Competition between segmental nerves at end-plates in rat gastrocnemius muscle during loss of polyneuronal innervation

Three-dimensional topography of the motor endplates of the rat gastrocnemius muscle

Spatial distribution of motor endplates affects the shape of the electrical activity recorded from muscle. In order to provide information for realistic models of action potential propagation within muscles, we assembled three-dimensional maps of the motor endplates of the rat medial gastrocnemius (MGM) and lateral gastrocnemius (MGL) muscles. The maps were assembled from histological cross sections stained for acetylcholinesterase activity. Within MGL, the motor endplates formed three columns along its longitudinal axis. Within MGM, the motor endplates were arranged in a leaf-like body that shifted obliquely from proximal to distal. As inferred from the proximo-distal distribution of the cross-sectional projection area, the majority of the motor endplates were concentrated in the middle of MGL and in the distal third of MGM. Regions of maximal motor endplate concentration are considered most suitable for injections of neuroactive substances, such as neuronal tracers. The assembled maps of the gastrocnemius muscles can be used as guides for such injections within the motor endplate zones.

Development of the neuromuscular junction in extraocular muscles of white Leghorn chicks

Relatively little is known about the development of the neuromuscular junction of extraocular muscles (EOMs). In recent years, chicks have been increasingly used as a developmental model in ophthalmological research. To utilize this model system for understanding the development and plasticity of the extraocular motor system, we investigated the structural changes that occur at the developing neuromuscular junction in the chick between embryonic day 14 (E14) and posthatch day 2 (P2). Axons and nerve terminals were visualized with fluorescent neurofilament antibodies and motor endplates with rhodamine-conjugated alpha-bungarotoxin. Nerve fibers and endplates were colabeled within the same tissue samples. Motor endplates (density, length, width, and area) were measured and numbers of axons per neuromuscular junction were counted using confocal and conventional microscopy. In P2 chicks, densities of motor endplates were significantly greater in the superior oblique muscle when compared with the superior rectus and lateral gastrocnemius muscle. EOMs showed a two- to threefold larger area of motor endplate size as compared to gastrocnemius muscle. Motor endplate size also differed among EOMs with the superior oblique muscle having endplates with a larger area than those of the superior rectus muscle. The period of synapse elimination was similar between EOM and gastrocnemius muscle. Synapse elimination began at about E18 and was completed by P2. By describing the normal morphological changes in developing EOMs, this study provides a baseline for future work to elucidate underlying molecular mechanisms that regulate EOM innervation and strength.

Management of infant brachial plexus injuries

Management of brachial plexus injuries is geared toward normalization of limb function, primarily through optimization of nerve regeneration and mechanical increase in elbow flexion and shoulder stabilization. Changes in the skeletal muscles and the osteous structures of the upper extremity are ongoing throughout the course of treatment, mandating continual assessment and aggressive rehabilitation. In patients who present too late for microsurgical intervention, irreversible changes take place in skeletal muscles, highlighting the importance of early referral. However, secondary procedures have been shown to be beneficial in older patients and in those whose primary procedures failed. Further advances in bionics and stem cell therapy may help replace the dynamic functional deficits of obstetric brachial plexus palsy.

Juvenile and adult rat neuromuscular junctions: density, distribution, and morphology

Anatomical and physiological differences in neuromuscular junctions (NMJs) between juvenile and adult muscle may partially explain the variability in clinical results following chemodenervation with botulinum toxin or nerve repair in children and adults. We evaluated NMJ density, distribution, and morphometry in biceps brachii and gastrocnemius muscles from juvenile and adult rats. Motor endplates were stained with Karnovsky-Roots methods, and NMJ density (number/gram muscle tissue) was determined. The NMJ morphometry was quantitated with alpha-bungarotoxin labeling using confocal microscopy. Juvenile rats had a greater NMJ density in both muscles compared with adult rats. Juveniles and adult rats had a similar NMJ distribution in both muscles. In juvenile muscles NMJs occupied approximately 50% of the surface area and 70% of the length, width, circumference, and gutter depth compared with adult muscles. Our study demonstrates that although NMJs are smaller, juvenile muscles have a higher NMJ density than do adult muscles with similar distributions. If these age-dependent differences in NMJ density are obtained in humans, they may account, at least in part, for the better recovery that occurs in children than adults after nerve repair and also suggest that the dosage of botulinum toxin (units per kg) for chemodenervation may need to be increased in juveniles.

