Electron-microscopic structure of denervated skeletal muscle

Why exercise builds muscles: titin mechanosensing controls skeletal muscle growth under load

Muscles sense internally generated and externally applied forces, responding to these in a coordinated hierarchical manner at different timescales. The center of the basic unit of the muscle, the sarcomeric M-band, is perfectly placed to sense the different types of load to which the muscle is subjected. In particular, the kinase domain of titin (TK) located at the M-band is a known candidate for mechanical signaling. Here, we develop a quantitative mathematical model that describes the kinetics of TK-based mechanosensitive signaling and predicts trophic changes in response to exercise and rehabilitation regimes. First, we build the kinetic model for TK conformational changes under force: opening, phosphorylation, signaling, and autoinhibition. We find that TK opens as a metastable mechanosensitive switch, which naturally produces a much greater signal after high-load resistance exercise than an equally energetically costly endurance effort. Next, for the model to be stable and give coherent predictions, in particular for the lag after the onset of an exercise regime, we have to account for the associated kinetics of phosphate (carried by ATP) and for the nonlinear dependence of protein synthesis rates on muscle fiber size. We suggest that the latter effect may occur via the steric inhibition of ribosome diffusion through the sieve-like myofilament lattice. The full model yields a steady-state solution (homeostasis) for muscle cross-sectional area and tension and, a quantitatively plausible hypertrophic response to training, as well as atrophy after an extended reduction in tension.

Neuromuscular junction maturation defects precede impaired lower motor neuron connectivity in Charcot-Marie-Tooth type 2D mice

Dominant mutations in GARS, encoding the essential enzyme glycyl-tRNA synthetase (GlyRS), result in a form of Charcot-Marie-Tooth disease, type 2D (CMT2D), predominantly characterized by lower motor nerve degeneration. GlyRS charges the amino acid glycine with its cognate tRNA and is therefore essential for protein translation. However, the underlying mechanisms linking toxic gain-of-function GARS mutations to lower motor neuron degeneration remain unidentified. The neuromuscular junction (NMJ) appears to be an early target for pathology in a number of peripheral nerve diseases and becomes denervated at later stages in two mouse models of CMT2D. We therefore performed a detailed longitudinal examination of NMJs in the distal lumbrical muscles and the proximal transversus abdominis (TVA) muscles of wild-type and Gars mutant mice. We determined that mutant lumbrical NMJs display a persistent defect in maturation that precedes a progressive, age-dependent degeneration. Conversely, the TVA remains relatively unaffected, with only a subtle, short-lived impairment in pre- and post-synaptic development and no reduction in lower motor neuron connectivity to muscle. Together, these observations suggest that mutant Gars is associated with compromised development of the NMJ prior to synaptic degeneration and highlight the neuromuscular synapse as an important site of early, selective pathology in CMT2D mice.

Neurotrophic factors improve motoneuron survival and function of muscle reinnervated by embryonic neurons

Motoneuron death can occur over several spinal levels with disease or trauma, resulting in muscle denervation. We tested whether cotransplantation of embryonic neurons with 1 or more neurotrophic factors into peripheral nerve improved axon regeneration, muscle fiber area, reinnervation, and function to a greater degree than cell transplantation alone. Sciatic nerves of adult Fischer rats were cut to denervate muscles; 1 week later, embryonic ventral spinal cord cells (days 14-15) were transplanted into the tibial nerve stump as the only source of neurons for muscle reinnervation. Factors that promote motoneuron survival (cardiotrophin 1; fibroblast growth factor 2; glial cell line-derived neurotrophic factor; insulin-like growth factor 1; leukemia inhibitory factor; and hepatocyte growth factor) were added to the transplant individually or in combinations. Inclusion of a single factor with the cells resulted in comparable myelinated axon counts, muscle fiber areas, and evoked electromyographic activity to cells alone 10 weeks after transplantation. Only cell transplantation with glial cell line-derived neurotrophic factor, hepatocyte growth factor, and insulin-like growth factor 1 significantly increased motoneuron survival, myelinated axon counts, muscle reinnervation, and evoked electromyographic activity compared with cells alone. Thus, immediate application of a specific combination of factors to dissociated embryonic neurons improves survival of motoneurons and the long-term function of reinnervated muscle.

Chapter 6: cubic membranes the missing dimension of cell membrane organization

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between “inside” and “outside”, survival, and death. Intracellular compartmentalization—governed by biomembranes as well—is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into “unusual”, highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Infant botulism, type F, presenting at 54 hours of life

We report a case of botulism in a 54-hour-old infant with rapidly progressive fulminant paralysis and rapid spontaneous recovery atypical for infant botulism. Clostridium baratii and type F botulinum neurotoxin were isolated from the patient's stool. This unique presentation with rapid recovery is consistent with pharmacokinetics of type F botulinum neurotoxin. Interestingly, a muscle biopsy also revealed pathologic changes early in the disease course. This article reports the youngest known case of infant botulism and only the third reported case of this disease caused by type F neurotoxin. Botulism should be considered in patients of any age with subacute or acute neuromuscular weakness.

Perisynaptic Schwann cells at the neuromuscular junction: nerve- and activity-dependent contributions to synaptic efficacy, plasticity, and reinnervation

Glial cells are increasingly recognized for their important contributions to CNS and PNS synaptic function. Perisynaptic Schwann cells, which are glial cells at the neuromuscular junction, have proven to be an exceptionally useful model for studying these roles. Recent studies have shown that they detect and reciprocally modulate synaptic efficacy in an activity-dependent manner in the short term. In addition, perisynaptic Schwann cells guide reinnervating nerve sprouts after deinnervation, and many important parameters of this are dependent on synapse activity. Thus, it is hypothesized that perisynaptic Schwann cells are key integrators in a continuum of synaptic efficacy, stability, and plasticity at the neuromuscular junction, which is important for maintaining and restoring synaptic efficacy.

