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.     

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 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.     

The formation of synapses in amphibian striated muscle during development.

1. A study has been made of the formation of synapses in developing reinnervated and cross-reinnervated amphibian twitch muscles which receive either a focal (iliofibularis) or a distributed (sartorius) innervation from 'en plaque' nerve terminals using histological, ultrastructural and electrophysiological techniques. 2. During the development of the tadpole through metamorphosis to the adult frog, the sartorius myofibres increased in length at about twice the rate of the iliofibularis myofibres, due to a fast rate of growth at their insertions on to the pelvic tendon. 3. The short iliofibularis and sartorius myofibres of young tadpoles (800 mum long) possessed only a single synapse and the iliofibularis myofibres did not receive any further innervation during development. However the sartorius myofibres received further transient innervation on the new muscle laid down during development at the fast growing pelvic insertion, until the distance between the original synapse formed on the myofibres and the synapse at the pelvic end of the muscle was about 12 mm. 4. During development synapses possessed either skewed, multimodal, or unimodal m.e.p.p. amplitude-frequency distributions; the intervals between m.e.p.p.s. were not distributed randomly according to a Poisson process, as m.e.p.p.s. of similar amplitudes tended to be separated by very short intervals; the unit-size e.p.p. had a similar amplitude-frequency distribution as the m.e.p.p.s. if these had a unimodal distribution. 5. Reinnervation or cross-reinnervation of the sartorius and the iliofibularis muscles in adults or at a late stage of development simply reconstituted the normal focal and distributed innervation patterns of the muscles, as found in the control muscles of the contralateral and unoperated legs. 6. These observations on synapse formation in amphibia are consistent with the hypothesis that during development the axon making the initial synaptic contact on the muscle cells induces a property over a length of muscle membrane adjacent to this site which makes it refractory to synapse formation; thus during reinnervation or cross-reinnervation of adult muscles this refractory property constrains synapse formation to these sites.     

The formation and regression of synapses during the re-innervation of axolotl striated muscles.

1. A study has been made of the formation and regression of synapses formed by spinal nerves 16 and 17 in axolotl hind-limb flexor muscles following the severing of nerve 16, using histological, ultrastructural and electrophysiological techniques. 2. Axolotl hind-limb flexor myofibres possessed 'en plaque' end-plates from either spinal nerve 16 or 17 or both at intervals of about 1000 micronm along their length; the myofibre's length constant was about 700 micronm allowing electrophysiological observations of at least two of these synapses during a single impalement; transmitter release at these synapses could be described by binomial statistics and in a given set of ionic conditions the binomial statistic parameter n was directly proportional to the size of the nerve terminals whilst the binomial statistic parameter p was invariant to changes in nerve terminal size. 3. The distribution of synapses formed by spinal nerves 16 and 17 in different sectors of the axolotl hind-limb flexor muscles was determined from a study of evoked end-plate potentials; the middle and proximal sectors of the flexor muscles contained myofibres which received an innervation from nerve 16 only, whereas the sectors surrounding these contained myofibres innervated either by nerve 16 or nerve 17 or by both nerves. 4. Six days following the severing of spinal nerve 16, evoked transmitter release from the synapses formed by this nerve had failed; transmission was subsequently recorded at a few synapses formed by nerve 17 in the middle and proximal sectors of the flexor muscles which are not normally innervated by this nerve and these synapses had a low n; during the succeeding four weeks the value of n at the synapses increased to a size about 70% that of the terminals normally formed by nerve 16 at these sites. 5. Four weeks after severing nerve 16, myofibres which possessed synapses formed by nerve 17 also possessed synapses from re-innervating nerve 16 and these were sometimes formed at the same synaptic sites as those occupied by nerve 17. 6. In the subsequent sixteen weeks, the n value of synapses formed by nerve 17 declined whilst the n values of synapses formed by re-innervating nerve 16 on the same myofibres matured to their control size. 7. It is suggested that on severing nerve 16 collateral sprouting of nearby intact nerve 17 occurs and these collateral sprouts innervate the denervated synaptic sites, although the sprouts arenot as well matched to the denervated synaptic sites as are the original nerve terminals; thus if nerve 16 returns it preferentially forms synapses at its original synaptic sites, and the collateral synapses formed by nerve 17 regress.     

