What is the role of the pre-synaptic region in myasthenia gravis (MG)?

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

Each motor neuron has an axon that branches distally to provide nerve terminals that innervate muscle fibers through the NMJ. Collectively, they are known as the motor unit. A muscle fiber is innervated by only one motor neuron with the exception of extraocular muscle fibers where single muscle fibers may receive multiple innervation. The motor nerve loses its myelin sheath as it approaches near the NMJ where it begins to divide into terminal branches. Each terminal branch of an axon, as it nears an individual muscle fiber, expands into a presynaptic terminal bouton that lies in a depression in the muscle membrane. A basement membrane overlies this terminal interface between the bouton and its muscle fiber, constituting part of the muscle end-plate. The terminal bouton has a number of subcellular components including neurotubules, neurofilaments, multiple mitochondria, and a large number of membrane-bound vesicles ranging from 300 to 500 Angstrom units in diameter, called synaptic vesicles. Each vesicle contains approximately 1 quantum of acetylcholine, which equals 10,000 molecules of acetylcholine. A single nerve terminal has approximately 200,000 synaptic vesicles. These vesicles are organized in 3 discrete groups.


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