Innervation and function of hind-limb muscles in the cat after cross-union of the tibial and peroneal nerves

Reverse End-to-Side Transfer to Ulnar Motor Nerve: Evidence From Preclinical and Clinical Studies

The disappointing outcomes of conventional nerve repair or grafting procedures for proximal ulnar nerve injuries have led the scientific community to search for better alternatives. The pronator quadratus branch of the anterior interosseous nerve has been transferred to the distal ulnar motor branch in a reverse end-to-side fashion with encouraging results. This transfer is now becoming commonly used as an adjunct to cubital tunnel decompression in patients with compressive ulnar neuropathy, underscoring the need for this knowledge transfer to the neurosurgical community. However, the mechanism of recovery after these transfers is not understood completely. We have reviewed the existing preclinical and clinical literature relevant to this transfer to summarize the current level of understanding of the underlying mechanisms, define the indications for performing this transfer in the clinic, and identify the complications and best practices with respect to the operative technique. We have also attempted to identify the major deficiencies in our current level of understanding of the recovery process to propose directions for future research.

A Model for Self-Organization of Sensorimotor Function: The Spinal Monosynaptic Loop

Recent spinal cord literature abounds with descriptions of genetic preprogramming and the molecular control of circuit formation. In this paper, we explore to what extent circuit formation based on learning rather than preprogramming could explain the selective formation of the monosynaptic projections between muscle spindle primary afferents and homonymous motoneurons. We adjusted the initially randomized gains in the neural network according to a Hebbian plasticity rule while exercising the model system with spontaneous muscle activity patterns similar to those observed during early fetal development. Normal connectivity patterns developed only when we modeled β motoneurons, which are known to innervate both intrafusal and extrafusal muscle fibers in vertebrate muscles but were not considered in previous literature regarding selective formation of these synapses in animals with paralyzed muscles. It was also helpful to correctly model the greatly reduced contractility of extrafusal muscle fibers during early development. Stronger and more coordinated muscle activity patterns such as observed later during neonatal locomotion impaired projection selectivity. These findings imply a generic functionality of a musculoskeletal system to imprint important aspects of its mechanical dynamics onto a neural network, without specific preprogramming other than setting a critical period for the formation and maturation of this general pattern of connectivity. Such functionality would facilitate the successful evolution of new species with altered musculoskeletal anatomy, and it may help to explain patterns of connectivity and associated reflexes that appear during abnormal development.

NEW & NOTEWORTHY A novel model of self-organization of early spinal circuitry based on a biologically realistic plant, sensors, and neuronal plasticity in conjunction with empirical observations of fetal development. Without explicit need for guiding genetic rules, connection matrices emerge that support functional self-organization of the mature pattern of Ia to motoneuron connectivity in the spinal circuitry.

Testing the effectiveness and the contribution of experimental supercharge (reversed) end-to-side nerve transfer

OBJECTIVE

Supercharge end-to-side (SETS) transfer, also referred to as reverse end-to-side transfer, distal to severe nerve compression neuropathy or in-continuity nerve injury is gaining clinical popularity despite questions about its effectiveness. Here, the authors examined SETS distal to experimental neuroma in-continuity (NIC) injuries for efficacy in enhancing neuronal regeneration and functional outcome, and, for the first time, they definitively evaluated the degree of contribution of the native and donor motor neuron pools.

METHODS

This study was conducted in 2 phases. In phase I, rats (n = 35) were assigned to one of 5 groups for unilateral sciatic nerve surgeries: group 1, tibial NIC with distal peroneal-tibial SETS; group 2, tibial NIC without SETS; group 3, intact tibial and severed peroneal nerves; group 4, tibial transection with SETS; and group 5, severed tibial and peroneal nerves. Recovery was evaluated biweekly using electrophysiology and locomotion tasks. At the phase I end point, after retrograde labeling, the spinal cords were analyzed to assess the degree of neuronal regeneration. In phase II, 20 new animals underwent primary retrograde labeling of the tibial nerve, following which they were assigned to one of the following 3 groups: group 1, group 2, and group 4. Then, secondary retrograde labeling from the tibial nerve was performed at the study end point to quantify the native versus donor regenerated neuronal pool.

