DEVELOPMENT OF THE NEUROMUSCULAR JUNCTION : II. Cytological and Cytochemical Studies on the Neuromuscular Junction of Dedifferentiating Muscle in the Regenerating Limb of the Newt Triturus

Following amputation of the limb of the newt, Triturus viridescens, muscle fibers dedifferentiate giving rise to mesenchymal cells. The earliest changes detected in neuromuscular junctions of dedifferentiating muscle fibers are the appearance of a few vacuoles and decrease in density of the terminal axoplasm. Later, synaptic vesicles become tightly clustered in the axon termination, and their content appears denser than normal. Then, vesicles diminish in number until few are seen in the ending. While these changes are occurring, the area of contact of nerve with muscle becomes smaller. Junctional folds persist only where the nerve maintains contact with muscle, but these are shorter than normal and appear as slight ridges on the muscle surface. Subsequently, the nerve withdraws from the muscle cell and is completely invested by Schwann cell cytoplasm, and all traces of junctional folds are lost at the former region of contact. Cholinesterase activity was localized with the thiolacetic acid-lead nitrate method. Even before marked morphological changes occur in the junction, DFP- and physostigmine-sensitive activity in the cleft between nerve and muscle is decreased in intensity. Activity continues to decrease as the area of nerve-muscle contact diminishes and junctional folds disappear. When the nerve has withdrawn from the muscle surface, only a few small deposits of lead are left in the intervening region. These results show that as muscle becomes less specialized during dedifferentiation, the neuromuscular junction also loses the cytological and cytochemical specializations associated with synaptic function.     

THE ULTRASTRUCTURE OF THE NEUROMUSCULAR JUNCTIONS OF MAMMALIAN RED, WHITE, AND INTERMEDIATE SKELETAL MUSCLE FIBERS

Distinct ultrastructural differences exist at the neuromuscular junctions of red, white, and intermediate fibers of a mammalian twitch skeletal muscle (albino rat diaphragm). The primary criteria for recognizing the three fiber types are differences in fiber diameter, mitochondrial content, and width of the Z line. In the red fiber the neuromuscular relationship presents the least sarcoplasmic and axoplasmic surface at each contact. Points of contact are relatively discrete and separate, and axonal terminals are small and elliptical. The junctional folds are relatively shallow, sparse, and irregular in arrangement. Axoplasmic vesicles are moderate in number, and sarcoplasmic vesicles are sparse. In the white fiber long, flat axonal terminals present considerable axoplasmic surface. Vast sarcoplasmic surface area is created by long, branching, closely spaced junctional folds that may merge with folds at adjacent contacts to occupy a more continuous and widespread area. Axoplasmic and sarcoplasmic vesicles are numerous. Both axoplasmic and sarcoplasmic mitochondria of the white fiber usually contain intramitochondrial granules. The intermediate fiber has large axonal terminals that are associated with the most widely spaced and deepest junctional folds. In all three fiber types, the junctional sarcoplasm is rich in free ribosomes, cisternae of granular endoplasmic reticulum, and randomly distributed microtubules.     

DEVELOPMENT OF THE NEUROMUSCULAR JUNCTION : I. Cytological and Cytochemical Studies on the Neuromuscular Junction of Differentiating Muscle in the Regenerating Limb of the Newt Triturus

Development of the neuromuscular junction on differentiating muscle was investigated in the regenerating limb of the newt Triturus. Motor end-plate formation begins when vesicle-filled axon terminations approach differentiating muscle cells that have reached the stage of a multinucleate cell containing myofibrils. Slight ridges or elevations occur on the muscle surface, and there is an increase in density of the cytoplasm immediately beneath the plasma membrane of the elevation. The axon becomes more closely approximated to the muscle cell and comes to lie in a shallow depression or gutter on the surface of the muscle. The surface ridges increase in length and constrict at their bases to form junctional folds. In the axon terminal, focal accumulations of vesicles are found where the axon contour projects slightly opposite the secondary synaptic clefts. Cholinesterase activity in the developing junctions was demonstrated by the thiolacetic acid-lead nitrate method. Enzymatic activity is not found on intercellular nerve fibers or the muscle surface prior to close approximation of axon endings and muscle. Eserine- and DFP-sensitive activity appears concurrently with morphological differentiation. Activity occurs in membranous tubulovesicles in the sarcoplasm subjacent to the neuromuscular junction and in association with the sarcolemma. The largest reaction deposits occur at the tips of the emerging junctional folds. Smaller and less numerous localizations occur on the axon membrane and within the axoplasm. It is concluded from these studies that the nerve endings have an inductive effect on both the morphological and chemical specializations of the neuromuscular junction.     

