Changes in chemosensitivity of developing chick muscle fibres in relation to endplate formation

1. The sensitivity to acetylcholine (ACh) of the fast posterior latissimus dorsi (PLD) and slow anterior latissimus dorsi (ALD) during embryonic development was studied and compared. The sensitivities were expressed as a ratio of the maximal tetanic tension and tension developed in response to ACh. 2. Up to the 17th day of incubation both muscles are sensitive to ACh to a similar extent; at the 18th day the sensitivity of the PLD muscle decreases and continues to do so until hatching and thereafter. 3. Since the decrease in sensitivity of PLD muscles takes place a few days after innervation, it is suggested that this is caused by activity of the motor nerve. To test this curare (dTc) and hemicholinium (HC-3), drugs that interfere with neuromuscular transmission, were injected into the yolk sac of the embryos when nervemuscle connections are usually established. In the curare and HC-3 treated embryos the desensitization of the PLD muscles did not take place. 4. The distribution of endplates on PLD muscles from drug treated 20-21 day old embryos was compared to that of untreated controls. Whereas control PLD muscles have only one band of endplates, muscles from curarized embryos and HC-3 treated embryos have several bands of endplates, and many muscle fibres with multiple innervation were found. 5. It is suggested that nerve fibres which make connections with PLD muscle fibres bring about a decline in chemosensitivity by releasing more transmitter, and thereby prevent further nerve muscle connections from being made along the same muscle fibre.     

Neuromuscular junctions in adult and developing fast and slow muscles

Functional changes that occur just before hatching in future fast muscles of the chicken are thought to be influenced by the pattern of innervation. We have compared the neuromuscular junctions of two fast muscles, the posterior latissimus dorsi (PLD) and the pectoralis, which differ in their myosin composition at 18 days in ovo. We have also presented new information on the neuromuscular junctions of the adult fast muscles and an adult slow muscle, the anterior latissimus dorsi (ALD). Both categories of adult muscles were heterogeneous, and there was little difference between endplates of the two fast muscles or between the fast and slow muscles. In contrast, there were significant structural differences between the two fast muscles during embryonic development. In early embryonic muscle fibers, which synthesize embryonic forms of myosin, individual motor endplates were contacted by multiple axon terminals. At 18 days in ovo, the majority of the neuromuscular junctions in the pectoralis continued to be multiterminal, whereas all but one of the terminals had been withdrawn from each endplate in the PLD. This single terminal had a unique form that distinguished it from the embryonic pectoralis and also from the two adult muscles. By 7 days after hatching, the neuromuscular junctions of both muscles had single terminals. They were different from the embryonic terminals, though not necessarily equivalent to adult terminals. The results show that multiple terminals persist at 18 days in ovo in the muscle that continues to express an embryonic myosin, but they have been withdrawn from the muscle that has lost this myosin. It is concluded, from combined data on the two muscles, that maturation of the neuromuscular junction during embryonic and late posthatch development is correlated with transitions in the myosin pattern and in contractile properties.     

Development of neuromuscular junctions of fast and slow muscles in the chick embryo: a light and electron microscopic study

Summary The morphogenesis of neuromuscular junctions (NMJ) was studied by electron microscopy in fast posterior and slow anterior latissimus dorsi muscles (PLD and ALD) of chick embryos. In 8 day embryos, the NMJ is already established in both types. In PLD at this stage, individual axons completely ensheathed by Schwann cell processes form NMJs with myotubes, whereas in ALD axon terminals lie adjacent to (and not separated by Schwann cell processes from) naked axons which are components of a nerve bundle. At 11–15 days, the number of profiles of axon terminals at each endplate increases in both PLD and ALD. In PLD, individual axon terminals are ensheathed by Schwann cells and often branch. In ALD, several axon terminals become ensheathed as a group by processes of a Schwann cell, along with a small number of adjacent naked axons. The individual axon terminals were confirmed by analysis of serial sections to originate from different preterminal axons. Thus, the increase in number of axon terminals in PLD may be due to extensive terminal branching, whereas in ALD it may be due to the arrival of other nerve fibres. From 16 days, each axon terminal in an endplate of ALD becomes individually ensheathed by Schwann cell processes. However, the property of polyneuronal innervation in each endplate is retained even in the adult muscle. The junctional sarcoplasm protrudes to separate individual axon terminals at certain developmental stages: the protuberances are thinner and more numerous in ALD than in PLD at 15–16 days.It is concluded that NMJ morphogenesis differs between PLD and ALD and that the differences reside mainly on the neuronal side.     

