Specificities of afferents reinnervating cat muscle spindles after nerve section

An assessment of co-contraction in reinnervated muscle

Aim: To investigate co-contraction in reinnervated elbow flexor muscles following a nerve transfer. Materials & methods: 12 brachial plexus injury patients who received a nerve transfer to reanimate elbow flexion were included in this study. Surface electromyography (EMG) recordings were used to quantify co-contraction during sustained and repeated isometric contractions of reinnervated and contralateral uninjured elbow flexor muscles. Reuslts: For the first time, this study reveals reinnervated muscles demonstrated a trend toward higher co-contraction ratios when compared with uninjured muscle and this is correlated with an earlier onset of muscle fatigability. Conclusion: Measurements of co-contraction should be considered within muscular function assessments to help drive improvements in motor recovery therapies.

There and back again: 50 years of wandering through terra incognita fusorum

This paper is in two parts: 'There', which is a review of some of the major advances in the study of spindle structure and function during the past 50 years, serving as an introduction to the symposium entitled 'Mechanotransduction, Muscle Spindles and Proprioception' held in Munich in July 2022; and 'And Back Again', presenting new quantitative morphological results on the equatorial nuclei of intrafusal muscle fibres and of the primary sensory ending in relationship to passive stretch of the spindle.

The making of a proprioceptor: a tale of two identities

Proprioception, the sense of body position in space, has a critical role in the control of posture and movement. Aside from skin and joint receptors, the main sources of proprioceptive information in tetrapods are mechanoreceptive end organs in skeletal muscle: muscle spindles (MSs) and Golgi tendon organs (GTOs). The sensory neurons that innervate these receptors are divided into subtypes that detect discrete aspects of sensory information from muscles with different biomechanical functions. Despite the importance of proprioceptive neurons in motor control, the developmental mechanisms that control the acquisition of their distinct functional properties and positional identity are not yet clear. In this review, we discuss recent findings on the development of mouse proprioceptor subtypes and challenges in defining them at the molecular and functional level.

Sensory nerve regeneration and reinnervation in muscle following peripheral nerve injury

Sensory afferent fibers are an important component of motor nerves and compose the majority of axons in many nerves traditionally thought of as "pure" motor nerves. These sensory afferent fibers innervate special sensory end organs in muscle, including muscle spindles that respond to changes in muscle length and Golgi tendons that detect muscle tension. Both play a major role in proprioception, sensorimotor extremity control feedback, and force regulation. After peripheral nerve injury, there is histological and electrophysiological evidence that sensory afferents can reinnervate muscle, including muscle that was not the nerve's original target. Reinnervation can occur after different nerve injury and muscle models, including muscle graft, crush, and transection injuries, and occurs in a nonspecific manner, allowing for cross-innervation to occur. Evidence of cross-innervation includes the following: muscle spindle and Golgi tendon afferent-receptor mismatch, vagal sensory fiber reinnervation of muscle, and cutaneous afferent reinnervation of muscle spindle or Golgi tendons. There are several notable clinical applications of sensory reinnervation and cross-reinnervation of muscle, including restoration of optimal motor control after peripheral nerve repair, flap sensation, sensory protection of denervated muscle, neuroma treatment and prevention, and facilitation of prosthetic sensorimotor control. This review focuses on sensory nerve regeneration and reinnervation in muscle, and the clinical applications of this phenomena. Understanding the physiology and limitations of sensory nerve regeneration and reinnervation in muscle may ultimately facilitate improvement of its clinical applications.

Calcium Homeostasis in Parvalbumin DRG Neurons is Altered After Sciatic Nerve Crush and Sciatic Nerve Transection Injuries

Reflex abnormalities mediated by proprioceptive sensory neurons after peripheral nerve injury (PNI) can limit functional improvement, leaving patients with disability that affects their quality of life. We examined post-injury calcium transients in a subpopulation of DRG neurons consisting primarily of proprioceptors to determine whether alterations in calcium homeostasis are present in proprioceptors, as has been documented in other DRG neurons after PNI. Using transgenic mice, we restricted expression of the calcium indicator GCaMP6s to DRG neurons containing parvalbumin (PV). Mice of both sexes were randomly assigned to sham, sciatic nerve crush, or sciatic nerve transection and resuture conditions. Calcium transients were recorded from ex-vivo preparations of animals at one of three post-surgery time points: 1-3 days, 7-11 days, and after 60 days of recovery. Results demonstrated that the post-PNI calcium transients of PV DRG neurons are significantly different than sham. Abnormalities were not present during the acute response to injury (1-3 days), but transients were significantly different than sham at the recovery stage where axon regeneration is thought to be underway (7-11 days). During late-stage recovery (60 days post-injury), disturbances in the decay time course of calcium transients in transection animals persisted, whereas parameters of transients from crush animals returned to normal. These findings identify a deficit in calcium homeostasis in proprioceptive neurons, which may contribute to the failure to fully recover proprioceptive reflexes after PNI. Significant differences in the calcium transients of crush versus transection animals after reinnervation illustrate calcium homeostasis alterations are distinctive to injury type.

Dermatosensory Peripheral Nerve Interfaces: Prevention of Pain Recurrence Following Sensory Neurectomy

Chronic pain is a significant health care problem. Many patients' pain can be linked to a neuropathic origin, diagnosed with a thorough history and physical examination, and confirmed with a diagnostic nerve block. There are new procedures designed to address neuropathic pain from symptomatic neuromas by providing physiologic targets for regenerating axons following neurectomy. Dermal wrapping of the end of a sensory nerve following transection, a technique called dermatosensory peripheral nerve interface, may provide an optimal environment to prevent neuroma pain and reduce chronic neuropathic pain.

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

Dysregulation of mechanosensory circuits coordinating the actions of antagonist motor pools following peripheral nerve injury and muscle reinnervation

Movement disorders observed following peripheral nerve injury and muscle reinnervation suggest discoordination in the activation of antagonist muscles. Although underlying mechanisms remain undecided, dysfunction in spinal reflex circuits is a reasonable candidate. Based on the well known role of reflex inhibition between agonist and antagonist muscles in normal animals, we hypothesized its reduction following muscle reinnervation, similar to that associated with other disorders exhibiting antagonist discoordination, e.g. spinal cord injury and dystonia. Experiments performed on acutely-decerebrated rats examined interactions of mechanosensory reflexes between ipsilateral muscles acting as mechanical antagonists at the ankle joint: ankle extensor, gastrocnemii (G) muscles (agonists) and ankle flexor, tibialis anterior (TA) muscle (antagonist). The force of agonist stretch reflex contraction was measured for its suppression or facilitation by concurrent conditioning stretch of the antagonist muscle. Data were compared between two groups of adult rats, an antagonist reinnervation group with TA muscle reinnervated and a control group with TA normally innervated. Results revealed a three-fold increase in reflex suppression in the antagonist reinnervation group, contrary to our predicted decrease. Reflex facilitation also increased, not only in strength, seven-fold, but also in its frequency of stochastic occurrence across stimulus trials. These observations suggest dysregulation in specific spinal reflex circuits as novel candidate origins of modified antagonist muscle coordination following peripheral nerve injury and muscle reinnervation.