Basal lamina directs acetylcholinesterase accumulation at synaptic sites in regenerating muscle

In skeletal muscles that have been damaged in ways which spare the basal lamina sheaths of the muscle fibers, new myofibers develop within the sheaths and neuromuscular junctions form at the original synaptic sites on them. At the regenerated neuromuscular junctions, as at the original ones, the muscle fibers are characterized by junctional folds and accumulations of acetylcholine receptors and acetylcholinesterase (AChE). The formation of junctional folds and the accumulation of acetylcholine receptors is known to be directed by components of the synaptic portion of the myofiber basal lamina. The aim of this study was to determine whether or not the synaptic basal lamina contains molecules that direct the accumulation of AChE. We crushed frog muscles in a way that caused disintegration and phagocytosis of all cells at the neuromuscular junction, and at the same time, we irreversibly blocked AChE activity. New muscle fibers were allowed to regenerate within the basal lamina sheaths of the original muscle fibers but reinnervation of the muscles was deliberately prevented. We then stained for AChE activity and searched the surface of the new muscle fibers for deposits of enzyme they had produced. Despite the absence of innervation, AChE preferentially accumulated at points where the plasma membrane of the new muscle fibers was apposed to the regions of the basal lamina that had occupied the synaptic cleft at the neuromuscular junctions. We therefore conclude that molecules stably attached to the synaptic portion of myofiber basal lamina direct the accumulation of AChE at the original synaptic sites in regenerating muscle. Additional studies revealed that the AChE was solubilized by collagenase and that it remained adherent to basal lamina sheaths after degeneration of the new myofibers, indicating that it had become incorporated into the basal lamina, as at normal neuromuscular junctions.     

Effect of castration and testosterone administration on the neuromuscular junction in the levator ani muscle of the rat

Summary The ultrastructure of the neuromuscular junction (n.m.j.) of the androgen-sensitive levator ani muscle was studied in normal adult male rats, in 8-month-old rats castrated at the age of one month and in castrated rats treated with testosterone propionate (TP). Castration does not result in significant changes of the n.m.j. The density of synaptic vesicles and the postsynaptic junctional folds remain practically normal in spite of marked atrophy of the muscle. TP administration for 7 days results in marked changes in preand postsynaptic structures. There is slow progressive depletion of synaptic vesicles, appearance of cisternae and coated vesicles in axon terminals, and coalescence of coated vesicles with the plasma membrane. Coated vesicles are also found inside Schwann cells and among junctional folds. Dense core vesicles appear both in the axon terminals and in the postsynaptic area. Collateral sprouting of terminal axons with the formation of new immature junctions is observed. After 35 days of TP administration depletion of synaptic vesicles continues. Glycogen -particles, mostly freely dispersed, occasionally seen in axon terminals 7 days after TP administration, subsequently increase in number. In the endplate zone of the muscle fibre increased protein synthesis is indicated by a rapid increase in ribosomes and irregularly located myofilaments and myofibrils. The appearance of n.m.j. after testosterone administration resembles that described after nerve stimulation; the degree of change is however less pronounced.The authors wish to acknowledge the skillful technical assistance of Mrs. L. Vedralová     

Studies of excitable membranes. II. A comparison of specializations at neuromuscular junctions and nonjunctional sarcolemmas of mammalian fast and slow twitch muscle fibers

Mammalian fast and slow twitch skeletal muscles are compared by freeze-fracture, thick and thin sectioning, and histochemical techniques using conventional and high voltage electron microscopy. Despite gross morphological differences in endplate structure visualized at relatively low magnifications in this sections, rat extensor digitorum longus (EDL) (fast twitch) and soleus (slow twitch) fibers cannot be distinguished on the basis of size, number, or distribution of molecular specializations of the pre- and postsynaptic junctional membranes exposed by freeze fracturing. Specializations in the cortex of the juxtaneuronal portions of the junctional folds are revealed by high voltage electron stereomicroscopy as a branching, ladder-like filamentous network associated with the putative acetylcholline receptor complexes. These filaments are considered to be involved in restricting the mobility of receptor proteins to the perineuronal aspects of the postynaptic membrane. Although the junctional membranes of both EDL and soleus appear similar, a differential specialization of the secondary synaptic cleft was noted. The extracellular matrix in the bottom of soleus clefts was observed as an ordered system of filamentous "combs," These filamentous arrays have not been detected in EDL junctions. Examination of the extrajunctional sarcolemmas of EDL and soleus reveal additional differences which may be correlated with variations in electrical and contractile properties. For example, particle aggregates termed "square arrays" previously described in the sarcolemmas of some fibers of the rat diaphragm were observed in large numbers in sarcolemmas of EDL fibers but were seldom encountered in soleus fibers. These gross compositional differences in the membranes are discussed in the light of functional differences between fiber types.     