Differentiation of electrical and contractile properties of slow and fast muscle fibres.

1. The development of the electrical and contractile properties of slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of the chick was studied. 2. At the earliest ages studied (14-16 days of incubation), the membrane resistance was similar in both muscles. Subsequently the membrane resistance of PLD decreased, and that of ALD increased. Electrical differentiation continued after hatching. 3. The contractile speeds of ALD and PLD muscles were similar in embryos at 14-16 days of incubation, and suddenly differentiated at 17-18 days. Unlike the electrical properties, contractile speeds changed little after hatching. 4. It is suggested that the different electrical properties of the membrane of the two types of muscle fibre develop as a result of the particular type of excitation imposed by their nerves.     

The influence of innervation on the differentiation of contractile speeds of developing chick muscles

1. The role of innervation of the differentiation of contractile speeds was studied in the slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscle of the chick. 2. These muscles become innervated during the 12th and 15th day of embryonic development. At this time both muscles contract and relax extremely slowly and their contractile speeds are very similar. By the 18th day their contraction and relaxation becomes more rapid. It is at this time that the contractile characteristics of both muscles also become very different from each other, ALD being about 3 times slower than PLD. Thus innervation percedes differentiation of contractile speeds by several days. 3. The influence of innervation on the contractile characteristics of developing slow and fast muscles was studied during muscle regeneration in adults. When a slow ALD muscle was minced and implanted in place of a fast PLD the newly regenerated ALD became innervated by a PLD nerve and resembled a fast PLD. Conversely, when PLD muscles were minced and replaced ALD the regenerated PLD was innervated by ALD nerve and became slow. 4. Histological examination revealed that the regenerated ALD became focally innervated, and the regenerated PLD multiply innervated. 5. Thus, the contractile speeds are not predetermined properties of the muscle fibre. Both contractile characteristic and the pattern of innervation of developing muscles are determined by the motor nerve.     

Fast and slow muscles of the chick after nerve cross-union

1. The multiply innervated anterior latissimus dorsi (ALD) and the focally innervated posterior latissimus dorsi (PLD) muscles of the chick were investigated 2-18 months after nerve cross-union.2. The fast PLD muscle re-innervated by the slow muscle nerve became supplied with ;en grappe' end-plates and responded to a single nerve volley with local potentials only. Control PLD muscles re-innervated by the original nerve, had ;en plaque' end-plates and responded to a single nerve volley by synchronous action potentials in the same way as normal muscles.3. In the slow ALD muscle re-innervated with the ;mixed' PLD nerve, the type of innervation and of electromyographic response remained practically unchanged, with the exception of transplanted ALD muscles supplied with PLD nerves where, in addition to local responses, propagated action potentials were registered electromyographically in response to single nerve volleys.4. ALD muscles of young chickens re-innervated both with an implanted purely fast muscle nerve and with the regenerated original nerve, had two types of innervation: ;en plaque' end-plates around the nerve implant, and multiple ;en grappe' end-plates in areas supplied with the ALD nerve. Accordingly, propagated action potentials were registered in response to single nerve volleys in regions near the implant, whereas local potentials were recorded in areas with original innervation.5. Contraction velocity was not substantially altered in PLD and ALD muscles after nerve cross-union.6. No changes were observed in the fine structure of muscle fibres in extrajunctional regions.     

