Histometric study of neuromuscular junction ultrastructure. I. Myasthenia gravis

The Neuromuscular Junction: Roles in Aging and Neuromuscular Disease

The neuromuscular junction (NMJ) is a specialized synapse that bridges the motor neuron and the skeletal muscle fiber and is crucial for conversion of electrical impulses originating in the motor neuron to action potentials in the muscle fiber. The consideration of contributing factors to skeletal muscle injury, muscular dystrophy and sarcopenia cannot be restricted only to processes intrinsic to the muscle, as data show that these conditions incur denervation-like findings, such as fragmented NMJ morphology and corresponding functional changes in neuromuscular transmission. Primary defects in the NMJ also influence functional loss in motor neuron disease, congenital myasthenic syndromes and myasthenia gravis, resulting in skeletal muscle weakness and heightened fatigue. Such findings underscore the role that the NMJ plays in neuromuscular performance. Regardless of cause or effect, functional denervation is now an accepted consequence of sarcopenia and muscle disease. In this short review, we provide an overview of the pathologic etiology, symptoms, and therapeutic strategies related to the NMJ. In particular, we examine the role of the NMJ as a disease modifier and a potential therapeutic target in neuromuscular injury and disease.

Nature and Action of Antibodies in Myasthenia Gravis

This article discusses antibodies associated with immune-mediated myasthenia gravis and the pathologic action of these antibodies at the neuromuscular junctions of skeletal muscle. To explain how these antibodies act, we consider the physiology of neuromuscular transmission with emphasis on 4 features: the structure of the neuromuscular junction; the roles of postsynaptic acetylcholine receptors and voltage-gated Na⁺ channels and in converting the chemical signal from the nerve terminal into a propagated action potential on the muscle fiber that triggers muscle contraction; the safety factor for neuromuscular transmission; and how the safety factor is reduced in different forms of autoimmune myasthenia gravis.

Myasthenia gravis Lambert-Eaton overlap syndrome

INTRODUCTION

To assess whether a myasthenia gravis (MG) Lambert-Eaton overlap syndrome (MLOS) exists.

METHODS

Case reports that met the universally accepted diagnostic criteria for MG and Lambert-Eaton myasthenic syndrome (LEMS) were sought through a PubMed search. Fifty-five possible cases of MLOS were identified.

RESULTS

Thirty-nine cases met the diagnostic criteria for MG and LEMS. Analysis of clinical features showed that these patients have common MG and LEMS symptoms: oculo-bulbar paresis and good response to anti-cholinesterase for MG and limb weakness and decreased or absent reflexes for LEMS. All had the classical LEMS pattern in the repetitive nerve stimulation test: low compound muscle action potential amplitude and incremental response > 60% with brief exercise or at high rate of stimulation. Eight patients had combined positive acetylcholine receptor antibody (AChR-ab) or muscle-specific kinase-ab and voltage-gated calcium channel- ab tests.

CONCLUSIONS

A myasthenia gravis Lambert-Eaton overlap syndrome (MLOS) does exist.

Neuromuscular transmission failure in myasthenia gravis: decrement of safety factor and susceptibility of extraocular muscles

An appropriate density of acetylcholine receptors (AChRs) and Na(+) channels (NaChs) in the normal neuromuscular junction (NMJ) determines the magnitude of safety factor (SF) that guarantees fidelity of neuromuscular transmission. In myasthenia gravis (MG), an overall simplification of the postsynaptic folding secondary to NMJ destruction results in AChRs and NaChs depletion. Loss of AChRs and NaChs accounts, respectively, for 59% and 40% reduction of the SF at the endplate, which manifests as neuromuscular transmission failure. The extraocular muscles (EOM) have physiologically less developed postsynaptic folding, hence a lower baseline SF, which predisposes them to dysfunction in MG and development of fatigue during "high performance" eye movements, such as saccades. However, saccades in MG show stereotyped, conjugate initial components, similar to normal, which might reflect preserved neuromuscular transmission fidelity at the NMJ of the fast, pale global fibers, which have better developed postsynaptic folding than other extraocular fibers.