Acetylcholine receptors in regenerating muscle accumulate at original synaptic sites in the absence of the nerve

We examined the role of nerve terminals in organizing acetylcholine receptors on regenerating skeletal-muscle fibers. When muscle fibers are damaged, they degenerate and are phagocytized, but their basal lamina sheaths survive. New myofibers form within the original basal lamina sheaths, and they become innervated precisely at the original synaptic sites on the sheaths. After denervating and damaging muscle, we allowed myofibers to regenerate but deliberately prevented reinnervation. The distribution of acetylcholine receptors on regenerating myofibers was determined by histological methods, using [125I] alpha-bungarotoxin or horseradish peroxidase-alpha-bungarotoxin; original synaptic sites on the basal lamina sheaths were marked by cholinesterase stain. By one month after damage to the muscle, the new myofibers have accumulations of acetylcholine receptors that are selectively localized to the original synaptic sites. The density of the receptors at these sites is the same as at normal neuromuscular junctions. Folds in the myofiber surface resembling junctional folds at normal neuromuscular junctions also occur at original synaptic sites in the absence of nerve terminals. Our results demonstrate that the biochemical and structural organization of the subsynaptic membrane in regenerating muscle is directed by structures that remain at synaptic sites after removal of the nerve.     

An ultrastructural study of the polyp and strobila of Atorella japonica (Cnidaria,Coronatae) with respect to muscles and nerves

Atorella japonica were observed by TEM to examine the nerve plexus in the capitulum of the polyp and the cross-striated muscle cells of the strobila. The nerve plexus included a number of neuromuscular junctions and many interneural synapses. Neuromuscular junctions contained two types of synaptic vesicle: clear and small (ca 75 nm diam.), and dense cored and large (ca 120 nm diam.). The first type of vesicle always appeared near the presynaptic membrane and the second type was distributed behind the former. In interneural synapses, two types of vesicle which were similar to neuromuscular synaptic vesicles were recognized. They were distributed in a pattern similar to that of the neuromuscular synaptic vesicles, but these vesicles were found on both sides of the two synaptic membranes.     

DEVELOPMENT OF THE NEUROMUSCULAR JUNCTION : III. Degeneration of Motor End Plates After Denervation and Maintenance In Vitro by Nerve Explants

To determine the effects of nerve explants on the integrity of motor end plates in vitro, cholinesterase activity and structure of end plates were compared in newt muscle denervated in vivo, cultured in the absence of nerve explants, and cultured in the presence of sensory ganglia. In neuromuscular junctions denervated in vivo or in vitro, the synaptic vesicles become clumped and fragmented. A few intact vesicles escape into the synaptic cleft. Axon terminals degenerate until they are left as residual bodies within the Schwann cell cytoplasm. Junctional folds on the muscle surface are reduced in height and are no longer evident once traces of axoplasm within the Schwann cell disappear. End plate cholinesterase activity is reduced as junctional folds are lost. When muscle is cultured in the presence of a sensory ganglion, the terminal axoplasm degenerates in the same manner but junctional folds persist on the muscle surface. Moderately intense cholinesterase activity remains in association with the junctional folds, so that normal motor end plates are maintained in the absence of innervation. These results show that degenerative changes in the structure of the motor end plate and loss of cholinesterase activity occurring in organ culture as a result of denervation can be retarded by nerve explants that do not directly innervate the muscle.     