Independent development of contractile properties and myosin light chains in embryonic chick fast and slow muscle

1. The contractile speeds and tetanus/twitch ratios of the slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles were studied during embryonic development and correlated with the type of myosin light chains present in these muscles as studied by one and two dimensional polyacrylamide gel electrophoresis. 2. At a time when the contractions of PLD were slow, i.e. in 15 day old embryos, the myosin light chains in this muscle were of the fast type. The slow contraction of this muscle may be due to incomplete and slow activation of the contractile elements. The tetanus/twitch ratio of muscles from 15 day old embryos is low and increases sharply with age. This increase could be due to the maturation of the internal membrane system, and occurs at about the same time as the increase in the speed of contraction. 3. ALD muscles contract slowly during all stages of development, although their tetanus/twitch ratio also increases with age. At 13 days they contain a mixture of fast and slow type myosin light chains and with increasing age the proportion of the slow type myosin light chains increases at the expense of the fast type. The slow time course of contraction of ALD is consistent with the presence of slow type myosin light chains. 4. The possibility that the synthesis of the slow type myosin light chains in ALD is induced by early motor activity in chick embryos is discussed.     

Early morphological changes in the striated muscles in normal and dystrophic chickens

Histological and histochemical analyses were performed on the anterior latissimus dorsi muscle (ALD, red muscle) and the posterior latissimus dorsi muscle (PLD, white muscle) in normal (line 412) and dystrophic chickens (line 413) from 19 day embryos to 6 weeks of age. PLD, the white muscle, in dystrophic chickens showed higher percentages of red and intermediate fibres than those of normal chickens during the early development of muscles. Increases of the oxidative enzyme activities and the numbers of NADH--TR formazan granules in the white fibres of PLD were already found at 1 week of age in dystrophic chicken. Fibre types, oxidative enzyme activities and NADH--TR formazan granules showed no differences in ALD between normal and dystrophic chickens. These results suggest that increases of oxidative enzyme activities and formazan granule numbers and incomplete fibre type differentiation in PLD of dystrophic chickens are early pathological processes in such birds.     

The formation of synapses in reinnervated and cross-reinnervated adult avian muscle

1. A study has been made of the formation of synapses in spontaneously reinnervated and cross-reinnervated anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles of adult fowls.2. Denervated ALD and PLD muscle fibres have a uniform and high sensitivity to iontophoretically applied acetylcholine (ACh). During early reinnervation the sensitivity distribution to ACh of the ALD muscle fibres begins to return to normal before synaptic potentials can be evoked. The normal ACh sensitivity distribution of PLD muscle fibres is also restored after reinnervation. After cross-reinnervation of the ALD and PLD muscles the ACh sensitivity distribution of many of the muscle fibres is again restored to normal.3. Reinnervating and cross-reinnervating ALD nerve terminals showed a greater than normal degree of facilitation of transmitter release when a test impulse was applied at various intervals after a conditioning impulse. Cross-reinnervating PLD nerve terminals showed facilitation of transmitter release rather than the normal depression in a conditioning-test impulse sequence.4. The distribution of nerve terminals over the surface of spontaneously reinnervated and cross-reinnervated ALD and PLD muscle fibres has been determined from an examination of the sensitivity distribution to applied ACh, the graded versus all-or-none nature of the evoked potential and the distribution of cholinesterase stained synapses.5. The results suggest that the innervation pattern of individual ALD and PLD muscle fibres is restored both after spontaneous reinnervation and cross-reinnervation.     

