Stability of motor-unit force thresholds in the decerebrate cat

To further test the hypothesis that some fixed property of motoneurons determines their recruitment order, we quantified the variation in force threshold (FT) for motoneurons recruited in muscle stretch reflexes in the decerebrate cat. Motor axons supplying the medial gastrocnemius (MG) muscle were penetrated with micropipettes and physiological properties of the motoneuron and its muscle fibers, i.e., the motor unit, were measured. FT, defined as the amount of MG force produced when the isolated motor unit was recruited, was measured from 20 to 93 consecutive stretch trials for 29 motor units. Trials were selected for limited variation in base force and rate of rise of force, which have been shown to covary with FT, and in peak stretch force, which gives some index of motor-pool excitability. Under these restricted conditions, large variation in FT would have been inconsistent with the hypothesis. Analysis of the variation in FT employed the coefficient of variation (CV), because of the tendency for FT variance and mean to increase together. We found that CV was distributed with a median value of 10% and with only 2 of 29 units exceeding 36%. Some of this variation was associated with measurement error and with intertrial fluctuations in base, peak, and the rate of change of muscle force. CV was not significantly correlated with motor-unit axonal conduction velocity, contraction time, or force. In three cases FT was measured simultaneously from two motor units in the same stretch trials. Changes in recruitment order were rarely observed (5 of 121 stretch trials), even when FT ranges for units in a pair overlapped. We suggest that the large variation in recruitment threshold observed in some earlier studies resulted not from wide variation in the recruitment ranking of motoneurons within one muscle, but rather from variation in the relative activity of different pools of motoneurons. Our findings are consistent with the hypothesis that recruitment order is determined by some fixed property of alpha-motoneurons and/or by some unvarying combination of presynaptic inputs that fluctuate in parallel.     

Self-reinnervated cat medial gastrocnemius muscles. II. analysis of the mechanisms and significance of fiber type grouping in reinnervated muscles

1. The technique of glycogen depletion was used to determine whether regenerating motor axons reestablish the normal regionalization of motor units (MUs) in the cat medial gastrocnemius (MG) muscle, 2) whether the extent of clumping between MU fibers and/or type grouping of muscle fibers progressively increases with a decrease in reinnervated MU numbers, and 3) whether the pattern of innervation can explain why MUs fail to increase significantly in size when the cut nerve is sutured directly to the muscle, even when few axons make functional connections. 2. Distributions of MU fibers were analyzed in 5 normal and 14 reinnervated cat MG muscles 4.5-16 mo after sectioning of its nerve and suturing of the proximal end to the distal nerve sheaths (N-N suture) or directly to the muscle fascia (N-M suture). Muscle unit distributions were quantified according to location, territory size, density, and extent of clumping between fibers from the same MU. 3. Normal MU fibers were regionalized within five regions along the muscle's longitudinal and transverse axes. Reinnervated MUs were located within similar regions, indicating that regenerating axons follow the major proximal nerve branches to restore normal compartmentalization. 4. Muscle unit fibers were diffusely scattered within discrete MU territories in normal muscles. Territory size tended to increase with MU size, whereas density of muscle unit fibers within the territory decreased. 5. Territories increased with MU size after N-N suture but were smaller and showed little size variation after N-M suture. The extent of muscle unit fiber clumping was inversely related to the number of reinnervated MUs. On average, the extent of clumping was substantially higher in muscles reinnervated after N-M suture. These results indicate that distal nerve sheaths facilitate proximal axon branching, which establishes MU territory size. Once the territory is established, motor axons branch distally to increase MU size, which in turn compensates for reduced MU numbers. 6. Muscles reinnervated by < 80% of the MUs exhibited fiber type grouping of type I fibers, and on average the extent of clumping was substantially higher in muscles reinnervated after N-M suture. With less innervation, type grouping increased inversely with the number of reinnervated MUs. However, for a similar number of MUs, type I fiber type grouping was substantially higher in muscle reinnervated after N-M suture. Type grouping therefore reflects muscle unit fiber clumping under conditions where MU size increased (N-N suture) or MU territory size decreased (N-M suture).     