Muscle spindle reinnervation using transplanted embryonic dorsal root ganglion cells after peripheral nerve transection in rats

OBJECTIVES

Muscle spindles are proprioceptive receptors in the skeletal muscle. Peripheral nerve injury results in a decreased number of muscle spindles and their morphologic deterioration. However, the muscle spindles recover when skeletal muscles are reinnervated with surgical procedures, such as nerve suture or nerve transfer. Morphological changes in muscle spindles by cell transplantation procedure have not been reported so far. Therefore, we hypothesized that transplantation of embryonic sensory neurons may improve sensory neurons in the skeletal muscle and reinnervate the muscle spindles.

MATERIALS AND METHODS

We collected sensory neurons from dorsal root ganglions of 14-day-old rat embryos and prepared a rat model of peripheral nerve injury by performing sciatic nerve transection and allowing for a period of one week before which we performed the cell transplantations. Six months later, the morphological changes of muscle spindles in the cell transplantation group were compared with the naïve control and surgical control groups.

RESULTS

Our results demonstrated that transplantation of embryonic dorsal root ganglion cells induced regeneration of sensory nerve fibre and reinnervation of muscle spindles in the skeletal muscle. Moreover, calbindin D-28k immunoreactivity in intrafusal muscle fibres was maintained for six months after denervation in the cell transplantation group, whereas it disappeared in the surgical control group.

CONCLUSIONS

Cell transplantation therapies could serve as selective targets to modulate mechanosensory function in the skeletal muscle.

VGLUT1 synapses and P-boutons on regenerating motoneurons after nerve crush

Stretch-sensitive Ia afferent monosynaptic connections with motoneurons form the stretch reflex circuit. After nerve transection, Ia afferent synapses and stretch reflexes are permanently lost, even after regeneration and reinnervation of muscle by motor and sensory afferents is completed in the periphery. This loss greatly affects full recovery of motor function. However, after nerve crush, reflex muscle forces during stretch do recover after muscle reinnervation and reportedly exceed 140% baseline values. This difference might be explained by structural preservation after crush of Ia afferent synapses on regenerating motoneurons and decreased presynaptic inhibitory control. We tested these possibilities in rats after crushing the tibial nerve (TN), and using Vesicular GLUtamate Transporter 1 (VGLUT1) and the 65 kDa isoform of glutamic acid-decarboxylase (GAD65) as markers of, respectively, Ia afferent synapses and presynaptic inhibition (P-boutons) on retrogradely labeled motoneurons. We analyzed motoneurons during regeneration (21 days post crush) and after they reinnervate muscle (3 months). The results demonstrate a significant loss of VGLUT1 terminals on dendrites and cell bodies at both 21 days and 3 months post-crush. However, in both cellular compartments, the reductions were small compared to those observed after TN full transection. In addition, we found a significant decrease in the number of GAD65 P-boutons per VGLUT1 terminal and their coverage of VGLUT1 boutons. The results support the hypothesis that better preservation of Ia afferent synapses and a change in presynaptic inhibition could contribute to maintain or even increase the stretch reflex after nerve crush and by difference to nerve transection.

Self-reinnervated muscles lose autogenic length feedback, but intermuscular feedback can recover functional connectivity

In this study, we sought to identify sensory circuitry responsible for motor deficits or compensatory adaptations after peripheral nerve cut and repair. Self-reinnervation of the ankle extensor muscles abolishes the stretch reflex and increases ankle yielding during downslope walking, but it remains unknown whether this finding generalizes to other muscle groups and whether muscles become completely deafferented. In decerebrate cats at least 19 wk after nerve cut and repair, we examined the influence of quadriceps (Q) muscles' self-reinnervation on autogenic length feedback, as well as intermuscular length and force feedback, among the primary extensor muscles in the cat hindlimb. Effects of gastrocnemius and soleus self-reinnervation on intermuscular circuitry were also evaluated. We found that autogenic length feedback was lost after Q self-reinnervation, indicating that loss of the stretch reflex appears to be a generalizable consequence of muscle self-reinnervation. However, intermuscular force and length feedback, evoked from self-reinnervated muscles, was preserved in most of the interactions evaluated with similar relative inhibitory or excitatory magnitudes. These data indicate that intermuscular spinal reflex circuitry has the ability to regain functional connectivity, but the restoration is not absolute. Explanations for the recovery of intermuscular feedback are discussed, based on identified mechanisms responsible for lost autogenic length feedback. Functional implications, due to permanent loss of autogenic length feedback and potential for compensatory adaptations from preserved intermuscular feedback, are discussed.

Some reminiscences on studies of age-dependent and activity-dependent degeneration of sensory and motor endings in mammalian skeletal muscle

I present here an overview of research on the biology of neuromuscular sensory and motor endings that was inspired and influenced partly by my educational experience in the Department of Zoology at the University of Durham, from 1971 to 1974. I allude briefly to neuromuscular synaptic structure and function in dystrophic mice, influences of activity on synapse elimination in development and regeneration, and activity-dependent protection and degeneration of neuromuscular junctions in Wld^S mice.

The innervation of the muscle spindle: a personal history

I present a brief review of current understanding of the innervation of the mammalian muscle spindle, from a personal historical perspective. The review begins with comparative studies on the numbers of spindle afferents and considers how their relative abundance may best be assessed. This is followed by an examination of the distribution and some functional properties of the motor innervation. The primary ending is the subject of the final section, in particular, I look at what can be learned from serial sectioning and volumetric reconstruction, and present new results on a model and simulations concerning sensory terminal deformation during stretch.

Complex impairment of IA muscle proprioceptors following traumatic or neurotoxic injury

The health of primary sensory afferents supplying muscle has to be a first consideration in assessing deficits in proprioception and related motor functions. Here we discuss the role of a particular proprioceptor, the IA muscle spindle proprioceptor in causing movement disorders in response to either regeneration of a sectioned peripheral nerve or damage from neurotoxic chemotherapy. For each condition, there is a single preferred and widely repeated explanation for disability of movements associated with proprioceptive function. We present a mix of published and preliminary findings from our laboratory, largely from in vivo electrophysiological study of treated rats to demonstrate newly discovered IA afferent defects that seem likely to make important contributions to movement disorders. First, we argue that reconnection of regenerated IA afferents with inappropriate targets, although often repeated as the reason for lost stretch-reflex contraction, is not a complete explanation. We present evidence that despite successful recovery of stretch-evoked sensory signaling, peripherally regenerated IA afferents retract synapses made with motoneurons in the spinal cord. Second, we point to evidence that movement disability suffered by human subjects months after discontinuation of oxaliplatin (OX) chemotherapy for some is not accompanied by peripheral neuropathy, which is the acknowledged primary cause of disability. Our studies of OX-treated rats suggest a novel additional explanation in showing the loss of sustained repetitive firing of IA afferents during static muscle stretch. Newly extended investigation reproduces this effect in normal rats with drugs that block Na(+) channels apparently involved in encoding static IA afferent firing. Overall, these findings highlight multiplicity in IA afferent deficits that must be taken into account in understanding proprioceptive disability, and that present new avenues and possible advantages for developing effective treatment. Extending the study of IA afferent deficits yielded the additional benefit of elucidating normal processes in IA afferent mechanosensory function.