Spinal nerve root repair and reimplantation of avulsed ventral roots into the spinal cord after brachial plexus injury

OBJECT

The authors review the first series of 10 cases in which injured intraspinal brachial plexus were surgically repaired. They describe the technique of spinal cord implantation or repair of ruptured nerve roots, as well as patient outcome.

METHODS

Spinal root repair/implantation was performed from 10 days to 9 months postinjury. There were nine male patients and one female patient. Postoperatively in most cases, regeneration of motor neurons from the spinal cord to denervated muscles could be demonstrated. The first signs of regeneration were noted approximately 9 to 12 months postoperatively. Useful function with muscle power of at least Medical Research Council Grade 3 occurred in three of 10 cases. Magnetic brain stimulation studies revealed a normal amplitude and latency from the cortex to reinnervated muscles on surgically treated and control sides. A certain degree of cocontraction between antagonistic muscles (for example, biceps-triceps) compromised function. With time there was a reduction of cocontractions, probably due to spinal cord plasticity. In these cases there was also, surprisingly, a return of sensory function, although the mechanism by which this occurred is uncertain. Sensory stimulation (thermal and mechanical) within the avulsed dermatomes was perceived abnormally and/or experienced at remote sites. There was some return of patients' sense of joint position.

CONCLUSIONS

A short time lag between the accident and the surgery was recognized as a significant factor for a successful outcome. Reimplantation of avulsed nerve roots may be combined with other procedures such as nerve transfers in severe cases of brachial plexus injury.

Synapse formation molecules in muscle and autonomic ganglia: the dual constraint hypothesis

In 1970 it was thought that if the motor-nerve supply to a muscle was interrupted and then allowed to regenerate into the muscle, motor-synaptic terminals most often formed presynaptic specializations at random positions over the surface of the constituent muscle fibres, so that the original spatial pattern of synapses was not restored. However, in the early 1970s a systematic series of experiments were carried out showing that if injury to muscles was avoided then either reinnervation or cross-reinnervation reconstituted the pattern of synapses on the muscle fibres according to an analysis using the combined techniques of electrophysiology, electronmicroscopy and histology on the muscles. It was thus shown that motor-synaptic terminals are uniquely restored to their original synaptic positions. This led to the concept of the synaptic site, defined as that region on a muscle fibre that contains molecules for triggering synaptic terminal formation. However, nerves in developing muscles were found to form connections at random positions on the surface of the very short muscle cells, indicating that these molecules are not generated by the muscle but imprinted by the nerves themselves; growth in length of the cells on either side of the imprint creates the mature synaptic site in the approximate middle of the muscle fibres. This process is accompanied at first by the differentiation of an excess number of terminals at the synaptic site, and then the elimination of all but one of the terminals. In the succeeding 25 years, identification of the synaptic site molecules has been a major task of molecular neurobiology. This review presents an historical account of the developments this century of the idea that synaptic-site formation molecules exist in muscle. The properties that these molecules must possess if they are to guide the differentiation and elimination of synaptic terminals is considered in the context of a quantitative model of this process termed the dual-constraint hypothesis. It is suggested that the molecules agrin, ARIA, MuSK and S-laminin have suitable properties according to the dual-constraint hypothesis to subserve this purpose. The extent to which there is evidence for similar molecules at neuronal synapses such as those in autonomic ganglia is also considered.