Polyneuronal innervation of skeletal muscle in new-born rats and its elimination during maturation.

1. The events taking place during the elimination of polyneuronal innervation in the soleus muscle of new-born rats have been studied using a combination of electrophysiological and anatomical techniques. 2. Each immature muscle fibre is supplied by two or more motor axons which converge on to a single end-plate. There was no sign of electrical coupling between muscle fibres receiving multiple synaptic inputs. By the end of the second week after birth virtually all muscle fibres are innervated by only a single motor axon. 3. The average tension produced by individual motor units, measured in terms of the percentage of the total muscle twitch tension, declined dramatically during the first 2 weeks after birth. During this period there was no significant change in the number of motor neurones innervating the soleus muscle. Thus, the disappearance of polyneuronal innervation reflects a decrease in the number of peripheral synapses made by each motor neurone. 4. The decline in motor unit size was delayed, but not ultimately prevented, by the early surgical removal of all but a few motor axons to the soleus muscle. This procedure also caused a delay in the removal of polyneuronal innervation involving the remaining motor units. 5. Following a crush of the soleus nerve in neonatal animals, regenerating axons usually returned to the original end-plates. Polyneuronal innervation was extensive at early stages of re-innervation and it disappeared during the second week after birth just as in normal muscles. 6. Cross-innervation of neonatal muscles by an implanted foreign nerve caused a rapid disappearance of cholinesterase at denervated original end-plates and in most fibres prevented re-innervation by the original nerve. In the small proportion of fibres that did become innervated through both the foreign and original nerves the end-plates were more than 1 mm apart, and both foreign and original nerve end-plates could persist indefinitely. 7. Many cross-innervated fibres received multiple inputs through the foreign nerve. Some foreign end-plates were separated by distances ranging up to 1 mm. Polyneuronal innervation through the foreign nerve was completely eliminated during maturation but over a slightly longer period than in normal muscles. Apparently the elimination process can act over a distance up to but not much more than 1 mm. 8. These observations suggest that there are several factors influencing the elimination of redundant inputs in immature muscles. Individual motor neurones appear to have an inherent tendency to withdraw the majority of their original complement of peripheral terminals. The determination of which particular synapses are to survive, however, seems to be made in the periphery by a selection among all the synapses that innervate a limited region of each muscle fibre. There may be a competitive interaction among synapses in which those belonging to smaller motor units are less likely to be eliminated, thereby leading to a relatively uniform size of the motor units in the soleus.     

The interaction between foreign and original motor nerves innervating the soleus muscle of rats.

1. The fibular nerve was transplanted on to the soleus muscle of the rats. Interruption of the original soleus nerve then permitted cross-innervation, and subsequently, over a period of weeks, re-innervation by the original nerve. 2. Individual muscle fibres were often innervated by both the original and the foreign nerve. The original and foreign end-plates were located in separate regions of the muscle. There were no indications that the original nerve could displace or repress the foreign innervation. 3. The extent of re-innervation by the original nerve depended upon the method of denervation. A single crush of the nerve was followed by virtually complete re-innervation, even of muscle fibres already innervated by the foreign nerve. When re-innervation was delayed by resection of a segment of the nerve only muscle fibres without foreign nerve innervation were re-innervated. Denervation by a simple nerve cut gave an intermediate result. 4. Re-innervation by the original nerve can take place without measurable extrajunctional sensitivity to ACh. 5. The original end-plate region could retain high and localized sensitivity to ACh for several months despite degeneration of its motor nerve terminal and activity of the muscle fibre. 6. Established foreign end-plates were re-innervated by the foreign nerve on muscle fibres with intact original innervation. 7. The factors controlling synapse formation in skeletal muscles are discussed.     