RESULTS

In phase I studies, a significantly increased neuronal regeneration in group 1 (SETS) compared with all other groups was observed, but with modest (nonsignificant) improvement in electrophysiological and behavioral outcomes. In phase II experiments, the authors discovered that secondary labeling in group 1 was predominantly contributed from the donor (peroneal) pool. Double-labeling counts were dramatically higher in group 2 than in group 1, suggestive of hampered regeneration from the native tibial motor neuron pool across the NIC segment in the presence of SETS.

CONCLUSIONS

SETS is indeed an effective strategy to enhance axonal regeneration, which is mainly contributed by the donor neuronal pool. Moreover, the presence of a distal SETS coaptation appears to negatively influence neuronal regeneration across the NIC segment. The clinical significance is that SETS should only employ synergistic donors, as the use of antagonistic donors can downgrade recovery.

Musculotendon adaptations and preservation of spinal reflex pathways following agonist-to-antagonist tendon transfer

Tendon transfer surgeries are performed to restore lost motor function, but outcomes are variable, particularly those involving agonist‐to‐antagonist muscles. Here, we evaluated the possibility that lack of proprioceptive feedback reorganization and musculotendon adaptations could influence outcomes. Plantaris‐to‐tibialis anterior tendon transfer along with resection of the distal third of the tibialis anterior muscle belly was performed in eight cats. Four cats had concurrent transection of the deep peroneal nerve. After 15–20 weeks, intermuscular length and force‐dependent sensory feedback were examined between hindlimb muscles, and the integrity of the tendon‐to‐tendon connection and musculotendon adaptations were evaluated. Three of the transferred tendons tore. A common finding was the formation of new tendinous connections, which often inserted near the original location of insertion on the skeleton (e.g., connections from plantaris toward calcaneus and from tibialis anterior toward first metatarsal). The newly formed tissue connections are expected to compromise the mechanical action of the transferred muscle. We found no evidence of changes in intermuscular reflexes between transferred plantaris muscle and synergists/antagonists whether the tendon‐to‐tendon connection remained intact or tore, indicating no spinal reflex reorganization. We propose the lack of spinal reflex reorganization could contribute the transferred muscle not adopting the activation patterns of the host muscle. Taken together, these findings suggest that musculotendon plasticity and lack of spinal reflex circuitry reorganization could limit functional outcomes after tendon transfer surgery. Surgical planning and outcomes assessments after tendon transfer surgery should consider potential consequences of the transferred muscle's intermuscular spinal circuit actions.

Axon pathfinding for locomotion

Motor neurons of the spinal cord are responsible for the assembly of neuromuscular connections indispensable for basic locomotion and skilled movements. A precise spatial relationship exists between the position of motor neuron cell bodies in the spinal cord and the course of their axonal projections to peripheral muscle targets. Motor neuron innervation of the vertebrate limb is a prime example of this topographic organization and by virtue of its accessibility and predictability has provided access to fundamental principles of motor system development and neuronal guidance. The seemingly basic binary map established by genetically defined motor neuron subtypes that target muscles in the limb is directed by a surprisingly large number of directional cues. Rather than being simply redundant, these converging signaling pathways are hierarchically linked and cooperate to increase the fidelity of axon pathfinding decisions. A current priority is to determine how multiple guidance signals are integrated by individual growth cones and how they synergize to delineate class-specific axonal trajectories.

Targeted Muscle Reinnervation for the Upper and Lower Extremity

Myoelectric devices are controlled by electromyographic signals generated by contraction of residual muscles, which thus serve as biological amplifiers of neural control signals. Although nerves severed by amputation continue to carry motor control information intended for the missing limb, loss of muscle effectors due to amputation prevents access to this important control information. Targeted Muscle Reinnervation (TMR) was developed as a novel strategy to improve control of myoelectric upper limb prostheses. Severed motor nerves are surgically transferred to the motor points of denervated target muscles, which, after reinnervation, contract in response to neural control signals for the missing limb. TMR creates additional control sites, eliminating the need to switch the prosthesis between different control modes. In addition, contraction of target muscles, and operation of the prosthesis, occurs in reponse to attempts to move the missing limb, making control easier and more intuitive. TMR has been performed extensively in individuals with high-level upper limb amputations and has been shown to improve functional prosthesis control. The benefits of TMR are being studied in individuals with transradial amputations and lower limb amputations. TMR is also being investigated in an ongoing clinical trial as a method to prevent or treat painful amputation neuromas.

