Species specific morphology of mammalian motor nerve terminals

Summary Motor nerve terminals in the diaphragm, extensor digitorum longus and soleus muscles of young and adult rat, hamster and guinea pig were studied with the light microscope after staining with methylene blue. In adult animals the nerve terminals are smaller in the diaphragm than in the two other muscles. In the rat and hamster the extensor digitorum longus and soleus terminals are of similar area, but the terminals in extensor digitorum longus are shorter. In the guinea pig the terminals are smaller and shorter in soleus than in extensor digitorum longus. The density of nerve terminal varicosities is lowest in the diaphragm in all three species. In the rat and hamster the density is higher in extensor digitorum longus than in soleus. In the guinea pig the converse is found. In all three muscles the density of varicosities is higher in the rat than in the hamster and guinea pig. The nerve terminal branches in the diaphragm are mostly organized in one group. In the rat and hamster the soleus terminal branches are more separated in groups than the extensor digitorum longus terminal branches. In the guinea pig the number of groups is almost the same in the two muscles. These muscle and species-specific differences appear already in very young animals.     

Postnatal development of rat motor nerve terminals

Summary The nerve terminals of neuromuscular junctions in the rat diaphragm, extensor digitorum longus muscle and soleus muscle have been studied in animals between 3 weeks and 2.5 years of age using methylene blue stain and light microscopy. Dimensions, structure and organization of the nerve terminals were shown to change during life at various rates in different muscles and postnatal periods. The area and length of the terminals increase in all three muscles until young adult age. Later these dimensions continue to increase in the extensor digitorum longus and soleus muscles. In the diaphragm only the length increases, and this occurs late in adult life. The area also increases in relation to the diameter of the corresponding muscle fiber. Adult soleus terminals are more elongated than terminals in the diaphragm and extensor digitorum longus muscle. During adult life the extension of nerve terminals in relation to muscle fiber length increases in the extensor digitorum longus and soleus muscles, but is almost unchanged in the diaphragm. The nerve terminal branches are mainly coarse and irregular in young animals, but possess varying numbers of varicosities in adult animals. The number of varicosities is high in the extensor digitorum longus muscle and low in the diaphragm. In old animals the number of varicosities tends to be reduced. With increasing age the nerve terminal branches become organized in distinct groups with increasing distance between the groups. This is prominent in the soleus.     

Demonstration of glutamate-like immunoreactivity at rat neuromuscular junctions by quantitative electron microscopic immunocytochemistry

Motor nerve terminals and adjacent structures in the extensor digitorum longus and soleus muscles of young adult rats were examined for their content of glutamate by means of quantitative, electron microscopic immunocytochemistry employing colloidal gold particles as markers. The level of glutamate immunoreactivity was stronger in the extensor digitorum longus terminals than in the soleus terminals. In both muscles the glutamate immunolabelling was stronger in the nerve terminals than in the synaptic clefts and the postsynaptic tissue separating the secondary clefts, but the differences were larger in the extensor digitorum longus than in the soleus muscle. The myofibrils of the soleus muscle were more densely labelled than those in the extensor digitorum longus muscle: The level of immunoreactivity was high in the Schwann cells of both muscles. By comparing the labelling intensity of motor nerve terminals with that of muscle fibres and hippocampal mossy fibres (compartments that have been analysed previously with respect to their glutamate content), the mean concentration of fixed glutamate in the extensor digitorum terminals was estimated to be in the range of 10–20 mmol/l. An association of glutamate immunoreactivity with synaptic vesicles was demonstrated in the most strongly labelled terminals. Whether these epitopes were localized in the interior of the vesicles or at their external surface could not be resolved with the present technique. These data indicate that motor nerve terminals contain glutamate, and that the enrichment of this amino acid is more pronounced in the terminals of the extensor digitorum longus muscle (a fast muscle) than in those of the soleus muscle (a slow muscle). A possible modulatory or trophic role of glutamate in the mammalian neuromuscular junction should be considered.     