The formation of synapses in striated muscle during development

1. A study has been made of the formation of synapses in developing striated muscles which receive either a focal (the rat hemidiaphragm) or a distributed (the avian anterior latissimus dorsi) innervation using histological, ultrastructural and electrophysiological techniques.2. In the developing diaphragm only a single synaptic contact was initially established at random along the length of the short (300 mum) myotubes by a single axon; in the developing ALD more than one synaptic contact could be established initially along the length of the long (2500 mum) myotubes by axons, but the distance between these was never less than 170 mum.3. Each synapse established by the initial axonal contact in either the diaphragm or the ALD subsequently received a multiple innervation from further exploring axons in the muscles, and all such additional innervation of muscle cells was constrained to the sites of the initial synaptic contacts; this multiple innervation of synaptic sites was lost in the subsequent 4 weeks.4. It is suggested that the axon forming the initial synaptic contact on myotubes induces a property over an adjacent length of myotube which makes its membrane refractory to synapse formation over this length; this characteristic length is longer for axons forming a focal innervation than it is for those forming distributed innervation.     

The roles of the synaptic basal lamina and of innervation in directing the accumulation of a synaptic molecule, mAb 3B6 antigen, in regenerating skeletal muscles

Summary We have recently described a novel nonhomogeneous distribution of a muscle synaptic molecule following denervation. Monoclonal antibody (mAb) 3B6 antigen, a molecule concentrated at endplate/junctional regions and myotendinous junctions in innervated muscles, appears in denervated muscles in restricted perijunctional regions that are continuous with and centered on endplates. In the present study we examine the roles of the synaptic basal lamina and of innervation in directing the accumulation of the molecule in newly formed regenerating muscle fibres. In denervated regenerating muscle fibres, mAb 3B6 antigen was associated with the plasma membrane and localized at former junctional and perijunctional regions. In those muscle fibres which displayed the perijunctional distribution, the molecule was preferentially colocalized with and centered on former endplate areas, Altogether, a preference for the localization of mAb 3B6 at former endplate regions was observed in 86–90% of denervated regenerating myofibres. A similar preference was observed in 97–99% of innervated regenerating muscle fibres. However, whereas 85.9% of denervated regenerating muscle fibres displayed a perijunctional distribution of the molecule, only 50.5% of innervated regenerating myofibres exhibited a perijunctional distribution. In addition, mAb 3B6 antigen was detected in the cytoplasm of most of the denervated regenerating myofibres but in none of the innervated ones. These results indicate that the basal lamina directs the preferential accumulation of mAb 3B6 antigen at original synaptic sites. Innervation, which is not a prerequisite for the expression of the molecule by regenerating muscle, down-regulates its overall production and presence in perijunctional regions.     

Innervation of muscle receptors in the cross-reinnervated soleus muscle of the cat

It has recently been reported (Gregory et al., J. Physiol., 331:367-383, 1982) that cutting a muscle nerve and letting it grow back into the muscle or cross-uniting the muscle with a foreign nerve results in major disruption of the normal response patterns of muscle spindles and tendon organs. Here we report observations on the structure of muscle receptors in cross-reinnervated and self-reinnervated soleus muscles in an attempt to detect abnormalities that might account for their disturbed function. Eight soleus muscles were reinnervated with the extensor digitorum longus nerve for periods up to 449 days and two were self-reinnervated. Following the physiological investigation, the muscle was fixed and stained according to the method of Barker and Ip (J. Physiol., 69:73P-74P, 1963). Spindles and tendon organs were teased from the muscle and photographed. In one cross-reinnervated muscle an attempt was made to isolate all receptors. About two-thirds of the normal number of spindles and tendon organs were found. Three categories of receptor were identified: normal, abnormal, and those having no visible nerve endings. There appeared to be little difference in degree of abnormality of receptors in self- and cross-reinnervated muscles. Of the 180 spindles, 3% were normal, 43% had no visible endings, and 54% had abnormal endings. Of 80 tendon organs, 38% were normally innervated, 33% were without visible innervation, and 29% had abnormal endings. We conclude that following long-term cross-reinnervation and self-reinnervation of soleus there is extensive disruption of the normal innervation pattern of both spindles and tendon organs which could account for their functional abnormalities.     