Synthesis and in vitro evaluation of new derivatives of 2-substituted-6-fluorobenzo[d]thiazoles as cholinesterase inhibitors

A series of novel cholinesterase inhibitors based on 2-substituted 6-fluorobenzo[d]thiazole were synthesised and characterised by IR, (1)H, (13)C and (19)F NMR spectroscopy and HRMS. Purity was checked by elemental analyses. The novel carbamates were tested for their ability to inhibit acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The toxicity of the most active compounds was investigated using a standard in vitro test with HepG2 cells, and the ratio between biological activity and toxicity was determined. In addition, the toxicity of the most active compounds was evaluated against MCF7 cells using the xCELLigence system. Structure-activity relationships reflecting the dependence of cholinesterase inhibitors on the lipophilicity of the compounds as well as on the Taft polar and steric substituent constants are discussed. The specific orientation of the inhibitors in the binding site of acetylcholinesterase was determined using molecular docking of the most active compound.

Preparation, in vitro screening and molecular modelling of symmetrical bis-quinolinium cholinesterase inhibitors--implications for early myasthenia gravis treatment

This paper describes the preparation and in vitro evaluation of 18 newly prepared bis-quinolinium inhibitors on human recombinant acetylcholinesterase (AChE) and human plasmatic butyrylcholinesterase (BChE). Their inhibitory (IC(50)) and was compared to the chosen standards ambenonium dichloride, edrophonium chloride, BW284c51 and ethopropazine hydrochloride. One novel compound was found to be a promising inhibitor of hAChE (in nM range) and was better than edrophonium chloride or BW284c51, but was worse than ambenonium chloride. This compound also showed selectivity towards hAChE and it was confirmed as a non-competitive inhibitor of hAChE by kinetic analysis. A molecular modelling study further confirmed its binding to the peripheral active site of hAChE via apparent π-π or π-cationic interactions.

Robotic-assisted thymectomy

Thymectomy is an established therapy for myasthenia gravis. Minimally invasive surgery for thymectomy has been reported, but not clearly shown to be equivalent to open resection. Robotic-assisted thymectomy may provide the benefit of a full resection of thymic tissue and anterior mediastinal tissue for the treatment of myasthenia gravis by a minimally invasive approach. We present a review of the experience of robotic thymectomy.

How myasthenia gravis alters the safety factor for neuromuscular transmission

Myasthenia gravis (MG), the most common of autoimmune myasthenic syndromes, is characterized by antibodies directed against the skeletal muscle acetylcholine receptors (AChRs). Endplate Na(+) channels ensure the efficiency of neuromuscular transmission by reducing the threshold depolarization needed to trigger an action potential. Postsynaptic AChRs and voltage-gated Na(+) channels are both lost from the neuromuscular junction in MG. This study examined the impact of postsynaptic voltage-gated Na(+) channel loss on the safety factor for neuromuscular transmission. In intercostal nerve-muscle preparations from MG patients, we found that endplate AChR loss decreases the size of the endplate potential, and endplate Na(+) channel loss increases the threshold depolarization needed to produce a muscle action potential. To evaluate whether AChR-specific antibody impairs the function of Na(+) channels, we tested omohyoid nerve-muscle preparations from rats injected with monoclonal myasthenogenic IgG (passive transfer model of MG [PTMG]). The AChR antibody that produces PTMG did not alter the function of Na(+) channels. We conclude that loss of endplate Na(+) channels in MG is due to complement-mediated loss of endplate membrane rather than a direct effect of myasthenogenic antibodies on endplate Na(+) channels.

Myasthenia gravis

Myasthenia gravis (MG) is a rare, autoimmune neuromuscular junction disorder. Contemporary prevalence rates approach 1/5,000. MG presents with painless, fluctuating, fatigable weakness involving specific muscle groups. Ocular weakness with asymmetric ptosis and binocular diplopia is the most typical initial presentation, while early or isolated oropharyngeal or limb weakness is less common. The course is variable, and most patients with initial ocular weakness develop bulbar or limb weakness within three years of initial symptom onset. MG results from antibody-mediated, T cell-dependent immunologic attack on the endplate region of the postsynaptic membrane. In patients with fatigable muscle weakness, the diagnosis of MG is supported by: 1. pharmacologic testing with edrophonium chloride that elicits unequivocal improvement in strength; 2. electrophysiologic testing with repetitive nerve stimulation (RNS) studies and/or single-fiber electromyography (SFEMG) that demonstrates a primary postsynaptic neuromuscular junctional disorder; and 3. serologic demonstration of acetylcholine receptor (AChR) or muscle-specific tyrosine kinase (MuSK) antibodies. Differential diagnosis includes congenital myasthenic syndromes, Lambert Eaton syndrome, botulism, organophosphate intoxication, mitochondrial disorders involving progressive external ophthalmoplegia, acute inflammatory demyelinating polyradiculoneuropathy (AIDP), motor neuron disease, and brainstem ischemia. Treatment must be individualized, and may include symptomatic treatment with cholinesterase inhibitors and immune modulation with corticosteroids, azathioprine, cyclosporine, and mycophenolate mofetil. Rapid, temporary improvement may be achieved for myasthenic crises and exacerbations with plasma exchange (PEX) or intravenous immunoglobulin (IVIg). Owing to improved diagnostic testing, immunotherapy, and intensive care, the contemporary prognosis is favorable with less than five percent mortality and nearly normal life expectancy.