Polyneural innervation in the psoas muscle of the developing rat

Polyneural innervation was studied in the psoas muscle in developing rats from P4 till P25 and at adult age, with the combined silver-acetylcholinesterase technique. Nerve endings were counted, and end-plates were measured. These data were compared with such data in the human. The end of polyneural innervation in the rat (around P20) and in the human (around 12 weeks postterm age) in both cases coincides with a transformation in motor behavior and postural control. The rat's psoas muscle at early stages is less heavily innervated than this muscle in the human. Up to three axons per motor end-plate were counted at P4, but in the human up to five axons at 25 weeks of post menstrual age. This difference might be related to the lower percentage of type I muscle fibers in the rat.

Stabilizing neuromuscular contacts increases motoneuron survival after neonatal nerve injury in rats

Following sciatic nerve crush at birth the rat soleus muscle is rendered permanently weak. This reduction in muscle force is caused by the loss of a proportion of its motoneurons. Furthermore, motoneurons that survive and reach the muscle fail to reoccupy a sufficient number of denervated muscle fibres to compensate for the loss of neurons. Both the loss of motoneurons and poor reinnervation may be due to the inability of the regenerating axons to establish and maintain neuromuscular contacts. Application of leupeptin, an inhibitor of a calcium-activated neutral protease and some serine proteases, is known to help in the maintenance of neuromuscular contacts during development and axonal sprouting. Here we examined whether protecting new neuromuscular contacts formed between regenerating axons and denervated muscle fibres after nerve injury, would influence the survival of motoneurons and improve muscle recovery. This study shows that in muscles treated with leupeptin the reduction in weight and force output after nerve crush at birth was significantly less than in those that were untreated. Moreover, the number of motor units in the leupeptin-treated muscles was significantly higher than in untreated muscles. Thus, treating regenerating nerve terminals with leupeptin during early stages of reinnervation rescues motoneurons and improves muscle recovery.

Evidence for compartmental identity in the development of the rat lateral gastrocnemius muscle

In adult rats, each neuromuscular compartment of the lateral gastrocnemius muscle (LG) is exclusively innervated by a primary branch of the LG nerve. In neonates, however, a small percentage of LG cells receives inputs from more than one primary nerve branch; these inputs are known as cross-compartmental. Cross-compartmental inputs are normally lost from the medial compartment of LG (LGm) by the 8th postnatal day. To investigate the mechanisms involved in the elimination of cross-compartmental inputs, muscle fibers in the LGm compartment were denervated by cutting the LGm nerve branch in 1-4 day old rat pups and in adult rats. We then assessed the degree of cross-compartmental innervation within the "denervated" compartment using intracellular recordings from neonatal muscle fibers or immunohistochemical staining for nerve cell adhesion molecule (N-CAM) and neurofilament protein in adult muscles. Following LGm axotomy in neonates, cross-compartmental innervation is more extensive than in controls and is present as late as 20 days after birth. Thus, in the absence of "native" LGm axons, neonatal cross-compartmental inputs proliferate by axonal sprouting and the formation of new synapses on vacant LGm fibers. In contrast, axotomized adults do not form new cross-compartmental inputs over the same time period. The differential response of neonates and adults to muscle nerve branch denervation is evidence for the existence of some form of compartment-specific recognition. We propose that compartmental identity either arises or becomes relatively more potent during ontogeny and normally acts selectively to eliminate foreign axons and deter the formation of new cross-compartmental inputs.

