Presynaptic dense bars at neuromuscular synapses of the lobster,Homarus americanus

Summary The threedimensional ultrastructure of presynaptic dense bars was examined by serial section electron microscopy in the excitatory neuromuscular synapses of the accessory flexor muscle in the limbs of larval, juvenile, and adult lobsters. The cross-sectional profile of the dense bar resembles an asymmetric hourglass, the part contacting the presynaptic membrane being larger than that projecting into the terminal. The bar has a height of 55–65 nm and varies in length from 75–600 nm. In its dimensions it resembles the dense projections in the synapses of the CNS of insects and vertebrates. The usual location of these dense bars is at well defined synapses, though a few are found at extrasynaptic sites either in the axon or terminal. In the latter case the bars are close to synapse-bearing regions, particularly in the larval terminals, suggesting that the extrasynaptic bars denote early events in synapse formation. In all cases the bars are intimately associated with electron lucent, synaptic vesicles located on either side, in the indentation of its hourglass-shaped cross sectional profile. The vesicles occur along the length of the bar and contact the presynaptic membrane. Consequently the dense bar may serve to align the vesicles at the presynaptic membrane prior to exocytosis. A similar role has been suggested for the presynaptic dense bodies at the neuromuscular junction of the frog, where synaptic vesicles form a row on either side of this structure.Supported by Muscular Dystrophy Association of Canada and NSERCC. Generous use of laboratory facilities at Woods Hole was provided by the late Fred Lang     

Differences in synaptic output between excitatory and inhibitory motoneurons in a crayfish muscle

A pair of antagonistic motoneurons, one excitatory and one inhibitory, innervates the distal accessory flexor muscle in the walking limb of the crayfish Procambarus clarkii. The number and size of synapses formed by these two axons on the muscle fibers (neuromuscular synapses) and on each other (axo-axonal synapses) were estimated using thin-section electron microscopy. Although profiles of nerve terminals of the two axons occur in roughly equal proportions, the frequency of occurrence of neuromuscular synapses differed markedly: 73% were excitatory and 27% were inhibitory. However, inhibitory synapses were 4–5 times larger than excitatory ones, and consequently, the total contact areas devoted to neuromuscular synapses were similar for both axons. Axo-axonal synapses were predominantly from the inhibitory axon to the excitatory axon (86%), and a few were from the excitatory axon to the inhibitory axon (14%). The role of the inhibitory axo-axonal synapse is presynaptic inhibition, but that of the excitatory axo-axonal synapse is not known. The differences in size of neuromuscular synapses between the two axons may reflect intrinsic determinants of the neuron, while the similarity in total synaptic area may reflect retrograde influences from the muscle for regulating synapse number.     

Synaptic plasticity at the crayfish opener neuromuscular preparation

The crayfish opener neuromuscular preparation exhibits most of the plasticities yet described for any synapse, including facilitation, long-term potentiation, presynaptic inhibition, and modulation. Since the presynaptic terminals and postsynaptic muscle fibers can both be intracellularly penetrated, one can now more easily examine the cellular/molecular bases for these plasticities. Data from such studies suggest that facilitation may be influenced by something other than residual free calcium and that presynaptic inhibition is produced by a conductance increase to chloride in the terminals of the excitor axon. Several drugs (ethanol, pentobarbital) have significant effects on these synaptic plasticities over concentration ranges which produce obvious behavioral effects in crayfish and mammals. Hence, this preparation should be a useful model system to determine cellular/molecular bases for various synaptic plasticities and the effects of drugs on these plasticities.     

Neuromuscular synapses on the dactyl opener muscle of the lobster Homarus americanus

The crustacean dactyl opener neuromuscular system has been studied extensively as a model system that exhibits several forms of synaptic plasticity. We report the ultrastructural features of the synapses on dactyl opener of the lobster (Homarus americanus) as determined by examination of serial thin sections. Several innervation sites supplied by an inhibitory motoneuron have been observed without nearby excitatory innervation, indicating that excitatory and inhibitory inputs to the muscle are not always closely matched. The ultrastructural features of the lobster synapses are generally similar to those described previously for the homologous crayfish muscle, with one major distinction: few dense bars are seen at the presynaptic membranes of these lobster synapses. The majority of the lobster neuromuscular synapses lack dense bars altogether, and the mean number of dense bars per synapse is relatively low. In view of the finding that the physiology of the lobster dactyl opener synapses is similar to that reported for crayfish, these ultrastructural observations suggest that the structural complexity of the synapses may not be a critical factor determining synaptic plasticity.This work was supported by funds from the University of Virginia Research and Development Committee.     

