Excerpt from Assessment of Neuromuscular Transmission

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Our current understanding of how the motor signal travels from nerve to muscle came to us through the efforts of clinical and basic scientists whose work represents an amazing triumph of human understanding. Equally amazing is the fact that we can analyze many aspects of these minute molecular and cellular processes in the clinic or at the bedside by using the relatively simple methods discussed in this article. This is truly one of the best examples of what is now called translational research (ie, application of basic science knowledge to the clinical situation).

The modern physician usually thinks of myasthenia gravis (MG) when hearing the term neuromuscular junction (NMJ). This is entirely appropriate because research on MG has always been at the center of our understanding of the NMJ. A brief history is discussed.

The first patient with MG ever to be described in medical writings may have been a Native American named Chief Opechankanough. Accounts of his weakness and ptosis, which improved with rest, were given by colonial physicians who examined him in the early 1600s and who sent accounts of his condition back to England. These accounts were not published in a formal medical journal. Chief Opechankanough was the uncle of a much more famous Native American named Matoaka (better known as Pocahontas). In 1672, Thomas Willis (known for the Circle of Willis) described and published the case of an English woman who also probably had MG. These physicians had no idea about the origin of the disease. Numerous cases were noted after that. In 1879, William Erb described several. In 1893, Samuel Goldflam gave excellent clinical descriptions. In 1895, Friedrich Jolly originated the term myasthenia gravis pseudoparalytica. He also pioneered one of the assessment methods discussed later; the method is now called repetitive stimulation, but some still call it the Jolly test. Still, there was little clue regarding the basic etiology of the condition.

The next great advance came in the 1930s when the English doctor, Mary Broadfoot Walker (one of the first British female physicians), showed that injections of physostigmine could temporarily increase the strength of persons with MG and reverse such symptoms as ptosis. She reported her findings in the Lancet in 1934. The modern Tensilon test, another assessment method described later, is essentially the same as Dr Walker's technique. Her work, especially when combined with the basic pharmacologic studies of such contemporaries as Dale and Feldberg on cholinergic neuromuscular transmission, was a clear indication that some aspect of cholinergic transmission at the nerve-muscle synapse was impaired in MG. However, the nature of the impairment and how it could be further assessed were still unknown.

About 30 years before Dr Walker's triumph, another clue along a different line was discovered—the immune system. In 1901, the famous pathologist Karl Weigert noted that lymphoid cells were present in muscle and other tissue of persons with MG and made the connection between MG and thymic hypertrophy and/or neoplasia. In 1905, neurologist E. Farquhar Buzzard noted collections of lymphocytes ("lymphorrhages") in muscle and other tissue of those with MG. In 1936, similar findings led clinician-scientist E. H. Norris to consider the thymus as a fundamental source of the pathology in MG. In fact, clinicians began to use thymectomy as a treatment of MG shortly after Weigert's and Buzzard's observations. At first, they operated for actual thymoma but later for thymic hyperplasia and ultimately for MG without any apparent thymic abnormality. In 1911, Ernst Ferdinand Sauerbruch, who then practiced in Zurich, was the first surgeon to use thymectomy for MG. Blalock, a famous American surgeon, also pioneered this procedure.

Throughout the first part of the 20th century, evidence connecting the thymus and immunity to MG continued to accumulate. In 1960, Dr John A. Simpson enunciated an interesting hypothesis: the cause of MG "is an ‘auto-immune' response of muscle in which an antibody to end-plate protein may be formed." Some later authors referred to this as "guessing it right," but it was far more than a guess. He had a deep understanding of disease processes and profound knowledge of other immunologic processes, which he brought to bear on the question. He thoroughly understood the neuromuscular assessment techniques of his day, and he had the overall insight and knowledge to make a brilliant extrapolation. Nonetheless, he really did not prove his case, and many other competing hypotheses remained active for almost 15 years. The two big points of contention were the following: (1) which side of the nerve-muscle junction contained the defect in MG—the presynaptic nerve endings or the postsynaptic motor endplate and (2) what was the fundamental etiology of the disease? Was it really autoimmune as Simpson speculated?