Effect of thyroid hormones on acetylcholinesterase mRNA levels in the slow soleus and fast extensor digitorum longus muscles of the rat

In the rat, the level of acetylcholinesterase messenger RNA in the typical slow soleus muscles is only about 20-30% of that in the fast extensor digitorum longus muscles. The expression of contractile proteins in muscles is influenced by thyroid hormones and hyperthyroidism makes the slow soleus muscle faster. The influence of thyroid hormones on the levels of acetylcholinesterase messenger RNA level in the slow soleus and fast extensor digitorum longus muscle of the rat was studied in order to examine the effect of thyroid hormones on muscle acetylcholinesterase expression. Hyperthyroidism was induced in rats by daily thyroid hormone injection or thyroid hormone releasing tablet implantation. Hind-limb suspension was applied to produce muscle unloading. Muscle denervation or reinnervation was achieved by sciatic nerve transection or crush. Acetylcholinesterase messenger RNA levels were analyzed by Northern blots and evaluated densitometrically. Hyperthyroidism increased the levels of acetylcholinesterase messenger RNA in the slow soleus muscles close to the levels in the fast extensor digitorum longus. The effect was the same in the unloaded soleus muscles. Acetylcholinesterase expression increased also in the absence of innervation (denervation), in the presence of changed nerve activation pattern (reinnervation), and under enhanced tonic neural activation of the soleus muscle (electrical stimulation). However, the changes were substantially smaller than those observed in the control soleus muscles. Enhancement of acetylcholinesterase expression in the soleus muscles by the thyroid hormones is, therefore, at last in part due to hormonal effect on the muscle itself. On the contrary, increased level of the thyroid hormones had no influence on acetylcholinesterase expression in the normal fast extensor digitorum longus muscles. However, some enhancing influence was apparent whenever the total number of nerve-induced muscle activations per day in the extensor digitorum longus muscle was increased. Thyroid hormones seem to be an independent extrinsic factor of acetylcholinesterase regulation in the slow soleus muscle.     

Effect of diffusion distance on measurement of rat skeletal muscle glucose transport in vitro.

The relationships between muscle size, diffusion distance, and glucose uptake were studied using the Type IIb epitrochlearis (13 +/- 1 mg intact), Type I soleus (25 +/- 1 mg), and mixed Type IIa/IIb extensor digitorum longus (25 +/- 1 mg) from 60-70 g rats. Using intact muscles, the relative rates of 3-O-methyl-glucose uptake in response to 2 mUml-1 insulin were soleus = epitrochlearis greater than extensor digitorum longus, a finding inconsistent with the fibre-type compositions and the relative GLUT-4 protein levels (soleus greater than extensor digitorum longus greater than epitrochlearis). To test whether these results were influenced by substrate diffusion limitations in the tubular muscles, soleus and extensor digitorum longus were split longitudinally from tendon to tendon into strips of comparable size (13 +/- 1 mg) to the epitrochlearis. Insulin-stimulated rates of 3-O-methyl-glucose uptake were significantly enhanced in the split soleus (+120%) and split extensor digitorum longus (+200%), but not in the epitrochlearis, with the relative rates being soleus greater than extensor digitorum longus greater than epitrochlearis. Diffusion distances of the split soleus and extensor digitorum longus, as reflected by [14C]mannitol space equilibration time, were markedly enhanced (by at least 50%) relative to the intact muscles, and were comparable to that of the epitrochlearis. These results indicate that when muscles of different size and/or shape are used for in vitro measurement of glucose transport, the muscle preparations used must have similar diffusion distances for physiologically meaningful comparisons to be made.     