Lesions of rat skeletal muscle after local block of acetylcholinesterase and neuromuscular stimulation

In skeletal muscle, a local increase of acetylcholine (ACh) in a few end plates has been hypothesized to cause the formation of contraction knots that can be found in myofascial trigger points. To test this hypothesis in rats, small amounts of an acetylcholinesterase inhibitor [diisopropylfluorophosphate (DFP)] were injected into the proximal half of the gastrocnemius muscle, and the muscle nerve was electrically stimulated for 30-60 min for induction of muscle twitches. The distal half of the muscle, which performed the same contractions, served as a control to assess the effects of the twitches without DFP. Sections of the muscle were evaluated for morphological changes in relation to the location of blocked end plates. Compared with the distal half of the muscle, the DFP-injected proximal half exhibited significantly higher numbers of abnormally contracted fibers (local contractures), torn fibers, and longitudinal stripes. DFP-injected animals in which the muscle nerve was not stimulated and that were allowed to survive for 24 h exhibited the same lesions but in smaller numbers. The data indicate that an increased concentration of ACh in a few end plates causes damage to muscle fibers. The results support the assumption that a dysfunctional end plate exhibiting increased release of ACh may be the starting point for regional abnormal contractions, which are thought to be essential for the formation of myofascial trigger points.

Expression of ciliary neurotrophic factor receptor in myasthenia gravis

In myasthenia gravis, high anti-nicotinic receptor (AChR) antibody titers are not always associated with severity of the disease, suggesting the involvement of other pathological effectors. We showed that ciliary neurotrophic factor receptor (CNTFR) gene expression was higher in muscle biopsy tissue from severely affected MG patients regardless of anti-nAChR antibody titer. This increase was also triggered in vitro by a seric factor from MG patients. CNTFR protein expression was decreased in muscles from seropositive MG patients only.Altogether, our data indicate that the alteration of CNTFR expression in some MG patients could contribute to the severity of the disease in a subgroup of patients.

T cell responses in EAMG-susceptible and non-susceptible mouse strains after immunization with overlapping peptides encompassing the extracellular part of Torpedo californica acetylcholine receptor alpha chain. Implication to role in myasthenia gravis of autoimmune T-cell responses against receptor degradation products

To study the role in myasthenia gravis (MG) of peptides resulting from acetylcholine receptor (AChR) degradation, we examined the ability of AChR peptides to induce T cell responses that are capable of cross-reacting with intact AChR. The studies were carried out in an experimental autoimmune MG (EAMG)-susceptible mouse strain [C57BL/6 (B6)] as well as in two non-susceptible strains [B6.C-H-2bm12 (bm12) and C3H/He]. A set of overlapping peptides encompassing the extracellular part (residues 1-210) of the alpha-chain of Torpedo californica (t) AChR were used, individually or in equimolar mixtures, as immunogens. In B6, immunization with peptides alpha45-60, alpha111-126, alpha146-162 and alpha182-198 gave T cells that responded in vitro to the correlate immunizing peptide. Only the T cells against the latter three peptides cross-reacted with tAChR. Peptide alpha146-162 exhibited the highest in vitro reaction with the immunizing peptide and cross-reaction with tAChR. T cells obtained by immunization of B6 with an equimolar mixture of the peptides responded in vitro to peptides alpha111-126, alpha146-162 and alpha182-198 and cross-reacted very strongly with tAChR. In bm12 and C3H/He, a number of peptides evoked, when used individually as immunogens, strong or moderate T cell responses that recognized in vitro the correlate immunizing peptide but cross-reacted poorly with tAChR. Immunization with the mixture of the peptides gave T cells that recognized several peptides in each strain butdid not cross-react with alpha146-162 or tAChR. The results indicate that the ability to recognize alpha146-162 or AChR by T cells against peptides resulting from receptor degradation can account for the susceptibility to, and aggravation of, MG in B6.