Dynamics of nuclei of muscle fibers and connective tissue cells in normal and denervated rat muscles

The mitotic activity in muscles of growing rats and the effect of denervation were studied by means of continuous infusion of 5-bromo-2-deoxyuridine (BRDU). Denervated muscles after 10 weeks contained 20 to 60% fewer muscle nuclei than normal; BRDU labeled about 25% of the nuclei of normal soleus and extensor digitorum longus (EDL) and of denervated EDL muscles but only 5% in the denervated soleus muscle. Labeled nuclei persisted in denervated but not in normal muscles. After the main growth period, the turnover of myonuclei was at most 1 to 2% per week. The behavior of connective tissue nuclei was similar to that in muscle fibers. Infusion of BRDU had no effect on contractile properties. It is suggested that the exceptionally rapid atrophy of the denervated rat soleus associated with loss of satellite cells was due to loss of myonuclei and differentiation and fusion of satellite cells. The cause may possibly be that the phase of postdenervation fibrillation is shorter than in other muscles.

Quantitative study of the effects of long-term denervation on the extensor digitorum longus muscle of the rat

BACKGROUND

In order to understand the cellular basis underlying the progressively poorer restorative capacity of long-term denervated muscle, we determined the effects of long-term denervation on the muscle fibers and satellite cell population of the rat extensor digitorum longus (EDL) muscle.

METHODS

In 36 male rats, the right hind legs were denervated, and EDL muscles were removed 2, 4, 7, 12, and 18 months later. Muscles were either fixed for electron microscopic analysis or were dissociated into individual muscle fibers for direct fiber counting or for confocal microscopic analysis.

RESULTS

The percentage of satellite cells rose from the 2.8% control value to 9.1% at 2 months of denervation; thereafter the percentage decreased to 1.1% at 18 months of denervation. The number of myonuclei per muscle fiber steadily declined from 410 in 4 month control muscle to 158 in 7 month denervated muscle. Up to 7 months of denervation, the total number of muscle fibers per muscle remained relatively constant at somewhat over 5,000. The calculated total satellite cell population in 4 month denervated EDL muscle was the same as that of controls at 65,000, but by 7 months of denervation it had declined to 21,000. With increasing time of denervation, the number of cross-sectional profiles of muscle fibers not containing nuclei rose from 14% in control muscle to 49% in 12 month denervated muscle. This was correlated with a pronounced regular clumping of the nuclei, with pronounced nonnucleated segments between nuclear clumps.

CONCLUSIONS

Increasing times of denervation are accompanied by a pronounced decline in the number of myonuclei per muscle fiber and an initial rise and subsequent fall in satellite cell number. These changes are correlated with a decreasing restorative ability of these muscles over the same periods of denervation. Further work on the proliferative capacity of the remaining satellite cells is necessary before firm quantitative conclusions can be made.

Comparison of muscle mass preservation in denervated muscle and transplanted muscle flaps after motor and sensory reinnervation and neurotization

The gracilis muscle model was used either as a denervated muscle in situ or as a transplanted flap in 273 rats to compare the trophic effects of muscle reinnervation and neurotization using sensory and motor nerves. The average gracilis muscle flap weighed 626 +/- 94 mg at the time of the initial procedure. Experimental muscles were examined 6 months following the procedure. In denervated, nontransplanted muscles, both motor nerve reinnervation and neurotization resulted in significantly preserved muscle mass, averaging 570 +/- 69 and 521 +/- 116 mg, respectively, compared with the denervated control average of 178 +/- 22 mg (p < 0.05). Sensory nerve reinnervation and neurotization produced much smaller trophic effects (p > 0.05). In transplanted gracilis free flaps, however, only direct reinnervation with motor or sensory nerves resulted in improved bulk preservation, with average weights of 313 +/- 83 and 327 +/- 91 mg compared with the control average of 201 +/- 76 mg (p < 0.05). Neither sensory nor motor neurotization was significantly effective in the free-flap model (p > 0.05). These data suggest that transplantation may alter the response of muscle to reinnervation.

Tubular networks in soleus muscle fibers of the rat following tenotomy. I. Spatial organization

Complex systems of tubules, 30-40 nm in diameter, were observed in myofibers of the soleus muscle of Wistar rats following experimental transection of the Achilles tendon. The appearance of these systems varied remarkably, depending upon the plane of sectioning. The spacial arrangement of the tubules was reconstructed by comparing several sections through individual systems. Such reconstructions revealed that they are networks of tubular elements arranged in an hexagonal pattern. A three-dimensional wire model was prepared, illustrating the spatial organization of the tubular systems. The model consists of four groups of lattices with a honeycomb-like arrangement. The lattices of each group are parallel to one another, and intersect those of the other groups at an inclination of 60 degrees.

Changes in mast-cell distribution in skeletal muscle after denervation

Motor nerves are known to govern the structure of skeletal muscle. In the normal guinea pig diaphragmatic muscle, we found that mast cells were predominantly located in the central tendon. Following denervation, these cells became more numerous in the muscle itself than in the tendon. Therefore, nervous trophic influences are exerted on other tissue elements besides those which sustain transmission and contraction. Ascertaining mast-cell distribution in biopsies may help to elucidate denervation effects in motor neuron disease.