Different effects of physical training on the morphology of motor nerve terminals in the rat extensor digitorum longus and soleus muscles

Summary The morphology of nerve terminals in the rat extensor digitorum longus and soleus muscles was studied with light microscopy in 13-week-old male animals after 6 weeks of treadmill running and compared with data from untrained controls. The terminals were stained with methylene blue. Physical training tended to increase the area and length of the nerve terminals in relation to the corresponding muscle fiber diameter, and to reduce the density of nerve terminal varicosities, but significant differences between the trained group and the control group were obtained only in the extensor digitorum longus muscle. The different degrees of effect on the nerve terminals in the two muscles may be due to different abilities to respond to the training, but may also be due to differences in work load caused by the training. The effect of training on extensor digitorum longus junctions may reflect some transformation from fast to slow morphological characteristics.     

Relative capabilities of sarcoplasmic reticulum in fast and slow mammalian skeletal muscles.

1. The calcium uptake capabilities of the sarcoplasmic reticulum (SR) of the fat-twitch muscles extensor digitorum longus (EDL) and tibialis anterior (TA) of the rat and the extensor digitorum longus of the cat have been compared with the same capabilities of the slow-twitch soleus muscles of the rat and cat. 2. For the ra the Vmax values of sarcoplasmic reticulum from tibialis anterior, extensor digitorum longus and from soleus muscles were 50, 51, and 10 micronmole Ca2+/g per minute, respectively. 3. For the extensor digitorum longus and soleus muscles of the cat the Vmax values were 34 and 5-6 micronmole Ca2+/g per minute, respectively. 4. These data were compared with mechanical data as reported in the literature for the same muscles. The relative calcium uptake capabilities of sarcoplasmic reticulum from slow and fast muscles corresponded closely to the relative rates of relaxation of these muscles.     

Adaptive range of myosin heavy chain expression in regenerating soleus is broader than in mature muscle

Summary In adult rat muscles experimentally exposed to various patterns of activation, expression of myosin heavy chain isoforms changes, but only within a certain adaptive range. It is characteristic and different in fast or slow muscles. This may be due either to different intrinsic properties of the myogenic cells of the two types of muscles or to extrinsic factors. To test these assumptions, either rat soleus or extensor digitorum longus muscles were injured and transplanted to the bed of the extensor digitorum longus muscle. They regenerated and were reinnervated by the extensor digitorum longus nerve. Expression of myosin heavy chain isoforms was demonstrated immunohistochemically and by in situ hybridization, and analysed by SDS-gel electrophoresis. Three months after cross-transplantation, regenerated soleus expressed all adult myosin heavy chain isoforms, including the myosin heavy chain-2B. The latter was detected in about 50% of muscle fibres and contributed about 10–20% of all myosin heavy chains. The same percentage of myosin heavy chain-2B was found in regenerated extensor digitorum longus. In this regard therefore, the adaptive range of the regenerated soleus muscle was not significantly different from that of the extensor digitorum longus regenerating under the same conditions. This indicates that restriction of the adaptive range in a mature soleus muscle is not due to intrinsic properties of its myogenic cells. It is probably imposed by an extrinsic factor leading to irreversible shut-down of individual myosin heavy chain genes. On the other hand, myosin heavy chain-1 expression was significantly greater in the regenerated soleus than in the extensor digitorum longus innervated by the same nerve. Myosin heavy chain-1 and myosin heavy chain-2B were co-expressed in some regenerated soleus muscle fibres.     

Topographic comparison of neuromuscular junctions in mouse slow and fast twitch muscles