Comparison of physiological and histochemical properties of motor units after cross-reinnervation of antagonistic muscles in the cat hindlimb

1. Physiological and histochemical properties of the cat ankle extensor muscles, the lateral and medial gastrocnemius, and the soleus were studied after cross-reinnervation by flexor motoneurons. 2. Tibial and common peroneal nerves were cut and cross-united in the popliteal fossa of 2- to 6-mo-old cats. Eighteen to 24 mo later, single motor units were isolated by dissection and stimulation of ventral root filaments and classified into four types: fast-twitch, fatigable (FF), fast-twitch with intermediate fatigue resistance (FI), fast-twitch, fatigue-resistant (FR), and slow, fatigue-resistant (S). Muscle fibers were classified as fast glycolytic (FG), fast, oxidative glycolytic (FOG), and slow oxidative (SO) on the basis of histochemical staining. 3. Although motor-unit force was normally well correlated with the size of the innervating motor axon in the cross-reinnervated muscles, the force of different unit types overlapped considerably. The reinnervated motor units also showed a higher than normal degree of fatigability. 4. The range of muscle unit forces in cross-reinnervated triceps surae muscles was the same as in the normally innervated triceps surae muscles. This range is 2-3 times greater than the flexor muscles, which the common peroneal nerve normally supplies. The range of contraction speed of units in the cross-reinnervated extensor muscles was comparable to that in the flexor muscles, consistent with a motoneuron-specific determination of muscle speed (28). 5. SO and FOG muscle fibers were found in all reinnervated triceps surae muscles, but FG fibers were only found in reinnervated medial gastrocnemius (MG) and lateral gastrocnemius (LG) muscles, consistent with previous findings of the resistance of soleus muscles to complete conversion (10, 16, 20, 21). Type grouping of muscle fibers was characteristic of the reinnervated muscles. 6. Reinnervated SO muscle fibers were larger than the corresponding fibers in normally innervated muscles as were the estimated number of muscle fibers innervated by slow motor axons. Nonetheless, the force generated by the S motor units remained relatively smaller than FR and FF units. The relative contributions of the number, cross-sectional area and specific tension to the force generation of reinnervated motor units are discussed.     

Effects of chronic stimulation on the size and speed of long-term denervated and innervated rat fast and slow skeletal muscles

This study seeks to identify the mechanisms which motoneurones use to control the contractile force and speed of skeletal muscles. We have stimulated directly slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles of adult rats intermittently at 100 Hz for 1-9 months. The muscles were either chronically denervated, denervated and reinnervated, or normally innervated. The stimulation started either immediately, or more commonly, after 1-9 months of denervation. Stimulation starting several months after denervation increased the mean maximum tetanic tension 37 times in SOL and eight times in EDL. These values represented 40 and 12% of the increases obtained by reinnervation after comparable periods of time. In denervated SOL and EDL muscles stimulated directly for more than 2 months, the mean isometric twitch contraction times were 13 and 12.7 ms, as in normal EDL muscles (13 ms). In innervated SOL muscles stimulated directly for 1-4 months, the mean twitch contraction times were 23.6 ms (normally innervated) and 19.2 ms (reinnervated), which were considerably shorter than in normal control SOL muscles (39.2 ms). Single motor unit recordings revealed that the natural (background) nerve impulse activity was essentially unaffected by the stimulation. Twitch contraction time and percentage of type II fibres in SOL muscles were related. The fastest muscles (denervated and stimulated) consisted of 100% type II fibres (with one exception), the second fastest (reinnervated and stimulated) of 70-50%, the third fastest (normally innervated and stimulated) of 45-0%, the second slowest (reinnervated) of 15-0%, and the slowest muscles (innervated controls) of 5-0% type II fibres.     

