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

Medial gastrocnemius muscles fatigue but do not atrophy in paralyzed cat hindlimb after long-term spinal cord hemisection and unilateral deafferentation

This study of medial gastrocnemius (MG) muscle and motor units (MUs) after spinal cord hemisection and deafferentation (HSDA) in adult cats, asked 1) whether the absence of muscle atrophy and unaltered contractile speed demonstrated previously in HSDA-paralyzed peroneus longus (PerL) muscles, was apparent in the unloaded HSDA-paralyzed MG muscle, and 2) how ankle unloading impacts MG muscle and MUs after dorsal root sparing (HSDA-SP) with foot placement during standing and locomotion. Chronic isometric contractile forces and speeds were maintained for up to 12 months in all conditions, but fatigability increased exponentially. MU recordings at 8-11½ months corroborated the unchanged muscle force and speed with significantly increased fatigability; normal weights of MG muscle confirmed the lack of disuse atrophy. Fast MUs transitioned from fatigue resistant and intermediate to fatigable accompanied by corresponding fiber type conversion to fast oxidative (FOG) and fast glycolytic (FG) accompanied by increased GAPDH enzyme activity in absolute terms and relative to oxidative citrate synthase enzyme activity. Myosin heavy chain composition, however, was unaffected. MG muscle behaved like the PerL muscle after HSDA with maintained muscle and MU contractile force and speed but with a dramatic increase in fatigability, irrespective of whether all the dorsal roots were transected. We conclude that reduced neuromuscular activity accounts for increased fatigability but is not, in of itself, sufficient to promote atrophy and slow to fast conversion. Position and relative movements of hindlimb muscles are likely contributors to sustained MG muscle and MU contractile force and speed after HSDA and HSDA-SP surgeries.

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.

Electrical stimulation of transplanted motoneurons improves motor unit formation

Motoneurons die following spinal cord trauma and with neurological disease. Intact axons reinnervate nearby muscle fibers to compensate for the death of motoneurons, but when an entire motoneuron pool dies, there is complete denervation. To reduce denervation atrophy, we have reinnervated muscles in Fisher rats from local transplants of embryonic motoneurons in peripheral nerve. Since growth of axons from embryonic neurons is activity dependent, our aim was to test whether brief electrical stimulation of the neurons immediately after transplantation altered motor unit numbers and muscle properties 10 wk later. All surgical procedures and recordings were done in anesthetized animals. The muscle consequences of motoneuron death were mimicked by unilateral sciatic nerve section. One week later, 200,000 embryonic day 14 and 15 ventral spinal cord cells, purified for motoneurons, were injected into the tibial nerve 10-15 mm from the gastrocnemii muscles as the only neuron source for muscle reinnervation. The cells were stimulated immediately after transplantation for up to 1 h using protocols designed to examine differential effects due to pulse number, stimulation frequency, pattern, and duration. Electrical stimulation that included short rests and lasted for 1 h resulted in higher motor unit counts. Muscles with higher motor unit counts had more reinnervated fibers and were stronger. Denervated muscles had to be stimulated directly to evoke contractions. These results show that brief electrical stimulation of embryonic neurons, in vivo, has long-term effects on motor unit formation and muscle force. This muscle reinnervation provides the opportunity to use patterned electrical stimulation to produce functional movements.

Neurobiology of peripheral nerve injury, regeneration, and functional recovery: from bench top research to bedside application

OBJECTIVES

We review the state-of-the-art neurobiology of nerve injury and regeneration, especially as it relates to return of useful function in patients who have sustained injuries to large nerve trunks such as the brachial plexus.

METHODS

This review focuses on research conducted in our laboratory at Ochsner and at other laboratories related to the neurobiology of nerve injury with emphasis on how some of the key findings from animal research help us understand the pathophysiology of poor functional recovery after nerve injury.