End-plate voltage-gated sodium channels are lost in clinical and experimental myasthenia gravis

This study examined the loss of voltage-gated Na+ channels as well as acetylcholine receptors (AChRs) from the end-plate region in patients with acquired myasthenia gravis (MG) and in rats with experimental autoimmune passively transferred MG (PTMG). Rats received a monoclonal IgG antibody directed against an extracellular epitope of the nicotinic acetylcholine receptor of muscle (AChR) to produce PTMG. At the end-plate border we examined miniature end-plate potentials (MEPPs), sodium current (INa) amplitude, and action potential (AP) properties; the latter two were also examined on the extrajunctional membrane. In the normal situation, the safety factor for neuromuscular transmission is ensured by the large INa at the end plate, which reduces the AP threshold. Among different fiber types, INa was largest for type IIb fibers and smallest for type I fibers. When end-plate border properties of fibers from 3 MG patients and 15 PTMG rats were compared with controls, INa was reduced, AP thresholds were higher, and rates of AP rise were reduced. Amplitudes of MEPPs and INa at the end plate indicated that loss of AChRs was greater than loss of Na+ channels in patients with MG and rats with PTMG; INa was reduced to about 60% of control values, whereas MEPPs were reduced to less than 30% of control values. On the extrajunctional membrane, INa and AP thresholds and rates of rise were similar for MG patients, PTMG rats, and controls. This evidence for loss of voltage-gated Na+ channels at the motor end plate in both patients with MG and in rats with PTMG reveals a hitherto unrecognized consequence of the end-plate damage initiated by the binding of complement-fixing IgG to end-plate AChRs.

Myasthenia gravis

Adult-onset myasthenia gravis is an acquired autoimmune disorder of neuromuscular transmission in which acetylcholine receptor antibodies attack the postsynaptic membrane of the neuromuscular junction. Although the cause of this disease is unknown, the role of immune responses in its pathogenesis is well established. Circulating acetylcholine receptor antibodies are present in 80% to 90% of patients with the generalized form of myasthenia gravis. Most patients have ptosis, diplopia, dysarthria and dysphagia. The weakness and fatigue worsen on exertion and improve with rest. Respiratory muscle and limb weakness are rare at the onset of the disease. For the past two decades, there has been considerable progress in understanding the diagnosis and management of myasthenia gravis. The diagnosis is based on clinical presentation, neurologic examination, and confirmation by means of electrophysiologic testing and immunologic studies. Myasthenia gravis mimics many neuromuscular diseases and even illnesses such as depression and chronic fatigue syndrome. One should always exclude drug-induced myasthenia gravis for all patients. With the introduction of new modalities of treatment, particularly immunosuppressive or immunomodulating drugs, plasma exchange and thymectomy, the morbidity and mortality of myasthenia gravis have decreased dramatically to the point that myasthenia gravis should not be considered as serious a disease as it once was. Although the several therapeutic options are usually effective and have meant independence in daily life to many patients with myasthenia gravis, well-designed, controlled, prospective studies are still lacking.

Myasthenia gravis-like syndrome induced by expression of interferon gamma in the neuromuscular junction

Abnormal humoral responses toward motor end plate constituents in muscle induce myasthenia gravis (MG). To study the etiology of this disease, and whether it could be induced by host defense molecules, we examined the consequences of interferon (IFN) gamma production within the neuromuscular junction of transgenic mice. The transgenic mice exhibited gradually increasing muscular weakness, flaccid paralysis, and functional disruption of the neuromuscular junction that was reversed after administration of an inhibitor of acetylcholinesterase, features which are strikingly similar to human MG. Furthermore, histological examination revealed infiltration of mononuclear cells and autoantibody deposition at motor end plates. Immunoprecipitation analysis indicated that a previously unidentified 87-kD target antigen was recognized by sera from transgenic mice and also by sera from the majority of human MG patients studied. These results suggest that expression of IFN-gamma at motor end plates provokes an autoimmune humoral response, similar to human MG, thus linking the expression of this factor with development of this disease.