The neuromuscular junctions of mammalian slow and fast twitch muscles are activated differently in vivo and show corresponding physiological differences in vitro, but the structural basis or consequences of these differences are relatively unexplored. Therefore, neuromuscular junctions of mouse fast (extensor digitorum longus) and slow (soleus) twitch muscles were compared by use of new scanning and light microscopy techniques. In both muscles, the endplate appeared as an elliptical area raised to a variable extent above the surrounding sarcolemma and containing the primary clefts. In most soleus endplates, this raised surface area was considerably higher and wider and about three times larger than in extensor digitorum longus. In addition, the primary cleft area was about two-fold greater in soleus than in extensor digitorum longus, even though cleft length was the same. The primary clefts formed either an elliptical shape along the outer margin of the endplate with inward-directed branches or a group of relatively rectilinear dendritic branches orthogonally oriented to one another. The latter type was most frequent in soleus and the elliptical type in extensor digitorum longus. Corresponding patterns of nerve terminal arborizations were seen by light microscopy. Although nerve terminal areas were the same in fast and slow muscles, in the former, numerous diverticulae significantly increased the length of the nerve terminal outline. The possible physiological significance of the different synaptic structure of slow and fast muscle is discussed.     

The growth-associated protein GAP-43 appears in dorsal root ganglion cells and in the dorsal horn of the rat spinal cord following peripheral nerve injury

When adult dorsal root ganglion cells are dissociated and maintained in vitro, both the small dark and the large light neurons show increases in the growth-associated protein GAP-43, a membrane phosphoprotein associated with neuronal development and plasticity. Immunoreactivity for GAP-43 appears in the cytoplasm of the cell bodies as early as 3.5 h post axotomy and is present in neurites and growth cones as soon as they develop. At early stages of culture (4 h to eight days) satellite/Schwann cells are also immunoreactive for GAP-43. Neurons in isolated whole dorsal root ganglion maintained in vitro become GAP-43-immunoreactive between 2 and 3 h after axotomy. It takes three days however, after cutting or crushing the sciatic nerve in adult rats in vivo, for GAP-43 immunoreactivity to appear in the axotomized dorsal root ganglion cells. GAP-43 immunoreactivity can be detected in the central terminals of primary afferent neurons in the superficial laminae of the dorsal horn of the lumbar enlargement four days after sciatic cut or crush. The intensity of the GAP-43 staining reaches a peak at 21 days and becomes undetectable nine weeks following crush injury and 36 weeks following sciatic nerve cut. The pattern of GAP-43 staining is identical to the distribution of sciatic small-calibre afferent terminals. Little or no staining is present in the deep dorsal horn, but GAP-43 does appear in the ipsilateral gracile nucleus 22 days after sciatic injury. In investigating the mechanism of GAP-43 regulation, blockade of axon transport in the sciatic nerve with vinblastine (10(-5) M-10(-4) M) or capsaicin (1.5%) was found to produce a pattern of GAP-43 immunoreactivity in the dorsal horn identical to that found with crush, while electrical stimulation of the sciatic nerve had no effect. Axotomy of primary sensory neurons or the interruption of axon transport in the periphery therefore acts to trigger GAP-43 production in the cell body. The GAP-43 is transported to both the peripheral and the central terminals of the afferents. In the CNS the elevated GAP-43 levels may contribute to an inappropriate synaptic reorganization of afferent terminals that could play a role in the sensory disorders that follow nerve injury.     

Ultraterminal sprouting in innervated and partially denervated adult and aged rat muscle

End plates from the extensor digitorum longus, soleus and diaphragm muscles of adult (10-month-old) and aged (25-month-old) rats were examined to determine whether aging affects the frequency of occurrence of ultraterminal sprouting, a form of terminal sprouting commonly associated with muscle denervation. There was a significant increase in the fraction of end plates exhibiting ultraterminal sprouts in the extensor digitorum longus muscle only; the number increased from 5.5% in 10-month-old animals to 23.0% in 25-month-old animals. To see whether age also influences the sprouting response to denervation, extensor digitorum longus muscles from 10-and 25-month-old animals were partially denervated by severing spinal nerve L5, and end plates were examined 4-7 and 10-14 days following denervation. Denervation induced a more profound increase in the percentage of end plates exhibiting ultraterminal sprouting in the 10- compared with 25-month-old animals. However, sprouting remained significantly greater in the 25- compared with the 10-month-old animals. Furthermore, following denervation the average ultraterminal sprout length was significantly greater in the 25- compared with the 10-month-old animals. There was no correlation between the extent of muscle denervation and the percentage of end plates exhibiting ultraterminal sprouts. End plates with ultraterminal sprouts had larger areas and contained fewer nerve terminal branches than end plates without these sprouts. It is suggested that the limited response to partial denervation in the 25-month-old extensor digitorum longus muscles may indicate that the aged extensor digitorum longus has already approached the maximum capacity for sprouting.     