The effect of acetylcholine on the function and structure of the developing mammalian neuromuscular junction

Isolated soleus muscles from rats aged 9–12 days were exposed to acetylcholine for 2 h in normal Krebs solution. This treatment caused changes in the ultrastructural appearance of the neuro-muscular junction and a significant reduction of axon profiles per endplate. Nevertheless, most neuro-muscular junctions remained functional, since the ratio of the indirectly to directly elicited contraction was not reduced.If muscles were exposed to acetylcholine in Krebs solutions containing 12m M Ca²⁺ instead of the normal 1.9 m M, the ultrastructural changes produced by acetylcholine were more severe, and the number of axon terminals per endplate was further reduced so that many endplates became completely denervated. This was also reflected in the impaired function of the nerve-muscle preparation; the ratio of the indirectly to directly elicited contraction decreased and about 40% of the muscles fibres became functionally denervated. Addition of curare to the incubating medium prevented the functional deterioration of the preparation.Addition of the protease inhibitors leupeptin and pepstatin protected the nerve terminals from the damaging effects of acetylcholine in Krebs solution containing 12m M Ca²⁺ and the number of axon profiles per endplate remained normal. The functional deterioration was also much reduced when protease inhibitors were included in the incubation medium.These results suggest that acetylcholine causes the activation and release of proteolytic enzymes in developing muscles. The response is mediated by calcium and may have a role in the removal of superfluous nerve-muscle contacts during development.     

The onset and progress of transformation of avian slow into fast muscles under neural influence

The slow anterior latissimus dorsi (ALD) muscles of newly hatched chickens were transposed and cross0innervated by the mixed, predominantly fast superior brachialis nerve, and investigated 2 to 15 months after the operation. Two months after the operation, myosin ATPase activity of the cross-innervated ALD muscles was still as low as in the control ALD, although the ultrastructure and the histochemical ATPase activity already showed a mixed fibre-type pattern with a predominance of fast -type fibres around the site of nerve implantation. The change of myosin properties of thw whole cross-innervated ALD did not occur until the third month after the operation. At that time, the myosin ATPase activity increased about 2.5 times and light chains of myosin of the fast type appeared in the electrophoretic pattern. The myosin ATPase activity attained 62% of the activity found in the control fast posterior latissimus dorsi muscles at three months; subsequently it remained at about this level reaching 68% 18 months after the operation. The results indicate that approximately two thirds of the cross-innervated ALD muscle fibres became changed towards the fast type under neural influence, whereas about one third remained slow, being re-innervated by the slow-type motor fibres of the implanted nerve.     

Freeze-fractured sarcoplasmic reticulum in adult and embryonic fast and slow muscles

Summary There is evidence to suggest that 8 nm calcium transport particles in the sarcoplasmic reticulum are involved in the regulation of twitch properties in adult muscles. We have studied ultrastructural characteristics of the sarcoplasmic reticulum in relation to previously defined physiological changes that take place in the normal course of development. The fast twitch posterior latissimus dorsi (PLD) and the slow tonic anterior latissimus dorsi (ALD) of the chicken were compared using the procedure of freeze-fracture. In the adult PLD, the sarcoplasmic reticulum was composed of longitudinal tubules, which gave rise to fenestrated cisternae at the centre of the H band and to terminal cisternae that form triads regularly at each A-I junction. In most of the fibres (85%), 8 nm intramembrane particles were closely packed in the concave fracture face (P-face). In the ALD, a tubular network with an open circular pattern extended the entire length of the A band and usually throughout the I band as well. Dyads or triads, which were infrequent, were often oriented obliquely. The density of intramembrane particles was low in the majority of the fibres, but there was a significant minority population (30%) in which particle density was relatively high. At 10 daysin ovo, when speed of contraction in both the ALD and PLD is slow, there was a circular configuration of sarcoplasmic reticulum components in both muscles, and particle density was low. Surprisingly, at 18 daysin ovo, when the rate of tension development and relaxation have reached nearly adult values in the fast PLD, this muscle, like the ALD, continued to exhibit a circular arrangement of sarcoplasmic reticulum tubules. The density of P-face particles, although greater than at 10 days, was still low relative to the adult PLD. Estimated values for the 18-day PLD were similar to those calculated for the adult slow muscle. Our observations, along with those of other investigators, suggest that abundant intramembrane particles may be related to the fast twitch properties of the adult PLD. However, they indicate that neither the pattern of membranes typical of the adult fast muscle nor the high content of calcium transport particles is required for the differentiation of fast twitch characteristics.     