Morphology of long-term denervated rat soleus muscle and the effect of chronic electrical stimulation

1. Rat soleus muscles were denervated for 6-10 months; some of these were chronically stimulated for the last 3-8 weeks before recording. The muscles were fixed at physiological lengths and embedded in epoxy resin. 2. Sections for light microscopy were stained with p-phenylenediamine. Denervation reduced the mean cross-sectional area of fibres to 3% of controls (peak at 20 microns2). The cross-sectional areas of the stimulated fibres had a peak at 70 microns 2. In light micrographs of denervated muscles, the total number of fibres appeared to be reduced; however not all fibres could be identified (see paragraph (3)). 3. In the electron microscope, many fibres showed disarrayed myofilaments. Few fibres had more or less normal cross-striations. Muscle fibres as small as 1 micron in diameter were seen. The smallest fibres did not contain myofilaments. Some unequivocally necrotic fibres were seen. 4. Most stimulated denervated fibres showed an almost normal sarcomere pattern. Scattered throughout the muscle were single fibres as small as 2 microns in diameter, but these were well organized and could be recognized in the light microscope. Few fibres were necrotic. Often fibres were serially arranged. Satellite cells were prominent. 5. It is concluded that in long-term denervated rat soleus the original fibres are lost and those seen are the result of repeated cycles of regeneration and necrosis. Stimulation maintains the sarcomeres and probably prevents secondary degeneration and necrosis.

The effect of denervation on the morphology of regenerating rat soleus muscles

This study examines the level to which muscle regeneration proceeds in the absence of innervation. Regeneration was monitored in rat soleus muscles following localised injection of a snake toxin, notexin. Muscles which had been concomittantly denervated were compared with those that were normally innervated. Until 3-4 days following toxin administration regeneration is identical in both groups. The muscles contain new myotubes in place of the degenerated "parent" fibres. Thereafter, the non-denervated muscles grow rapidly and by 28 days their myofibres attain the size of those from the contralateral controls. Growth of denervated regenerating muscles, however, is retarded and is superseded by a gradual atrophy. In such muscles we further identify ultrastructural abnormalities from 7 days post-injection. These a re loss of individual myosin filaments and the presence of immature and abnormal configurations of the transverse system and triads. We, thus, conclude that innervation is an obligatory requirement for the restoration of normal myofibrillar and sarcotubular morphology, as well as growth, but is not necessary for the neo-formation of myofibres.

Muscle fibre loss and reinnervation after long-term denervation

Cutaneous pectoris muscles of frog (Rana temporaria) were investigated 19.5-40 months after denervation. On whole mounts a heavy reduction in size and number of muscle fibres is noticed; in two muscles studied with semithin and ultrathin sections the number of remaining muscle fibres is 149 and around 120, while one of the contralateral muscles contains 250 and control muscles of equal sized frogs between 220 and 320 (n = 18) fibres. By electron microscopy muscle fibres undergoing degeneration or phagocytosis can be seen (3 of 20 muscle fibres present in a single ultrathin cross-section). On the other hand several profiles contained within one common basal lamina sheath are present in 14 of 20 fibres, indicating satellite cell proliferation. In one preparation 40 months after denervation not a single muscle fibre or axon is present, suggesting that eventually, without nerve supply, muscle fibres entirely disappear. Upon spontaneous reinnervation or implantation of the hypoglossal nerve 16 months after denervation, synapses are formed with the remaining muscle fibres. When studied 3.5-24 months after nerve implantation muscles innervated by few axons only (less than 10, 10-20 axons) contain a low number of muscle fibres (mean 44 +/- 41 SD, n = 6), while all muscles with a larger number of axons have more than 150 muscle fibres (n = 6). This indicates that unless large numbers of axons regenerate and/or when reinnervation is delayed muscle fibre loss continues to occur. The presence in one muscle of motor axons but only six muscle fibres 24 months after nerve implantation indicates that muscle fibre loss cannot be reversed, or recovery is extremely slow. This observation is interpreted as evidence for the exhaustibility of the satellite cell pool.

On the effect of muscle activity on the end-plate membrane in denervated mouse muscle

1. Mouse soleus muscles were denervated and some of them were chronically stimulated. Sixteen to twenty-one days later, the number of junctional acetylcholine receptors (AChR) and their metabolic stability were examined by measuring binding of 125I-alpha-bungarotoxin, their gating properties by analysis of acetylcholine-induced current fluctuations and the ultrastructure of the end-plate membrane by electron microscopy. 2. In agreement with other studies on inactive muscles, no effect of denervation on junctional AChR number could be resolved. However, some of the fast-gating 'adult' AChR channels had been replaced by slowly gating fetal AChRs, their half-life was lowered to 38 h and the folding of the end-plate membrane was reduced. 3. These changes were prevented in denervated but stimulated active muscles: the junctional AChR population remained homogeneously 'adult', the half-life of junctional AChRs was 13 days and folding of the end-plate membrane remained comparable to that in control muscles. 4. The significance of these results is discussed with respect to the role of muscle activity in end-plate development.

Expression of ACh-activated channels and sodium channels by messenger RNAs from innervated and denervated muscle

Xenopus oocytes were used to express polyadenylated messenger RNAs (mRNAs) encoding acetylcholine receptors and voltage-activated sodium channels from innervated and denervated skeletal muscles of cat and rat. Oocytes injected with mRNA from denervated muscle acquired high sensitivity to acetylcholine, whereas those injected with mRNA from innervated muscle showed virtually no response. Hence the amount of translationally active mRNA encoding acetylcholine receptors appears to be very low in normally innervated muscle, but increases greatly after denervation. Conversely, voltage-activated sodium currents induced by mRNA from innervated muscle were about three times larger than those from denervated muscle; this result suggests that innervated muscle contains more mRNA coding for sodium channels. The sodium current induced by mRNA from denervated muscle was relatively more resistant to block by tetrodotoxin. Thus a proportion of the sodium channels in denervated muscle may be encoded by mRNAs different from those encoding the normal channels.