Influence of muscle cell substrates on differentiation of intrafusal fiber types in neonatal rats

Rat muscle spindles contain one nuclear bag2, one nuclear bag1, and two nuclear chain fibers. The three different types of intrafusal fiber in spindles may be a reflection of concomitant changes in proportions of slow primary, slow/fast secondary, and fast secondary myotubes during the period of spindle development. We examined whether experimentally altering the available muscle substrates would impact the intrafusal fiber type composition of spindles. De novo formation of spindles in muscles devoid of primary myotubes was induced by crushing the nerve to the medial gastrocnemius muscle in newborn rats and administering nerve growth factor for ten days afterwards. Encapsulated fibers of the reinnervated muscles examined one month after nerve crush had myofibrillar adenosine triphosphatase and myosin heavy chain profiles similar to normal bag2, bag1, or chain intrafusal fibers. However, spindles in reinnervated muscles contained fewer fibers than controls. Most experimental spindles contained chain and/or bag1 fibers, the two fiber types which ordinarily arise during secondary myogenesis. In contrast, bag2 fibers, fibers that normally form concomitant with primary myogenesis, were absent from nearly 90% of spindles in reinnervated muscles. The paucity of bag2 fibers may reflect the absence of primary myotubes, whereas the prevalence of chain and/or bag1 fibers may reflect that secondary myotubes or myofibers that descended from the secondary myotubes were the principal muscle substrates available for spindle formation in the nerve-crushed muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the electromyogram of two major hindlimb muscles during locomotor development in the rat

The development of the electromyogram (EMG) of tibialis anterior (TA) and medial gastrocnemius (GM) during locomotion was studied in normal rats from the onset of quadruped walking (postnatal day 10, P10) until P42. The objectives were to relate signal properties of the EMG and coordination of muscle activity to functional development of the hindlimb, which proceeds rapidly around P15. Both the EMG characteristics and the activation pattern showed marked changes with age. Initially, the EMG bursts were irregular and protracted. The activity level in the two muscles, in particular in GM, seemed to be low. Until P14, the motor units of GM showed a tendency towards synchronization. The EMG of TA consisted of an adult-like interference pattern from the youngest age studied. Although co-contraction of TA and GM was sometimes observed until P14, reciprocal activation of the muscles was evident at all ages. The timing of the alternating pattern became more accurate with age. The activity level in both muscles increased markedly from P15. These changes were reflected in the form factor (coefficient of variability) of the EMG and, to a lesser extent, in the power spectra. The time course of these changes bears a close temporal relationship to the development of locomotion. These results suggest that the degree of muscle activation is a decisive factor with respect to locomotor development. It is proposed that this is related to the maturation of supraspinal control.

Tenotomy delays both synapse elimination and myogenesis in rat lateral gastrocnemius

To investigate a possible relationship between synapse elimination and myogenesis, we examined both phenomena during the first 2 weeks of postnatal life in the rat lateral gastrocnemius muscle. Synapse elimination and myogenesis occur simultaneously. Sixty per cent of the number of fibers observed in adult muscles is generated during the first 10 days of postnatal life; during this time, the majority of muscle cells in lateral gastrocnemius also become singly innervated. We delayed synapse elimination by cutting the tendon of insertion of lateral gastrocnemius (tenotomy) on the day of birth. Both synapse elimination and postnatal myogenesis were slowed by tenotomy. Tenotomized muscles contained fewer detectable cells than unoperated contralateral control muscles. These results suggest that synapse elimination may be altered by altering postnatal muscle fiber addition.

The development of locomotion in the rat

The development of free walking was studied in rats between postnatal days 10 and 20. Spontaneous quadruped walking with the ventral surface of the body off the floor was first observed at postnatal day 11. Locomotion remained clumsy and invariably slow during the next few days, but a rapid transformation into the mature pattern of locomotion occurred around postnatal day 15. This transformation involved changes in quantitative parameters of locomotion as well as a change in the movement pattern of the hindlimb. A swimming-like movement characterized by abduction, rotation and hyperextension of the paw was replaced by the digitigrade adult pattern without marked rotation. Joint angle trajectories of the major joints during the step cycle changed considerably during the transitional period. The results, which are to serve as a framework for ongoing research into the effects of early undernutrition and movement restriction upon motor function, are discussed in the perspective of developmental changes in the nervous and musculoskeletal system.