The role of muscle activity in the differentiation of neuromuscular junctions in slow and fast chick muscles

Summary The differentiation of neuromuscular junctions of multiply innervated, slow, anterior latissimus dorsi (ALD) and focally innervated, fast, posterior latissimus dorsi (PLD) muscles was studied in normal and curarized chick embryos. At 16 days of incubation, fibres of both muscles are contacted by several axon profiles, the number of which falls with age. In 18-day-old embryos individual endplates in ALD are usually contacted by three axon profiles, whereas in PLD, endplates are contacted only by a single large terminal profile. At this time, there is already a significant accumulation of cell organelles in the postsynaptic area.Treatment of embryos with curare during the 7th and 12th day of incubation delays the differentiation of the neuromuscular junction in both muscles. The paralysis dramatically affects the decrease of the number of axon profiles at individual endplates in both muscles. At 16 days the number of axon profiles was greater in embryos treated with curare than in the untreated controls. At 18 days when the number of axon profiles normally decreases, the endplates of both types of curarized muscles have an even greater number of axon profiles than at 16 days. Endplates in curarized PLD had up to 13 and in curarized ALD up to 12 axon profiles. The effects of curare gradually wore off and when the movements of the embryos again became more vigorous, the normal differentiation of neuromuscular junctions continued. At 21 days of incubation many embryos recover from curare and show endplates of normal appearance in both muscles. These results suggest that activity of the muscle is essential for the maturation of the neuromuscular junctions.     

The formation of synapses in reinnervated mammalian striated muscle

1. The hemidiaphragm of the adult rabbit has a single band of end-plates running around the middle of the muscle. A study has been made of the formation of synapses during spontaneous reinnervation of this muscle, using histological, ultrastructural and electrophysiological techniques.2. Following spontaneous reinnervation, silver-stained nerve terminals were found in association with cholinesterase-stained end-plates only in the region of the muscle corresponding to the original innervation band.3. The regenerated nerve terminals were observed with the electronmicroscope in positions overlying or adjacent to the old synaptic folds.4. Spontaneous miniature end-plate potentials and evoked synaptic potentials were recorded only in the middle of the muscle fibres after reinnervation.5. The growth of the regenerating axons was not oriented towards the end-plate zone but followed muscle fibres and blood vessels in random directions.6. It is concluded that, in adult mammalian striated muscle, the old end-plate region is preferentially reinnervated as a consequence of some special property of the muscle fibre at this site.     

Influence of phospholipase C on some electrical properties of the skeletal muscle membrane

1. A study was made of the effects of phospholipase C (PhC) on the resting membrane potential, input resistance, action potential and acetylcholine sensitivity of innervated and chronically denervated single muscle fibres of the rat.2. In doses higher than 1.5 mug/ml. PhC significantly increased the ionic permeability of the muscle membrane (indicated by a fall in input resistance) and reduced the resting membrane potential of innervated fibres. The ;fast' or ;white' extensor digitorum longus muscle was the most sensitive and the ;slow' or ;red' soleus the least sensitive muscle. A similar difference was observed among ;fast' and ;slow' muscles of the chicken, the posterior latissimus dorsi being far more sensitive than the ;slow' anterior latissimus dorsi muscle. Chronically denervated muscles were more resistant to these actions of PhC than innervated ones but even after denervation the ;fast' muscles remained more sensitive than the ;slow' muscles.3. The action-potential generating mechanism in the ;fast' and ;slow' muscle was completely and irreversibly blocked by 1.5 mug/ml. of PhC within 1 hr. Furthermore the innervated and chronically denervated muscles were equally sensitive to this effect of PhC. The enzyme caused a gradual increase in the threshold for excitation, reduction in the rate of rise and the amplitude of the action potential. The input resistance and the resting membrane potential were not reduced by this dose of PhC.4. Acetylcholine sensitivity of chronically denervated muscles was not affected by PhC in doses that abolished the electrical excitability of the membrane. When PhC reduced the input resistance and the resting membrane potential a decrease was also observed in the response to applied acetylcholine.5. The results suggest that PhC by acting at the polar heads of membrane phospholipids interferes with the ionic carrier mechanism which generates the action potential. The differences in sensitivity between ;fast' and ;slow' muscles and between innervated and chronically denervated ones are tentatively explained on the basis of heterologous membrane phospholipids and/or variations in their stereochemical arrangement.     