Effects of vagotomy on the ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis)

The ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis) was studied after bilateral cervical vagotomy and survival times of 1, 3, 5, 7, 10, 21 and 28 days. During the first week after vagotomy, a few atrial cells showed a reduction in the sarcoplasm, crowding of the myofibrils, peripheral dispersion and reduced intercristal density of the mitochondria and increased sarcoplasmic reticulum and glycogen particles. In some profiles, there was increased electron density and granularity at the I bands and the intercalated discs. The number of such affected cells increased in the subsequent days such that by 21 to 28 days about 50% of the cells were estimated to be affected. During the latter stages further changes included, the degradation of the myofilaments and increased electron density, disorganisation and disintegration of the digital extensions at the intercalated discs. Throughout the experiments there was a leucocytic infiltration, more evident in the longer survival times.

Network and lamellar structures in the tail muscle fibers of the metamorphosing anuran tadpole

Networks of regularly arranged tubular units and lamellar structures were observed in the degenerating tail muscle fibers of spontaneously metamorphosing anuran tadpoles. These networks appeared to be similar to those previously found in the skeletal muscle of other animals under abnormal conditions. They stained clearly with ruthenium red (RR) and a continuity with the transverse tubular system (T tubules) of triads was clearly observed. The diameter of the tubular unit, 20-25 nm, was almost equal to that of the T tubules of the intact tail muscle fibers, indicating the network structures were probably formed by T tubules connecting together. In the early stages of metamorphosis, networks in the tetragonal configuration were observed within the end region of the muscle fibers. At the climax of metamorphosis, well-developed networks in which the tubular units were arranged in a hexagonal pattern were seen in various regions of the fibers. Other observed lamellar structures may originate from lateral elements of the triads. The formation of the network structure is discussed.

Chronic denervation of rat hemidiaphragm: maintenance of fiber heterogeneity with associated increasing uniformity of myosin isoforms

During several months of denervation, rat mixed muscles lose slow myosin, though with variability among animals. Immunocytochemical studies showed that all the denervated fibers of the hemidiaphragm reacted with anti-fast myosin, while many reacted with anti-slow myosin as well. This has left open the question as to whether multiple forms of myosin co-exist within individual fibers or a unique, possibly embryonic, myosin is present, which shares epitopes with fast and slow myosins. Furthermore, one can ask if the reappearance of embryonic myosin in chronically denervated muscle is related both to its re-expression in the pre-existing fibers and to cell regeneration. To answer these questions we studied the myosin heavy chains from individual fibers of the denervated hemidiaphragm by SDS PAGE and morphologically searched for regenerative events in the long term denervated muscle. 3 mo after denervation the severely atrophic fibers of the hemidiaphragm showed either fast or a mixture of fast and slow myosin heavy chains. Structural analysis of proteins sequentially extracted from muscle cryostat sections showed that slow myosin was still present 16 mo after denervation, in spite of the loss of the selective distribution of fast and slow features. Therefore muscle fibers can express adult fast myosin not only when denervated during their differentiation but also after the slow program has been expressed for a long time. Light and electron microscopy showed that the long-term denervated muscle maintained a steady-state atrophy for the rat's life span. Some of the morphological features indicate that aneural regeneration events continuously occur and significantly contribute to the increasing uniformity of the myosin gene expression in long-term denervated diaphragm.

The muscle satellite cell: a review

Since the first reports of satellite cells in 1961, considerable knowledge has accumulated concerning their phylogenetic distribution and their location, morphology, and function. There is no doubt that satellite cells are capable of undergoing mitosis and that they have considerable motility. These cells function as the progenitors of the myofiber nuclei that must be added during normal (postnatal) growth of muscle. In muscle undergoing or attempting to undergo regeneration, the satellite cell functions as a myogenic stem cell to produce myoblasts that line up and fuse within the scaffolding of the remnant basal lamina or migrate into the interstitium to produce neofibers . A number of problems remain to be solved concerning the regulation of satellite cell function. At this time it is equivocable whether or not the presumptive myoblast and the satellite cell are functionally identical and at the same stage of myogenic differentiation. Apparently there is species variation in terms of the ability of myotubes from embryonic myogenic cells and satellite cells to synthesize protein. The mechanism(s) by which a wide variety of stimuli activate satellite cells is not known, nor is the mechanism(s) by which satellite cells become inactive during the latter stages of growth and adulthood known. Mitogenic factors are present in damaged muscle; but the specific characteristics of these factors and their mechanism of activation are also unknown. Hormones are certainly involved in the regulation of proliferation and differentiation of myogenic cells, but whether presumptive myoblasts and satellite cells or their myotubes respond similarly to hormones in culture has not been adequately examined. Greater understanding of these mechanisms will increase the possibility of total muscle recovery from severe injury or disease. Such knowledge would also have particular application to the production of meat animals and to a greater understanding of the growth process in general.