A stereological comparison of the muscle-tendon junctions of fast and slow fibers in the chicken

Transmission of contractile tension from skeletal muscle fibers to connective tissue elements is thought to occur at the muscle-tendon junctions, specialized regions at the extreme ends of the fibers. Previous work has suggested that the structure of this region may be quantitatively modified to match the contractile properties of the fibers. Using scanning electron microscopy, transmission electron microscopy, and stereological analysis, we have analyzed the three-dimensional structure, and have quantitatively compared the muscle-tendon junctions, of slow and fast fibers of the anterior (ALD) and posterior (PLD) latissimus dorsi muscles of the chicken. The ends of ALD and PLD fibers are found to be structurally different in some respects but to be similar with respect to their surface specializations, which are believed to function in the transmission of tension. Quantitative analysis of these specializations indicates that, when referred to similar cross-sectional areas of myofilaments, the fast fibers of the PLD have approximately 40% more surface area devoted to force transmission than do the slow fibers of the ALD. These observations are consistent with the idea that the amount of cell surface specialized for force transmission is related to the functional properties of the muscle fiber.     

Structure, distribution and innervation of muscle spindles in avian fast and slow skeletal muscle

Muscle spindles in 2 synergistic avian skeletal muscles, the anterior (ALD) and posterior (PLD) latissimus dorsi, were studied by light and electron microscopy to determine whether morphological or quantitative differences existed between these sensory receptors. Differences were found in the density, distribution and location of muscle spindles in the 2 muscles. They also differed with respect to the morphology of their capsules and intracapsular components. The slow ALD possessed muscle spindles which were evenly distributed throughout the muscle, whereas in the fast PLD they were mainly concentrated around the single nerve entry point into the muscle. The muscle spindle index (number of spindles per gram wet muscle weight) in the ALD was more than double that of its fast-twitch PLD counterpart (130.5±2.0 vs 55.4±2.0 respectively, n=6). The number of intrafusal fibres per spindle ranged from 1 to 8 in the ALD and 2 to 9 in the PLD, and their diameters varied from 5.0 to 16.0 μm and 4.5 to 18.5 μm, respectively. Large diameter intrafusal fibres were more frequently encountered in spindles of the PLD. Unique to the ALD was the presence of monofibre muscle spindles (12.7% of total spindles observed in ALD) which contained a solitary intrafusal fibre. In muscle spindles of both the ALD and PLD, sensory nerve endings terminated in a spiral fashion on the intrafusal fibres in their equatorial regions. Motor innervation was restricted to either juxtaequatorial or polar regions of the intrafusal fibres. Outer capsule components were extensive in polar and juxtaequatorial regions of ALD spindles, whereas inner capsule cells of PLD spindles were more numerous in juxtaequatorial and equatorial regions. Overall, muscle spindles of the PLD exhibited greater complexity with respect to the number of intrafusal fibres per spindle, range of intrafusal fibre diameters and development of their inner capsules. It is postulated that the differences in muscle spindle density and structure observed in this study reflect the function of the muscles in which they reside.     

The formation of synapses in reinnervated and cross-reinnervated striated muscle during development