Ocular myasthenia: a protean disorder

Ocular myasthenia is a localized form of myasthenia clinically involving only the extraocular, levator palpebrae superioris, and/or orbicularis oculi muscles. Ocular manifestations can masquerade as a variety of ocular motility disorders, including cranial nerve and gaze palsies. A history of variable and fatiguable muscle weakness suggests this diagnosis, which may be confirmed by the edrophonium (Tensilon) test and acetylcholine receptor antibody titer. Anticholinesterases, corticosteroids and other immunosuppressive agents, and other therapeutic modalities, including thymectomy and plasmapheresis, are used in treatment. As the pathophysiology of myasthenia has been elucidated in recent years, newer treatment strategies have evolved, resulting in a much more favorable prognosis than several decades ago. This review provides historical background, pathophysiology, immuno-genetics, diagnostic testing, and treatment options for ocular myasthenia, as well as a discussion of drug-induced myasthenic syndromes.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Determination of the lengths of nonisotropic linear features in micrographs

The method described was devised to facilitate rapid and reasonably accurate estimations of the length of a nonisotropic linear feature in a micrograph. It arose from studies in which we were determining the length of the Purkinje cell layer in each of a set of serial sections through the rat cerebellum. The Purkinje cell layer appears as a linear feature in such sections and the simplest and most rapid method of estimating the length of this type of feature is to count the number of intersections that it makes with a series of equally spaced parallel test lines (see, e.g., Weibel, E.R., 1979: Stereological Methods Vol. 1, Practical Methods for Biological Morphometry). In many sections, the Purkinje cell layer was markedly nonisotropic, and the length obtained by this method varied very considerably depending on the orientation of the section relative to the test lines. The present method employs two orthogonal sets of parallel test lines, and the length of the feature is estimated from the square root of the sum of the squares of the counts of the number of intersections with each of the two sets of lines. The result obtained varies very little with the relative orientations of the feature and the test grid and a good estimate of the length can be obtained from the counts from a single random placement of the grid on the section. It has been found that the technique can be carried out efficiently and reliably by relatively inexperienced personnel, and the results are obtained more rapidly than when alternative methods for estimating dimensions of nonisotropic features are used.

Motor endplate involvement in the extraocular muscles of the myotonic rat

The motor endplates from the extraocular muscles of rats administered 20,25-diazacholesterol were examined by electron microscopy. Many anomalies were found at various levels of the junctional complex. Electron microscopy showed disrupted sole-plate nuclei, disruptions of the junctional sarcoplasm, atypical sarcoplasmic extensions, simplified postsynaptic areas, and interposition of Schwann cell cytoplasm between axonal terminal and muscle fiber with a reduplication of basement membrane, as well as atrophic and disrupted axonal terminals. These data indicate that the motor endplate is affected as well as the muscle fiber in myotonic dystrophy.

[Muscular biopsy in myasthenia gravis: histochemical and morphometric study of 4 cases]

Myasthenia gravis is a neuromuscular disease characterized by excessive fatigability of muscle function and particularly involves muscles innervated by the cranial nerves. It is believed that the defect is in the neuromuscular junction. The authors studied histochemical and morphometric findings in 4 patients with myasthenia gravis showing that the type II fibres were significantly smaller than the type I fibres.

Transplacental transmission of experimental autoimmune myasthenia gravis. A morphological study

Using neonates born from experimental autoimmune myasthenic rabbits, the authors demonstrated antibodies to acetylcholine receptor (AChR) in the newborn sera. By radioimmunoassay, antibody titers of 1-day-old neonates were roughly one seventh to one ninth of the mothers. At 8 weeks postpartum, the antibody was no longer detectable. Ultrastructural observations of the intercostal muscles of the neonates revealed two types of changes. The first type was degenerative alterations in the postsynaptic membrane. The second type of change, which was morphometrically analyzed, was immaturity of postsynaptic membrane structure with underdeveloped secondary synaptic clefts. After 28 days postpartum, these changes were not visible, thereby indicating that the process is reversible as the antibody titer decreases. These results suggested the possibility that the antibody to AChR, transferred transplacentally, arrested the development of postsynaptic structure, although reversible, by blocking of the receptor sites in the end-plate.