Topographic organization of the peripheral projections of the trigeminal ganglion in the fetal rat

Retrograde tracing with true blue (TB) and diamidino yellow (DY) was used to determine the topography of the peripheral projections of the trigeminal (V) ganglion in rats on embryonic day 16 (E-16; E-0 was the day of conception). On E-16, the earliest age at which we were able to accomplish retrograde tracing successfully, the topographic organization of the V ganglionic projection to the periphery was quite adult-like. Cells projecting to the vibrissa pad were restricted to the ophthalmic-maxillary portion of the ganglion, with those innervating dorsal row follicles located medially and those supplying ventral row follicles located laterally. Injections of tracer into ophthalmic skin and/or the cornea labeled cells that were tightly clustered in the most dorsal and anteromedial portion of the ophthalmic-maxillary region. Injections of tracer into the lower jaw or the skin just rostral to the ear labeled cells that were restricted to the lateral, mandibular part of the ganglion. None of the combinations of injections we carried out resulted in large numbers of double-labeled V ganglion cells. Injection of TB into the vibrissa pad and DY into the upper lip produced a small number of double-labeled ganglion cells. This was also the case for paired injections of TB and DY into the lower jaw and lip, respectively. No more than 15 such cells were observed in a ganglion. These findings suggest that the substantial cell death that has been reported to occur in prenatal V ganglion development (Davies and Lumsden, 1984) is probably not involved in the correction of major peripheral targeting errors by the axons of V ganglion cells.

Compartmental and topographical distributions of axons in nerves to the amphibian (Bufo marinus) glutaeus muscle

The present work seeks to determine if axons to an amphibian muscle are segregated in nerve trunks between the spinal cord and muscle according to their primary nerve destination or their topographical projection in the muscle. The distribution of axons to different compartments and subcompartments of the amphibian (Bufo marinus) glutaeus muscle has been determined in transverse sections of spinal and limb nerves after retrogradely labelling the axons with horseradish peroxidase. Glutaeus axons were dispersed widely through spinal nerves 8 and 9 but loosely gathered together in one quadrant of the sciatic nerve after passing through the lumbar plexus. Glutaeus axons became tightly clustered to the exclusion of other axons along the length of the triceps femoris nerve after it divides from the sciatic nerve. Furthermore, axons destined for one of the two glutaeus primary nerve branches segregate from those of the other branch at the level of the triceps femoris nerve before the glutaeus nerve forms. On the other hand, motoneurones that subserve a primary branch are not segregated, but are found throughout the rostrocaudal extent of the glutaeus motoneurone pool. Injection of horseradish peroxidase under the epimysium of either the ventral or the dorsal surfaces of the glutaeus muscle labelled motoneurones preferentially in either the rostral or caudal part of the motoneurone pool, respectively. This confirms studies that have shown a topographical projection from the spinal motoneurone pool onto the glutaeus muscle. However, there was no segregation of dorsally projecting axons in the glutaeus and primary nerve branches. Thus, glutaeus axons segregate according to their muscle compartmental projections well before entering the muscle, but they show no organization in nerves with respect to their topographical projections within a compartment.

Use of formamide increases the number of detectable inputs to polyneuronally innervated mammalian skeletal muscle

We evaluated the extent of polyneuronal innervation in neonatal rat lateral gastrocnemius muscle with intracellular recording techniques using both formamide and d-tubocurarine as paralytic agents. We detected more polyneuronal innervation during the first postnatal week using formamide than d-tubocurarine. Both the average number of inputs per muscle cell and the percentage of polyinnervated cells were greater when formamide, rather than d-tubocurarine, was used to diminish muscle contraction. The difference in the extent of polyneuronal innervation detected using the two methods declines during the first postnatal week and the number of inputs observed with formamide during the second postnatal week does not differ from that seen with d-tubocurarine.