CONCLUSIONS

Published research on the neurobiology of nerve injury and regeneration strongly suggests that chronic Schwann cell denervation, chronic neuronal axotomy, and misdirection of regenerating axons into wrong endoneurial tubes are primarily responsible for poor functional recovery. The effect of muscle denervation atrophy is secondary. Experimental therapeutic strategies (which we are currently investigating in our laboratory at Ochsner) to combat these 3 neurobiologic phenomena have the potential to improve the return of function in patients who have sustained nerve injuries.

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.

Properties of medial gastrocnemius motor units and muscle fibers reinnervated by embryonic ventral spinal cord cells

Severe muscle atrophy occurs after complete denervation. Here, Embryonic Day 14-15 ventral spinal cord cells were transplanted into the distal tibial nerve stump of adult female Fischer rats to provide a source of neurons for muscle reinnervation. Our aim was to characterize the properties of the reinnervated motor units and muscle fibers. Some reinnervated motor units contracted spontaneously. Electrical stimulation of the transplants at increasing intensity produced an average (+/- SE) of 7 +/- 1 electromyographic and force steps. Each signal increment represented the excitation of another motor unit. These reinnervated units exerted an average force of 12.0 +/- 1.5 mN, strength similar to that of control fatigue-resistant units. Repeated transplant stimulation depleted 17% of the muscle fibers of glycogen, an indication of some functional reinnervation. Reinnervated (glycogen-depleted), denervated (no cells transplanted), and control fibers were of histochemical type I, IIA, or IIB. Fibers of the same type were grouped after reinnervation. The proportion of fiber types also changed. Reinnervated fibers were primarily type IIA, whereas most fibers in denervated and control muscles were type IIB. Reinnervated fibers of each type had significantly larger cross-sectional areas than the corresponding fiber types in denervated muscles. These data suggest that neurons with different properties can reside in the unusual environment of the adult rat peripheral nerve, make functional connections with muscle, specify muscle fiber type, and reduce the amount that each type atrophies.

Force-frequency and fatigue properties of motor units in muscles that control digits of the human hand

Modulation of motor unit activation rate is a fundamental process by which the mammalian nervous system encodes muscle force. To identify how rate coding of force may change as a consequence of fatigue, intraneural microstimulation of motor axons was used to elicit twitch and force-frequency responses before and after 2 min of intermittent stimulation (40-Hz train for 330 ms, 1 train/s) in single motor units of human long finger flexor muscles and intrinsic hand muscles. Before fatigue, two groups of units could be distinguished based on the stimulus frequency needed to elicit half-maximal force; group 1 (n = 8) required 9.1 +/- 0.5 Hz (means +/- SD), and group 2 (n = 5) required 15.5 +/- 1.1 Hz. Twitch contraction times were significantly different between these two groups (group 1 = 66. 5 ms; group 2 = 45.9 ms). Overall 18% of the units were fatigue resistant [fatigue index (FI) > 0.75], 64% had intermediate fatigue sensitivity (0.25 </= FI </= 0.75), and 18% were fatigable (FI < 0. 25). However, fatigability and tetanic force were not significantly different among groups. Therefore unlike findings in some other mammals, fast-contracting motor units were neither stronger nor more susceptible to fatigue than slowly contracting units. Fatigue, however, was found to be greatest in those units that initially exerted the largest forces. Despite significant slowing of contractile responses, fatigue caused the force-frequency relation to become displaced toward higher frequencies (44 +/- 41% increase in frequency for half-maximal force). Moreover, the greatest shift in the force-frequency relation occurred among those units exhibiting the largest force loss. A selective deficit in force at low frequencies of stimulation persisted for several minutes after the fatigue task. Overall, these findings suggest that with fatigue higher activation rates must be delivered to motor units to maintain the same relative level of force. Questions regarding classification of motor units and possible mechanisms by which fatigue-related slowing might coexist with a shift in the force-frequency curve toward higher frequencies are discussed.

Contractile Properties of Human Motor Units: Is Man a Gat?