Strategies to promote peripheral nerve regeneration: electrical stimulation and/or exercise

Enhancing the regeneration of axons is often considered to be a therapeutic target for improving functional recovery after peripheral nerve injury. In this review, the evidence for the efficacy of electrical stimulation (ES), daily exercise and their combination in promoting nerve regeneration after peripheral nerve injuries in both animal models and in human patients is explored. The rationale, effectiveness and molecular basis of ES and exercise in accelerating axon outgrowth are reviewed. In comparing the effects of ES and exercise in enhancing axon regeneration, increased neural activity, neurotrophins and androgens are considered to be common requirements. Similarly, there are sex-specific requirements for exercise to enhance axon regeneration in the periphery and for sustaining synaptic inputs onto injured motoneurons. ES promotes nerve regeneration after delayed nerve repair in humans and rats. The effectiveness of exercise is less clear. Although ES, but not exercise, results in a significant misdirection of regenerating motor axons to reinnervate different muscle targets, the loss of neuromuscular specificity encountered has only a very small impact on resulting functional recovery. Both ES and exercise are promising experimental treatments for peripheral nerve injury that seem to be ready to be translated to clinical use.

α2-chimaerin is required for Eph receptor-class-specific spinal motor axon guidance and coordinate activation of antagonistic muscles

Axonal guidance involves extrinsic molecular cues that bind growth cone receptors and signal to the cytoskeleton through divergent pathways. Some signaling intermediates are deployed downstream of molecularly distinct axon guidance receptor families, but the scope of this overlap is unclear, as is the impact of embryonic axon guidance fidelity on adult nervous system function. Here, we demonstrate that the Rho-GTPase-activating protein α2-chimaerin is specifically required for EphA and not EphB receptor signaling in mouse and chick spinal motor axons. Reflecting this specificity, the loss of α2-chimaerin function disrupts the limb trajectory of extensor-muscle-innervating motor axons the guidance of which depends on EphA signaling. These embryonic defects affect coordinated contraction of antagonistic flexor-extensor muscles in the adult, indicating that accurate embryonic motor axon guidance is critical for optimal neuromuscular function. Together, our observations provide the first functional evidence of an Eph receptor-class-specific intracellular signaling protein that is required for appropriate neuromuscular connectivity.

The longitudinal spinal cord injury: lessons from intraspinal plexus, cauda equina and medullary conus lesions

Spinal nerve root avulsion injury interrupts the transverse segmental spinal cord nerve fibers. There is degeneration of sensory, motor, and autonomic axons, loss of synapses, deterioration of local segmental connections, nerve cell death, and reactions among non neuronal cells with central nerve system (CNS) scar formation, i.e., a cascade of events similar to those known to occur in any injury to the spinal cord. This is the longitudinal spinal cord injury (SCI). For function to be restored, nerve cells must survive and there must be regrowth of new nerve fibers along a trajectory consisting of CNS growth-inhibitory tissue in the spinal cord as well as peripheral nervous system (PNS) growth-promoting tissue in nerves. Basic science results have been translated into a successful surgical strategy to treat root avulsion injuries in man. In humans, this technique is currently the most promising treatment of any spinal cord injury, with return of useful muscle function together with pain alleviation. Experimental studies have also identified potential candidates for adjunctive therapies that, together with surgical replantation of avulsed roots after brachial plexus and cauda equina injuries, can restore not only motor but also autonomic and sensory trajectories to augment the recovery of neurological function. This is the first example of a spinal cord lesion that can be treated surgically, leading to restoration of somatic and autonomic activity and alleviation of pain.

