What is the anatomy of myasthenia gravis (MG)?

Updated: Aug 27, 2018
  • Author: Abbas Jowkar, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

The neuromuscular junction (NMJ) serves as a transducer and amplifier to the peripheral nerve’s relatively small electrical current using a chemical signal subserved by neurotransmitter ACh. This in turn produces an electrical current of sufficient intensity and proper location such that it initiates a propagating action potential in the muscle fiber. Thus, the NMJ is considered a chemical synapse that functions as an electrical-chemical-electrical link.

In MG, autoantibodies (immunoglobulin G [IgG1]) develop against nicotinic acetylcholine postsynaptic receptors at the NMJ of skeletal muscles. [1, 2] The reasons for this development are unknown, although it is clear that certain genotypes are more susceptible. [11] To understand MG, it is necessary to become familiar with the normal anatomy and physiology of the NMJ (see the image below). A full appreciation of the normal function of the NMJ is needed to understand the principles underlying diagnostic testing and the mechanisms of therapeutic interventions. 

Normal neuromuscular junction showing a presynapti Normal neuromuscular junction showing a presynaptic terminal with a motor nerve ending in an enlargement (bouton terminale): Synaptic cleft and postsynaptic membrane with multiple folds and embedded with several acetylcholine receptors.

The nerve terminal of the motor nerve enlarges at its end to form the so-called terminal bouton or terminal bulb. This bulbous nerve terminal lies within a groove or indentation along the somatic muscle fiber. The NMJ can be subdivided into 3 major areas: presynaptic region, synpatic space, and post-synaptic membrane. Disorders of neuromuscular transmission can arise from abnormalities in each of these locations.


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