In the 1970s, a tremendous amount of information came together about the neuromuscular junction. Use of a snake toxin, alpha-bungarotoxin, which bound specifically to certain portions of the acetylcholine receptor (AChR) was pioneered by pharmacologists C. C. Chang, C. Y. Lee, and L. F. Tseng. Pioneering work by clinician-scientists such as Andrew Engel, Douglas Fambrough, Edward Lambert, Vanda Lennon, Daniel Drachman, Eric Stalberg, Joze Trontelj, Jan Ekstedt, John Newsom-Davis, David Richman, Marjorie Seybold, and Klaus Toyka helped tie together the emerging immunologic/neurophysiologic linkages that hinted strongly at the postsynaptic immunologic nature of the disease. The group at the Salk Institute, especially Jim Patrick, Jon Lindstrom, and their associates, raised antibodies to various subunits of the receptor and ultimately showed that the disease could be reproduced by antibodies to AChR.

Largely as a result of the scientific focus on the neuromuscular junction related to MG and other neuromuscular junction diseases, today we have a highly detailed knowledge of the basic science behind neuromuscular transmission. Although there are still aspects that are not known, the essential framework is well known to most first- or second-year medical students. As a simple summary, the major steps are listed below.

1. The nerve action potential arrives at the axon terminal. Voltage-gated calcium channels open, and calcium ions enter the neuron.
2. A biochemical cascade ensues causing the vesicles, which contain acetylcholine (ACh), to fuse with the cell membrane, releasing Ach into the synaptic cleft. This cascade requires a synaptic fusion complex that is composed of SNARE proteins. The acronym SNARE is short for soluble NSF attachment factor, and NSF means N-ethylmaleimide sensitive fusion protein, or N-ethylmaleimide sensitive factor. SNARE proteins are involved in the release of numerous neurotransmitters in many species from mammals through yeast, and they are part of a large superfamily of proteins. For ACh release at the neuromuscular junction, some of the key SNARE proteins are synaptobrevin, SNAP-25 (synaptosomal-associated protein of 25 kd), and syntaxin I.
3. The ACh diffuses across the synaptic cleft and binds to the nicotinic AChRs.
4. The AChRs, which are sodium ion channels, open and allow sodium ions to flow into the muscle cell. Several other nearby membrane proteins including agrin, rapsyn, and muscle-specific tyrosine kinase (MUSK) also influence the function and positioning of the AChRs.
5. This inrush of sodium (plus an exit of potassium through potassium channels) depolarizes the muscle cell membrane. If depolarization is sufficient, it triggers a regenerative muscle action potential that spreads throughout the cell into the transverse tubules releasing calcium from the sarcoplasmic reticulum and initiating the process of muscle contraction.
6. After a certain time, the ACh comes off of the AChR and acetylcholinesterase degrades it into an acetate group and choline. The choline is taken directly back into the neuron where it is combined with another acetate to reform acetylcholine, which is repackaged into a vesicle.

Perhaps as amazing as our ability to understand what goes on at the myoneural synapse, is the fact that we can even assess many parts of this detailed molecular process right in our medical offices using techniques and devices that are not exotic or costly.

The clinician now has at least 4 ways of assessing neuromuscular transmission:

1. History and physical examination: Though the phrase neuromuscular assessment calls to mind electrical techniques and laboratory methods, the history and physical examination are extremely valuable assessment methods. Therefore, some aspects of the bedside or office examination that the average non-neurologic physician might not think about are discussed.
2. Pharmacologic methods
3. Neurophysiologic testing
4. Neuroimmunologic testing

In the following sections, how these 4 types of assessments apply to MG is described in detail. In so doing, the major features of the assessment methods are documented. Then, many of the other neuromuscular junction disorders and how the assessment methods apply to them are discussed.

Please click here to view the full topic text: Assessment of Neuromuscular Transmission