Misdirection of regenerating axons and functional recovery following sciatic nerve injury in rats

Poor functional recovery found after peripheral nerve injury has been attributed to the misdirection of regenerating axons to reinnervate functionally inappropriate muscles. We applied brief electrical stimulation (ES) to the common fibular (CF) but not the tibial (Tib) nerve just prior to transection and repair of the entire rat sciatic nerve, to attempt to influence the misdirection of its regenerating axons. The specificity with which regenerating axons reinnervated appropriate targets was evaluated physiologically using compound muscle action potentials (M responses) evoked from stimulation of the two nerve branches above the injury site. Functional recovery was assayed using the timing of electromyography (EMG) activity recorded from the tibialis anterior (TA) and soleus (Sol) muscles during treadmill locomotion and kinematic analysis of hindlimb locomotor movements. Selective ES of the CF nerve resulted in restored M-responses at earlier times than in unstimulated controls in both TA and Sol muscles. Stimulated CF axons reinnervated inappropriate targets to a greater extent than unstimulated Tib axons. During locomotion, functional antagonist muscles, TA and Sol, were coactivated both in stimulated rats and in unstimulated but injured rats. Hindlimb kinematics in stimulated rats were comparable to untreated rats, but significantly different from intact controls. Selective ES promotes enhanced axon regeneration but does so with decreased fidelity of muscle reinnervation. Functional recovery is neither improved nor degraded, suggesting that compensatory changes in the outputs of the spinal circuits driving locomotion may occur irrespective of the extent of misdirection of regenerating axons in the periphery.

Nerve regeneration in the peripheral nervous system versus the central nervous system and the relevance to speech and hearing after nerve injuries

Schwann cells normally form myelin sheaths around axons in the peripheral nervous system (PNS) and support nerve regeneration after nerve injury. In contrast, nerve regeneration in the central nervous system (CNS) is not supported by the myelinating cells known as oligodendrocytes. We have found that: 1) low frequency electrical stimulation can be used to elevate cAMP thereby promoting regeneration of CNS axons and 2) a conditioning lesion, created by a crush of the peripheral branch of the dorsal root ganglion sensory neurons along with a simultaneous cut of these axons in the CNS, promotes even greater neural outgrowth than electrical stimulation. The effectiveness of the lesion results from both an acceleration of axon outgrowth and an increase in the rate of axon growth. However, electrical stimulation remains a more viable treatment of nerve injuries to stimulate regeneration and has been successfully used to promote development of the auditory pathways in children with severe to profound deafness who use cochlear implants. Without nerve regeneration, there is only a random reinnervation of affected muscles. An example occurs when the laryngeal nerve attempts to reinnervate the vocal cords after injury, causing deficits in speech. Synkinesis occurs when reinnervation of antagonistic muscles effectively paralyze the vocal cords and, in turn, severely compromises speech. The misdirection of laryngeal nerve reinnervation can be alleviated surgically by strategies favoring inspiratory abduction.

LEARNING OUTCOMES

Readers of this article will gain an understanding of (1) the potential for axon regeneration in the central nervous system and (2) problems and possible solutions for random reinnervation of laryngeal muscles for speech.

The resilience of the size principle in the organization of motor unit properties in normal and reinnervated adult skeletal muscles

Henneman's size principle relates the input and output properties of motoneurons and their muscle fibers to size and is the basis for size-ordered activation or recruitment of motor units during movement. After nerve injury and surgical repair, the relationship between motoneuron size and the number and size of the muscle fibers that the motoneuron reinnervates is initially lost but returns with time, irrespective of whether the muscles are self- or cross-reinnervated by the regenerated axons. Although the return of the size relationships was initially attributed to the recovery of the cross-sectional area of the reinnervated muscle fibers and their force per fiber, direct enumeration of the innervation ratio and the number of muscle fibers per motoneuron demonstrated that a size-dependent branching of axons accounts for the size relationships in normal muscle, as suggested by Henneman and his colleagues. This same size-dependent branching accounts for the rematching of motoneuron size and muscle unit size in reinnervated muscles. Experiments were carried out to determine whether the daily amount of neuromuscular activation of motor units accounts for the size-dependent organization and reorganization of motor unit properties. The normal size-dependent matching of motoneurons and their muscle units with respect to the numbers of muscle fibers per motoneuron was unaltered by synchronous activation of all of the motor units with the same daily activity. Hence, the restored size relationships and rematching of motoneuron and muscle unit properties after nerve injuries and muscle reinnervation sustain the normal gradation of muscle force during movement by size-ordered recruitment of motor units and the process of rate coding of action potentials. Dynamic modulation of size of muscle fibers and their contractile speed and endurance by neuromuscular activity allows for neuromuscular adaptation in the context of the sustained organization of the neuromuscular system according to the size principle.Key words: motor unit size, motor unit recruitment, innervation ratio, reinnervation.

