Reinnervation of muscle fiber basal lamina after removal of myofibers. Differentiation of regenerating axons at original synaptic sites.
TL;DR: Within the terminals, the synaptic organelles line up opposite periodic specializations in the myofiber's BL, demonstrating that components associated with the BL play a role in organizing the differentiation of the nerve terminal.
Abstract: Axons regenerate to reinnervate denervated skeletal muscle fibers precisely at original synaptic sites, and they differentiate into nerve terminals where they contact muscle fibers. The aim of this study was to determine the location of factors that influence the growth and differentiation of the regenerating axons. We damaged and denervated frog muscles, causing myofibers and nerve terminals to degenerate, and then irradiated the animals to prevent regeneration of myofibers. The sheath of basal lamina (BL) that surrounds each myofiber survives these treatments, and original synaptic sites on BL can be recognized by several histological criteria after nerve terminals and muscle cells have been completely removed. Axons regenerate into the region of damage within 2 wk. They contact surviving BL almost exclusively at original synaptic sites; thus, factors that guide the axon's growth are present at synaptic sites and stably maintained outside of the myofiber. Portions of axons that contact the BL acquire active zones and accumulations of synaptic vesicles; thus by morphological criteria they differentiate into nerve terminals even though their postsynaptic targets, the myofibers, are absent. Within the terminals, the synaptic organelles line up opposite periodic specializations in the myofiber's BL, demonstrating that components associated with the BL play a role in organizing the differentiation of the nerve terminal.
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TL;DR: The extent to which the NMJ is a suitable model for development of neuron-neuron synapses is considered, and an additional set of cues biases synapse formation in favor of appropriate partners.
Abstract: We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.
1,492 citations
TL;DR: It is indicated that MuSK responds to a critical nerve-derived signal (agrin), and in turn activates signaling cascades responsible for all aspects of synapse formation, including organization of the postsynaptic membrane, synapse-specific transcription, and presynaptic differentiation.
869 citations
Cites background from "Reinnervation of muscle fiber basal..."
...Recent evidence suggests that a 43 kDa cytosignaling molecules might be embedded in the extracel- plasmic protein, known as rapsyn, anchors AChRs to a lular matrix (Sanes et al., 1978; Burden et al., 1979; subsynaptic cytoskeleton complex, probably via interMcMahan and Slater, 1984; Kuffler, 1986)....
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...…in the absence of the underlying muscle suggests that muscle-derived signals may be embed- receptors localized in the postsynaptic membrane (Altiok et al., 1995; Moscoso et al., 1995; Zhu et al., 1995).ded in synaptic basal lamina, like agrin and neuregulin (Sanes et al., 1978)....
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TL;DR: This review describes the cytological and molecular architecture of the neuromuscular junction, making three main points: that chemical synapses are designed for rapid, focal transmission of information; that this task is performed by highly specialized preand postsynaptic domains that lie in precise juxtaposition across the synaptic cleft; and that many of the components forming these domains have now been isolated and characterized.
827 citations
TL;DR: The mechanism of cholinergic neurotransmission requires the rapid inac tivation of acetylcholine, which exists in all classes of vertebrates and is characterized in horse serum by Stedman et al (1932), who called it choli nesterase.
Abstract: The mechanism of cholinergic neurotransmission requires the rapid inac tivation of acetylcholine (Dale 1914). Loewi & Navratil showed in 1926 that acetylcholine can be destroyed by an enzyme that exists in aqueous extracts of frog tissues. An esterase that specifically hydrolyzes choline esters was characterized in horse serum by Stedman et al (1932), who called it choli nesterase. It was later found that blood cells also contain a high level of acetylcholine-hydrolyzing activity (Stedman & Stedman 1935). Alles & Hawes (1940) subsequently found that in human blood the serum and cell enzymes are qualitatively different; Mendel et al (1943a) showed that, al though the serum enzyme hydrolyzes butyrylcholine or propionylcholine faster than acetylcholine [as already noted by Stedman et al (1932)], the cell-bound enzyme acts preferentially on acetylcholine, at low substrate concentration. The particulate enzyme also presents a characteristic excess substrate inhibition, so that its activity varies, in a bell-shaped manner, as a function of substrate concentration (Mendel & Rudney 1943). These two activities exist in all classes of vertebrates. The serum enzyme (Be 3.1.1.8) varies somewhat in its specificity, notably in the relative rates of hydrolysis of propionylcholine and butyrylcholine (Augustinsson 1959a,b). The serum enzyme was called originally "nonspecific" cholineste rase, or "pseudocholinesterase" (Mendel et al 1943, Mendell & Rudney 1943). In contrast, the erythrocyte enzyme (EC 3.1.1.7) was considered the
799 citations
TL;DR: The sections in this article are: Motor Unit Types: Histochemical Profiles and Ultrastructural Correlations, Anatomical Considerations, and Control of Muscular Action: Recruitment and Rate Modulation.