STUDIES OF EXCITABLE MEMBRANES : I. Macromolecular Specializations of the Neuromuscular Junction and the Nonjunctional Sarcolemma

The neuromuscular junctions and nonjunctional sarcolemmas of mammalian skeletal muscle fibers were studied by conventional thin-section electron microscopy and freeze-fracture techniques. A modified acetylcholinesterase staining procedure that is compatible with light microscopy, conventional thin-section electron microscopy, and freeze-fracture techniques is described. Freeze-fracture replicas were utilized to visualize the internal macromolecular architecture of the nerve terminal membrane, the chemically excitable neuromuscular junction postsynaptic folds, and the electrically excitable nonjunctional sarcolemma. The nerve terminal membrane is characterized by two parallel rows of 100–110-Å particles which may be associated with synpatic vesicle fusion and release. On the postsynpatic folds, irregular rows of densely packed 110–140-Å particles were observed and evidence is assembled which indicates that these large transmembrane macromolecules may represent the morphological correlate for functional acetylcholine receptor activity in mammalian motor endplates. Differences in the size and distribution of particles in mammalian as compared with amphibian and fish postsynaptic junctional membranes are correlated with current biochemical and electron micrograph autoradiographic data. Orthogonal arrays of 60-Å particles were observed in the split postsynaptic sarcolemmas of many diaphragm myofibers. On the basis of differences in the number and distribution of these "square" arrays within the sarcolemmas, two classes of fibers were identified in the diaphragm. Subsequent confirmation of the fiber types as fast- and slow-twitch fibers (Ellisman et al. 1974. J. Cell Biol. 63[2, Pt. 2]:93 a. [Abstr.]) may indicate a possible role for the square arrays in the electrogenic mechanism. Experiments in progress involving specific labeling techniques are expected to permit positive identification of many of these intriguing transmembrane macromolecules.     

MYONEURAL JUNCTIONS OF TWO ULTRASTRUCTURALLY DISTINCT TYPES IN EARTHWORM BODY WALL MUSCLE

The longitudinal muscle of the earthworm body wall is innervated by nerve bundles containing axons of two types which form two corresponding types of myoneural junction with the muscle fibers Type I junctions resemble cholinergic neuromuscular junctions of vertebrate skeletal muscle and are characterized by three features: (a) The nerve terminals contain large numbers of spherical, clear, ~500 A vesicles plus a small number of larger dense-cored vesicles (b) The junctional gap is relatively wide (~900 A), and it contains a basement membrane-like material, (c) The postjunctional membrane, although not folded, displays prominent specializations on both its external and internal surfaces The cytoplasmic surface is covered by a dense matrix ~200 A thick which appears to be the site of insertion of fine obliquely oriented cytoplasmic filaments The external surface exhibits rows of projections ~200 A long whose bases consist of hexagonally arrayed granules seated in the outer dense layer of the plasma membrane The concentration of these hexagonally disposed elements corresponds to the estimated concentration of both receptor sites and acetylcholinesterase sites at cholinergic junctions elsewhere. Type II junctions resemble the adrenergic junctions in vertebrate smooth muscle and exhibit the following structural characteristics: (a) The nerve fibers contain predominantly dense-cored vesicles ~1000 A in diameter (b) The junctional gap is relatively narrow (~150 A) and contains no basement membrane-like material, (c) Postjunctional membrane specialization is minimal. It is proposed that the structural differences between the two types of myoneural junction reflect differences in the respective transmitters and corresponding differences in the mechanisms of transmitter action and/or inactivation.     

THE SARCOPLASMIC RETICULUM, THE T SYSTEM, AND THE MOTOR TERMINALS OF SLOW AND TWITCH MUSCLE FIBERS IN THE GARTER SNAKE

Twitch and slow muscle fibers, identified morphologically in the garter snake, have been examined in the electron microscope. The transverse tubular system and the sarcoplasmic reticulum are separate entities distinct from each other. In twitch fibers, the tubular system and the dilated sacs of the sarcoplasmic reticulum form triads at the level of junction of A and I bands. In the slow fibers, the sarcoplasmic reticulum is severely depleted in amount and the transverse tubular system is completely absent. The junctional folds of the postsynaptic membrane of the muscle fiber under an "en grappe" ending of a slow fiber are not so frequent or regular in occurrence or so wide or so long as under the "en plaque" ending of a twitch fiber. Some physiological implications of these differences in fine structure of twitch and slow fibers are discussed. The absence of the transverse tubular system and reduction in amount of sarcoplasmic reticulum, along with the consequent disposition of the fibrils, the occurrence of multiple nerve terminals, and the degree of complexity of the post junctional folds of the sarcolemma appear to be the morphological basis for the physiological reaction of slow muscle fibers.     