Structural plasticity at crustacean neuromuscular synapses

Crustacean motor axons innervate muscle fibers via a multiplicity of synaptic terminals which release small but variable amounts of transmitter. Differences in release performance appear to be correlated with the size of synaptic contacts and presynaptic dense bars (active zones). These structural parameters proliferate via sprouting from existing synaptic terminals and relocate to ever more distal sites during development and growth of an identified axon. Moreover, alterations in number of synaptic contacts and active zones occur in adults following stimulation or decentralization, demonstrating structural plasticity of crustacean neuromuscular synapses.     

THREE-DIMENSIONAL ULTRASTRUCTURE OF THE CRAYFISH NEUROMUSCULAR APPARATUS

The synapse-bearing nerve terminals of the opener muscle of the crayfish Procambarus were reconstructed using electron micrographs of regions which had been serially sectioned. The branching patterns of the terminals of excitatory and inhibitory axons and the locations and sizes of neuromuscular and axo-axonal synapses were studied. Excitatory and inhibitory synapses could be distinguished not only on the basis of differences in synaptic vesicles, but also by a difference in density of pre- and postsynaptic membranes. Synapses of both axons usually had one or more sharply localized presynaptic "dense bodies" around which synaptic vesicles appeared to cluster. Some synapses did not have the dense bodies. These structures may be involved in the physiological activity of the synapse. Excitatory axon terminals had more synapses, and a larger percentage of terminal surface area devoted to synaptic contacts, than inhibitory axon terminals. However, the largest synapses of the inhibitory axon exceeded in surface area those of the excitatory axon. Both axons had many side branches coming from the main terminal; often, the side branches were joined to the main terminal by narrow necks. A greater percentage of surface area was devoted to synapses in side branches than in the main terminal. Only a small fraction of total surface area was devoted to axo-axonal synapses, but these were often located at narrow necks or constrictions of the excitatory axon. This arrangement would result in effective blockage of spike invasion of regions of the terminal distal to the synapse, and would allow relatively few synapses to exert a powerful effect on transmitter release from the excitatory axon. A hypothesis to account for the development of the neuromuscular apparatus is presented, in which it is suggested that production of new synapses is more important than enlargement of old ones as a mechanism for allowing the axon to adjust transmitter output to the functional needs of the muscle.     

Structural definition of the neuromuscular system in the swimming-paddle opener muscle of blue crabs

Blue crabs are excellent swimmers, using their highly modified last pereiopods as sculling paddles. Hence, the hypertrophied paddle opener muscle was examined for adaptations of its motor innervation by an excitor and a specific inhibitor axon. The muscle has a uniform composition of slow fibers with long (6-12 microm) sarcomere lengths. Individual fibers are richly innervated with approximately two-thirds excitatory and one-third inhibitory innervation. The profuse excitatory innervation reflects the high activity levels of this motoneuron in swimming. Adaptation to sustained activity associated with swimming is also reflected in the motor nerve terminals by a high concentration of energy source, which is equally divided between glycogen granules and mitochondria, the former providing a more rapid source of energy. The excitor axon makes predominantly neuromuscular synapses, but also a few synapses onto the inhibitor axon. The location of these excitatory axoaxonal synapses suggests regional modulation of the inhibitor axon. The specific inhibitor axon makes less than two-thirds of its synapses with the muscle fiber, regulating contraction via postsynaptic inhibition. The remaining inhibitory synapses are onto the excitor axon, signaling very strong presynaptic inhibition. Such presynaptic inhibition will effectively decouple the opener muscle from the stretcher muscle even though both are innervated by a single excitor axon.     