A comparison of fibrillation in denervated skeletal muscle of the anaesthetized rat and guinea-pig

Summary We have used intracellular recordingin vivo to study fibrillation (spontaneous repetitive membrane activity) in extensor digitorum longus (fast twitch) and soleus (slow twitch) muscles of the anaesthetized rat and guinea-pig denervated for periods of about 10 to 60 days. The proportion of fibres fibrillating in the guinea-pig soleus was greater than 50% in most animals up to the longest period of denervation (65 days). Fibrillation was rarely found in rat soleus after three weeks of denervation. Its incidence in the extensor digitorum longus muscles of both species was intermediate. The mean frequency of fibrillation was higher in guinea-pig extensor digitorum longus (16 Hz) and soleus (8 Hz) than in the rat extensor digitorum longus (3 Hz) and soleus (2 Hz). The resting membrane potentials of the denervated muscles were less than normal and correlated inversely with the frequency of fibrillation but not with the incidence of fibrillation: in rat soleus, many fewer fibres were fibrillating at a given membrane potential than in the other three muscles. The incidence of fibrillation was compared with previously reported tensions of the four denervated muscles and was found to have the same rank order. We suggest that fibrillation may reduce atrophy (and hence tension loss) of denervated muscle, which may have implications for artificial stimulation. Fibrillation frequency was directly related to changes in twitch speed of the four muscles after denervation.     

Transformation of contraction speed in muscle following cross-reinnervation; dependence on muscle size

Summary The characteristics of isometric contractions and the force-velocity relation were studied in flexor digitorum longus, flexor hallucis longus and soleus muscles of the cat,in situ, at 37° C and with nerve stimulation. The two flexors were identified as typical fast twitch muscles and the soleus as a typical slow twitch muscle. Following self-reinnervation, both fast and slow muscles retained, to a large extent, their basic contraction characteristics. The soleus muscle, when cross-reinnervated with the nerve of either flexor hallucis longus muscle or extensor digitorum longus muscle exhibited a more complete slow-to-fast transformation than when cross-reinnervated with the nerve of flexor digitorum longus muscle. The flexor digitorum longus muscle underwent a greater degree of fast-to-slow transformation than the flexor hallucis longus muscle, when each was cross-reinnervated with the soleus nerve. The data previously reported for sarcomere shortening velocities of the cross-reinnervated muscles in the rat, the rabbit and the cat are reviewed in the light of present findings. It is found that the discrepancies obtained between species and between different muscles in the same species, with respect to the degree of muscle-speed transformation following cross-reinnervation, are correlated with the differences in the size-ratio of the muscles used in the cross-reinnervation procedure.     

Cross-innervation of fast and slow-twitch muscles by motor axons of the sural nerve in the mouse

The extensor digitorum longus (EDL) or soleus muscles of adult mice were cross-innervated by the sural nerve (SN) and deprived of their original innervation. The number and sizes of motor units and the location of endplates in these muscles were studied 1.5 to 16 months later. In the EDL muscle, the SN cross-innervated the original endplates. Very few ectopic endplates were seen, even when the nerve was implanted well outside of the original endplate area. Only 3% of the fibres were polyneuronally innervated. In the soleus muscle, however, the SN formed large numbers of ectopic endplates whether the nerve was implanted in the original endplate zone or outside of it. In addition, 20% of the muscle fibres were polyneuronally innervated. The SN cross-innervated both EDL and soleus muscles completely. There was no preference for a particular group of the SN motoneurones since all the cross-innervated muscles were innervated by all SN motor axons and the motor unit sizes of the SN were similar in the cross-innervated EDL and soleus muscles. It is concluded that intrinsic properties of a muscle determine the ability to form ectopic synapses. The distribution of the motor unit sizes is determined by the particular pool of motoneurones which innervates the muscle.     