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.     

Innervation by Nerve Implants of “Fast” and “Slow” Skeletal Muscles of the Rat

The time course of innervation by nerve implants of the fast contracting flexor digitorum longus and the slow contracting soleus muscles were studied by the recorcling of isometric twitch tensions in response to stimulation of the implanted nerve and by histochemical visualization of newly-formed end-plates. When a foreign motor nerve was implanted into a muscle 30 days prior to sectioning of the original nerve supply, synaptic contacts were established 4–8 days after the section of the nerve, while, with simultaneous implantation and denervation, 16 days were required for the implanted nerve to innervate the muscle. Implantation of the nerve which normally innervates the flexor digitorum longus muscle into the soleus altered the contraction and relaxation speeds of the soleus muscle towards those of a fast contracting muscle, the change being apparent as soon as 4–8 days following sectioning of the original nerve. In muscles with nerve implants and intact normal innervation hypertrophy occurred, amounting to 20–30 per cent within one month.     

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.     

An electrophysiological and morphological study of normal and denervated chicken latissimus dorsi muscles.

1. Some electrophysiological and morphological properties of 'fast' (singly innervated) and 'slow' (multiply innervated) muscle fibres were studied in normal and denervated posterior and anterior latissimus dorsi muscles of the young chickens. 2. Normal singly and multiply innervated muscle fibres are capable of generating action potentials which in all qualitative respects are similar. 3. The action potentials of multiply innervated muscle fibres are of lower amplitude and slower maximum rate of rise than action potentials in singly innervated muscle fibres. 4. Denervation causes the resting membrane potential and the maximum rate of rise of the action potential to fall. The changes are greater in singly innervated than in multiply innervated fibres, but in neither case are as great as in mammalian skeletal muscle fibres after surgical denervation. 5. In neither singly nor multiply innervated muscle fibres do the action potentials generate any 'resistance' to tetrodotoxin as a result of denervation. 6. The diameter of multiply innervated fibres is increased after denervation, but it is reduced in singly innervated fibres. The number of myofilaments increases in multiply innervated fibres, but decreases in single innervated fibres. In both types of muscle fibre the volume fraction of myofibrils is decreased. 7. In the singly innervated muscle fibres there is an increase in the volume fraction of mitochondria. 8. In the singly innervated muscle fibres, there is some rearrangement of the membrane systems in that some of the transversely orientated triads are replaced by longitudinally orientated dyads.     

Differentiation of motor nerve terminals formed in the absence of muscle fibres

During reinnervation of frog skeletal muscle, axons form functional nerve terminals at original synaptic sites on denervated myofibres. When muscle is damaged as well as denervated, myofibres decompose but their sheaths of basal lamina (BL) survive. Despite the absence of myofibres, axons regenerate to contact BL and there acquire clusters of synaptic vesicles and membrane-associated dense patches that resemble active zones; BL regulates this differentiation. We show here that these BL-associated axonal segments appear smaller and contain fewer active zones than terminals on intact myofibres in the same preparation. However, terminals formed on BL sheaths are capable of activity-dependent recycling of synaptic vesicles (demonstrated by tracer uptake), and bear an antigen normally present in terminals but not preterminal axons (demonstrated by immunofluorescence). Thus, axons can acquire functional and biochemical, as well as morphological, characteristics of normal motor nerve terminals in the absence of a postsynaptic cell.