Regenerated synaptic terminals on a crayfish slow muscle identify with transplanted phasic or tonic axons

Phasic or tonic nerves transplanted onto a denervated slow superficial flexor muscle in adult crayfish regenerated synaptic connections that displayed large or small excitatory postsynaptic potentials (EPSPs), respectively, suggesting that the neuron specifies the type of synapse that forms (Krause et al., J Neurophysiol 80:994-997, 1998). To test the hypothesis that such neuronal specification would extend to the synaptic structure as well, we examined the regenerated synaptic terminals with thin serial section electron microscopy. There are distinct differences in structure between regenerated phasic and tonic innervation. The phasic nerve provides more profuse innervation because innervation sites occurred more frequently and contained larger numbers of synaptic terminals than the tonic nerve. Preterminal axons of the phasic nerve also had many more sprouts than those of the tonic nerve. Phasic terminals were thinner and had a lower mitochondrial volume than their tonic counterparts. Phasic synapses were half the size of tonic ones, although their active zone-dense bars were similar in length. The density of active zones was higher in the phasic compared with the tonic innervation, based on estimates of the number of dense bars per synapse, per synaptic area, and per nerve terminal volume. Because these differences mirror those seen between phasic and tonic axons in crayfish muscle in situ, we conclude that the structure of the regenerated synaptic terminals identify with their transplanted axons rather than with their target muscle. Therefore, during neuromuscular regeneration in adult crayfish, the motoneuron appears to specify the identity of synaptic connections.     

Fine structure of comparable synapses in a mature and larval lobster muscle

Neuromuscular synapses from the single excitor axon to the proximal accessory flexor muscle (PAFM) was studied by serial section electron microscopy in a 1st stage larval (< 0.1 g) and a large adult (6.8 kg) lobster. The adult innervation of a lateral and a medial fiber, physiologically identified as low- and high-output respectively, was similar in the number and mean size of synapses but had significantly larger pre-synaptic dense bars for the high-output synapses. This correlation between quantal transmitter output and pre-synaptic dense bars and the appearance of exocytotic profiles along the dense bars strongly implicates the bars as active sites of transmitter release. Moreover the mature innervation is differentiated on the basis that the percentage of dense bar area to synaptic area is 9% for the low-output type compared to 22% for its high-output counterpart. In the larval PAFM the excitatory axon has not proliferated many branches and the innervation is therefore localized to groups of fibers in the lateral, medial and central regions of the muscle rather than to individual fibers. The lateral and medial sites of innervation representing putative low- and high-output types respectively (because of their location) do not differ in the size and number of pre-synaptic dense bars thereby suggesting a similarity in quantal synaptic transmission. However the percentage of dense bar area to synaptic area is 40% for the lateral site compared to 67% for the medial site. Since this is a trend mimicking the mature innervation it shows an early stage in the differentiation of low-and high-output synapses. Furthermore the main axon provides half of the total innervation in the larval PAFM but none in the adult thereby demonstrating a restructuring of multiterminal innervation.     

Heterogeneity of excitatory synapses at the ends of single muscle fibers in lobster,Homarus americanus

Crustacean neuromuscular synapses arising from a single excitor axon are known to be well differentiated among different muscle fibers but little is known about their condition along single fibers. Focal recording techniques were used to examine the quantal transmitter release and facilitation properties of synapses in the single excitatory innervated distal accessory flexor muscle of the lobster, Homarus americanus. Synapses were reliably differentiated with respect to quantal output so that those located near the tendon end were 1.15–4.12 times greater than those at the opposite, exoskeletal end (p < 0.01, paired t-test). Regional differences were also seen in the amount of facilitation determined from twin pulse experiments. The fine structural basis for these differences was determined by serial section electron microscopy of 10-μm segments at each end to ensure that the area of focal recording was sampled. No quantitative differences were found in the terminals or synapses in the two regions. Instead, the physiological diversity was correlated with number and size of presynaptic dense bars. Thus, the tendon end had a greater number and larger mean surface area of dense bars compared to the exoskeletal end. This heterogeneity of excitatory multiterminal innervation is correlated with the axonal branching pattern. Thus, the main axon and the larger primary axon branches lie in close proximity to the tendon end of the muscle fibers, whereas the exoskeletal end is innervated by smaller secondary and tertiary axonal branches. This proximity to the large axonal branches of the higher quantal output synapses at the tendon end may be regulated by some neural influence including a timing of innervation and/or access to greater amounts of metabolites in the larger branches which may be conducive to forming high-output synapses.     