Effect of diffusion distance on measurement of rat skeletal muscle glucose transport in vitro

The relationships between muscle size, diffusion distance, and glucose uptake were studied using the Type II b epitrochlearis (13 ± 1 mg intact), Type I soleus (25± 1 mg), and mixed Type II a/II b extensor digitorum longus (25 ± 1 mg) from 60–70 g rats. Using intact muscles, the relative rates of 3-O-methyl-glucose uptake in response to 2 mUml^-1 insulin were soleus = epitrochlearis > extensor digitorum longus, a finding inconsistent with the fibre-type compositions and the relative GLUT-4 protein levels (soleus > extensor digitorum longus > epitrochlearis). To test whether these results were influenced by substrate diffusion limitations in the tubular muscles, soleus and extensor digitorum longus were split longitudinally from tendon to tendon into strips of comparable size (13 ± 1 mg) to the epitrochlearis. Insulin-stimulated rates of 3-O-methyl-glucose uptake were significantly enhanced in the split soleus (+120%) and split extensor digitorum longus (+200%), but not in the epitrochlearis, with the relative rates being soleus > extensor digitorum longus > epitrochlearis. Diffusion distances of the split soleus and extensor digitorum longus, as reflected by [¹⁴C]mannitol space equilibration time, were markedly enhanced (by at least 50%) relative to the intact muscles, and were comparable to that of the epitrochlearis. These results indicate that when muscles of different size and/or shape are used for in vitro measurement of glucose transport, the muscle preparations used must have similar diffusion distances for physiologically meaningful comparisons to be made.     

Changes produced by chronic denervation in the temperature-dependent isometric contractile characteristics of rat fast and slow twitch skeletal muscles.

1. Influence of temperature (range 20-35 degrees C) on the isometric contractile properties of normal and chronically denervated fast-twitch extensor digitorum longus and slow-twitch soleus muscles of the rat have been studied in vitro. 2. The times to peak twitch tension of denervated muscles were longer than those of normal in both types of muscle. The denervated muscles were, however, identifiable as 'fast-twitch' or 'slow-twitch' throughout the entire temperature range. 3. The twitch tension/tetanic tension ratios of the denervated muscles at 35 degrees C were significantly higher than those of the normal muscles. 4. The twitch tension of the normal extensor digitorum longus muscles increased whereas that of the normal soleus muscles decreased with cooling from 35 to 20 degrees C. Such a qualitative difference did not exist between the denervated extensor digitorum longus and soleus muscles. The twitch tensions of both denervated muscles decreased with cooling. 5. Cooling to 20 degrees C resulted in a greater fall in tetanic tension in the denervated than in the normal muscles.     

Effects of perchlorate on myofibrillar calcium sensitivity in rat skinned skeletal muscles

The effects of perchlorate (1–20 mm ) on myofibrillar calcium responsiveness have been tested in Triton X-100-skinned fibre bundles from rat soleus (slow-twitch) and extensor digitorum longus (fast-twitch) skeletal muscles. In extensor digitorum longus and soleus, perchlorate dose-dependently shifted the pCa (-log[Ca²⁺])/tension relationship towards lower free calcium concentration (sensitizing effect) and maximal tension was unchanged. The degree of sensitization was greater in extensor digitorum longus than in soleus bundles. Reversibility after exposure to 12 mm perchlorate was complete in soleus but not in extensor digitorum longus muscles. In fact, the ‘return’ pCa/tension relationship in extensor digitorum longus was shifted to higher free calcium concentration (desensitizing effect) compared with control. Perchlorate (12 mm ) also enhanced myofibrillar calcium responsiveness of frog semitendinosus skinned skeletal fibres. Assuming a passive distribution of perchlorate across the sarcolemma, this sensitizing effect is probably not involved in perchlorate-induced potentiation of contractile responses of intact muscles and thereby supports the specificity of perchlorate as an agonist of the excitation/calcium release sequence in skeletal muscle fibres.     