Altered electromyographic activity pattern of rat soleus muscle transposed into the bed of antagonist muscle

Human patients suffering from motor paralysis of the leg can learn, to some extent, how to use the transposed antagonistic muscle in place of the damaged or ineffective muscle. Experiments on animals showed opposite results, although in a few experiments the functional reorganization of the activity of the transposed muscle was not excluded. In our experiments, we performed transposition of the soleus (Sol) with a preserved innervation into the bed of the removed extensor digitorum longus (EDL) in 6-d-old pups and 3-month-old rats. The locomotor and reflex EMG activity of the transposed Sol (trSol) was recorded in chronic experiments 3-4 months later. Our results showed that the EMG activity of the Sol might be modified when the muscle is transposed into the bed of the antagonistic muscle EDL. The modification consisted of an additional burst of activity during the swing phase and of reflex response of the muscle to the plantar flexion. This modification was present in all animals operated on at 6 d of age and in two of six adults. After chronic experiments, we excluded the possibility that the flexor-like activity was induced by additional innervation of the trSol by the cut end of the EDL nerve. We suggest that the observed modifications of activity of trSol might be caused by readjustment of the spinal network organization to the new functional demands. Two factors might be responsible for this readjustment: the immaturity of the nervous system at the moment of surgery and preserved afferent innervation of the transposed muscle.

Increased neuromuscular activity reduces sprouting in partially denervated muscles

The effects of increasing neural activity on sprouting remain unclear and controversial. In a rat model of partial denervation of skeletal muscles, we investigated the effect of neuromuscular activity on sprouting. Rat hindlimb muscles were partially denervated by avulsion of either L4 or L5 spinal root. Immediately after partial denervation, the rats were divided into three groups: (1) normal caged activity, (2) running exercise on wheels, 8 hr daily, and (3) functional electrical stimulation (FES) of sciatic nerves, 20 Hz for 8 hr daily. At 1 month, muscle unit (MU) enlargement was quantitated electrophysiologically and histochemically. MU twitch force was increased by four- to fivefold by partial denervation in extensively denervated tibialis anterior (TA) and medial gastrocnemius (MG) and by approximately twofold in moderately denervated plantaris (PL) and soleus (SOL). For the extensively denervated TA and MG muscles, MU enlargement, measured electrophysiologically, declined significantly after an average of 1757 +/- 310 m/d running exercise and daily FES for 1 month. The detrimental effects on MU enlargement were much less but significant in the moderately denervated PL and did not reach statistical significance in the moderately denervated SOL muscle. Histochemical evaluation of sprouting showed a reduction in the number of sprouts in the extensively denervated TA muscle, but not the moderately denervated PL and SOL muscles, by increased neuromuscular activity. Thus, increased neuromuscular activity is detrimental primarily in muscles that are extensively denervated, and the MUs are smaller than under conditions in which the muscles experience normal physiological levels of activation.

Increased neuromuscular activity reduces sprouting in partially denervated muscles