Lack of fiber type selectivity during reinnervation of neonatal rabbit soleus muscle

Fast and slow contracting fibers in neonatal mammalian skeletal muscle are each innervated in a highly specific manner by motor neurons of the corresponding type, even at an age when polyinnervation is widespread. Chemospecific recognition is a possible mechanism by which this pattern of innervation could be established. We have investigated this possibility by studying the degree of specificity during reinnervation of rabbit soleus muscle following nerve crush on Postnatal Day 1 or 4. We assayed fiber type composition by measuring the twitch rise times of motor units within 2 days of the onset of functional reinnervation (5-6 days after nerve crush). In contrast to the broad, bimodal distribution of single motor unit twitch rise times seen in normal muscles, motor units in reinnervated muscles yielded a narrower, unimodal distribution of rise times. Rise times of reinnervated units were intermediate to those of normal fast and slow units, suggesting that reinnervated units were composed of a mixture of fast and slow contracting fibers. An alternative possibility, that specific reinnervation was masked by contractile dedifferentiation of muscle fibers, was examined by maintaining a transmission blockade induced by botulinum toxin poisoning for an equivalent interval. Twitch rise times of treated motor units exhibited the distinctly bimodal distribution characteristic of normal muscles, suggesting that muscle fibers can retain contractile diversity during a transient period of denervation. We carried out computer simulations to estimate the amount of rise time diversity induced by varying degrees of specificity during reinnervation. Based on this analysis, we conclude that there is little if any selective reinnervation of muscle fiber types at the ages studied.

The development of topographical maps and fibre types in toad (Bufo marinus) glutaeus muscle during synapse elimination

1. The toad glutaeus muscle consists of two muscle compartments. A study has been made of the topographical distribution of motor units in these compartments, in relation to the fibre types which arise during different stages of development. 2. Monoclonal antibodies to myosin allowed the distribution of fibre types to be determined. In mature muscles (from toads of greater than 30 g body weight) clusters of type 5 (tonic) fibres were found exclusively at the dorsal surface of the muscle, surrounded by a layer of type 3 (slow-twitch) fibres. A homogeneous layer of type 2 (fast-twitch red) fibres was found beneath this dorsal rind of slow and tonic fibres. The rest of the muscle, including the ventral surface, consisted of a mosaic of type 1 (fast-twitch white) and type 2 fibres. 3. Glycogen-depletion methods, together with the myosin antibodies, allowed the distribution of single motor units and their fibre types to be determined. In mature muscles, axons originating from rostral spinal cord possessed muscle units located in a band extending from the ventral surface to beyond the middle of the muscle; these units consisted of 78% type 1 and 22% type 2 fibres found amongst the mosaic of type 1 and type 2 fibres. Intermediate axons possessed muscle units located primarily in the middle and dorsal half of the muscle. These units consisted mostly of type 2 fibres (29% type 1, 71% type 2) also found amongst the mosaic of type 1 and type 2 fibres. Thus rostral and intermediate units were of mixed fibre type, with type 1 fibres predominating in the former units and type 2 in the latter. Caudal axons possessed muscle units located mostly in the homogeneous layer of type 2 fibres, beneath the dorsal rind of tonic fibres; these units were almost always composed entirely of type 2 fibres. 4. The distribution of single motor units and their fibre types were determined for the caudal axons during development. In juvenile animals (toads of about 10 g body weight) the dorsal rind of tonic and slow fibres, together with the underlying homogeneous layer of type 2 fibres, were still present, but the rest of the muscle to the ventral surface consisted almost entirely of type 1 fibres. Caudal axons innervated the type 2 fibre layer at the dorsal surface as they do in mature animals. 5. The glutaeus in post-metamorphic toads (0.15 g body weight) had only a small number of tonic and slow-twitch fibres in the very dorsal layer of cells; the muscle was largely type 1.(ABSTRACT TRUNCATED AT 400 WORDS)

The Effect of Inhibiting the Calcium Activated Neutral Protease, on Motor Unit Size after Partial Denervation of the Rat Soleus Muscle