Factors affecting the susceptibility of different strains of mice to experimental myasthenia gravis

Mice immunized with purified AChR (T. californica) invariably form anti-AChR antibodies and often develop a condition of extreme muscular weakness and flaccid paralysis. Pharmacological, physiological, and ultrastructural studies indicate that the pathophysiology of EMG in the mouse closely resembles that of human MG. The single episode of muscular weakness typically found in mouse EMG differs from the acute phase of rat EMG in that macrophages and other phagocytes do not appear to play an active role in the destruction of the neuromuscular junction. The frequency of paralysis in mice immunized with AChR is highly strain dependent and is not attributable to polymorphisms with respect to susceptibility to cholinergic blockade. The incidence of paralysis does not correlate with the magnitude of the humoral response to either T. californica or mouse AChR. Because both paralyzed and nonparalyzed mice form antibodies which are able to increase the rate of both junctional and extrajunctional AChR degradation, the mere presence of antibodies reactive with cell surface antigenic determinants of AChR is not sufficient for the induction of paralysis. While it is still possible that antibody-induced degradation of AChR may be necessary for the induction of paralysis, these studies rule out the possibility that antigenic modulation of AChR is sufficient to account for the induction of paralysis in mouse EMG. In the present studies alleles of the two loci were identified which significantly effect the probability with which mice immunized with AChR can be expected to become paralyzed, the MHC and the IgCH region. Because one genotype, H-2b, Ig-1b segregated with high susceptibility to EMG in four strains derived from three dissimilar backgrounds, these studies strongly suggest that susceptibility to the development of paralysis is a heritable trait determined by regions of the mouse genome which regulate immune responsiveness.

Choline acetyltransferase in skeletal muscle from patients with myasthenia gravis

Acetylcholine synthesis in homogenates of human intercostal muscle was measured by a radiochemical method. Choline acetyltransferase activity in control muscle was about 20 nmol . g-1 . h-1. The enzyme was found only in the endplate area of the muscle. At high substrate concentrations its activity was overshadowed by the acetylcholine synthesizing activity of a different enzyme not saturated by 10 mM-choline. The nonspecific enzyme was present at and away from the endplate area. Choline acetyltransferase in parasternal samples of intercostal muscle from myasthenia gravis patients was about 2.5 times higher than in samples, taken from a more lateral location, of control patients, but the Km for choline was not altered (0.24 mM). It is suggested that in myasthenia gravis the shortage of acetylcholine receptors is partially compensated for by increased synthesis, storage, and release of the transmitter.

Studies of human myasthenia gravis: electrophysiological and ultrastructural evidence compatible with antibody attachment to acetylcholine receptor complex

Neuromuscular junctions from patients with early onset and chronic myasthenia gravis were examined by electrophysiological and ultrastructural techniques. Acetylcholine (AcCh) sensitivities were reduced by 34-63% in early onset myasthenia and 60-80% in chronic myasthenia. Ultrastructural analysis revealed that virtually all junctional folds of the early onset patients were intact but that the AcCh-receptor-rich crests of these folds were uniformly covered by an attached layer of 30 X 70 A particles arranged in small tufts or rosettes. In chronic myasthenic endplates, however, junctional fold crests were destroyed, apparently being replaced by vesicular membrane debris similarly labeled by tufts of 30 X 70 A particles. Thus, the initial reduction in junctional AcCh sensitivity observed in early onset myasthenia gravis may be attributed at least in part to in situ masking or inactivation of AcCh receptors, whereas the marked decrease in AcCh sensitivity observed in the chronic myasthenic patient may represent a combination of two factors: (a) in situ masking of AcCh receptors and (b) destruction of the receptor-containing crests of the junctional folds. These observations are compatible with an autoimmune etiology of myasthenia gravis initially involving an apparent antibody attachment to one or more components of the functional AcCh receptor complex, followed by systematic destruction and removal of junctional folds by both humoral and cell-mediated autoimmune responses.

Morphological observations on motor end-plates in rabbits with experimental myasthenia