The effects of increasing neural activity on sprouting remain unclear and controversial. In a rat model of partial denervation of skeletal muscles, we investigated the effect of neuromuscular activity on sprouting. Rat hindlimb muscles were partially denervated by avulsion of either L4 or L5 spinal root. Immediately after partial denervation, the rats were divided into three groups: (1) normal caged activity, (2) running exercise on wheels, 8 hr daily, and (3) functional electrical stimulation (FES) of sciatic nerves, 20 Hz for 8 hr daily. At 1 month, muscle unit (MU) enlargement was quantitated electrophysiologically and histochemically. MU twitch force was increased by four- to fivefold by partial denervation in extensively denervated tibialis anterior (TA) and medial gastrocnemius (MG) and by approximately twofold in moderately denervated plantaris (PL) and soleus (SOL). For the extensively denervated TA and MG muscles, MU enlargement, measured electrophysiologically, declined significantly after an average of 1757 +/- 310 m/d running exercise and daily FES for 1 month. The detrimental effects on MU enlargement were much less but significant in the moderately denervated PL and did not reach statistical significance in the moderately denervated SOL muscle. Histochemical evaluation of sprouting showed a reduction in the number of sprouts in the extensively denervated TA muscle, but not the moderately denervated PL and SOL muscles, by increased neuromuscular activity. Thus, increased neuromuscular activity is detrimental primarily in muscles that are extensively denervated, and the MUs are smaller than under conditions in which the muscles experience normal physiological levels of activation.

Nerve transfer to the median nerve using parts of the ulnar and radial nerves in the rabbit--effects on motor recovery of the median nerve and donor nerve morbidity

In this study, motor re-innervation of the median nerve by transfer of one-third, one-half, and two-thirds of either the agonistic ulnar nerve or the antagonistic radial nerve was investigated in both extremities of 20 rabbits. Recipient median nerve: Muscle contraction force of the flexor digitorum sublimus muscle after a one-third and a one-half of the ulnar nerve transfer achieved an average of 75 and 97% muscle power respectively as compared to conventional end-to-end neurorrhaphy. Muscle contraction force after one-third or one-half of the radial nerve transfer was significantly lower (36%). Donor nerves: Extensor carpi radialis muscle or flexor carpi ulnaris muscle contraction force 6 months postoperatively demonstrated a significant decrease after a one-half ulnar nerve and a two-thirds ulnar or radial nerve transfer, but not after a one-third transfer of either radial or ulnar nerves. Histologically, the number of axons in the re-innervated median nerve and both donor nerves distal to the coaptation site seemed to follow variable patterns. It was concluded that in the rabbit use of one-third of the agonistic ulnar nerve for re-innervation of the median nerve results in useful motor recovery with negligible donor site morbidity. Clinically, this technique may offer an alternative option for proximal nerve injuries or for free functioning muscle transplantations.

Fast to slow transformation of denervated and electrically stimulated rat muscle

1. Denervated fast extensor digitorum longus (EDL) muscles of adult rats were stimulated electrically for up to 4 months with a slow pattern resembling the activity in soleus (Sol) motor units and examined with antibodies against myosin heavy chains (MHCs). 2. The normal EDL contained, on average, 45% type IIB, 29% type IIX, 23% type IIA and 3% type I fibres. All type IIB and almost all type IIX fibres disappeared during the first 3 weeks of stimulation. They were replaced by type IIA and type I fibres, whose percentages increased to about 75 and 15, respectively. Type IIA fibres remained at 75% for nearly 2 months and were then gradually replaced by type I fibres during the next 2 months. The transformation occurred sequentially in the order IIB/IIX-->IIA-->I, the first step (IIB/IIX-->IIA) occurring after a short delay (2 weeks) and the last step (IIA-->I in originally IIB or IIX fibres) after a long delay (> 2 months). During the transformation coexpression of MHCs occurred. 3. It appears that the transformation to type I fibres occurred in pre-existing type II fibres since no signs of fibre damage or regeneration were observed. 4. Normal EDL was also stimulated through an intact nerve with the same pattern for up to 37 days. The effects on fibre type distributions were identical to those observed in the denervated EDL. The result indicated that the Sol-like pattern of evoked muscle activity, rather than nerve-derived trophic influences or denervation per se, was primarily responsible for the fast to slow transformation.

Neural control of locomotion; The central pattern generator from cats to humans

In the last years it has become possible to regain some locomotor activity in patients suffering from an incomplete spinal cord injury (SCI) through intense training on a treadmill. The ideas behind this approach owe much to insights derived from animal studies. Many studies showed that cats with complete spinal cord transection can recover locomotor function. These observations were at the basis of the concept of the central pattern generator (CPG) located at spinal level. The evidence for such a spinal CPG in cats and primates (including man) is reviewed in part 1, with special emphasis on some very recent developments which support the view that there is a human spinal CPG for locomotion. Copyright 1997 Elsevier Science B.V.