Fusimotor-free spindles in reinnervated muscles of neonatal rats treated with nerve growth factor

Crushing the nerve to the medial gastrocnemius muscle in newborn rats and administering nerve growth factor afterwards results in a reinnervated muscle containing supernumerary muscle spindles. The structure and innervation of 88 spindles in the reinnervated muscles were reconstructed from serial thick and thin transverse sections at 30-35 days after the nerve crush, and compared to those of five control spindles. The spindles consisted of one to four small-diameter encapsulated fibers with features of nuclear chain intrafusal fibers, or infrequently a nuclear bag intrafusal fiber. Some of the spindles were located within a capsule that also contained an extrafusal fiber. Each spindle was innervated by an afferent with features of the primary afferent. The density of secondary afferents was lower in reinnervated muscles than in controls. Endplates were observed on extrafusal fibers in the experimental muscles, attesting to restoration of skeletomotor (alpha) innervation after the nerve crush. However, 78% of the experimental spindles were entirely devoid of efferent innervation. The remainder received either one or two fusimotor (gamma) axons or a skeletofusimotor (beta) axon, compared to the six to eight motor axons that innervated control spindles. The presence of supernumerary spindles composed of fibers that resemble normal intrafusal fibers in the absence of motor innervation suggests that afferents alone can induce the formation and subsequent differentiation of intrafusal fibers in nerve-crushed muscles of neonatal rats. In addition, the paucity of gamma innervation in nerve-crushed muscles suggests that immature gamma neurons are more susceptible than spindle afferents or alpha efferents to cell death after axotomy at birth.     

Isolation and characterisation of a diverse family of Arabidopsis two and three-fingered C2H2 zinc finger protein genes and cDNAs

The motor nerve transplantation (MNT) technique is used to transfer an intact nerve into a denervated muscle by harvesting a neurovascular pedicle of muscle containing motor endplates from the motor endplate zone of a donor muscle and implanting it into a denervated muscle. Thirty-six adult New Zealand White rabbits underwent reinnervation of the left long peroneal (LP) muscle (fast twitch) with a motor nerve graft from the soleus muscle (slow twitch). The right LP muscle served as a control. Reinnervation was assessed using microstimulatory single-fiber electromyography (SFEMG), alterations in muscle fiber typing and grouping, and isometric response curves. Neurofilament antibody was used for axon staining. The neurofilament studies provided direct evidence of nerve growth from the motor nerve graft into the adjacent denervated muscle. Median motor endplate jitter was 13 microsec preoperatively, and 26 microsec at 2 months, 29.5 microsec at 4 months, and 14 microsec at 6 months postoperatively (p < 0.001). Isometric tetanic tension studies showed a progressive functional recovery in the reinnervated muscle over 6 months. There was no histological evidence of aberrant reinnervation from any source outside the nerve pedicle. Isometric twitch responses and adenosine triphosphatase studies confirmed the conversion of the reinnervated LP muscle to a slow-type muscle. Acetylcholinesterase studies confirmed the presence of functioning motor endplates beneath the insertion of the motor nerve graft. It is concluded that the MNT technique achieves motor reinnervation by growth of new nerve fibers across the pedicle graft into the recipient muscle.     

Lateral rectus whole muscle and motor unit contractile measures with the extraocular muscles intact

Both extracellular and intracellular stimulation of single motoneurons were shown to be similarly effective and consistent in eliciting contractile responses in single lateral rectus muscle motor units. The whole muscle was activated by stimulating the sixth nerve in the brain stem. Both whole muscle and motor unit contractile characteristics, under isometric conditions, were found to remain consistent regardless of whether this extraocular muscle was detached or left attached to the globe. In addition, whole muscle twitch and maximum tetanic tension evoked by sixth nerve stimulation was significantly less than would be predicted by the linear summation of individual motor unit twitch and maximum tetanic tensions.     