Normal distribution of VGLUT1 synapses on spinal motoneuron dendrites and their reorganization after nerve injury

Peripheral nerve injury induces permanent alterations in spinal cord circuitries that are not reversed by regeneration. Nerve injury provokes the loss of many proprioceptive IA afferent synapses (VGLUT1-IR boutons) from motoneurons, the reduction of IA EPSPs in motoneurons, and the disappearance of stretch reflexes. After motor and sensory axons successfully reinnervate muscle, lost IA VGLUT1 synapses are not re-established and the stretch reflex does not recover; however, electrically evoked EPSPs do recover. The reasons why remaining IA synapses can evoke EPSPs on motoneurons, but fail to transmit useful stretch signals are unknown. To better understand changes in the organization of VGLUT1 IA synapses that might influence their input strength, we analyzed their distribution over the entire dendritic arbor of motoneurons before and after nerve injury. Adult rats underwent complete tibial nerve transection followed by microsurgical reattachment and 1 year later motoneurons were intracellularly recorded and filled with neurobiotin to map the distribution of VGLUT1 synapses along their dendrites. We found in control motoneurons an average of 911 VGLUT1 synapses; ~62% of them were lost after injury. In controls, VGLUT1 synapses were focused to proximal dendrites where they were grouped in tight clusters. After injury, most synaptic loses occurred in the proximal dendrites and remaining synapses were declustered, smaller, and uniformly distributed throughout the dendritic arbor. We conclude that this loss and reorganization renders IA afferent synapses incompetent for efficient motoneuron synaptic depolarization in response to natural stretch, while still capable of eliciting EPSPs when synchronously fired by electrical volleys.

Sensory reinnervation of muscle spindles after repair of tibial nerve defects using autogenous vein grafts

Motor reinnervation after repair of tibial nerve defects using autologous vein grafts in rats has previously been reported, but sensory reinnervation after the same repair has not been fully investigated. In this study, partial sensory reinnervation of muscle spindles was observed after repair of 10-mm left tibial nerve defects using autologous vein grafts with end-to-end anastomosis in rats, and functional recovery was confirmed by electrophysiological studies. There were no significant differences in the number, size, or electrophysiological function of reinnervated muscle spindles between the two experimental groups. These findings suggest that repair of short nerve defects with autologous vein grafts provides comparable results to immediate end-to-end anastomosis in terms of sensory reinnervation of muscle spindles.

Long-term neuromechanical results of selective tibial neurotomy in patients with spastic equinus foot

BACKGROUND

The neuromechanical consequences of tibial neurotomy have not been extensively studied.

METHODS

Fifteen patients were evaluated before and after selective tibial neurotomy (after 2 months and after 15 months) by means of clinical, neurophysiological [tendon (T) reflexes, Hoffmann (H) reflexes and maximum motor response, Mmax] and mechanical parameters (passive stiffness of plantar flexors at the ankle). The neurotomy concerned the soleus (100 % of cases), gastrocnemius (20 % of cases), posterior tibial (60 % of cases) and flexor digitorum longus (47 % of cases) nerves.

RESULTS

Neurotomy provided more than 90 % improvement of clinical spasticity scores, 20 % improvement of walking scores and the angle of passive dorsiflexion (APDF) of the ankle (mean angle: 7°), temporary reduction of the soleus Mmax (18 % at 2 months with return to the preoperative value at 15 months), and lasting reduction of the soleus Hmax/Mmax (68 % at 2 months, 78 % at 15 months) and T/Mmax (84 % at 2 months, 80 % at 15 months). M and H responses of the gastrocnemius (whether or not they were included in the neurotomy) were not modified, while T/Mmax decreased to the same degree as for soleus. Passive stiffness was lastingly decreased from 64.0 Nm/rad to 49.0 Nm/rad (2 months) and 49.5 Nm/rad (15 months).

CONCLUSION

Selective tibial neurotomy of the soleus nerve induces long-term reduction of reflex hyperexcitability and passive stiffness of plantar flexors in spastic patients, with no lasting impairment of motor efferents. In parallel, it modifies the tendon reflexes of synergistic muscles (gastrocnemius) not concerned by the neurotomy.

Permanent central synaptic disconnection of proprioceptors after nerve injury and regeneration. I. Loss of VGLUT1/IA synapses on motoneurons

Motor and sensory proprioceptive axons reinnervate muscles after peripheral nerve transections followed by microsurgical reattachment; nevertheless, motor coordination remains abnormal and stretch reflexes absent. We analyzed the possibility that permanent losses of central IA afferent synapses, as a consequence of peripheral nerve injury, are responsible for this deficit. VGLUT1 was used as a marker of proprioceptive synapses on rat motoneurons. After nerve injuries synapses are stripped from motoneurons, but while other excitatory and inhibitory inputs eventually recover, VGLUT1 synapses are permanently lost on the cell body (75-95% synaptic losses) and on the proximal 100 μm of dendrite (50% loss). Lost VGLUT1 synapses did not recover, even many months after muscle reinnervation. Interestingly, VGLUT1 density in more distal dendrites did not change. To investigate whether losses are due to VGLUT1 downregulation in injured IA afferents or to complete synaptic disassembly and regression of IA ventral projections, we studied the central trajectories and synaptic varicosities of axon collaterals from control and regenerated afferents with IA-like responses to stretch that were intracellularly filled with neurobiotin. VGLUT1 was present in all synaptic varicosities, identified with the synaptic marker SV2, of control and regenerated afferents. However, regenerated afferents lacked axon collaterals and synapses in lamina IX. In conjunction with the companion electrophysiological study [Bullinger KL, Nardelli P, Pinter MJ, Alvarez FJ, Cope TC. J Neurophysiol (August 10, 2011). doi:10.1152/jn.01097.2010], we conclude that peripheral nerve injuries cause a permanent retraction of IA afferent synaptic varicosities from lamina IX and disconnection with motoneurons that is not recovered after peripheral regeneration and reinnervation of muscle by sensory and motor axons.