Delayed hypoglossal-facial nerve suture after predegeneration of the peripheral facial nerve stump improves the innervation of mimetic musculature by hypoglossal motoneurons

Surgical reconstruction of the facial nerve is common clinical practice following destruction of the intracranial facial nerve. Delayed hypoglossal-facial anastomosis (HFA) is the procedure of choice, although the effect of delay on outcome remains unclear. To study the effect of delayed anastomosis on reinnervation, we sutured the proximal stump of a freshly transected hypoglossal nerve of Wistar rats to the distal stump of the ipsilateral facial nerve, which had been transected 7-56 days earlier. Animals that had received HFA without delay served as the control group. Forty days after HFA, horseradish peroxidase (HRP) was injected into the whisker pad; 2 days later, the animals were killed. Reinnervation was assessed by determining the proportion of labeled neuronal cell bodies in the brainstem. The control group had 68% reinnervation of these muscles by hypoglossal neurons and had 32% reinnervation by facial neurons. When the distal facial nerve had been allowed to degenerate for 7 days before HFA, reinnervation of the hypoglossal nerve decreased to 54%, and reinnervation by the facial nerve increased to 46%. However, after a delay of 10-56 days, the hypoglossal fraction increased and stabilized at 77%, and the facial motoneuron fraction decreased to 23%. The presence of new neuromuscular junctions was confirmed by HRP labeling of motor end plates in vivo and by electromyography. We conclude that, under the conditions of hypoglossal-facial crossed nerve suture, the predegeneration of the distal stump of a transected facial nerve enhances the reinnervation of facial muscles by hypoglossal axonal sprouts.

Locomotion of the hindlimbs after neurectomy of ankle flexors in intact and spinal cats: model for the study of locomotor plasticity

To study the potential plasticity of locomotor networks in the spinal cord, an important issue for locomotor rehabilitation after spinal injuries, we have investigated the locomotor performance of cats before and after a unilateral denervation of the ankle flexors tibialis anterior (TA) and extensor digitorum longus (EDL) both in cats with intact spinal cord and after spinalization. The effects of the inactivation of the ankle flexors were studied in three cats with intact spinal cord during periods of 4-7 wk. Cats adapted their locomotor performance very rapidly within a few days so that the locomotor behavior appeared to be unchanged practically. However, kinematic analyses of video records often revealed small but consistent increase in knee and/or hip flexion. These changes were accompanied by some increase in the amplitude of knee and hip flexor muscle activity. Cats maintained a regular and symmetrical walking pattern over the treadmill for several minutes. Two of these cats then were spinalized at T13 and studied for approximately 1 mo afterward. Whereas normally cats regain a regular and symmetrical locomotor pattern after spinalization, these cats had a disorganized and asymmetrical locomotor pattern with a predominance of knee flexion and absence of plantar foot contact of the denervated limb. Another cat first was spinalized and allowed to recuperate a regular symmetrical locomotor performance. Then it also was submitted to the same unilateral ankle flexor inactivation and studied for approximately 50 days. The cat maintained a well-organized symmetrical gait although there was almost no ankle flexion on the denervated side. There was no exaggerated knee hyperflexion and gait asymmetry as seen in the two previous cats spinalized only after they had adapted to the denervation of ankle flexors. It is concluded that, after muscle denervation, locomotor adaptation is achieved through changes occurring at different levels. Because cats spinalized after adaptation to the neurectomy had an asymmetrical locomotor pattern dominated by hyperflexion, it is suggested that the spinal circuitry has been modified during the adaptive process, presumably through the action of corrective supraspinal inputs. Indeed spinal cats do not normally display such abnormal hyperflexions, and neither did the one cat denervated after spinalization. On the other hand, because the modified locomotor pattern in the spinal state is not functional and contains only some aspects of the compensatory response seen before spinalization, it is suggested that the complete functional adaptation observed in intact cats after peripheral nerve lesions may depend on changes occurring at the spinal and the supraspinal levels.