Synaptic differentiation between two phasic motoneurons to a crayfish fast muscle

Synaptic differentiation among crustacean phasic motoneurons was investigated by characterizing the synaptic output and nerve terminal morphology of the two axons to the adductor exopodite muscle in the crayfish uropod. The muscle is of the fast type with short sarcomeres (2–3 μm) and a low thin to thick filament number (6:1). On single muscle fibers, excitatory postsynaptic potentials generated by the large-diameter axon are significantly larger than those by the small-diameter axon suggesting a presynaptic origin for these differences. Nerve terminals arising from these two axons have typical phasic features, filiform shape and a low (6–8%) mitochondrial density. Synaptic contacts are similar in size between the two axons as is the length and number of active zone dense bars at these synapses. The large-diameter axon, however, exhibits a twofold larger area of nerve terminal than the small-diameter axon resulting in a higher density of synapses per muscle fiber. Hence, differences in synaptic density may in part account for differences in synaptic output between these paired phasic axons. Electronic Publication     

Structure of allotransplanted ganglia and regenerated neuromuscular connections in crayfish

In adult crayfish, Procambarus clarkii, motoneurons to a denervated abdominal superficial flexor muscle regenerate long-lasting and highly specific synaptic connections as seen from recordings of excitatory postsynaptic potentials, even when they arise from the ganglion of another crayfish. To confirm the morphological origins of these physiological connections we examined the fine structure of the allotransplanted tissue that consisted of the third abdominal ganglion and the nerve to the superficial flexor muscle (the fourth ganglion and the connecting ventral nerve cord were also included). Although there is considerable degeneration, the allotransplanted ganglia display intact areas of axon tracts, neuropil, and somata. Thus in both short (6–8 weeks) and long (24–30 weeks) term transplants approximately 20 healthy somata are present and this is more than the five axons regenerated to the host muscle. The principal neurite and dendrites of these somata receive both excitatory and inhibitory synaptic inputs, and these types of synaptic contacts also occur among the dendritic profiles of the neuropil. Axon tracts in the allotransplanted ganglia and ventral nerve cord consist largely of small diameter axons; most of the large axons including the medial and lateral giant axons are lost. The transplanted ganglia have many blood vessels and blood lacunae ensuring long-term survival. The transplanted superficial flexor nerve regenerates from the ventral to the dorsal surface of the muscle where it has five axons, each consisting of many profiles rather than a single profile. This indicates sprouting of the individual axons and accounts for the enlarged size of the regenerated nerve. The regenerated axons give rise to normal-looking synaptic terminals with well-defined synaptic contacts and presynaptic dense bars or active zones. Some of these synaptic terminals lie in close proximity to degenerating terminals, suggesting that they may inhabit old sites and in this way ensure target specificity. The presence of intact somata, neuropil, and axon tracts are factors that would contribute to the spontaneous firing of the transplanted motoneurons. © 1996 John Wiley & Sons, Inc.     

Proliferation and relocation of developing lobster neuromuscular synapses

The development of multiterminal innervation from a single identifiable excitatory motoneuron to the lobster distal accessory flexor muscle (DAFM) was studied by serial section electron microscopy. The number, size, and location of neuromuscular synapses and presynaptic dense bars within the peripheral branching pattern of the axon was determined in cross sections of the DAFM in 1st (24-hr-old)-, 4th (2-week-old)-, and 12th (1-year-old)-stage lobsters. The mean size of synapses remains fairly constant in these three stages but synaptic density, i.e., the number of synapses per unit length of fiber, increased more than 20-fold between the 1st and 4th stages and more than 5-fold between the 4th and 12th stages. Synaptic surface area per fiber length showed a parallel increase. Consequently there is a proliferation of synapses along the length of individual muscle fibers during primary development. Furthermore from the 1st stage where only a few fibers are innervated, synapses proliferate to many more fibers in the 4th and to all fibers in the 12th stage. The neuromuscular synapses are distributed in different proportions within the axonal branching pattern in the three stages. Based on the number and size of synapses and presynaptic dense bars, the main axon and primary branches provide almost equal amounts of innervation in the 1st stage. With further branching in the 4th stage, the main axon accounts for only 20–25% of the innervation; the primary branches for 45% and other finer branches the remainder. By the 12th-stage synapses are found only on branches other than the main axon and its primary offshoots. There is therefore a shift in innervation from the main axon to the primary branches and then to the finer branches during primary development. This shift in innervation involves the formation of new synaptic terminals and the restructuring of existing ones into axonal areas. In this way the multiterminal innervation arising from an identifiable motoneuron is remodeled.     