Permanent central synaptic disconnection of proprioceptors after nerve injury and regeneration. II. Loss of functional connectivity with motoneurons

Regeneration of a cut muscle nerve fails to restore the stretch reflex, and the companion paper to this article [Alvarez FJ, Titus-Mitchell HE, Bullinger KL, Kraszpulski M, Nardelli P, Cope TC. J Neurophysiol (August 10, 2011). doi:10.1152/jn.01095.2010] suggests an important central contribution from substantial and persistent disassembly of synapses between regenerated primary afferents and motoneurons. In the present study we tested for physiological correlates of synaptic disruption. Anesthetized adult rats were studied 6 mo or more after a muscle nerve was severed and surgically rejoined. We recorded action potentials (spikes) from individual muscle afferents classified as IA like (*IA) by several criteria and tested for their capacity to produce excitatory postsynaptic potentials (EPSPs) in homonymous motoneurons, using spike-triggered averaging (STA). Nearly every paired recording from a *IA afferent and homonymous motoneuron (93%) produced a STA EPSP in normal rats, but that percentage was only 17% in rats with regenerated nerves. In addition, the number of motoneurons that produced aggregate excitatory stretch synaptic potentials (eSSPs) in response to stretch of the reinnervated muscle was reduced from 100% normally to 60% after nerve regeneration. The decline in functional connectivity was not attributable to synaptic depression, which returned to its normally low level after regeneration. From these findings and those in the companion paper, we put forward a model in which synaptic excitation of motoneurons by muscle stretch is reduced not only by misguided axon regeneration that reconnects afferents to the wrong receptor type but also by retraction of synapses with motoneurons by spindle afferents that successfully reconnect with spindle receptors in the periphery.

Recovery of proprioceptive feedback from nerve crush

Sensorimotor functions are restored by peripheral nerve regeneration with greater success following injuries that crush rather than sever the nerve. Better recovery following nerve crush is commonly attributed to superior reconnection of regenerating axons with their original peripheral targets. The present study was designed to estimate the fraction of stretch reflex recovery attributable to functional recovery of regenerated spindle afferents. Recovery of the spindle afferent population was estimated from excitatory postsynaptic potentials evoked by muscle stretch (strEPSPs) in motoneurons. These events were measured in cats that were anaesthetized, so that recovery of spindle afferent function, including both muscle stretch encoding and monosynaptic transmission, could be separated from other factors that act centrally to influence muscle stretch-evoked excitation of motoneurons. Recovery of strEPSPs to 70% of normal specified the extent of overall functional recovery by the population spindle afferents that regained responsiveness to muscle stretch. In separate studies, we examined recovery of the stretch reflex in decerebrate cats, and found that it recovered to supranormal levels after nerve crush. The substantial disparity in recovery between strEPSPs and stretch reflex led us to conclude that factors in addition to recovery of spindle afferents make a large contribution in restoring the stretch reflex following nerve crush.

Permanent reorganization of Ia afferent synapses on motoneurons after peripheral nerve injuries

After peripheral nerve injuries to a motor nerve, the axons of motoneurons and proprioceptors are disconnected from the periphery and monosynaptic connections from group I afferents and motoneurons become diminished in the spinal cord. Following successful reinnervation in the periphery, motor strength, proprioceptive sensory encoding, and Ia afferent synaptic transmission on motoneurons partially recover. Muscle stretch reflexes, however, never recover and motor behaviors remain uncoordinated. In this review, we summarize recent findings that suggest that lingering motor dysfunction might be in part related to decreased connectivity of Ia afferents centrally. First, sensory afferent synapses retract from lamina IX, causing a permanent relocation of the inputs to more distal locations and significant disconnection from motoneurons. Second, peripheral reconnection between proprioceptive afferents and muscle spindles is imperfect. As a result, a proportion of sensory afferents that retain central connections with motoneurons might not reconnect appropriately in the periphery. A hypothetical model is proposed in which the combined effect of peripheral and central reconnection deficits might explain the failure of muscle stretch to initiate or modulate firing of many homonymous motoneurons.

Sensory protection of rat muscle spindles following peripheral nerve injury and reinnervation

BACKGROUND

Skeletal muscle structure and function are dependent on intact innervation. Prolonged muscle denervation results in irreversible muscle fiber atrophy, connective tissue hyperplasia, and deterioration of muscle spindles, specialized sensory receptors necessary for proper skeletal muscle function. The protective effect of temporary sensory innervation on denervated muscle, before motor nerve repair, has been shown in the rat. Sensory-protected muscles exhibit less fiber atrophy and connective tissue hyperplasia and maintain greater functional capacity than denervated muscles. The purpose of this study was to determine whether temporary sensory innervation also protects muscle spindles from degeneration.

METHODS

Rat tibial nerve was transected and repaired with either the saphenous or the original transected nerve. Negative controls remained denervated. After 3 to 6 months, the electrophysiologic response of the nerve to stretch in the rat gastrocnemius muscle was measured (n = 3 per group). After the animals were euthanized, the gastrocnemius muscle was removed, sectioned, stained, and examined for spindle number (n = 3 per group) and morphology (one rat per group). Immunohistochemical assessment of muscle spindle innervation was examined in four additional animals.

RESULTS

Significant deterioration of muscle spindles was seen in denervated muscle, whereas in muscle reinnervated with the tibial or the saphenous nerve, spindle number and morphology were improved. Histologic and functional evidence of spindle reinnervation by the sensory nerve was obtained.

CONCLUSION

These findings add to the known means by which motor or sensory nerves exert protective effects on denervated muscle, and further promote the use of sensory protection for improving the outcome after peripheral nerve injury.

A comparative analysis of the encapsulated end-organs of mammalian skeletal muscles and of their sensory nerve endings

The encapsulated sensory endings of mammalian skeletal muscles are all mechanoreceptors. At the most basic functional level they serve as length sensors (muscle spindle primary and secondary endings), tension sensors (tendon organs), and pressure or vibration sensors (lamellated corpuscles). At a higher functional level, the differing roles of individual muscles in, for example, postural adjustment and locomotion might be expected to be reflected in characteristic complements of the various end-organs, their sensory endings and afferent nerve fibres. This has previously been demonstrated with regard to the number of muscle-spindle capsules; however, information on the other types of end-organ, as well as the complements of primary and secondary endings of the spindles themselves, is sporadic and inconclusive regarding their comparative provision in different muscles. Our general conclusion that muscle-specific variability in the provision of encapsulated sensory endings does exist demonstrates the necessity for the acquisition of more data of this type if we are to understand the underlying adaptive relationships between motor control and the structure and function of skeletal muscle. The present quantitative and comparative analysis of encapsulated muscle afferents is based on teased, silver-impregnated preparations. We begin with a statistical analysis of the number and distribution of muscle-spindle afferents in hind-limb muscles of the cat, particularly tenuissimus. We show that: (i) taking account of the necessity for at least one primary ending to be present, muscles differ significantly in the mean number of additional afferents per spindle capsule; (ii) the frequency of occurrence of spindles with different sensory complements is consistent with a stochastic, rather than deterministic, developmental process; and (iii) notwithstanding the previous finding, there is a differential distribution of spindles intramuscularly such that the more complex ones tend to be located closer to the main divisions of the nerve. Next, based on a sample of tendon organs from several hind-foot muscles of the cat, we demonstrate the existence in at least a large proportion of tendon organs of a structural substrate to account for multiple spike-initiation sites and pacemaker switching, namely the distribution of sensory terminals supplied by the different first-order branches of the Ib afferent to separate, parallel, tendinous compartments of individual tendon organs. We then show that the numbers of spindles, tendon organs and paciniform corpuscles vary independently in a sample of (mainly) hind-foot muscles of the cat. Grouping muscles by anatomical region in the cat indicated the existence of a gradual proximo-distal decline in the overall average size of the afferent complement of muscle spindles from axial through hind limb to intrinsic foot muscles, but with considerable muscle-specific variability. Finally, we present some comparative data on muscle-spindle afferent complements of rat, rabbit and guinea pig, one particularly notable feature being the high incidence of multiple primary endings in the rat.