The morphology of motor end-plates in rabbits immunized with Torpedo nicotinic acetylcholine receptor (nAChR) has been studied by light and electron microscopy. Rabbits were studied either after one period of paralysis, some in parallel with electrophysiological recordings of MEPPs and EPPs and of Naja naja alpha-neurotoxin binding properties or after recovery followed by a second paralysis. Changes in the sub-neural apparatus were noted after cholinesterase staining only in the latter group. Ultrastructurally, however, most end-plates in both groups contained a wide range of abnormalities. Many were similar in appearance to those observed in human myasthenia gravis (MG). This further supports the theory that immunized rabbits can be used as a model for myasthenia gravis. In the rabbits with 1 period of paralysis an acute stage of influence on the neuromuscular junction seemed to be present while simplified motor end-plates typical for human MG were mostly found in rabbits with 2 periods of paralysis. Short post-synaptic folds in conjugation with thickeneed membrane-bound vesicles at their tops, inside the basement membrane, were frequently observed. These were interpreted as if the crests of the folds containing nAChR had degenerated and had been budded off. If so, a large number of receptor sites had been lost which would be one possible explanation for the lowered capacity of the muscles to bind Naja naja alpha-neurotoxin. Membrane thickenings with projections and striations were interpreted as reflecting ACh receptors and were observed in the post-junctional membrane without proximity to the nerve terminal. The degeneration of the top of the post-synaptic folds and the occurrence of receptors at other locations within the motor end-plate will result in a widened distance between the nerve terminal and the receptors, which can explain previous interpretations of a presynaptic defect in MG.

The results to be expected from electrical testing in the diagnosis of myasthenia gravis

The purpose of this study was to derive numerical data concerning the probability of diagnosing myasthenia gravis by electrical tests, employing repetitive stimulation of motor nerves. This was done by recording the compound muscle action potential (CMAP) from 80 patients with clear-cut myasthenia gravis, using a number of different, reportedly useful, testing techniques. In 95% of these patients, the diagnosis could be documented by the careful application of rather simple methods, providing several muscles, including a proximal one such as deltoid, were studied.

Ultrastructural studies on the acetylcholine receptor at motor end plates of normal and pathologic muscles

The acetylcholine receptors at the mammalian motor end plate have been counted and their distribution there determined ultrastructurally. Electron microscope autoradiography applied to muscles labeled with alpha-(3H)bungarotoxin was used for this purpose. The receptors are distributed asymmetrically along the postsynaptic membrane, being concentrated at the fold crests-that portion nearest to the presynaptic membrane. The density of receptor sites at that region is estimated to be 20,000-25,000 per mum2 of membrane surface. This density holds for the several species and muscle types thus far examined and appears to be a constant parameter of the motor end plate. It determines the limit of responsiveness to acetylcholine. By contrast, the enzyme acetylcholinesterase is found to be distributed evenly over the folds and may reside in the intersynaptic matrix. When mouse diaphragm end plates from dystrophic animals or animals following 5-day denervation were similarly examined, no significant alterations in either the density or the distribution of the receptor sites were found. Similarly, dystrophic muscles in chickens possess an unaltered number of receptors at their end plates. A model is outlined to correlate receptor and cholinesterase concentrations with known aspects of transmitter release. These findings may have relevance to some of the electrophysiological abnormalities seen in myasthenia gravis.

The role of humoral and cellular immune factors in neuromuscular block in myasthenia gravis

The neuromuscular block of myasthenia gravis appears to be due to decreased and abnormal responsiveness of the motor end plates to transmitter, and perhaps a decreased number of functioning end plates. The presence in myasthenic patients of serum globulins that bind to a bariety of cellular and subcellular components, and of thymic abnormalities, has encourage the search for humoral and cellular immune factors that may be responsible for the anatomic and functional defects. Attempts to demonstrate neuromuscular blocking activity in the serum have been inconclusive. While 30% to 48% of myasthenic patients have globulins that react with muscle membrane, striations, ribonucleporotein, and thymic epithelial cells, these globulins have not been shown to react with the end plate or acetylcholine receptor, or to impair neuromuscular transmission. Antinuclear, rheumatoid, antimitochondrial antithyroid, anti-smooth-muscle, and anti-gastric-parietal cell factors are found in the serum of 3% to 16% of myasthenic patients, but these are much more commonly present in other diseases. However, antibodies to acetylcholine receptor from electric tissue, which were recently reported in the serum of three fourths of myasthenic patients, may prove to be more directly related to the neuromuscular block. While the majority of myasthenic patients have thymic abnormalities, including thymoma in 9% of patients and hyperplastic thymus in 66% of patients, the remaining 25% of patients have normal, involuted, or undetectable thymus. Thymectomy has a favorable effect in about two thirds of myasthenic patients, but about one third of patients have no benefit. Thirty-two patients have been described who developed myasthenia gravis after total thymectomy and presumably in the absence of the thymus. Thymus lymphocytes of myasthenic patients have some differences from those of normal subjects, including a greater proportion of B cells, but their significance is not known. Attempts to demonstrate neuromuscular blocking activity in the thymus of myasthenic patients have been inconclusive. Blood lymphocytes of myasthenic patients also have some differences from those of normal subjects, including a lower proportion of T cells. The proportion of both T and B cells increased following thymectomy. While studies on the immunological reactivity of lymphocytes from myasthenic patients have shown some differences from those of normal subjects, neuromuscular blocking activity has not been demonstrated in these cells or in their extracts. There is increasing evidence that the neuromuscular block of myasthenia gravis is due to alteration of the acetylcholine receptor. The recent reports of antibodies to acetylcholine receptor in the serum of myasthenic patients suggests that these may be responsible for the neuromuscular block, but such action, and the cause of antibody release, remain to be determined.