1. A study has been made of the formation of synapses in reinnervated and cross-reinnervated developing striated muscles which normally receive either a focal or distributed innervation, using histological, ultrastructural and electrophysiological techniques.2. The focally innervated mammalian tibialis anterior muscle, denervated soon after birth, was reinnervated at both the original end-plates as well as on the new muscle added during the period of denervation; but not on the muscle present at the time of denervation. Nearly all the synapses which had formed, other than at the original end-plates, disappeared by 6 weeks post-natal.3. The avian anterior latissimus dorsi muscle (ALD), which receives a distributed innervation, was denervated during the first week post-hatched, and became reinnervated both at the original synaptic sites as well as on the new muscle added during the period of denervation; all these synapses were spaced approximately 200 mum apart along the length of individual muscle cells.4. The myofibres of the ALD muscle cross-reinnervated at hatching with the superior brachialis nerve, which contains fast motor axons that normally form a focal innervation, were each focally innervated by a single ;en plaque' terminal; these synapses had the same electrical properties as normal synapses formed by fast motor axons.5. Many of the myofibres of the avian posterior latissimus dorsi (PLD), which normally receive a focal innervation, received a distributed innervation from ;en grappe' terminals when cross-reinnervated with the ALD nerve at hatching.6. It is suggested that during development the nerve type determines the pattern of synapses over an effector; this is achieved by the nerve, after forming the initial synaptic contact, making the rest of the muscle cell membrane refractory to further synapse formation for some distance, this distance being determined by the nerve type.     

Characterization of Ca2+- and Sr2+-activated tension in functionally skinned chicken fibers of normal and dystrophic skeletal and normal cardiac muscle

The Ca²⁺ and Sr²⁺ activation of tension in functionally skinned chicken fibers of normal and dystrophic skeletal and normal cardiac muscle were studied. The muscles studied can be separated into two groups based upon their Ca²⁺ and Sr²⁺ sensitivities: those which are significantly more sensitive to Ca²⁺ than to Sr²⁺, pectoralis and posterior latissimus dorsi (PLD), and those which show no Ca²⁺/Sr²⁺ sensitivity difference, cardiac and anterior latissimus dorsi (ALD). This suggests that there is more than one type of Ca²⁺ site involved in Ca²⁺ control of muscle contraction in different muscle types and suggests that ALD and cardiac muscle may be controlled by a different type of binding site than PLD and pectoralis muscle. Dystrophic ALD and PLD muscles showed little change in their Ca²⁺ and Sr²⁺ sensitivities from those of normal muscles in contrast to the pectoralis which showed a decrease in both Ca²⁺ and Sr²⁺ sensitivity (approaching that of PLD) with the onset of dystrophy. Similarly, upon SDS polyacrylamide gel electrophoresis, dystrophic ALD and PLD muscles showed no difference in contractile proteins from those of normal muscles, in contrast to pectoralis muscle where the appearance of a 36,000 dalton protein band correlated with the onset of dystrophy and the changes in the Ca²⁺/Sr²⁺ activation properties of this muscle. The contractile protein band pattern of normal and dystrophic PLD and dystrophic pectoralis muscle were similar including the presence of the 36,000 dalton protein.     

Differentiation of membrane systems during development of slow and fast skeletal muscle fibres in chicken

Summary The disposition of transverse (T) tubules, sarcoplasmic reticulum (SR) and T-SR junctions (triads) and the width of Z lines are matched to contractile properties in adult muscle fibres. We have studied the development of the membrane systems in the slow anterior (ALD) and the fast posterior (PLD) latissimus dorsi of the chicken in ovo (E14–E21) and after hatching (D1–D30). T tubules, SR, triads and Z lines were visualized using DiIC16[3] labelling for confocal microscopy and either Ca-osmium-ferrocyanide or standard procedures for electron microscopy. Anterior latissimus dorsi and PLD have similar, slow twitches in early development (E14–E16), but PLD suddenly becomes faster starting at E17–E18. We find that in coincidence with the differentiation of faster contraction properties (starting at E18–E19) density of triads is significantly higher and width of Z lines is narrower in PLD. The SR also begins to acquire fibre-type specific characteristics at this time. Early development of T tubules, on the other hand, is quite similar in the two muscles. Peripherally-located, longitudinally-oriented T tubules, and the first T networks crossing the fibre center appear earlier in ALD (E14–E15 and E16) than in PLD (E14–E16 and E17), but have similar dispositions. The final fibre-type specific distribution of T tubules is achieved after hatching. Some T tubules-rich fibres in the ALD, presumably future fast fibres, develop extensive T tubule networks at early stages. Location of triads at the Z line in pectoralis occurs in three steps: an initial location of longitudinally oriented triads at the A-I junction; a subsequent move to the Z lines and finally a rotation to a transverse orientation.     