Age-related physiological and morphological changes of muscle spindles in rats

Age-related physiological and morphological changes of muscle spindles were examined in rats (male Fischer 344/DuCrj: young, 4-13 months; middle-aged, 20-22 months; old, 28-31 months). Single afferent discharges of the muscle spindles in gastrocnemius muscles were recorded from a finely split dorsal root during ramp-and-hold (amplitude, 2.0 mm; velocity, 2-20 mm s(-1)) or sinusoidal stretch (amplitude, 0.05-1.0 mm; frequency, 0.5-2 Hz). Respective conduction velocities (CVs) were then measured. After electrophysiological experimentation, the muscles were dissected. The silver-impregnated muscle spindles were teased and then analysed using a light microscope. The CV and dynamic response to ramp-and-hold stretch of many endings were widely overlapped in old rats because of the decreased CV and dynamic response of primary endings. Many units in old rats showed slowing of discharge during the release phase under ramp-and-hold stretch and continuous discharge under sinusoidal stretch, similarly to secondary endings in young and middle-aged rats. Morphological studies revealed that primary endings of aged rat muscle spindles were less spiral or non-spiral in appearance, but secondary endings appeared unchanged. These results suggest first that primary muscle spindles in old rats are indistinguishable from secondary endings when determined solely by previously used physiological criteria. Secondly, these physiological results reflect drastic age-related morphological changes in spindle primary endings.

Metabosensitive Afferent Fiber Responses after Peripheral Nerve Injury and Transplantation of an Acellular Muscle Graft in Association with Schwann Cells

Studies dedicated to the repair of peripheral nerve focused almost exclusively on motor or mechanosensitive fiber regeneration. Poor attention has been paid to the metabosensitive fibers from group III and IV (also called ergoreceptor). Previously, we demonstrated that the metabosensitive response from the tibialis anterior muscle was partially restored when the transected nerve was immediately sutured. In the present study, we assessed motor and metabosensitive responses of the regenerated axons in a rat model in which 1 cm segment of the peroneal nerve was removed and immediately replaced by an autologous nerve graft or an acellular muscle graft. Four groups of animals were included: control animals (C, no graft), transected animals grafted with either an autologous nerve graft (Gold Standard-GS) or an acellular muscle filled with Schwann Cells (MSC) or Culture Medium (MCM). We observed that (1) the tibialis anterior muscle was atrophied in GS, M(SC) and M(CM) groups, with no significant difference between grafted groups; (2) the contractile properties of the reinnervated muscles after nerve stimulation were similar in all groups; (3) the metabosensitive afferent responses to electrically induced fatigue was smaller in M(SC) and MCM groups; and (4) the metabosensitive afferent responses to two chemical agents (KCl and lactic acid) was decreased in GS, M(SC) and M(CM) groups. Altogether, these data indicate a motor axonal regeneration and an immature metabosensitive afferent fiber regrowth through acellular muscle grafts. Similarities between the two groups grafted with acellular muscles suggest that, in our conditions, implanted Schwann cells do not improve nerve regeneration. Future studies could include engineered conduits that mimic as closely as possible the internal organization of uninjured nerve.

Central suppression of regenerated proprioceptive afferents

Long after a cut peripheral nerve reinnervates muscle and restores force production in adult cats, the muscle does not respond reflexively to stretch. Motivated by the likelihood that stretch areflexia is related to problems with sensing and controlling limb position after peripheral neuropathies, we sought to determine the underlying mechanism. Electrophysiological and morphological measurements were made in anesthetized rats having one of the nerves to the triceps surae muscles either untreated or cut and immediately rejoined surgically many months earlier. First, it was established that reinnervated muscles failed to generate stretch reflexes, extending observations of areflexia to a second species. Next, multiple elements in the sensorimotor circuit of the stretch reflex were examined in both the PNS and CNS. Encoding of muscle stretch by regenerated proprioceptive afferents was remarkably similar to normal, although we observed some expected abnormalities, e.g., increased length threshold. However, the robust stretch-evoked sensory response that arrived concurrently at the CNS in multiple proprioceptive afferents produced synaptic responses that were either smaller than normal or undetectable. Muscle stretch failed to evoke detectable synaptic responses in 13 of 22 motoneurons, although electrical stimulation generated monosynaptic excitatory postsynaptic potentials that were indistinguishable from normal. The ineffectiveness of muscle stretch was not attributable therefore to dysfunction at synapses made between regenerated Ia afferents and motoneurons. Among multiple candidate mechanisms, we suggest that centrally controlled neural circuits may actively suppress the sensory information encoded by regenerated proprioceptive afferents to prevent recovery of the stretch reflex.

La réparation nerveuse périphérique : 30 siècles de recherche

INTRODUCTION

Nerve injury compromises sensory and motor functions. Techniques of peripheral nerve repair are based on our knowledge regarding regeneration. Microsurgical techniques introduced in the late 1950s and widely developed for the past 20 years have improved repairs. However, functional recovery following a peripheral mixed nerve injury is still incomplete.

STATE OF ART

Good motor and sensory function after nerve injury depends on the reinnervation of the motor end plates and sensory receptors. Nerve regeneration does not begin if the cell body has not survived the initial injury or if it is unable to initiate regeneration. The regenerated axons must reach and reinnervate the appropriate target end-organs in a timely fashion. Recovery of motor function requires a critical number of motor axons reinnervating the muscle fibers. Sensory recovery is possible if the delay in reinnervation is short. Many additional factors influence the success of nerve repair or reconstruction. The timing of the repair, the level of injury, the extent of the zone of injury, the technical skill of the surgeon, and the method of repair and reconstruction contribute to the functional outcome after nerve injury.

CONCLUSION

This review presents the recent advances in understanding of neural regeneration and their application to the management of primary repairs and nerve gaps.