Glycosaminoglycan supplementation promotes nerve regeneration and muscle reinnervation

This study shows that treatment of rats with exogenous glycosaminoglycans stimulates peripheral nerve regeneration, increases the abundance of mRNAs for myelin proteins and promotes muscle reinnervation. After the sciatic nerve had been crushed the number of regenerating axons in the distal stump was markedly and highly significantly increased by glycosaminoglycan treatment throughout the experimental period. The increased number of axons was correlated with increased axon and fibre (axon+myelin) diameter. The abundance of mRNAs for P0 protein and myelin basic protein of regenerating nerves was also affected by treatment with glycosaminoglycans. The increase in mRNA was also observed in the contralateral unlesioned nerve. Such a phenomenon did not occur in saline-treated rats. Glycosaminoglycan treatment markedly increased the number of muscle fibres reinnervated and accelerated the restoration of muscle twitch tension elicited by nerve stimulation. The effect was particularly evident during the early stages (16 and 21 days after nerve crush) of muscle reinnervation.     

1998 ISEK Congress Keynote Lecture: Motor units: how many, how large, what kind? International Society of Electrophysiology and Kinesiology

There are now at least nine methods for motor unit number estimation (MUNE) in living human muscles. All methods are based on the comparison of an average single motor unit potential (or twitch) with the response of the whole muscle. Such estimations have been performed for proximal and distal muscles of the arm and leg in healthy subjects and in patients with various neuromuscular disorders. In healthy subjects there is a loss of motor units which is most evident in distal muscles and after the age of 60 years. Substantial losses of motor units have been measured in patients with ALS, post-polio symptoms, and diabetic peripheral neuropathies. In contrast, normal MUNEs have been found in approximately half of patients with persisting obstetric brachial palsies. The sizes of motor units show considerable variations within the same muscle and also between muscles; very large units are usually present in severe partial denervation. Although many motor unit properties are largely governed by motoneurons, some exhibit less plasticity in humans than in other mammals.     

Recruitment stability in masseter motor units during isometric voluntary contractions

Recruitment of single motor units (SMUs) of the masseter muscle was studied using macro representation (MacroRep) as the indicator of motor unit size. When subjects followed a slow isometric force ramp, units were usually recruited in order of MacroRep size. However, pooling the data from repeated ramps in the same subject resulted in a weak relationship between MacroRep size and force recruitment threshold, probably due to marked variations in the relative contributions of the jaw muscles, and varying levels of cocontraction, in the development of total bite force in each ramp. The force recruitment thresholds of individual SMUs showed marked variability, but recruitment threshold stability was improved when expressed as a percentage of maximum surface electromyographic (SEMG) activity in the ipsilateral masseter. Therefore the SEMG recruitment threshold was concluded to be a more stable and accurate indicator of the SMU's position in the recruitment hierarchy in a given muscle. It was concluded that SMUs in masseter are recruited according to the size principle, and that when investigating recruitment in jaw muscles, SEMG recruitment threshold should be used in preference to force recruitment threshold.     

The effect of denervated muscle and Schwann cells on axon collateral sprouting

The effects of denervated muscle and Schwann cells on collateral sprouting from peripheral nerve were studied in the peroneal and tibial nerves of 48 Sprague-Dawley rats. Three groups were prepared. In group MSW (muscle-Schwann cell-window), the peroneal nerves were transected 3 mm below the sciatic bifurcation. The proximal stumps were sealed in a blocked tube to prevent regeneration and the distal stumps were implanted into denervated muscle cells that were wrapped around the ipsilateral tibial nerve, which had a window of perineurium resected. Schwann cells from the ipsilateral sural nerve were implanted into the muscle. Group MS (muscle-Schwann cell) was similar to group MSW, except that the tibial nerve perineurium was kept intact. In group MW (muscle-window), the muscle was prepared without Schwann cells and the tibial nerve perineurium was windowed. S-100 immunostain was used to identify the Schwann cells surviving 1 week after transplantation. After 16 weeks of regeneration, horseradish peroxidase tracer was used to label motor neurons and sensory neurons reinnervating the peroneal nerve. Myelinated axons of the reinnervated peroneal nerves were quantified with the Bioquant OS/2 computer system (R&M Biometrics, Nashville, TN). A mean of 169 motor neurons in group MSW, 64 in group MW, and 26 in group MS reinnervated the peroneal nerve. In the dorsal root ganglion, the mean number of labeled sensory neurons was 1,283 in group MSW, 947 in group MS, and 615 in group MW. The mean number of myelinated axons in the reinnervated peroneal nerve was 1,659 in group MSW, 359 in group MS, and 348 in group MW. Reinnervated anterolateral compartment muscles in group MSW were significantly heavier than those in group MS or MW. This study demonstrates that the transplantation of denervated muscle and Schwann cells promotes motor and sensory nerve collateral sprouting through a perineurial window.     