A major goal in neuroscience is to understand how the CNS controls posture and movement in humans. This requires an understanding of individual human motor unit properties and how they interact within the muscle to perform different tasks. This article describes differences and similarities between the contractile properties of human motor units and those of the cat prototype medial gastrocnemius (MG) muscle, on which so many studies have been conducted. The article describes the methods available for measuring human motor unit properties and their limitations, and it discusses how far the behavior of whole muscles can be predicted from their histochemistry. It questions the extent to which human motor units conform to the conventional criteria by which S (slow, fatigue resistant), FR (fast but fatigue resistant) and FF (fast, fatigable) unit types are usually classified. An important difference between human and cat MG data is that weak human motor units are not necessarily slow, nor strong ones fast; that is, generally, human unit force is not correlated with contractile speed. Also, unlike cat MG, the few human muscles studied so far contain few if any FF units but a high proportion of units with intermediate fatigue resistance (Flnt). These apparently aberrant human properties, however, are also found in other cat and rat muscles. Thus, cat MG may not be the best model for motor unit behavior generally. Finally, the influence of human motor unit properties on force output by recruitment and/or rate coding is discussed.

Incomplete rematching of nerve and muscle properties in motor units after extensive nerve injuries in cat hindlimb muscle

1. Motor units were characterized in partially denervated or completely denervated and reinnervated cat medial gastrocnemius (MG) muscles where the number of innervating motor axons was severely reduced to determine (1) to what extent the nerve and muscle properties are rematched in enlarged motor units, (2) whether the normal size relationships between axon size, unit tetanic force and contractile speed are re-established, and (3) whether the type of nerve injury and/or repair affects the re-establishment of nerve and muscle properties. 2. Single MG units were sampled in (1) partially denervated muscles and in reinnervated muscles after either (2) crushing or (3) transecting the nerve and suturing its proximal end to either the distal nerve stump (N-N), or (4) directly to the muscle fascia (N-M). 3. The majority (75-88 %) of motor units in all muscles were classified as S (slow), FR (fast fatigue resistant), FI (fast fatigue intermediate) and FF (fast fatigable). However, there was an increased number of FI and unclassifiable motor units compared to normal. These results suggest that motor unit properties are not entirely regulated by the reinnervating motoneurone. 4. Despite more overlap in the range of unit force between different motor unit types the tetanic force of each type increased in all muscles when reinnervated by few (< 50 %) motor axons. This increase in unit force was due to an expansion in motor unit innervation ratio. 5. The normal relationships between axon size, unit tetanic force, and contractile speed were re-established in all muscles except when reinnervated by < 50 % of their normal complement of motor units after N-M suture. This lack of correlation was due to the reduced fast glycolytic (FG) fibre size and the proportionately greater increase in force of the S units. 6. After reinnervation the ranges in fibre cross-sectional area within single FF units were very similar to those found within the entire FG fibre population. 7. These results show that when few axons make functional connections in partially denervated or reinnervated muscles the normal relationships between axon size and motor unit contractile properties are re-established provided the nerves regenerate within the distal nerve sheath. This rematching of motoneurone size and motor unit contractile properties occurs primarily because the size of the motor axon governs the number of muscle fibres it supplies.

Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. I. Muscle and motor unit properties