Tremor in the human hand following peripheral nerve transection and reinnervation

Slow movements and position holding by the digits are both characterised by 8-10 Hz tremor which appears to be centrally generated. Denervation and subsequent reinnervation lead to significant alterations in peripheral connectivity and reflex organisation. We have tested the hypothesis that 8-10 Hz tremor is present in the digits of subjects following a complete nerve lesion. The frequency content of abduction and adduction movements was recorded in 12 index fingers and nine little fingers reinnervated subsequent to a complete ulnar nerve transection. An optical position laser transducer was used to measure digital movements, minimising mechanical interference to the system. Concurrently, surface electromyograms (EMG) were also recorded from first dorsal interosseus muscles (1DI) and abductor digiti minimi brevis (ADMB) muscles for index and little fingers, respectively. The maximal voluntary contraction (MVC) of the reinnervated muscles varied from 5.9% to 100% of those of the unimpaired, contralateral hands. The subjects performed abduction-adduction movements of the index and little fingers and a position holding task. Significant peaks in PSD curves of acceleration and rectified integrated EMG traces were identified. Tremor in the 8-10 Hz range was evident in both the acceleration and EMG signals for the majority of digits during both the slow movement and position holding tasks. These findings demonstrate the robust nature of these 8-10 Hz oscillations, even following the significant changes in peripheral connectivity of muscle and nerve resulting from nerve transection and reinnervation.

Long term course of the H reflex after selective tibial neurotomy

OBJECTIVES

This study was conducted to evaluate the long term clinical and electrophysiological outcome by recording the H reflex in a consecutive series of six patients treated by selective tibial neurotomy for spastic equinus foot.

METHOD

The amplitudes of Hmax reflexes, Mmax responses, and Hmax:Mmax ratio were recorded in six patients with chronic lower limb spasticity, before and after surgery, at day 1 and 8 months and 24 months after selective tibial neurotomy. The passive range of movement, the stretch reflex score according to the Tardieu scale, the osteoarticular and tendon repercussions, and the quality of motor control of dorsiflexion were evaluated preoperatively and postoperatively.

RESULTS

At the end of the study, all patients presented a reduction of equines. Gait and Tardieu's score of spasticity had improved in all patients. Active dorsiflexion of the ankle was unchanged in four patients, but two improved by 5 degrees to 12 degrees. In five cases, fascicular resection of the superior nerve to soleus was, alone, sufficient to reduce spastic equinus foot, without recurrence, for a mean follow up of 28 months. Two patients were reoperated on, one for remaining spasticity related to an underestimated spasticity of the gastrocnemius muscles, and the other for painful claw toes. Hmax, Mmax, and Hmax:Mmax ratios were significantly lower the day after surgery. The reduction of Hmax and Hmax/Mmax ratio remained stable over time and was still statistically significant two years after the operation. However, the value of Mmax eight months postoperatively was no longer significantly different from the preoperative value.

CONCLUSION

This study shows the long term efficacy of the selective tibial neurotomy as treatment of spastic equinus foot. Neurotomy confined to fibres supplying the soleus muscle is sufficient in most cases and acts by decreasing sensory afferents without significant long term motor denervation.

Changes in afferent activities from tibialis anterior muscle after nerve repair by self-anastomosis

In order to study sensory nerve plasticity after nerve injury and repair, recordings were made from afferent axons innervating the tibialis anterior muscle in rats under several different experimental conditions. In two groups of rats, reinnervation of the denervated tibialis anterior was examined 2.5 months (group A) and 7 months (group B) after section, along with self-anastomosis of the common peroneal nerve. The other rats (group C) were examined 2.5 months after the nerve was cut and ligatured to its stumps to avoid axonal regeneration. No evoked potentials and no activation in response to any test agent were found in group C rats. We found a significant increase in the proportion of group I-II fibers and a significant decrease in group IV fibers in the group B rats when compared with group A (P < 0.05 and P < 0.01) and control animals (P < 0.01 and P < 0.01). A higher conduction velocity was measured in group IV fibers in group B rats when compared with group A (P < 0.01) and the controls (P < 0.01). The proportion of afferent units showing an optimal discharge in response to tendon vibration at 70 Hz (range 0-100 Hz) was higher in groups A and B (72.2 and 80%, respectively) than in the controls (36.8%). The response of muscle afferents to KCl (1-20 mM) and lactic acid (0.5-3 mM) concentrations was markedly depressed in group A rats (P < 0.05), whereas it was restored and even accentuated in group B animals when compared with the controls (P < 0.05). Electrically induced fatigue (3 min, 10 Hz) significantly activated (P < 0.05) muscle afferents only in controls. The present study indicates that after self-anastomosis of a cut hindlimb muscle nerve, sensory innervation was markedly modified in the direction of enhanced mechanosensitivity to high-frequency tendon vibration and depressed metabosensitivity.

Coculture of rat embryonic proprioceptive sensory neurons and myotubes

With the aim to study the cellular mechanism underlying the process of muscle spindle regeneration, dorsal root ganglia (DRG) neurons derived from 16-day rat embryos were cocultured with developing myotubes in a compartmentalized culture device. To accomplish the selective survival and neurite formation of the proprioceptive subpopulation, the neurotrophic factor, neurotrophin-3, was added to the culture medium. It appeared that the proprioceptive DRG neurons could develop specialized, Ia afferent terminal-like contacts with myotubes. However, these interactions were scarce and did not result in the induction of differentiation of the contacted myotubes into intrafusal fibers as normally occurs during in vivo development. The present coculture setup apparently lacks appropriate regulatory factors essential for the proper matching of sensory axons and intrafusal fiber precursors and the induction of a functional sensory myoneural connection.

Axonal regeneration

Axons damaged in a peripheral nerve are often able to regenerate from the site of injury along the degenerate distal segment of the nerve to reform functional synapses. Schwann cells play a central role in this process. However, in the adult mammalian central nervous system, from which Schwann cells are absent, axonal regeneration does not progress to allow functional recovery. This is due to inhibitors of axonal growth produced by both oligodendrocytes and astrocytes and also to the decreased ability of adult neurons to extend axons during regeneration compared to embryonic neurons during development. However once provided with a substrate conducive to axonal growth, such as a peripheral nerve graft, many central neurons are able to regenerate axons over long distances. Over the past year this response has been utilised in experimental models to produce a degree of behavioural recovery.

Reconstruction of tonic vibration reflex in the biceps brachii reinnervated by transferred intercostal nerves in patients with brachial plexus injury

The transfer of intercostal nerves to musculocutaneous nerves has been performed to reconstruct elbow flexion in patients with brachial plexus injury. In nine of 15 such patients, 2-3 Hz tapping of the distal tendon of the biceps muscle reinnervated by the transferred intercostal nerves (IC-biceps) induced a mode in the correlogram between tap and EMG pulse of IC-biceps. In five of the mode-positive patients, tapping of various frequencies induced gradual augmentation of integrated EMG of IC-biceps. This reflex was consistent with the tonic vibration reflex (TVR) in normal controls. Conduction velocity and frequency property of the reflex were compatible with the speculation that rapid-conducting muscle afferents (group Ia or II or both) reinnervate mechanoreceptors, such as muscle spindles. The clinical significance of muscle sensory reinnervation is not clear; however, the reconstruction of TVR following this operation is worthy in that it confirms the specific sensory reinnervation of denervated muscle in humans.