The orderly recruitment of motor units of the masseter and temporal muscles during voluntary isometric contraction in man

1. The contractile properties of the motor units of the masseter and temporal muscles of human subjects were studied during voluntary isometric contractions, using a method previously employed to examine units of a small hand muscle. 2. Over the range of forces studied (0-6 kg), the units of both muscles were recruited in an orderly fashion, with a nearly linear relationship between the voluntary force at which units were recruited and their measured twitch tensions. 3. The range of contraction times (25-90 msec) was similar to that observed for the hand muscle. In some subjects it seemed that small units, recruited at low forces, exhibited shorter contraction times.     

Summation of extraocular motor unit tensions in the lateral rectus muscle of the cat

Contractile measures on 67 single muscle units in the cat lateral rectus muscle were made in response to motoneuron stimulation. Simultaneous activation of four to five additional units, using muscle nerve stimulation, allowed an examination of unit force summation. Linear force addition was found in 73% of the units, while 25% added only about half of their twitch force to the twitch force of the nerve-activated units. "Nonadditive" units had significantly weaker twitch tensions than the units which added linearly. Lengthening or shortening the whole muscle, from maximal isometric settings, reduced whole muscle twitch tension as well as muscle unit tension. Injury to the lateral rectus muscle did not significantly alter whole muscle tension. These findings suggest that the known serial and branching arrangement of these muscle fibers, as well as the complex interfiber matrix, may help explain the force reduction in some muscle units and the whole muscle's resistance to insult.     

123I-MIBG myocardial scintigraphy in diabetic patients: relationship with 201Tl uptake and cardiac autonomic function

The purpose of this study is to investigate the effects of aging on the human stretch reflexes. The EMG and torque responses of the stretch reflex of the wrist flexors were evoked by ramp-and-hold mechanical perturbations. The stretch reflexes were recorded at seven test conditions with different stretch velocity and muscle preload. The test results from young and older healthy adult subjects were compared. In average, the absolute amplitude of the short-latency (20-40 ms) EMG (recorded from flexor carpi radialis) reflex response was significantly lower in the older group. If the data were normalized and expressed in percentage of the maximal voluntary EMG activity, however, this group difference was not significant. There was no change in the reflex gain of the short-latency reflex with aging. For the long-latency (50-90 ms) EMG reflex response, both the normalized amplitude and the reflex gain were significantly enhanced with aging, probably through supraspinal mechanisms. There was no significant difference in the threshold velocity for the evoked EMG reflexive activities between age groups. There were also no changes in the reflexive wrist flexion torque with aging. These results suggested that the number of motor units recruited during the stretch reflex activity declined with aging although the percentage of motor units recruited was not affected by aging. It is concluded that the central regulating mechanisms of the spinal motoneuron excitability are not compromised by aging. The automatic gain compensation phenomenon is also preserved with aging.     

The response of Golgi tendon organs to single motor unit contractions

1. Cross-correlation analysis has been used to quantify the responses of cat soleus tendon organs to repetitive twitch contractions of: (a) different motor units within the muscle, (b) single motor units at different muscle lengths, and (c) single motor units when the pulse-train pattern of stimulation delivered to the motor unit axon was altered. 2. Ib afferents were observed which responded to each of several hundred successive motor unit twitches with identical numbers of spikes and with relatively invariant latencies. 3. The present results show that tendon organs are sensitive to subtle alterations in motor unit twitch wave form and amplitude, and that this sensitivity is reflected in the precise timings of their afferent discharge. 4. Examination of these tendon organ responses indicates that the forces produced by single motor units couples to the receptor capsule are well above threshold. Calculations based on these results, and earlier soleus motor unit and muscle fibre data, suggest that the absolute force threshold for tendon organs may be as little as 4 mg, which is less than the estimated minimum twitch force generated by individual soleus muscle fibres. 5. Considering the number of tendon organs in a muscle, and the likelihood that every motor unit is connected with at least one receptor, the sensitivity of tendon organs ensures that every twitch of every motor unit will be reflected in the population of afferent signals projecting to the spinal cord.     