Abstract: The sections in this article are:
1
Motor Unit Types
1.1
Muscle Fiber Types: Histochemical Profiles and Ultrastructural Correlations
1.2
Motor Unit Types: Physiological Profiles in Experimental Animals
1.3
Motor Units in Human Muscle
1.4
Stability of Motor Unit Types
1.5
Developmental Considerations
1.6
Skeletofusimotor Units
2
Anatomical Considerations
2.1
Anatomy of Muscle Units
2.2
Anatomy of Motor Nuclei
2.3
Motoneuron Anatomy in Relation to Unit Type
2.4
Electrophysiological Properties Intrinsic to Motoneurons
2.5
Organization of Synaptic Input
2.6
Control of Motoneuron Excitability: Interactive Factors
3
Control of Muscular Action: Recruitment and Rate Modulation
3.1
Motor Unit Recruitment
3.2
Precision and Stereotypy in Recruitment Process
3.3
Output Modulation by Rate and Pattern of Motoneuron Firing
3.4
Recruitment or Rate and Pattern Modulation?
4
Summary and Concluding Comments
711 citations
References
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TL;DR: During stimulation the intracellular compartments of this synapse change shape and take up extracellular protein in a manner which indicates that synaptic vesicle membrane added to the surface during exocytosis is retrieved by coated vesicles and recycled into new synaptic vESicles by way of intermediate cisternae.
Abstract: When the nerves of isolated frog sartorius muscles were stimulated at 10 Hz, synaptic vesicles in the motor nerve terminals became transiently depleted. This depletion apparently resulted from a redistribution rather than disappearance of synaptic vesicle membrane, since the total amount of membrane comprising these nerve terminals remained constant during stimulation. At 1 min of stimulation, the 30% depletion in synaptic vesicle membrane was nearly balanced by an increase in plasma membrane, suggesting that vesicle membrane rapidly moved to the surface as it might if vesicles released their content of transmitter by exocytosis. After 15 min of stimulation, the 60% depletion of synaptic vesicle membrane was largely balanced by the appearance of numerous irregular membrane-walled cisternae inside the terminals, suggesting that vesicle membrane was retrieved from the surface as cisternae. When muscles were rested after 15 min of stimulation, cisternae disappeared and synaptic vesicles reappeared, suggesting that cisternae divided to form new synaptic vesicles so that the original vesicle membrane was now recycled into new synaptic vesicles. When muscles were soaked in horseradish peroxidase (HRP), this tracerfirst entered the cisternae which formed during stimulation and then entered a large proportion of the synaptic vesicles which reappeared during rest, strengthening the idea that synaptic vesicle membrane added to the surface was retrieved as cisternae which subsequently divided to form new vesicles. When muscles containing HRP in synaptic vesicles were washed to remove extracellular HRP and restimulated, HRP disappeared from vesicles without appearing in the new cisternae formed during the second stimulation, confirming that a one-way recycling of synaptic membrane, from the surface through cisternae to new vesicles, was occurring. Coated vesicles apparently represented the actual mechanism for retrieval of synaptic vesicle membrane from the plasma membrane, because during nerve stimulation they proliferated at regions of the nerve terminals covered by Schwann processes, took up peroxidase, and appeared in various stages of coalescence with cisternae. In contrast, synaptic vesicles did not appear to return directly from the surface to form cisternae, and cisternae themselves never appeared directly connected to the surface. Thus, during stimulation the intracellular compartments of this synapse change shape and take up extracellular protein in a manner which indicates that synaptic vesicle membrane added to the surface during exocytosis is retrieved by coated vesicles and recycled into new synaptic vesicles by way of intermediate cisternae.
2,032 citations
"Reinnervation of muscle fiber basal..." refers background in this paper
...In normal (5, 11, 22, 34, 37) and reinnervated ~ muscles, vesicles and active zones are concentrated directly opposite the mouths of the junctional folds that periodically indent the postsynaptic membrane....
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TL;DR: Ruthenium red stains intracellular lipid droplets revealing lamellae, and stains myelin forms grown from crude egg lecithin but cannot penetrate deeply, and is localized in extracellular materials which have an important mechanical function.
Abstract: The inorganic dye, ruthenium red, stains extracellular materials in animal tissues which probably are acidic mucopolysaccharides. It complements other techniques, its advantages being fine grain, high resolution and good contrast. Localization is shown in mouse and rat muscle, heart, lung and intestine, frog cartilage and cells scraped from oral epithelium of human beings. Attention is paid to collagen bundles, the cell/collagen interface and particularly the myotendinal junction, cartilage matrix and agar gel, desmosomes, intestinal microvilli, erythrocytes and vascular endothelium, nerve fibers and the T-system of striated muscle. Although ruthenium red generally is excluded by plasma membranes, it penetrates giving intracellular density, if the membrane is broken. Even when the cell membrane is intact, exceptions occur with selective staining of the T-tubules or the sarcoplasmic sacs depending upon the state of contraction of the muscle cell, and with intracellular staining of certain nuclei and epithelial cells. Ruthenium red stains intracellular lipid droplets revealing lamellae, and stains myelin forms grown from crude egg lecithin but cannot penetrate deeply. It is localized in extracellular materials which have an important mechanical function. Its exclusion by cell membranes permits tracing tortuous cellular invaginations and those exceptions to its exclusion invite a comparison of the localization of the dye with the function of the cell.