Changes in PAD patterns of group I muscle afferents after a peripheral nerve crush

In the anesthetized cat we have analyzed the changes in primary afferent depolarization (PAD) evoked in single muscle spindle and tendon organ afferents at different times after their axons were crushed in the periphery and allowed to regenerate. Medial gastrocnemius (MG) afferents were depolarized by stimulation of group I fibers in the posterior biceps and semitendinosus nerve (PBSt), as soon as 2 weeks after crushing their axons in the periphery, in some cases before they could be activated by physiological stimulation of muscle receptors. Two to twelve weeks after crushing the MG nerve, stimulation of the PBSt produced PAD in all MG fibers reconnected with presumed muscle spindles and tendon organs. The mean amplitude of the PAD elicited in afferent fibers reconnected with muscle spindles was increased relative to values obtained from Ia fibers in intact (control) preparations, but remained essentially the same in fibers reconnected with tendon organs. Quite unexpectedly, we found that, between 2 and 12 weeks after crushing the MG nerve, stimulation of the bulbar reticular formation (RF) produced PAD in most afferent fibers reconnected with muscle spindle afferents. The mean amplitude of the PAD elicited in these fibers was significantly increased relative to the PAD elicited in muscle spindle afferents from intact preparations (from 0.08 +/- 0.4 to 0.47 +/- 0.34 mV). A substantial recovery was observed between 6 months and 2.5 years after the peripheral nerve injury. Stimulation of the sural (SU) nerve produced practically no PAD in muscle spindles from intact preparations, and this remained so in those afferents reconnected with muscle spindles impaled 2-12 weeks after the nerve crush. The mean amplitude of the PAD produced in afferent fibers reconnected with tendon organs by stimulation of the PBSt nerve and of the bulbar RF remained essentially the same as the PAD elicited in intact afferents. However, SU nerve stimulation produced a larger PAD in afferents reconnected with tendon organs 2-12 weeks after the nerve crush (mean PAD changed from 0.05 +/- 0.04 to 0.32 +/- 0.17 mV). The results obtained indicate that the PAD patterns of the afferent fibers reconnected with muscle spindle and tendon organ afferents are changed after crushing their axons in the periphery: stimulation of the bulbar RF appears to produce larger PAD in fibers reconnected with muscle spindles, and stimulation of cutaneous afferents produces larger PAD in fibers reconnected with tendon organs. It is suggested that these alterations in the patterns of PAD of muscle afferents result from central changes in the balance of excitatory and inhibitory influences acting on the segmental pathways mediating the PAD. Although the functional role of these changes has not been established, they may reflect compensatory changes aimed to adjust information arising from damaged afferents.

The static sensitivity of tendon organs during recovery from nerve injury

An investigation has been carried out into the physiological properties of tendon organs and their interactions with motor units following two types of nerve injury: nerve crush and nerve transection followed by suture repair. Recovery from nerve crush was very successful: 6 weeks after the injury 60% of the tendon organ-motor unit interactions (n = 62) evoked normal or near-normal patterns of afferent discharge but with reduced firing rates. After 10 weeks of recovery 81% of the interactions (n = 43) were normal. The main abnormality observed was a phasic-only pattern of discharge. The overall reductions in firing rate during early recovery may be attributable to the lower contractile forces generated by the reinnervated muscle units, while the phasic-only responses may also represent immaturity of the transduction mechanism of the regenerated afferent axons. Following nerve transection the quality of recovery was much lower and a range of abnormal, as well as normal patterns of response were observed. For all the afferents studied, both types of response were recorded, suggesting that although there may be changes in the sensitivity of the afferents to muscle contraction, the abnormal responses more probably reflect changes in the form of the mechanical input rather than abnormalities of the transduction process.

Repairing spinal roots after brachial plexus injuries

The problems of repairing spinal roots after brachial plexus avulsion injuries are discussed in the light of current surgical diagnosis and treatment. An advancing understanding of the cellular mechanisms of nerve regeneration and progress in surgical technology indicates a possibility for the repair at least of ventral roots with grafts which may be of neural or non-neural origin. Enhancement of the regenerative properties may further be made possible by the application of neurotrophic factors at the repair site or centrally. The short and long-term implications of current research into these methods are discussed.

Median nerve neurotization by peripheral nerve grafts directly implanted into the spinal cord: anatomical, behavioural and electrophysiological evidences of sensorimotor recovery

Over the years, peripheral nerve grafts, a favorable environment to support axonal elongation, have given rise to increasing interest as a possible solution for promoting spinal cord repair. In the experiments described here, following an avulsion injury of the rat brachial plexus, the median nerve was repaired by a peripheral nerve graft (PN) inserted directly into the dorsal side of the spinal cord. Eight months later the animals were submitted to behavioral tests, electrophysiological and histological studies. Regrowth of axons from both motoneurons and ganglionic neurons was demonstrated following a single superficial dorsal implantation of a PN. Sensorimotor peripheral reinnervation allowed most of the studied animals to recover enough flexor activity for grasping. Reinnervation was achieved even without prior root avulsion suggesting that the presence of a PN is sufficient to induce sprouting in the spinal cord from axotomized and non-axotomized neurons.

Sensory reinnervation of muscle receptor in human

This report describes sensory reinnervation of muscle receptors in humans. In this study, sensory function was investigated in fourteen patients with brachial plexus injury, whose musculocutaneous nerve was surgically sutured with intercostal nerves for reconstruction of elbow flexion. Tapping the intercostal nerve-reinnervated brachial biceps muscle (IC-biceps) induced somatosensory evoked potentials (IC-biceps SEP) in four patients. Tapping also induced reflex IC-biceps activity in all IC-biceps SEP (+) patients, but not in eight out of nine IC-biceps SEP (-) patients. Results show that muscle afferent fibers in the intercostal nerves reinnervated mechanoreceptors in IC-biceps, and induced both cortical sensory activity and muscle activity that indicated the classical stretch reflex.

Location and completeness of reinnervation by two types of neurons at a single target: the feline muscle spindle

Muscle spindles from the tenuissimus muscle of the cat were examined microscopically to assess the precision and completeness of reinnervation of intrafusal muscle fibers by efferent and afferent neurons. Positions of motor and sensory nerve terminals were charted relative to the cross-sectional area enclosed by the outer capsule of the spindle. Profiles of nerve endings were measured for normally innervated and reinnervated spindles. The tenuissimus was deprived of innervation by freezing its nerve, sometimes in conjunction with either spinal ganglion removal or ventral rhizotomy. Sensory and motor terminals occupied separate locales along the length of normal muscle spindles. Nerve terminals of efferent and afferent neurons were located in appropriate positions along the length of spindles when axons of both types of neurons regrew together and when either category of axon regenerated alone. Precise reinnervation of muscle spindles occurred in spite of a diminished diameter of intrafusal fibers. Repopulation of the spindle with motor endings was less complete than that by sensory endings, based on the proportion and size of the regenerated terminals. We conclude that under optimal conditions for axonal regrowth, efferent and afferent neurons reinnervate their respective regions along intrafusal muscle fibers but motor lags sensory reinnervation within the spindle. The mechanism by which positional specificity happens during reinnervation of intrafusal fibers requires neither an interaction between terminals of the two types of neurons nor target cells of normal bulk.