A quantitative ultrastructural analysis of satellite cells in denervated fast and slow muscles of the mouse

Mononucleated cells located between the external lamina and sarcolemma of denervated muscle fibers within the extensor digitorum longus (EDL) and soleus muscles of adult mice were quantified and examined ultrastructurally from 3 to 65 days after ligating and removing a section of the sciatic nerve. During the first 2 weeks postdenervation, mononucleated cells in denervated muscles were morphologically indistinguishable from satellite cells observed in control muscles. With time, however, many of these satellite-like cells appeared more active as evidenced by a decrease in their nucleocytoplasmic ratio and an increase in their mean percentage of euchromatin material. The number of satellite cells (expressed as a ratio of satellite cell nuclei to satellite cell nuclei plus myonuclei) did not increase significantly until 30 days postdenervation, at which time the mean percentage for the soleus muscle had risen from a control value of 4.1-8.5%, and for the EDL from 1.2-4.1%. Small-diameter, presumably regenerating, myofibers were occasionally observed but only after 30 days denervation. The ultrastructural evidence plus comparisons of euchromatin distributions between myonuclei and satellite cell nuclei support the concept that an increase in the number of satellite-like cells during denervation is more likely due to satellite cell proliferation than to the formation of mononucleated fragments utilizing preexisting myonuclei.

Morphometric ultrastructural study of the regeneration of soleus muscle in hypokinesia

The aim of the present study was a morphometric ultrastructural evaluation of regenerating soleus muscle fibers after denervation under conditions of hypokinesia. It was found that in rats kept under conditions of hypokinesia degenerative changes in the soleus muscle after denervation are more pronounced than in the control. On the 36th day after denervation of muscle, symptoms of regeneration of muscle fibers were observed. It was more pronounced in the control than in the experimental groups. Denervation and immobilization applied simultaneously causes more severe degeneration and atrophy of muscle fibers than does the action of only one of these factors.

Neurotrophic control of channel properties at neuromuscular synapses of rat muscle

1. Ectopic neuromuscular synapses formed when the fibular nerve was implanted into the proximal part of rat soleus muscle and the soleus nerve was cut. The gating properties of acetylcholine (ACh) receptor channels in the newly formed ectopic and in the denervated original end-plates were examined at various stages of ectopic synapse formation.2. At ectopic end-plates the apparent mean open time of ACh receptor channels changes during synaptic development. Channels in immature ectopic end-plates, examined 1 week after cutting the soleus nerve, have apparent mean open times of approximately 4 ms (at -70 mV, 22 degrees C), similar to those of the extrasynaptic ACh receptor channels of completely denervated fibres. The channel gating of mature ectopic end-plates, examined 3-7 weeks after nerve section, is characterized by a mean open time of approximately 1 ms and resembles that found in normal end-plates of adult fibres.3. The conversion of end-plate channel gating occurs during the second and third week of synapse formation. During this period two discrete classes of channels with different gating behaviour are present in the ectopic end-plate.4. Examination of ectopic end-plates in the electron microscope reveals that junctional folds begin to appear in the subsynaptic membrane during the period of channel conversion.5. At the denervated original end-plates of ectopically innervated fibres the apparent mean open time of ACh receptor channels remains similar to that of normally innervated end-plates. Original end-plates retain the normal synaptic class of channel for at least 42 days after denervation. At this time, most of the ACh receptors present originally in the membrane have been replaced by newly inserted receptors.6. At former end-plates of completely denervated fibres ACh activates two classes of channels, even when most of the ACh receptors originally present in the end-plate have been replaced by new receptors.7. The results show that during synapse formation a neurally controlled conversion of ACh receptor channels occurs about 2-3 weeks after establishment of the nerve muscle contact. Thereafter end-plate channel properties are independent of neural influences. These observations are consistent with a mechanism of channel conversion whereby the nerve modifies not the ACh receptor channel itself, but another constituent of the end-plate membrane which determines the gating properties of end-plate channels.

Chronic denervation of rat diaphragm: selective maintenance of adult fast myosin heavy chains

After long-term denervation in mixed rat muscles there is a selective loss of slow myosin. The bidimensional electrophoretic pattern of light chains and the results of preliminary studies on heavy chains have left open the question of whether the nature of the residual fast-like myosin is of the immature or adult type. We have further investigated chronically denervated myosin by (1) electrophoresis in nondissociating conditions; (2) acidic electrophoresis of the heavy chains; and (3) proteolytic mapping of the heavy chains. These techniques clearly distinguish adult myosins from those present in immature muscles. Using these criteria, myosin from the chronically denervated diaphragm is of an adult type, even though the presence of trace amounts of embryonic myosin cannot be excluded. The contractile properties also indicate that chronically denervated hemidiaphragm is more similar to an adult fast muscle than to an immature muscle. The selective maintenance after long-term denervation of fast myosin in adult muscle provides good evidence of the independence of the genetic expression of myosin from a direct neural chemotrophic control.

Neural and non-neural acetylcholine in the rat diaphragm

The compartmentation of acetylcholine (ACh) and of choline acetyltransferase in the rat diaphragm was analysed by measuring their contents in muscle segments containing endplates (e.p.) and endplate-free segments (non-e.p.) at different times following section of the phrenic nerve. In addition ACh release was determined before and after denervation. Freshly dissected hemidiaphragms contained about 125 pmol of ACh; more than 90% of this was localized in the e.p. portion. Between 10 and 18 h after denervation the ACh content of the e.p. portion decreased by 80% and its ACh concentration became approximately equal to that in the non-e.p. region, whose ACh content did not change. Spontaneous release of ACh was reduced by denervation and ACh release evoked by 50 mM KC1 was practically abolished. Choline acetyltransferase activity in freshly dissected preparations was about 30 nmol of ACh per gram per hour, Km 0.5 mM. About 65% of the enzyme disappeared in the first 24 h and the remaining 35% between 24 and 50 h after denervation. A different enzyme capable of ACh synthesis was found in the muscle fibres; its activity did not decrease after denervation. It is concluded that about 70% of the ACh in the diaphragm is contained in the motor nerve terminals, about 10% in the intramuscular nerve fibres and the remainder in the muscle fibres, and that about 65% of choline acetyltransferase is in the motor terminals and 35% in the nerve fibres.