This study of cat medial gastrocnemius (MG) muscle and motor unit (MU) properties tests the hypothesis that the normal ranges of MU contractile force, endurance, and speed are directly associated with the amount of neuromuscular activity normally experienced by each MU. We synchronously activated all MUs in the MG muscle with the same activity (20 Hz in a 50% duty cycle) and asked whether conversion of whole muscle contractile properties is associated with loss of the normal heterogeneity in MU properties. Chronically implanted cuff electrodes on the nerve to MG muscle were used for 24-h/day stimulation and for monitoring progressive changes in contractile force, endurance, and speed by periodic recording of maximal isometric twitch and tetanic contractions under halothane anesthesia. Chronic low-frequency stimulation slowed muscle contractions and made them weaker, and increased muscle endurance. The most rapid and least variable response to stimulation was a decline in force output of the muscle and constituent MUs. Fatigue resistance increased more slowly, whereas the increase in time to peak force varied most widely between animals and occurred with a longer time course than either force or endurance. Changes in contractile force, endurance, and speed of the whole MG muscle accurately reflected changes in the properties of the constituent MUs both in extent and time course. Normally there is a 100-fold range in tetanic force and a 10-fold range in fatigue indexes and twitch time to peak force. After chronic stimulation, the range in these properties was significantly reduced and, even in MU samples from single animals, the range was shown to correspond with the slow (type S) MUs of the normal MG. In no case was the range reduced to less than the type S range. The same results were obtained when the same chronic stimulation pattern of 20 Hz/50% duty cycle was imposed on paralyzed muscles after hemisection and unilateral deafferentation. The findings that the properties of MUs still varied within the normal range of type S MUs and were still heterogeneous despite a decline in the variance in any one property indicate that the neuromuscular activity can account only in part for the wide range of muscle properties. It is concluded that the normal range of properties within MU types reflects an intrinsic regulation of properties in the multinucleated muscle fibers.

Innervation ratio and motor unit force in large muscles: a study of chronically stimulated cat medial gastrocnemius

1. The present study uses chronic low frequency stimulation of cat medial gastrocnemius (MG) muscle to investigate the relative contribution of innervation ratio to the wide range of motor unit force in large mammalian muscles by reducing the normal variation in muscle fibre cross-sectional area and specific force. 2. Isometric force recordings from isolated and physiologically characterized motor units were made 42-240 days after stimulation. Innervation ratio, fibre area and fibre type (I, II A, II B) were determined in one glycogen-depleted motor unit per muscle. 3. After 42 days of stimulation, all motor units were non-fatigable and were classified as either slow (S) or fast-fatigue resistant (FR). Despite the absence of fast-fatigable (FF) motor units, all three muscle fibre types were present, as identified according to their myofibrillar ATPase reactivity. After 143 days, all motor units and muscle fibres were classified as type S and type I, respectively. 4. A rapid decline in muscle and motor unit force to 30% of normal values after 42 days of chronic stimulation was accounted for by a reduction in muscle fibre area. Fibre areas did not change further with longer periods of stimulation but type II fibres were converted to type I. All stimulated muscle fibres were the size of normal type I fibres; the size of the fibres within single motor units covered the full range of the muscle fibre population. 5. In long-term stimulated muscles (> 100 days) when all muscle fibres were type I and all motor units type S, only differences in innervation ratio could account for the remaining range in motor unit force. Estimates of this range from the minimum and maximum values recorded and from values of tetanic force between the 5th and 95th percentiles indicate that the range in innervation ratio in the MG muscles is at least 15-fold and may be as large as 38-fold. Enumerations of glycogen-depleted muscle fibres from single motor units were consistent with this explanation. 6. The findings provide evidence that there is a wide range of innervation ratios in large muscles, which can account for the large range in motor unit forces in the muscles. Since motor unit force and innervation ratio vary with motoneurone size, these studies provide further support that the size of the peripheral field of innervation of motoneurones is related to their size.

Isometric contractile function following nerve grafting: a study of graft storage

In order to assess the effects of storage on nerve grafts, the isometric contractile function of the gastrocnemius muscle was assessed 14 months following sciatic nerve autografting in the rat. Three-centimeter sciatic nerve grafts were stored at either 5 degrees C or 22 degrees C for 6 h, 24 h, or 3 weeks in an organ transplant solution. Muscle mass and maximal force in the fresh control graft group returned to 47% and 36% of normal levels, respectively, which was similar to stored grafts. Storage at 5 degrees C was superior to 22 degrees C and there was no decrement in contractile function in grafts stored up to 3 weeks at 5 degrees C. These findings suggest that the storage of nerve grafts is a feasible technique that might be applied to nerve allografts, thus permitting elective reconstruction of large peripheral nerve gaps.