The Neuromuscular Junction and Myasthenia Gravis
The neuromuscular junction is a specialized region where motor nerves chemically synapse with muscle fibers. Each neuromuscular junction is composed of a motor nerve terminal, synaptic space, and the highly-folded endplate region of the skeletal muscle fiber. The neurotransmitter acetylcholine (ACh) is synthesized and stored in vesicles in the motor nerve terminal. When a motor nerve action potential is propagated to the motor nerve terminal, voltage-gated calcium channels in active zones open. The resulting calcium influx initiates exocytosis of ACh which diffuses across the narrow synaptic cleft to bind with acetylcholine receptors (AChR). ACh binding opens ion channels with movement of sodium into the muscle fiber at the endplate. This sodium influx generates an end-plate potential. When the endplate potential amplitude reaches or exceeds the threshold level, a muscle fiber action potential is generated with spread of depolarization from the endplate region to both ends of the muscle fiber. The process of excitation-contraction coupling is thereby initiated, and the muscle fiber contracts. Acetylcholinesterase attached to the endplate basal lamina inactivates ACh by hydrolysis to choline and acetic acid. Choline then undergoes reuptake by the presynaptic nerve terminal for ACh resynthesis.
- Novel Challenges in Myasthenia Gravis
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International Standard Book Number 13 (ISBN-13)