A freeze-fracture study of the anterior and posterior latissimus dorsi muscles of the chicken

The sarcolemma, sarcoplasmic reticulum (SR), and T system of the anterior (tonic) and posterior (fast twitch) latissimus dorsi muscles of the chicken have been examined by the freeze-fracture technique, and quantitative data on the P and E fracture faces have been obtained. The fractured plasma membranes reveal (a) profiles of surface caveolae, (b) randomly distributed intramembranous particles ranging in size from 40–100 Å in diameter, and (c) orthogonal assemblies composed of groups of 60 Å particles in close association, and differences with respect to all three structures are present between the tonic (ALD) and fast twitch (PLD) muscles. In the ALD muscle, the surface caveolae are more uniformly distributed and have smaller openings than in the PLD muscle; the former muscle also has a two-fold higher caveolae density than the latter muscle. The intramembranous particles are more numerous in the ALD than in the PLD muscle in both fracture faces, but the orthogonal assemblies are fewer. The functional significance of these differences in the two fiber types are discussed. The fractured membranes of the SR have intramembranous particles (IMP's) approximately 80 Å in diameter, with a two-fold higher packing density in the PLD than in the ALD muscle. This difference is present in both the longitudinal and cisternal components of the SR. In addition, there are collar-like expansions (CLE's) in the SR of the ALD muscle which are particularly poor in intramembranous particles. These particles are considered to represent Ca²⁺ transport ATP-ase, and the reduced density of IMP's could be a significant factor in the low calcium uptake and slow relaxation characteristics of the ALD muscle.     

Developmental change in choline acetyltransferase activity in nerve endings of latissimus dorsii muscles in the chick embryo: influence of chronic spinal cord stimulation

Choline acetyltransferase (CAT) activity of chick latissimus dorsii muscles was studied during embryonic development and at post-hatching states. CAT activity was always higher in anterior (ALD) than in posterior (PLD) muscles. At embryonic stages, chronic spinal cord stimulation at a low rhythm did not modify CAT activity in ALD nerve endings but caused a transient increase in PLD terminals. This increase in CAT activity seems to be related to an acceleration of neuronal maturation rather than to the occurrence of the multiple innervation that results from the central stimulation.     

Muscle length affects the architecture and pattern of innervation differently in leg muscles of mouse, guinea pig, and rabbit compared to those of human and monkey muscles

The innervation pattern and fascicular anatomy of muscles of different lengths in mouse, guinea pig, rabbit, macaque monkey and human legs were analyzed. Neuromuscular junctions, muscle tendon junctions and ends of intrafascicularly terminating fibers were stained for acetylcholinesterase, and fascicle lengths measured. A high correlation between increasing fascicle length and increasing number of neuromuscular junctions was found, with non-primate (mouse, guinea pig, rabbit) and primate (macaque monkey, human) muscles forming two discrete groups. In non-primates, muscles with a single endplate band, fascicles were always shorter than 35 mm, fixing the limit of fiber length served by one neuromuscular junction. Muscles with fascicles longer than this had multiple discrete bands of motor endplates crossing their width at regular intervals. An increase in muscle length across or within species corresponded to an equivalent, standard increase of 10-12 mm fascicle length per motor endplate band. All human and monkey leg muscles, with the exception of gracilis and sartorius, were singly innervated and all muscle fibers ran the full distance from tendon to tendon. Singly innervated primate muscle fibers were up to 140 mm long whereas the mean distance between endplate bands in the two multiply innervated muscles was also considerably greater than in non-primates. These data indicate that allometric effects of increasing fascicle length, are distinct in common laboratory animals and two primates, when architecture and pattern of innervation are compared.