Recovery of original nerve supply after hypoglossal-facial anastomosis causes permanent motor hyperinnervation of the whisker-pad muscles in the rat

Hypoglossal-facial anastomosis (HFA), used in humans for the treatment of facial palsy, was experimentally performed in adult female Wistar rats. The time course of facial reinnervation and the extent of the new motor nerve supply of the vibrissal muscles that develops after HFA were estimated by counting all motoneurons in the brainstem labeled by injection of horseradish peroxidase (HRP) into the whisker pad; muscle innervation by motor endplates was not studied. In untreated animals, HRP injection labels 1,254 +/- 54 (mean +/- S.D.; n = 6) motoneurons, localized exclusively in the lateral subdivision of the facial nucleus. Immediately following HFA, this number drops to zero. The first HRP-labeled motoneurons appear in the hypoglossal nucleus at 28 days postoperation (dpo) and at 56 dpo their number reaches 1,096 +/- 48. Unexpectedly, the facial nerve, whose proximal stump has been left as blind end during surgery, additionally sends axons to the facial periphery. This resprouting is first detected at 42 dpo with HRP-marked neurons throughout the facial nucleus lacking somatotopic organization. The number of these labeled neurons also rises with time, and at 56 dpo, a total of 1,797 +/- 142 facial and hypoglossal motoneurons, that is, 43% more motoneurons than in normal animals, supplies the whisker pad. This hyperinnervation, that is, the projection of more motoneurons into the target muscle than under normal conditions--further increases to 1,978 +/- 92 motoneurons at 224 dpo and may provide a new animal model for studying the competitive relationships between motoneurons in their search for peripheral targets.

Patterns of reinnervation and motor unit recruitment in human hand muscles after complete ulnar and median nerve section and resuture

Following complete ulnar or above-elbow median nerve sections, there was no significant correlation between motor unit size (twitch amplitude) and recruitment threshold, as assessed by spike triggered averaging. This absence of orderly recruitment was attributed to misdirection of motor axons during regeneration. Following median nerve section at wrist level, where the reinnervated muscles have more synergistic actions, orderly recruitment by size appeared to be re-established. Thus, the size principle of motor unit recruitment can be re-established after nerve section in humans, if motor axons innervate their original muscles or ones with closely synergistic functions.

Organization of motor units following cross-reinnervation of antagonistic muscles in the cat hind limb

Peripheral reorganization of nerve and muscle properties was studied following misdirection of regenerating axons to foreign muscles. The tibial nerve, which innervates all of the distal extensor muscles, was cross-united with the common peroneal nerve, which innervates all of the distal flexor muscles, in one hind limb of seven 2-6 month old cats. After 18-24 months the properties of the motor units in the reinnervated triceps surae muscles were studied by dissection and stimulation of the ventral root filaments. The normal size relationships were re-established in reinnervated medial gastrocnemius (m.g.) and lateral gastrocnemius-soleus (l.g.s.) muscles in so far as motor unit force was directly correlated with the extracellularly recorded amplitude of the axon potential and with contractile speed. The range of sizes of motor units in reinnervated m.g. and l.g.s. muscles was similar to normal, both in terms of the muscle unit size (determined by measuring tetanic tension) and axon size (determined from the amplitude of the extracellularly recorded action potential). The amplitude of the axon potentials in the crossed nerves was weakly correlated with calculated conduction velocities because of the variation in the conduction delays across the suture line. As axon potential amplitude is a direct function of axon size at the recording site, it provided a reliable measure of regenerating axon size. Motor units were classified as slow (s.), fast fatigue resistant (f.r.), fast intermediate (f.i.) or fast fatigable (f.f.) by their contractile speed and fatigue properties. Tetanic tensions generally increased from slow to fast units with s. less than f.r. less than f.i. less than f.f. in reinnervated and normally innervated muscles, but the overlap between different unit types was considerably greater in reinnervated muscles. Thus, peripheral reorganization of nerve and muscle properties according to size can occur in reinnervated muscles, irrespective of the source of the regenerating nerves.