Rat soleus muscles were partially denervated by removal of the L5 ventral ramus at either 4 - 6 days or 17 - 19 days. Local application of leupeptin, a potent inhibitor of the calcium activated neutral protease to these operated muscles, resulted in a significantly greater maximal tetanic tension and motor unit size, when compared to untreated partially denervated muscles. This was achieved in the 4 - 6 day operated animals by an increased number of terminals and in the 17 - 19 day old animals by increased number of axonal sprouts that maintain contact with muscle fibres. In both groups of operated animals in the leupeptin treated muscles large numbers of motor units were able to maintain or achieve an expanded territory, whilst the size of the largest motor unit did not appear to be increased. It is proposed that leupeptin exerts its effect by inhibiting the degradative action of the neuronal calcium activated neutral protease on the axonal cytoskeleton. Such inhibition may act to prevent or decrease the degradation of cytoskeletal structures in the nerve terminal, and so provide protection for weak terminals at a synapse and growth cones of sprouting axons following partial denervation.

On the role of the 200-kDa neurofilament protein at the developing neuromuscular junction

To examine whether the 200-kDa neurofilament protein (200K NFP) is involved in mechanically stabilizing axons, we studied the developmental appearance of immunoreactivity to nonphosphorylated and phosphorylated 200K NFP at the neuromuscular junction. Polyinnervated rat muscle fibers become singly innervated during the first 3 weeks of postnatal life through the process of synapse elimination. If production or post-translational modification of the 200K NFP is actively involved in imparting mechanical stability on neuromuscular synapses, then the selective presence of this protein in only one of several axons at each developing end plate region might make that one axon selectively resistant to elimination. The remaining axons would then be eliminated. Immunoreactivity to the 200K NFP is present on Gestational Day 14 and can be seen in more than one preterminal axon in the end plate region of a muscle fiber during the period of synapse elimination. These results suggest that the 200K NFP is present and phosphorylated early in development and, although the 200K NFP may increase the mechanical stability of axons, this increased stability does not determine the final outcome of synapse elimination.

Synaptic competition and the elimination of polyneuronal innervation following reinnervation of adult frog sartorius muscles

The elimination of polyneuronal innervation (synapse elimination) that occurs following reinnervation was studied in sartorius muscles of adult Rana pipiens. The percentage of neuromuscular junctions that were polyneuronally innervated declined from 47% at 40-80 days after nerve crush to 22% at greater than 250 days after nerve crush. We measured the size, synaptic strength, and position of competing nerve terminals at identified dually innervated neuromuscular junctions at these two different periods of synapse elimination. Our goal was to determine if any of these parameters play a role in the competition between nerve terminals that ultimately results in the elimination of polyneuronal innervation. Our data support the hypothesis that polyneuronal innervation will persist if competing nerve terminals are of similar synaptic efficacies but will be eliminated if the competing terminals are of different synaptic efficacies. We also tested, but failed to find any evidence, that the spatial proximity of competing nerve terminals at the same synaptic site influences the elimination of polyneuronal innervation.

The formation of topographical maps in developing rat gastrocnemius muscle during synapse elimination