Some observations on the fine structure of the giant synapse in the stellate ganglk of the squid,Doryteuphis bleekeri

Summary The fine structure of the synapse between the second-order giant fibre and the third order-giant fibre of the squid Doryteuphis bleekeri was studied by means of electron microscope. In the synaptic region, the two giant fibres are arranged side by side. Many small processes from the third-order giant fibre penetrate the common sheath which separats the adjacent giant axons making synaptic contact with the second order giant axon.The contact surface consists of opposing two plasma membranes of adjacent axons separated by a narrow space of 20–30 m in width. The synaptic membranes are more electron dense and thicker than the other part of the axon membrane. The synaptic vesicles are concentrated exclusively in the presynaptic axon.The fine structural differences between giant synapse in the stellate ganglion of the squid and the giant-to-motor giant synapse of the crayfish were discussed.This work was supported by Grant Number B-3348 from the National Institutes of Health, United States Public Health Service, Department of Health, Education and Welfare.     

Depressing Effects of Caffeine at Crayfish Neuromuscular Synapses I. Dosage Response and Ca<Superscript>++</Superscript> Gradient Effects

The response of crayfish synaptic terminals to drugs began to be studied to characterize the terminal’s physiological characteristics. Caffeine, the first drug to be studied, was selected to enhance synaptic transmission because of its ability to increase calcium release from internal stores. 1. The largest excitor neuron to the superficial flexor muscle system of Procambarus clarkii was stimulated at 10 Hz while recording junction potentials from several lateral muscle fibers. 2. Caffeine unexpectedly decreased synaptic transmission in this system in a dosage-dependent manner. The depressing effect of caffeine was observed at 5 mM caffeine and junction potentials disappeared completely at 50 mM. Washing the preparation in fresh control Ringers did not restore the amplitudes of the junction potentials. 3. Changes in extracellular calcium concentrations delayed or depressed the caffeine effect depending on the calcium gradient across the membrane or the caffeine dosage. The data suggest that calcium is involved in caffeine’s response in this system in a way yet to be determined.     

Fiber composition of the distal accessory flexor muscle in several decapod crustaceans

The fiber composition of the distal accessory flexor muscle (DAFM) and the branching pattern of its excitor axon were compared in several species of crabs, in the lobster and the crayfish. The muscle is composed exclusively of long sarcomere (> 6 μm) fibers and therefore of the slow type. In all the crab species, except one, there is a distal to proximal gradient of fibers with increasing sarcomere lengths; this gradient is reverse in lobsters and crayfish. A proximal to distal gradient of increasing fiber diameters occurs in the DAFM of all crab species but not in the lobster and crayfish, in which all the fibers are approximately equal in diameter. The single excitatory axon traverses the width of the DAFM and gives off primary branches on either side in the lobster and crayfish but on only one side in crabs. The hypothesis that the axonal branching pattern may govern the regional distribution of fibers with differing sarcomere lengths in proposed.     

Crustacean Neuromuscular Mechanisms: Functional Morphology of Nerve Terminals and the Mechanism of Facilitation

Two aspects of crustacean neuromuscular physiology are discussed:(1) the ultrastructural identification of the excitatory andinhibitory nerve terminals, and (2) the characteristics of,and the possible mechanisms for, facilitation. The first problem was studied in crayfish opener muscle whichhas one excitatory and one inhibitory axon. One of the nerveswas stimulated in the presence of DNP until synaptic transmissionfailed; the preparations were then fixed for electron microscopy.Whenever the excitatory nerve was stimulated, the terminalswith round synaptic vesicles were depleted while nearby terminalswith smaller elongate vesicles were normal. When the inhibitorynerve was stimulated, the converse was true. The possible reasons for the diversity in crustacean neuromuscularproperties are discussed. Large EPSP's with a high quantal content(m), appear to be produced by terminals which are invaded bya propagated spike. Small EPSP's (small m) appear to be producedby terminals which don't spike and which are depolarized bya decrementally conducted potential. There is an inverse relationshipbetween m and the amount of facilitation. The physiologicalbasis for facilitation is discussed; previous hypotheses arefound wanting and a new one is proposed, that of slow depolarization.     