Effects of different methods of peripheral nerve repair on the number and distribution of muscle afferent neurons in rat dorsal root ganglion

The effects of three methods of peripheral nerve repair and normal controls were compared with respect to the number and distribution of muscle afferent nerve cell bodies from the extensor digitorum longus muscle in the dorsal root ganglia of the rat. Nerves were repaired using one of three methods: 1) direct epineurial suture; 2) a three-strand cable graft; or 3) a coaxially aligned freeze-thawed autologous skeletal muscle graft. In all cases the number and distribution of muscle afferent nerve cells were estimated 300 days after nerve repair. Retrograde transport of horseradish peroxidase, injected into the extensor digitorum longus muscle, was used to identify muscle afferent nerve cell bodies in dorsal root ganglia exposed at laminectomy after fixation. It was found that all methods of nerve repair were associated with a change in both the number and distribution of labeled muscle afferent cell bodies. The number of labeled muscle afferent cells was significantly different from normal controls in all methods of repair, but was not significantly different among the three methods compared. On the other hand, cell size and cell distribution departed significantly more from normal after either of the grafting procedures than after direct repair. There was no significant difference between the two grafting techniques. These results are discussed with respect to their mechanistic and clinical interpretation.

Recovery of denervated muscle receptors following treatments to accelerate nerve regeneration

Nerve regeneration promoters offer the possibility of enhancing recovery following nerve injury by increasing the numbers of regenerating axons and decreasing the period of tissue denervation. To date, recent studies have concentrated on evaluating the action of such promoters on the nerve itself and on the restoration of motor function. This study examines afferent regeneration by evaluating the recovery of muscle spindles and tendon organs after nerve injury and treatment with alpha-melanocyte stimulating hormone or nerve stimulation. Treatment was found to be effective in enhancing short-term recovery after nerve-crush injury in terms of increasing the rate of functional and morphological recovery. Following nerve section, there was an increase in the number of reinnervating axons compared with control values.

An Organotypic Spinal Cord - Dorsal Root Ganglion - Skeletal Muscle Coculture of Embryonic Rat. I. The Morphological Correlates of the Spinal Reflex Arc

The cytoarchitecture of a spinal cord - dorsal root ganglion - skeletal muscle tissue coculture system was investigated at the level of the light microscope using a number of different staining techniques. In these cultures central synapses between dorsal root ganglion (DRG) cells and interneurons in the ventral spinal cord and between DRG cells and motoneurons were visualized by parvalbumin immunostaining and by intracellular horseradish peroxidase (HRP) filling of DRG cells. Skeletal muscle fibres regenerated in vitro first into multinucleated myotubes, and around day 8 in vitro into well differentiated muscle fibres with regular cross-striation. At the same time newly formed motor endplates could be visualized using acetylcholinesterase staining. The axons of motoneurons could be traced retrogradely by local application of HRP to the regenerated muscle fibres. The motor axons sometimes gave off collaterals reminiscent of Renshaw collaterals at about 300 microm from the axon hillock. Intracellular filling to motoneurons with HRP revealed that only a minority of the motoneurons within a culture had reached their appropriate target. Comparing the dendrograms of the motoneurons which had innervated muscles to those which had not suggested that motoneurons innervating muscle tissue had more complex dendritic trees and larger somata than those which did not innervate muscle tissue. Peripheral neurites of parvalbumin-immunoreactive DRG cells coiling around regenerated muscle fibres could be demonstrated in these cultures. These probably correspond to that part of the sensory muscle spindle apparatus which developed in vivo. However, only a few of the several hundred DRG cells found in every culture were parvalbumin-immunoreactive, suggesting that the actual number of Ia and II afferents within the population of DRG cells in culture is very small. This study demonstrates that all the neural elements necessary for the segmental spinal reflexes develop and can be maintained for several weeks in vitro.

Plasticity of sympathetic reflex organization following cross-union of inappropriate nerves in the adult cat

1. The present study has investigated the reflex organization of sympathetic neurones and its control of autonomic effector organs following nerve injury and repair. A well-defined population of vasoconstrictor neurones supplying blood vessels of the hairy skin was forced to innervate a territory that contained some appropriate, but mainly inappropriate autonomic effector organs. For this purpose the central stump of the cut sural nerve was sutured to the peripheral stump of the cut tibial nerve 11-12 months prior to the terminal experiment. 2. The activity of postganglionic sympathetic neurones was recorded from fine strands of the sural nerve proximal to the nerve lesion. Using a laser-Doppler device cutaneous blood flow was measured in the hairless skin of the hindpaw that was now reinnervated by the sural nerve. The results show a qualitative change of the reflex organization of sympathetic neurones following cross-union of these nerves. 3. Stimulation of arterial chemoreceptors by ventilating the animals with a hypoxic gas mixture (8% O2 in N2 for 3-8 min) increased the activity in twelve out of thirteen strands containing postganglionic sympathetic fibres. The increase of sympathetic activity contrasts with results from normal animals where systemic hypoxia causes a reflex decrease of activity in postganglionic fibres of the sural nerve. 4. Reflex changes of sympathetic activity were closely followed by corresponding changes of cutaneous blood flow. Systemic hypoxia produced vasoconstriction in operated animals in contrast to the vasodilatation observed in normal animals. 5. We conclude that the reflex organization of sympathetic neurones can change qualitatively following nerve lesion when sympathetic neurones regenerate and supply inappropriate target tissues. This long-lasting change reflects the plasticity of the autonomic nervous system and can produce a sustained abnormal control of reinnervated autonomic effector organs.

Aberrant reinnervation

Although great emphasis is placed on providing a satisfactory conduit for regeneration of peripheral axons after nerve repair, the quality of functional restoration is influenced as much by the quality as the quantity of axonal regeneration. Misdirected regeneration is so commonly encountered that motor axons appear to enter and regenerate to muscles in an almost random manner. Thus, when there are several choices, as usually is the case with more proximal nerve or plexus repairs, misdirected reinnervation accounts in many incidences for a poor quality of functional restoration. The regenerative capacities of type I and type II motor axons appear to differ. Proprioceptors and other sensory axons have been shown to reinnervate inappropriate end organs. Consequently, deranged central reflex modulation and disturbed orderly recruitment of motor units according to the size principle also contributes to this problem. Central re-education or adaptation to misdirected regeneration does not occur to any appreciable extent.