Denervation changes in normal and myasthenia gravis human muscle fibres during organ culture

1. Human intercostal nerve-muscle obtained from normal and myasthenia gravis affected patients has been organ cultured for up to 5 weeks at 23 degrees C. In addition normal nerve-muscle has been cultured for up to 2 weeks at 36 degrees C. Muscle fibres had normal resting and overshooting action potentials. Input resistances dropped markedly after 21 days at 23 degrees C or 10 days at 36 degrees C.2. Muscle fibre action potentials became partially resistant to tetrodotoxin (10(-7) g/ml.) after culturing for 8 days at 36 degrees C.3. Extrajunctional acetylcholine (ACh) sensitivity was compared in fresh and cultured muscle. Fresh normal fibres possessed extrajunctional ACh sensitivity covering several hundred micrometres around the end-plate and at the muscle-tendon junction. Myasthenia gravis affected fibres had reduced extrajunctional sensitivity at the end-plate and no detectable ACh sensitivity near the tendon.4. Less than one third of normal muscle bundles showed an increased area of ACh sensitivity after several days in organ culture. Under the same conditions myasthenic muscle bundles did not show an increase in extrajunctional ACh sensitivity,5. M.e.p.p.s were present for 2-4 days in normal fibres cultured at 36 degrees C. In normal and myasthenic muscles cultured at 23 degrees C m.e.p.p.s disappeared after 6-8 days and re-appeared in some fibres (50%) after approximately two weeks in organ culture. These m.e.p.p.s were abolished by curare and increased in frequency by hypotonic solution suggesting they are due to the release of ACh-packets from Schwann cells.6. Electron microscopic examination of cultured human muscle indicates that the disappearance of m.e.p.p.s corresponds with degeneration of nerve terminals.7. In muscle bundles shown to possess m.e.p.p.s after 13-14 days, the synaptic gutter, which had been vacated by the nerve terminal, was usually occupied by a Schwann cell or projections of Schwann cell cytoplasm. This indicates that the Schwann cell at denervated human end-plates may be capable of releasing packets of ACh.8. It is concluded that the organ culture system described here is suitable for studying normal and diseased human muscle fibres. Using this system we find that the denervation changes which follow nerve transection appear to be similar in most respects at normal and myasthenic end-plates.

The effect of tetraphenylporphinesulfonate (TPPS) on muscle end-plates in mice. An ultrastructural and quantitative study

Motor end-plates were studied in mice at various intervals after a single injection of a synthetic porphyrin, tetraphenylporphinesulfonate (TPPS). Ultrastructurally, excess accumulation of neurofilaments constituted the earliest abnormality. These were followed by atrophy of many nerve terminals and their separation from the postsynaptic area by interposed Schwann cells. Five to 8 months after the injection some postsynaptic areas showed denervation and atrophy. These progressive changes in the nerve terminals were accompanied by secondary changes in the subneural apparatus. Morphometric analysis revealed marked atrophy of the end-plates and significant swelling of preterminal axons. The present findings are suggestive of partial denervation of muscle, occurring after the injection of a synthetic porphyrin.

Changes in mechanical properties of the inferior oblique muscle of the rabbit after denervation

The inferior oblique muscle (IO) of the rabbit was denervated. The mechanical properties of the muscles were determined at 35 degrees C in vitro 2--76 days after the operation. After denervation the muscles exhibited a considerable hypertrophy. The cross-sectional area of muscles denervated 30 days or longer grew to about twice as innervated controls. The length-tension relationship of passive or activated long-term denervated IO differed from normal in a higher stiffness. After denervation the time parameters of single twitches and tetanic contractions evoked by massive stimulation were prolonged, the fusion frequency was decreased, and the fatigue resistance was increased. During the first week after cutting the motor nerve both twitch and tetanic tensions decreased drastically. A minimum was reached at the end of the second weak. In the subsequent weeks the tension development was increased again, but per unit cross-sectional area it was always smaller than in innervated muscles. After denervation the twitch:tetanus ratio was increased. Cooling to 25 degrees C was followed by an increase in time parameters of single twitches and tetanic contractions and by a depression of twitch and tetanic tensions. Following a repetitive stimulation denervated IO showed a posttetanic depression of the single twitch.

Effects of acute and chronic denervation on release of acetylcholinesterase and its molecular forms in rat diaphragms

Hemidiaphragms were removed from rats at various times after intrathoracic transection of the left phrenic nerve and were incubated in organ baths containing 1.5 ml of oxygenated, buffered physiologic saline solution, with added glucose and bovine serum albumin. After incubation, the acetylcholinesterase (AChE: EC 3.1.1.7) activities of the bath fluid and of the muscle were determined. Innervated left hemidiaphragms were found to release 107 units of AChE over a 3-h period, corresponding to 1.9% of their total AChE activity. Denervation led to a rapid loss of AChE from the muscle coincident with a transient increase in the outpouring of enzyme activity into the bath fluid. Thus, 1 day after nerve transection the left hemidiaphragm contained only 68% of the control amounts of AChE activity, but released 140% as much as control. After 3 or 4 days of denervation, the AChE activity of the diaphragm stabilized at 35% of the control value. Release also fell below control by this time, but not as far. One week after denervation the release, 69 units per 3 hr, correspond to 3.3% of the reduced content of AChE activity in the muscle, indicating that denervation caused an increase in the proportion of AChE released. Sucrose density gradient ultracentrifugation showed that 10S AChE accounted for more than 80% of the released enzyme activity at all times. The results did not rule out the possibility, however, that the released enzyme originally stemmed from 4S or 16S AChE in the diaphragms.

