindles differ in their myosin composition. After selective motor denervation intrafusal muscle fibres develop mature ultrastructural characteristics. In order to evaluate the role of fusimotor innervation on the maturation of the myosin composition of intrafusal muscle fibres we have examined with immunohistochemical techniques i) the postnatal development of muscle spindles in new-born rats and in 7-21 day old rats; ii) muscle spindles in the EDL of 21-day-old rats de-efferented at birth. For the characterization of myosins in intrafusal fibres we used three myosin antisera: antipectoral myosin, antiheart myosin and antiheart myosin adsorbed with muscle powder from the soleus muscle of guinea pig. We show in this study that during development intrafusal fibres change immunoreactivity and that in the absence of motor innervation bag fibres do not fully develop the myosin characteristics of control spindles. We conclude that the maturation of bag1 and bag2 fibres apparently requires next to the inductive influence of sensory axon terminals the presence and activity of fusimotor axons.

Survival of Pacinian corpuscles after denervation in adult rats

The ultrastructure of Pacinian corpuscles located on the crural interosseous membrane was studied in adult rats 6h to 10 months after transection of the right sciatic nerve. Axon terminals degenerated one day after transection and were engulfed and resorbed by cells of the inner core within one week. The axial space left after removal of the axonal debris was closed by the lamellae of the inner core. The main structural features of the inner core and capsule remained preserved after denervation throughout the period of study. The denervated inner cores, however, became atrophic 10 months after neurotomy, their mean diameter being reduced by 17.5% compared with that of contralateral control corpuscles. The number of capsular lamellae was unaltered, and perineurial pathways of the peripheral nerve stump remained preserved. Schwann cells proliferated and formed Büngner bands during the first month after denervation, but retracted their processes and became atrophic at later stages after neurotomy. Survival of Pacinian corpuscles after long-term denervation in adult rats is in contrast to their rapid degeneration within several days after nerve section in neonates.

Histochemical profiles of cat intrafusal muscle fibers and their motor innervation

Muscle spindles were examined histochemically in serial transverse sections of cat tenuissimus muscles. The myofibrillar adenosine triphosphatase (ATPase) staining reaction was used to identify nuclear bag1, bag2 and nuclear chain intrafusal muscle fibers. Regional differences in ATPase staining occurred along the bag1 and bag2 fibers but not along the chain fibers. All intrafusal fiber types displayed regional variability in staining for nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR). Motor nerve terminals were demonstrated along the poles of bag1, bag2 and chain fibers by staining for cholinesterase (ChE). There was no consistent spatial correlation between the intensity of regional ATPase staining along the bag fibers and location, number or type of motor endings. However, most ChE deposits occurred in intrafusal fiber regions that displayed the greatest NADH-TR variability. Some fiber poles or whole intrafusal fibers were devoid of any ChE deposits but their ATPase and NADH-TR content was comparable to that of fibers bearing ChE deposits. The observations suggested that motor nerve fibers per se may not play a major role in determining the histoenzymatic content of intrafusal fibers.