1. The rat lateral gastrocnemius muscle (LG) is a complex of four muscle compartments, each defined in terms of its unique innervation by a single primary nerve branch of the muscle nerve. A study has been made of the topographical distribution of motor units in the medial compartment of the LG (LGM) both before and after the loss of polyneuronal innervation that accompanies development. 2. Glycogen depletion methods showed that the distribution of single motor units depended on the rostro-caudal origins of their axons in the spinal cord: rostral axons possessed motor units almost exclusively confined to the medial half of the LGM; intermediate axons possessed motor units primarily in the intermediate and lateral part of the LGM; caudal axons possessed motor units that were not restricted to any particular part of the LGM. 3. Myosin ATPase staining showed that about 80% of the LGM consists of type II A fibres, whilst the remainder are type II B. Physiological determination of the contractile properties of motor units indicated two classes of units: those that were relatively fatigue resistant and did not show a sag property (like fast-twitch, fatigue-resistant fibres or FR) and those that were relatively fatigable and did show a sag property (like fast-twitch, fatigable fibres or FF). 4. Glycogen depletion was also used to determine the distribution of motor units in the LGM at 7 days post-natal, when most fibres still receive a polyneuronal innervation. The LGM primary nerve branch innervated a confined sub-volume of muscle fibres which is similar to the mature pattern. However, rostral axons possessed motor units that extended into the lateral half of the LGM, a position from which they are excluded in the adult. 5. These observations suggest that the axons of rostral and intermediate units form a topographical map within adult FR motor units (type II A fibres) in the LGM. The results suggest that competition between axon terminals for synaptic sites plays a role in the elimination of inappropriately positioned terminals and subsequent emergence of the topographical map.

The role of synapse elimination in the establishment of neuromuscular compartments

To investigate the specificity of development of initial neuromuscular connections, we examined the compartmental distribution of synapses in neonatal rat lateral gastrocnemius (LG) muscle. Initial neuromuscular connections might be restricted to the compartmental territories present in adults; alternatively, synapse elimination could establish the compartments from a less precise pattern of innervation. We examined 46 pups of ages 0 to 14 postnatal days using a variety of techniques. The principle method was evoked electromyographic (EMG) activity in response to nerve stimulation. The nerve branch to one neuromuscular compartment was cut and the remainder of the nerve was stimulated. The presence of EMG activity was used to identify the areas of muscle contracting in response to nerve stimulation. After cutting a particular branch, EMG activity generally could not be recorded from the denervated compartment. These results indicate that the pattern of innervation at birth is essentially compartment-specific, and that neuromuscular compartments are not shaped from some less precise pattern by postnatal synapse elimination. The factors which operate prenatally to determine this high degree of specificity in neuromuscular connectivity seen at the time of birth, however, remain unknown.

Loss of polyneuronal innervation and establishment of a topographical map in the glutaeus muscle of Bufo marinus during generation of secondary muscle cells

The development of synaptic connections to the toad (Bufo marinus) glutaeus magnus from segmental nerves 8 (N8) and 9 (N9) was determined in the postmetamorphic period. Three different-size toads were studied: small (0.3-2.0 g), medium-size (5-15 g) and large (greater than 20 g). The number of cells in the glutaeus increased about 9-fold during development; this involved the appearance and subsequent maturation of secondary fibres throughout the muscle. The glutaeus in small toads, which consisted almost entirely of primary fibres, was innervated to a similar extent by N8 and N9 as assessed by tetanic contraction measurements. During late development there was a progressive increase in the percentage of the muscle innervated by N9 and a decrease in the percentage innervated by N8. This change in the segmental innervation was accompanied by changes in the innervation of the ventral glutaeus as assessed by intracellular recording. In small toads this surface of the muscle was innervated predominantly by N8, with N9 frequently appearing as a low-efficacy terminal on dually innervated fibres. With further development there was a progressive reduction in the percentage of dually innervated fibres and a concomitant decrease in the percentage innervation of the entire ventral glutaeus by N8. These results suggest that the topographical projection is established by the initial distribution of N9 terminals on the primary fibres of the muscle. The multiple innervation of newly generated fibres and the on-going process of terminal elimination results in N9 terminals, many of which were initially weak, preserving their position in the muscle. This occurs at the expense of N8 terminals, whose relative incidence declines during development. The competitive advantage of N9 motoneurones may be due to their greater capacity to lay down axon collaterals and preferentially innervate newly generated fibres; alternatively N9 terminals may displace N8 terminals, which were initially more efficacious, from dually innervated fibres. Secondary muscle fibres generated throughout the muscle are thus incorporated into an increasingly precise topographical map.