Hypertrophic smooth muscle. I. Size and shape of cells, occurrence of mitoses

An extensive hypertrophy of the muscle coat develops in the small intestine of the guinea pig oral to an experimental stenosis. The profiles of smooth muscle cells become larger and irregular in shape. From the analysis of serial sections the arrangement of the muscle cells is less orderly than in control muscles. Many muscle cells are split into two or more branches over part of their length. The average cell volume is 3--4 times that of control muscle cells; the cell surface increases less dramatically and, in spite of the appearance of deep invaginations of the cell membrane, the surface-to-volume ratio falls from 1.4 to 0.8. The average cell length is only slightly increased compared with controls. Smooth muscle cells in mitosis are observed in all the hypertrophic muscles examined, in both muscle layers; in the circular musculature they occur mainly found in the middle part of the layer.

The fine structure of denervated and reinnervated muscle spindles: morphometric study of intrafusal muscle fibers

The fine structure of normal, denervated, and reinnervated muscle spindles in lower lumbrical muscles of rats was studied morphometrically at time intervals ranging from 3-14 months. In control spindles, the mean transverse area of mitochondria was estimated to be more than twice as large in nuclear chain than in typical nuclear bag fibers. Following denervation, there was a severe decrease of the mean number and transverse area of mitochondria, and a moderate, but statistically significant decrease of the mean transverse area of intrafusal muscle fibers (IMFs) despite an increase of the number of IMFs. At 12-14 months of reinnervation, changes of the transverse areas of IMFs were statistically insignificant, but the mean values for the mitochondria were incompletely restored. At 4 x 3 months, after fourfold repeated crush injuries to the nerve, most of the values estimated (transverse area of mitochondria; number, shape, and transverse area of IMFs and nuclei) tended to approach those in denervated rather than in reinnervated IMFs. The differences of the reactions of intra- and extrafusal muscle fibers following complete motor and sensory denervation appeared to be in accordance with their normal dimensional dissimilarities.

Changes in the satellite cells of growing muscle following denervation

Satellite cells exhibit a number of distinct morphological changes after denervation which appear to be a direct response to nerve section or altered functional state of the muscle fibers. These changes appear generally related to increased movement and overall activation of the cells. After longer periods of denervation many satellite cells appear to separate from their fibers and become free cells in the interstitial space. It is proposed that this mechanism provides a cellular source for the small-diameter, immature fibers dispersed throughout the muscle after two to three weeks. Although neo-formation of myofibers appears to be a feature of denervated growing muscle, an increase in total fiber population was not observed. It is suggested that in the absence of a viable nerve supply the new fibers degenerate.

Properties of isolated adult rat muscle fibres maintained in tissue culture

1. Adult rat skeletal muscles were dissociated by collagenase treatment and trituration, and the isolated muscle fibres were maintained in vitro for 2-3 weeks. At various stages, the fibres were examined physiologically and morphologically. 2. The isolated fibres underwent some changes characteristic of muscle denervated in vivo. For instance, input resistance increased and extrajunctional acetylcholine (ACh) receptors appeared. In addition, the beginning stages of apparent muscle fibre fragmentation were observed. 3. In other respects, the cultured isolated fibres behaved differently than in vivo denervated fibres. Fibrillation developed only occasionally in vitro. The onset of ACh supersensitivity was slower (6 days) than after denervation in vivo (2-3 days). Some fibres developed localized regions of destriation, which apparently was due to loss of in-register alignment of myofibrils.

Physiological properties of dissociated muscle fibres obtained from innervated and denervated adult rat muscle

1. Adult rat flexor digitorum brevis muscles were dissociated by treatment with collagenase and trituration. Several hundred isolated fibres were obtained from each muscle. 2. Most isolated fibres appeared to be intact as judged by some morphological and physiological criteria, although resting membrane potentials were about -60 mV, which is somewhat lower than normal. 3. A small percentage of the muscle fibres were branched. 4. Acetylcholine sensitivity was measured iontophoretically. The sensitivity fell abruptly outside the margin of the end-plate. Extrajunctional sensitivty was detected on all fibres, and declined smoothly away from the end-plate to an undetectable level over a distance of about 200 micron. On a few fibres, ACh sensitivity was mapped circumferentially from the end-plate. It appeared to decline with distance in a manner similar to the longitudinal sensitivity gradient. 5. Fibres dissociated from muscles denervated a week earlier were sensitive to ACh everywhere on their surfaces.

Effects of long term denervation on smooth muscle of the chicken expansor secundariorum

Denervation of the expansor secundariorum muscle of the adult and 2 week chicken, by sectioning the brachial plexus, resulted in an approximate twofold increase in dry weight over 8 weeks. Unlike skeletal muscle, no ultrastructural changes were exhibited by the smooth muscle cells for a period of up to 5 months post denervation. No evidence of hypertrophy of the individual muscle cells was observed, but following colchicine treatment a definite increase in the number of mitotic figures was noted within muscle bundles indicating that the increase in dry weight of the expansor muscle is due to hyperplasia of the smooth muscle cells. The results are discussed in relation to in vitro studies of the interaction of sympathetic nerves with smooth muscle.