Does spasticity contribute to walking dysfunction after stroke?

OBJECTIVES: Clinically, it is assumed that spasticity of the calf muscles interferes with walking after stroke. The aim was to examine this assumption by evaluating the contribution of spasticity in the gastrocnemius muscle to walking dysfunction in an ambulant stroke population several months after stroke. METHODS: Fourteen stroke patients who were able to walk independently and 15 neurologically normal control subjects were recruited. Both resting and action stretch reflexes of the gastrocnemius muscle were investigated under conditions that simulated walking. Resting tonic stretch reflexes were measured to assess spasticity whereas action tonic stretch reflexes were measured to assess the possible contribution of spasticity to gait dysfunction. RESULTS: Two thirds of the stroke patients exhibited resting tonic stretch reflexes which indicate spasticity, whereas none of the control subjects did. However, the stroke patients exhibited action tonic stretch reflexes that were of similar magnitude to the control subjects, suggesting that their reflex activity during walking was not different from that of control subjects. Furthermore, there was no evidence that the action stretch reflex in the stroke patients contributed a higher resistance to stretch than the control subjects. CONCLUSIONS: Whereas most of the stroke patients exhibited spasticity when measured both clinically and physiologically, they did not exhibit an increase in resistance to dorsiflexion due to exaggerated action tonic stretch reflexes. It is concluded that it is unlikely that spasticity causes problems in walking after stroke in ambulant patients. Therefore, it seems inappropriate to routinely reduce or inhibit the reflex response to improve functional movement in stroke rehabilitation. Factors other than spasticity should be considered when analysing walking after stroke, so that appropriate treatment is provided to patients.     

Properties of motor units of the frog sartorius muscle

1. The mechanical properties of single motor units in the sartorius muscle of the frog Litoria aurea were examined during single shock and repetitive stimulation of motor axons. 2. The tetanic tension developed by motor units lay in the range 1-40% of whole muscle tension with two peaks in the distribution, in the range 5-10% and 25-30%. The large units had briefer times-to-peak for the twitch than the small units and were more readily fatigued during prolonged repetitive stimulation. 3. Histological examination of the muscle gave a count of 620 muscle fibres with a diameter range of 28-128 mum. Cholinesterase stained preparations showed that the majority of muscle fibres had several nerve terminals (mean 3, range 1-5). 4. Muscle fibres received their multiple innervation from different axons (polyneuronal) or branches of the same axon (multiterminal). The presence of polyneuronal innervation of muscle fibres was confirmed by a comparison of the tensions when each of a pair of motor units was stimulated alone and when they were stimulated together. The tension excess, or overlap, was up to 60% when expressed in terms of the tension developed by either unit alone. Motor units developing similar amounts of tension tended to show more overlap in their innervation than units with very different tensions. 5. An estimate of the amount of multiterminal innervation gave variable results but could account for up to 60% of a motor unit's tension. No correlation could be detected between the values for multiterminal innervation and any other measured parameter. However, it is argued that because of the limitations of the measurements the existence of a relationship between the extent of multiterminal or polyneuronal innervation and the mechanical properties of the motor unit cannot be excluded.     

Control of torque direction by spinal pathways at the cat ankle joint

To study the biomechanics of the calcaneal tendon's complex insertion onto the calcaneus, we measured torque-time trajectories exerted by the triceps surae and tibialis anterior muscles in eight unanesthetized decerebrate cats using a multi-axis force-moment sensor placed at the ankle joint. The ankle was constrained to an angle of 110 degrees plantarflexion. Muscles were activated using crossed-extension (XER), flexion (FWR), and caudal cutaneous sural nerve (SNR) reflexes. Torque contributions of other muscles activated by these reflexes were eliminated by denervation or tenotomy. In two animals, miniature pressure transducers were implanted among tendon fibers from the lateral gastrocnemius (LG) muscle that insert straight into the calcaneus or among tendon fibers from the medial gastrocnemius (MG) that cross over and insert on the lateral aspect of calcaneus. Reflexively evoked torques had the following directions: FWR, dorsiflexion and adduction; SNR, plantarflexion and abduction; and XER, plantarflexion and modest abduction or adduction. The proportion of abduction torque to plantarflexion torque was always greater for SNR than XER; this difference was about 50% of the magnitude of abduction torque generated by tetanic stimulation of the peronei. During SNR, pressures were higher in regions of the calcaneal tendon originating from MG than regions originating from LG. Similarly, pressures within the MG portion of the calcaneal tendon were higher during SNR than during XER, although these two reflexes produced matched ankle plantarflexion forces. Selective tenotomies and electromyographic recordings further demonstrated that MG generated most of the torque in response to SNR, while soleus, LG, and MG all generated torques in response to XER.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. II. Motoneuron properties