869 citations
"Reinnervation of muscle fiber basal..." refers methods in this paper
...The innermost layer, a typical lipid-rich, osmophilic plasma membrane, is coated by a thin, carbohydrate-rich glycocalyx, which can be rendered electron dense by any of a number of stains (40), including the ruthenium red-osmium mixture (28) used in this study....
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TL;DR: Findings indicate that the stained layer is a "cell coat" located outside the plasma membrane, since the cell coat is also stained by colloidal thorium, a technique for detection of acidic carbohydrates, this structure presumably contains not only glycoprotein but also acidic residues.
Abstract: Periodic acid-silver methenamine, a fairly specific technique for glycoprotein detection, was used to stain a variety of rat tissues, in the hope of confirming the existence of a carbohydrate-rich "cell coat" at the surface of mammalian cells. It was found that nearly all cells are coated by a thin layer of stained material. Around fibrocytes and migrating blood cells, the layer is uniform and merges with the ground substance. In the nervous system, cells and processes are surrounded with a layer whose density increases in synaptic clefts. Around epithelial cells, the layer outlines apical microvilli, follows lateral interspaces, and extends between cells and basement membrane. The layer is continuous with the middle plate of desmosomes and can be followed within the wide portion of terminal bars. In contrast, staining usually vanishes when two adjacent plasma membranes fuse to form tight junctions. These findings indicate that the stained layer is a "cell coat" located outside the plasma membrane. Since the cell coat is also stained by colloidal thorium, a technique for detection of acidic carbohydrates, this structure presumably contains not only glycoprotein(s) but also acidic residues. The carbohydrates may play a role in holding cells together and in controlling the interactions between cells and environment.
652 citations
TL;DR: It is proposed that the localization of cholinesterases in myocardium at the ultrastructural level should be taken into account in considering the possible functions of these myocardial enzymes, and it is hoped that knowledge of their localization will open up new avenues of approach in considering their physiological role inMyocardium.
Abstract: A method has been developed for localizing sites of cholinesterase activity in rat cardiac muscle by electron microscopy. The method utilizes thiocholine esters as substrates, and is believed to be dependent on the reduction of ferricyanide to ferrocyanide by thiocholine released by enzymatic activity. The ferrocyanide thus formed is captured by copper to form fine, electron-opaque deposits of copper ferrocyanide, which sharply delineate sites of enzymatic activity at the ultrastructural level. Cholinesterase activity in formalin-fixed heart muscle was localized: (a) in longitudinal elements of the sarcoplasmic reticulum, but not in the T, or transverse, elements; and (b) in the A band, with virtually no activity noted in the M band, or in the H zone. The I band was also negative. No activity was detected in the sarcolemma, or in invaginations of the sarcolemma at the level of the Z band. The perinuclear element of the sarcoplasmic (endoplasmic) reticulum was frequently strongly positive. Activity at all sites was completely abolished by omitting the substrates, or by inhibition with eserine 10-4 M and diisopropylfluorophosphate 10-5 M. Eserine 10-5 M completely inhibited reaction in the sarcoplasmic reticulum, and virtually abolished that in the A band. These observations, together with the use of the relatively specific substrates and suitable controls to eliminate non-enzymatic staining, indicate that cholinesterase activity was being demonstrated. The activity in rat heart against different substrates was that of non-specific cholinesterases, in accordance with biochemical data. The activity in the A band was considered to be probably due to myosincholinesterase. It is proposed that the localization of cholinesterases in myocardium at the ultrastructural level should be taken into account in considering the possible functions of these myocardial enzymes, and it is hoped that knowledge of their localization will open up new avenues of approach in considering their physiological role in myocardium, which at present is not definitely known.
481 citations
"Reinnervation of muscle fiber basal..." refers methods in this paper
...To demonstrate cholinesterase (ChE) at neuromuscular junctions, Karnovsky's histochemical stain was applied to the muscle after glutaraldehyde and before osmium fixation (25, 27)....
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413 citations
"Reinnervation of muscle fiber basal..." refers background in this paper
...The folds persist in denervated but undamaged muscle for many months (6, 27, 45)....
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...(6) (8) (8) o I I I 0 5 10 15 20 2'5 3'0 DAYS AFTER DAMAGE AND DENERVATION...
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...If, however, the nerve is crushed at the same time that the muscle is cut, nerve terminals degenerate and are phagocytized by Schwann ceils, as occurs in denervated but undamaged muscle (6, 35)....
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