Morphological differences between excitatory and inhibitory nerve terminals in cockroach coxal muscles

Two morphologically distinct types of neuromuscular junction on the coxal leg muscles of the cockroach, Periplaneta americana, which have been physiologically described as innervated by fast, slow and inhibitory nerve fibers, have been found. In one type of neuromuscular junction the axon terminal contains many round clear synaptic vesicles and contacts several sarcoplasmic extensions from the muscle fiber. The muscle processes adhere to the axon terminal for a short distance (short contact or SC type). The axon terminal of the other type of neuromuscular junction directly contacts the muscle fiber and no extensions of the muscle fiber are formed. The contact region is comparatively long (long contact or LC type). The nerve terminal contains many polymorphic synaptic vesicles. From a correlation of the present morphological findings and the previous physiological results, it may be suggested that the SC type of nerve terminal represents both fast and slow nerve terminals and the inhibitory terminal is of the LC type.     

Ultrastructural comparison of the Drosophila larval and adult ventral abdominal neuromuscular junction

Drosophila melanogaster has recently emerged as model system for studying synaptic transmission and plasticity during adulthood, aging and neurodegeneration. However, still little is known about the basic neuronal mechanisms of synaptic function in the adult fly. Per se , adult Drosophila neuromuscular junctions should be highly suited for studying these aspects as they allow for genetic manipulations in combination with ultrastructural and electrophysiological analyses. Although different neuromuscular junctions of the adult fly have been described during the last years, a direct ultrastructural comparison with their larval counterpart is lacking. The present study was designed to close this gap by providing a detailed ultrastructural comparison of the larval and the adult neuromuscular junction of the ventrolongitudinal muscle. Assessment of several parameters revealed similarities but also major differences in the ultrastructural organisation of the two model neuromuscular junctions. While basic morphological parameters are retained from the larval into the adult stage, the analysis discovered major differences of potential functional relevance in the adult: The electron‐dense membrane apposition of the presynaptic and postsynaptic membrane is shorter, the subsynaptic reticulum is less elaborated and the number of synaptic vesicles at a certain distance of the presynaptic membrane is higher.     

Unusual neuromuscular junctions in the heart of the crayfish (Procambarus clarkii)

Summary The neuromuscular junctions in the crayfish heart were studied with the electron microscope and were classified into two types based on the characteristics of the post-synaptic side. Type I junction was characterized by a mazy post-synaptic apparatus which has been referred to in this work as the junctional envelope, consisting of the cytoplasmic processes and/or lamellae of the muscle cell. Type II junction on the other hand, lacked the junctional envelope. The nerve terminals in both Type I and Type II junctions contained two types of synaptic vesicles: large granular and small agranular vesicles, which were about 1000 Å and 450 Å in diameter respectively. The physiological significance of these neuromuscular junctions and the nature of their synaptic vesicles are discussed.Acknowledgement. The author wishes to express sincere gratitude to Prof. T.Yamamoto for the kind encouragement and guidance during the course of this study.The presence of this unusual neuromuscular junction, coupled with the histological characteristics of heart muscles themselves (Komuro, 1968), may be involved in the different physiological properties of the crustacean heart. This subject will be discussed in a later publication by the author.     

Intense ultraterminal sprouting from motor nerves and ultrastructural aspects of the neuromuscular junction and non-junctional sarcolemma of the soleus (slow-twitch) muscle in motor endplate disease in the mouse

Motor end-plate disease (med) in the mouse is an hereditary defect of the neuromuscular system, with partial functional denervation and muscle inactivity in late stages of the disease. Motor end-plate disease is characterized by an intense ultraterminal sprouting of the motor nerves from swollen nerve terminal branches in the soleus muscle. At the ultrastructural level, the neuromuscular junctions extend to very wide territories, often outside the original motor end-plate, in regions where the nerve sprouts are in simple apposition to the muscle fiber, with no secondary synaptic folds. The nerve terminals are rich in neurofilaments and poor in synaptic vesicles.Freeze fracture analysis of the pre-synaptic and post-synaptic membrane specializations fails to reveal any important structural alteration which could suggest a defect in acetylcholine release or in muscle membrane excitability. However, the non-junctional sarcolemmal specializations (the so-called ‘square arrays’) arc found with a frequency slightly higher than in normal muscle.The nerve abnormalities at the neuromuscular junction may be either a consequence of muscle inactivity or the morphological expression of some primary nerve abnormality. Further studies of the soleus muscle at early stages of the disease may provide evidence in favor of either possibility.     