Chronic stimulation (for 2-3 mo) of the medial gastrocnemius (MG) muscle nerve by indwelling electrodes renders the normally heterogeneous MG muscle mechanically and histochemically slow (type SO). We tested the hypothesis that motoneurons of MG muscle thus made type SO by chronic stimulation would also convert to slow phenotype. Properties of all single muscle units became homogeneously type SO (slowly contracting, nonfatiguing, nonsagging contraction during tetanic activation). Motoneuron electrical properties were also modified in the direction of type S, fatigue-resistant motor units. Two separate populations were identified (on the basis of afterhyperpolarization, rheobase, and input resistance) that likely correspond to motoneurons that had been fast (type F) or type S before stimulation. Type F motoneurons, although modified by chronic stimulation, were not converted to the type S phenotype, despite apparent complete conversion of their muscle units to the slow oxidative type (type SO). Muscle units of the former type F motor units were faster and/or more powerful than those of the former type S motor units, indicating some intrinsic regulation of motor unit properties. Experiments in which chronic stimulation was applied to the MG nerve cross-regenerated into skin yielded changes in motoneuron properties similar to those above, suggesting that muscle was not essential for the effects observed. Modulation of group Ia excitatory postsynaptic potential (EPSP) amplitude during high-frequency trains, which in normal MG motoneurons can be either positive or negative, was negative in 48 of 49 chronically stimulated motoneurons. Negative modulation is characteristic of EPSPs in motoneurons of most fatigue-resistant motor units. The general hypothesis of a periphery-to-motoneuron retrograde mechanism was supported, although the degree of control exerted by the periphery may vary: natural type SO muscle appears especially competent to modify motoneuron properties. We speculate that activity-dependent regulation of the neurotrophin-(NT) 4/5 in muscle plays an important role in controlling muscle and motoneuron properties.     

Recovery of muscle after different periods of denervation and treatments

Three aspects of reinnervation and recovery of skeletal muscle following various periods of denervation were investigated: (1) the effect of duration of denervation; (2) the effect of hyperthyroidism on recovery; and (3) whether the muscle or the nerve limits recovery. The rat medial gastrocnemius (MG) nerve was cut and then resutured after 0, 3, 7, 21, or 56 days. In a second group of animals, the MG muscle was denervated and, in addition, the animal received triiodothyronine (T3) supplementation during reinnervation. The third group of animals had the denervated MG muscle reinnervated by a larger number of newly transected foreign axons. The force produced by the reinnervated muscle depends on the period that the muscle was denervated. Recovery was impaired when the period of denervation exceeded 7 days. T3 treatment did not benefit the return of force production, nor did providing the muscle with a larger number of newly transected axons.     

Adaptations of rat lateral gastrocnemius motor units in response to voluntary running

This study investigated the effects of 12 wk of voluntary wheel running on motor units from rat lateral gastrocnemius. Motor units were isolated via ventral root splitting (L5) from active or sedentary rats and were classified into slow, fast-fatigue-resistant, and fast-fatigable (FF) units. An overall increase in mean motor unit tetanic tension (35%) was accompanied by a decrease in mean motor unit fatigue resistance (-10%). These adaptations were localized in the fast unit population but varied among fast motor unit subtypes. The overall increase in tetanic force was due primarily to increases in fast-fatigue-resistant units (300%), whereas changes in fatigue resistance (-43%) were confined to FF units. However, the changes seen with activity may have been partly obscured by classifying fast motor units based on fatigability, since a significant increase in tetanic force accompanied by a decreased twitch one-half relaxation time was apparent in units falling in the midrange of the tetanic force continuum and included a number of FF units. These data provide direct demonstration of nonuniform motor unit adaptations subsequent to increases in normal functional activity.