Membrane properties underlying spontaneous activity of denervated muscle fibres

We have examined the events underlying the initiation of spontaneous action potentials (fibrillation) in fibres of previously denervated rat diaphragm maintained in organ culture for up to 10 days.1. Based on discharge pattern, two classes of spontaneously active fibres were found: rhythmically discharging fibres, and fibres in which action potentials occur at irregular intervals.2. Sites of action potentials initiation were located by exploration along the fibre length with two independent extracellular recording electrodes. The majority of sites of origin in both regular and irregular fibres were at the former end-plate zone; however, there was no region along the length that could not, at least in some fibres, be a site of origin.3. Intracellular recording at or near sites of origin of action potential discharge showed two types of initiating events. Irregularly discharging fibres were brought to threshold by discrete depolarizations of up to 15 mV in amplitude, while regularly occurring action potentials were associated with oscillations of the membrane potential.4. Discrete depolarizations (called fibrillatory origin potentials or f.o.p.s) at sites of origin in irregularly discharging fibres have the following properties: (a) random occurrence and nearly constant amplitude outside a refractory period during which both amplitude and probability of a second f.o.p. are reduced; (b) associated inward current flow which is localized to about 100 mum or less along the fibre length, and (c) dependence of amplitude and frequency on membrane potential.5. Oscillation of membrane potential found at sites of origin of action potential discharge in regular fibres also occurred locally along the fibre length and was sensitive to changes in membrane potential.6. Both f.o.p.s and oscillations of membrane potential were reversibly abolished by low Na(+)-Ringer fluid or tetrodotoxin.7. Neither type of initiating event was appreciably affected by concentrations of D-tubocurarine which blocked extrajunctional sensitivity to acetylcholine.8. We conclude that spontaneous action potentials under these conditions arise from a localized Na(+)-conductance change in the membrane of the active fibre; this conductance change is distinct from the increased Na(+)-conductance which follows the interaction of acetylcholine with its receptor. Spontaneous activity in single, denervated muscle fibres is cyclical and self-inhibiting (Purves & Sakmann, 1974); thus the Na(+)-conductance change underlying the initiation of spontaneous action potentials is affected by muscle fibre activity.

Physiological and morphological identification of hypothalamic magnocellular neuroendocrine cells in goldfish preoptic nucleus

1. Intracellular recordings were made from antidromically identified neurones in the goldfish preoptic nucleus and Procion Yellow was ejected from the recording pipette, marking these magnocellular neuroendocrine cells diffusely, for histological identification.2. In confirmation of earlier studies these preoptic neuroendocrine cells showed resting membrane potentials of 47 mV, action potentials up to 85 mV, action potentials of long duration (3.9 msec) occurring in two steps, long-lasting hyperpolarizing after-potentials and orthodromic driving from olfactory input.3. Magnocellular neuroendocrine cells exhibited all-or-none jumps to shorter antidromic latencies as pituitary stimulus strength increased, suggesting multiple branching of the ;axones' either near the preoptic nucleus or within the neural lobe.4. I find three morphological types of neuroendocrine cells throughout the magnocellular part of the preoptic nucleus. Cell Type I is a large (37 mum), multipolar neurone, 48 mum from the ependyma, with fine ;dendrites' projecting into the lateral hypothalamus and within the preoptic nucleus, with multiple branched ;axones'. Cell Type II is a large (31 mum), multipolar neurone, 24 mum from the ependyma, with a coarse ;dendrite' to the ependyma and fine ;dendrites' within the preoptic nucleus, with limited ;axonal' branching. Cell Type III is a small (18 mum), multipolar neurone, 46 mum from the ependyma, with fine ;dendritic' processes distributed within the preoptic nucleus, with limited ;axonal' branching.5. I conclude that magnocellular neuroendocrine cells show electrical membrane properties of other central neurones with both physiological and morphological evidence for multiple ;axonal' branching. The three identifiable neuroendocrine cell types (I, II, III) are distributed widely within the anatomical limits of the preoptic nucleus, pars magnocellularis, with each type receiving ;specific' input connexions and with unique output pathways. I suggest that these three types of neuroendocrine cells may be related to the ;cellular' secretion of ;specific' neurohypophysial hormones and neurophysins.

Electrically induced release of acetylcholine from denervated Schwann cells

1. Focal electrical stimulation of Schwann cells at the end-plates of denervated frog muscles elicited slow depolarizations of up to 30 mV in the muscle fibres. This response is referred to as a Schwann-cell end-plate potential (Schwann-e.p.p.).2. Repeated stimulation sometimes evoked further Schwann-e.p.p.s, but they were never sustained for more than 30 pulses. Successive e.p.p.s varied in amplitude and time course independently of the stimulus.3. The Schwann-e.p.p.s were reversibly blocked by curare, suggesting that they result from a release of acetylcholine (ACh) by the Schwann cells.4. ACh release by electrical stimulation did not seem to occur in quantal form and was not dependent on the presence of calcium ions in the external medium; nor was it blocked by tetrodotoxin.5. Stimulation which caused release of ACh also resulted in extensive morphological disruption of the Schwann cells, as seen with both light and electron microscopy.6. It is concluded that electrical stimulation of denervated Schwann cells causes break-down of the cell membrane and releases ACh, presumably in molecular form.

Age changes in cross striated muscle of the rat

1. Senile muscle atrophy is characterized by a marked reduction in the frequency of spontaneous transmitter release with no electrophysiological evidence of denervation.2. In spite of the reduced number of muscle fibres, there is no ultrastructural evidence for denervation at the end-plates. There is agglutination of synaptic vesicles, neurotubules and filaments, thickening of the basement membrane, widening of the primary synaptic cleft, and irregular branching of the junctional infoldings, but no axonal degeneration.3. The contractile process in senile muscles is slowed down as is indicated by a prolongation of contraction time, latency period, maximum rate of twitch tension and relaxation time.4. The muscle fibres show proliferation of the T system and increased SR but no fragmentation as is observed in denervation atrophy.5. Senile muscle atrophy thus presents some specific features affecting both pre- and post-synaptic structures, related to a very slow process of deterioration of the neuromuscular contact.