Alterations in neuromuscular junction morphology during fast-to-slow transformation of rabbit skeletal muscles

Summary Chronic low frequency stimulation of motor nerves results in transformation of muscle fibre phenotype from fast- to slow-twitch. We examined the light and electron microscopic structure of neuromuscular junctions in normally fast twitch muscles, tibialis anterior and extensor digitorum longus of rabbit after 3 weeks of stimulation to determine whether synaptic structure is also modified during fibre type transformation. Neuromuscular junctions of stimulated and unstimulated (control) tibialis anterior and extensor digitorum longus muscles and unstimulated slow twitch soleus muscle were visualized with rhodamine-conjugated -bungarotoxin. Video light microscopic images of neuromuscular junctions were digitized to allow quantification of their surface areas, perimeters, lengths and widths. Three weeks of stimulation resulted in a decrease in the maximal velocity of muscle fibre shortening and augmentation of mitochondrial volume in fast muscles, demonstrating the efficacy of the stimulation protocol employed in altering muscle fibre phenotype. Neuromuscular junctions of control tibialis anterior and extensor digitorum longus are thin, compact, and continuous, with complex branching patterns. In contrast, those of slow-twitch soleus are thicker and discontinuous. Neuromuscular junctions in control tibialis anterior and extensor digitorum longus are larger than those in soleus. Three weeks of stimulation causes a marked decrease in the size of neuromuscular junctions in tibialis anterior and extensor digitorum longus, as reflected in the significant reduction in neuromuscular junction surface area, length and width. Electron microscopy of these junctions suggests that secondary postsynaptic folds in stimulated muscles are more closely spaced. Also, axon terminals of stimulated muscles appear to contain more densely packed synaptic vesicles and mitochondria than controls. Decreases in neuromuscular junction dimensions can be partly explained by muscle fibre atrophy. However, the decrease in neuromuscular junction size is proportionately greater than that of muscle fibre diameter in both muscles, indicating that factors other than fibre atrophy may contribute to the reduced neuromuscular junction size in stimulated muscles. Neuromuscular junctions of stimulated tibialis anterior and extensor digitorum longus muscles exhibit some features characteristic of normal soleus neuromuscular junctions, indicating structural adaptations consistent with the altered muscle fibre phenotype. On the other hand, neuromuscular junctions of 3 week stimulated tibialis anterior and extensor digitorum longus and their synaptic branches remain as thin and continuous as those of unstimulated controls, suggesting that the transformation of neuromuscular junctions towards a morphology characteristic of slow muscle, is only partial. These results demonstrate that an altered pattern of impulse activity causes significant synaptic remodelling in adult rabbit skeletal muscles.     

A quantitative study of satellite cells in regenerated soleus and extensor digitorum longus muscles

Satellite cells quantitated in the rat soleus and extensor digitorum longus (EDL) muscles following a complete regeneration returned to “normal” percentages of myofiber nuclei in both muscles 3 months after injury. Following cross-transplantation, the percentage of satellite cell nuclei in the EDL regenerated in the soleus bed was indistinguishable from the percentage in the soleus. Likewise, the soleus muscle regenerated in the EDL bed had a satellite cell percentage characteristic of the EDL. These results suggest that (1) the proportion of satellite cells is reestablished in a regenerated muscle and (2) the innervating nerve determines the proportion of satellite cell nuclei in a muscle.