The motor end plate in myasthenia gravis and in experimental autoimmune myasthenia gravis. A quantitative ultrastructural study

Neuromuscular junction ultrastructure in rat forelimb digit extensor muscle was sequentially and quantitatively investigated in experimental autoimmune myasthenia gravis (EAMG). Experimental animals received a single dose of highly purified eel-electroplax acetylcholine receptor protein plus complete Freund's adjuvant and B. pertussis organisms. During the first 7 days (latent period) after immunization the experimental end plates remained normal. Between day 7 and 11 (acute phase) mononuclear cells infiltrated those regions of muscle where the end plates were located and the was sudden degeneration of the postsynaptic regions with splitting away of the abnormal junctional folds from the underlying muscle fibers. Macrophages entered the gaps arising between the muscle fibers and the separating postsynaptic folds and removed the degenerating folds by phagocytosis. The nerve terminals were displaced from their usual location but maintained their structural integrity. After day 11 (chronic phase) the inflammatory reaction subsided and the nerve terminals returned to the highly simplified postsynaptic regions. Subsequently the postsynaptic folds were reconstituted and again they degenerated. The degeneration involved especially the tips of the folds where acetylcholine receptor sites are concentrated. Immature junctions with poorly differentiated postsynaptic regions and nerve sprouts near end plates were also observed. In two animals that relapsed on day 27 and 33, respectively, degeneration of the postsynaptic folds was more intense than in the remaining animals that had not relapsed during the chronic phase. Morphometric analysis of the end plates demonstrated significant decreases in the postsynaptic membrane length, in the postsynaptic membrane density and in the postsynaptic to presynaptic membrane length ratio in chronic EAMG. In addition, the concentration of synaptic vesicles in the nerve terminals was increased in acute and chronic EAMG while the nerve terminal area was decreased in acute EAMG. The alterations in the nerve terminal may be secondary to the postsynaptic changes. The postsynaptic region is the primary target of the autoimmune reaction in EAMG and the ultrastructural and morphometric abnormalities of the end plate in the chronic phase of the syndrome closely resemble those which have been observed in human myasthenia gravis.

Intercostal muscle biopsy in human neuromuscular disease. Histochemical and electron microscopic studies

External intercostal muscle biopsies were examined histochemically and by electron microscopy. The use of this muscle allowed correlation with physiological and pharmacological studies on the same specimens. Changes observed in musclar dystrophy and motor neurone disease resembled those previously described in biopsied limb muscle and underline the particular usefulness of this preparation in the study of human neuromuscular disease.

The pathology of the muscle spindle in myasthenia gravis

Abnormalities in the morphology and motor innervation of the muscle spindles are described in 4 autopsied cases of myasthenia gravis. There were changes consistent with motor denervation in 5 of 118 spindles examined in sectioned material. In 65 teased, silver-impregnated spindles there was proliferation of the fusimotor innervation due to axonal sprouting occurring in the endings themselves. Motor end-plates on the intrafusal muscle fibres showed "dystrophic" changes similar to those previously described in extrafusal muscle in this disease. The sensory innervation was normal. The possible signifance of these findings is discussed.

Ultrastructure of the syndrome of continuous muscle fibre activity

The ultrastructure of muscle and the myoneural junction of a man of 60 suffering from the syndrome of continuous muscle fibre activity was studied. This syndrome is manifested by disturbances of walking, muscle weakness, permanent muscle contractions and involuntary movements. The myoneural junction was hypertrophied and showed ramifications of the secondary clefts. The presynaptic nerve ending contained no synaptic vesicles. The relationship of these findings to the disease is discussed.