The fine structure of a multiterminal innervation of an insect muscle

The detailed structure of nerve branches, neuromuscular junctions, and muscle fibers of a multiterminal innervation of cockroach abdominal muscle has been studied with the electron microscope. The muscle fiber is of the banded myofibril type; with paired mitochondria and abundant endoplasmic reticulum. The peripheral nerve branches are multiaxonal with large central axon and several small peripheral tunicated axons. Tracheoblasts closely accompany the nerve branches. The multiple neuromuscular junctions show typical axonal vesicles, muscle aposynaptic granules, and close plasma membrane apposition with no interposition of basement membrane material.     

The Fine Structure of a Multiterminal Innervation of an Insect Muscle

The detailed structure of nerve branches, neuromuscular junctions, and muscle fibers of a multiterminal innervation of cockroach abdominal muscle has been studied with the electron microscope. The muscle fiber is of the banded myofibril type; with paired mitochondria and abundant endoplasmic reticulum. The peripheral nerve branches are multiaxonal with large central axon and several small peripheral tunicated axons. Tracheoblasts closely accompany the nerve branches. The multiple neuromuscular junctions show typical axonal vesicles, muscle aposynaptic granules, and close plasma membrane apposition with no interposition of basement membrane material.     

Acetylcholinesterase activity in nerve endings of tails of Rana japonica and R. catesbeiana during metamorphosis

Summary In anuran tadpole tails, the myelinated motor nerve fibers branch in the myoseptum to innervate both red and white muscle fibers at, or near, their ends. There are no significant ultrastructural differences between the nerve endings of the two types of muscle fibers.Intense acetylcholinesterase reaction product was observed in synaptic clefts and junctional folds, as well as in transverse tubules. As metamorphosis proceeded, the junctional folds of the nerve endings disappeared, however, acetylcholinesterase reaction product was still observed in the synaptic clefts. As muscle fibers began to degenerate, nerve endings began to separate from them. However, after nerve endings were completely separated from the surfaces, degenerated muscle fibers, synaptic and cored vesicles were still well preserved although no acetylcholinesterase reaction product was found. It seems clear that the mechanism of the muscle degeneration in the tadpole tail during metamorphosis is not the result of the degeneration of its nerve endings.     

Ultrastructure of Muscle Spindles (Mechanoreceptors) of Rat m. soleus after Support Unloading and Its Combination with Hypergravity

We used electron microscopy to evaluate the effect of support unloading of m. soleus in adult Wistar rats (restrained in antiorthostatic position for 23–24 h/day within 24 days) on the ultrastructure of the intrafusal fibers and motor neuromuscular junctions of the muscle spindles, as well as the efficiency of intermittent hypergravity (+2G_Z; 1 h/day for 19 days in a centrifuge in hypokinetic cages) as a countermeasure used in conditions of support unloading of this muscle. In the absence of support on the hind limbs, most of intrafusal fibers of m. soleus preserved the typical ultrastructure, while the axon terminals of the neuromuscular junctions accumulated a lot of synaptic vesicles (including large vesicles); the coated vesicles were absent due to unloading of the muscle and its muscle spindles (no contractions of the intrafusal fibers). A short-term effects of hypergravity at the background of support unloading of m. soleus mostly induced static loading of the muscle inducing different responses of the intrafusal fibers in different regions of the muscle spindles: local lysis of myofilaments was observed in single intrafusal fibers of the equatorial and intracapsular motor regions, while myofibrils remained intact in most fibers in the intra- and extracapsular regions of the spindles. The revealed adaptive response of the intrafusal fibers is, on the one hand, due to their specific innervation and ultrastructure and, on the other hand, to positive effect of hypergravity on the motor and extracapsular regions of the muscle spindles. Hypergravity decreased the number of synaptic vesicles and induced appearance of the coated vesicles in the axon terminals of the neuromuscular junctions of the intrafusal fibers in the animals restrained in antiorthostatic position (support unloading of m. soleus), which is due to increased functional load of the muscle. The ultrastructure of the muscle spindles adequately reflected the functional status of the postural m. soleus both during support unloading and support unloading combined with hypergravity load.