The fine structure of neuromuscular junctions and contact zones between body wall muscle cells of Ascaris lumbricoides (var. suum)

Summary Neuromuscular junctions and close membrane apposition between body wall muscle cells of Ascaris lumbricoides (var. suum) have been examined with the light and electron microscopes. It was found that the body wall muscle cells send out elongate processes from their basal, myofibril containing portion to terminate on dorsal and ventral nerves. When observed with the aid of the electron microscope the neuromuscular junctions were seen to consist of several muscle cell processes in apposition to a single axon. The intersynaptic cleft was approximately 350–500 Å wide. Both the axolemma and sarcolemma were triple layered membranes which were 75–80 Å thick. Electron dense patches were observed at intervals on the apposed membranes which were due to increased thickness of the inner membrane leaflets of axolemma and sarcolemma. Muscle cell membranes, at the level of the neuromuscular junction, were in close apposition resulting in an apparently five-layered membrane complex which was 170–210 Å thick. The sarcolemmata in these regions were separated by 10–50 Å. Presynaptic axons contained mitochondria, microtubules which were 180–270 Å in diameter, and two, morphologically distinct types and sizes of synaptic vesicles. One was 200–600 Å in diameter, with a single, triple-layered membrane bounding a center of low electron density. The other was 600–1200 Å in diameter, with a single, triple-layered membrane bounding a central, electron dense granule of 500–800 Å size.The functional significances of the close membrane appositions between body wall muscle cells and of the two types of synaptic vesicles found at the neuromuscular junctions of Ascaris lumbricoides were discussed with respect to their possible role in neuromuscular physiology.Supported by U.S.P.H.S. Grant No. NB-01528 and Research Career Development Award No. 9-K3-NB-15255. — The author wishes to express his grateful appreciation for the excellent technical assistance given by Miss Gabrielle Rouiller during the course of this investigation.     

The efferent innervation of the genital chamber by an identified serotonergic neuron in the female cricket Acheta domestica

Summary The serotonergic innervation of the genital chamber of the female cricket, Acheta domestica, has been investigated applying anti-serotonin (5-HT) immunocyto-chemistry at both light- and electron-microscopic levels as well as using conventional electron microscopy. Whole mount and pre-embedding chopper techniques of immuno-cytochemistry reveal a dense 5-HT-immunoreactive network of varicose fibers in the musculature of the genital chamber. All of these immunoreactive fibers originate from the efferent serotonergic neuron projecting through the nerve 8v to the genital chamber (Hustert and Topel 1986; Elekes et al. 1987). At the electron-microscopic level, 5-HT-immunoreactive nerve terminals, which contain small (50–60 nm) and large ( 100 nm) agranular vesicles as well as granular vesicles (100nm), contact the muscle fibers or the sarcoplasmic processes without establishing specialized neuromuscular connections. In addition to the 5-HT-immunoreactive axons, two types of immunonegative axons can also be found in the musculature. By use of conventional electron microscopy, three ultrastructurally distinct types of axon processes can be observed, one of which resembles 5-HT-immunoreactive axons. While the majority of the varicosities do not synapse on the muscle fibers, terminals containing small (50–60 nm) agranular vesicles occasionally form specialized neuromuscular contacts. It is suggested that the 5-HTergic innervation plays a non-synaptic modulatory role in the regulation circular musculature in the genital chamber of the cricket, while the musculature as a whole may be influenced by both synaptic and modulatory mechanisms.Fellow of the Alexander von Humboldt-Stiftung     

Asymmetry of motor impulses and neuromuscular synapses produced in crayfish claws by unilateral immobilization

Summary The fast axon supplying the closer muscle in crayfish (Procambarus clarkii) normally fires few impulses and generates large excitatory postsynaptic potentials (EPSPs) that fatigue rapidly with repeated stimulation. When the dactyl of one claw is immobilized in the closed position, impulse production in the fast axon decreases on the immobilized side and increases on the contralateral side. On the immobilized side, EPSPs become larger but more readily depressed with repeated stimulation, while converse changes occur on the contralateral side.In order to establish whether the smaller number of impulses on the immobilized side was responsible for the changes in EPSPs, extra impulses were generated in the fast axon of immobilized claws by implanting electrodes in the claw. Raising the impulse production to equal or exceed that of the contralateral side did not prevent the changes in EPSPs produced by immobilization. Thus, it is probable that changes in the level of synaptic input to central parts of the fast closer excitor neuron are mainly responsible for altered physiological properties of peripheral synapses, rather than the fast axon's impulse traffic per se.