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Posts: 89
Joined: 19 Dec 2017 13:21
Full Name: Lakshmi Venkataraman
Name of Your College/Medical School: SRMC&RI Porur Chennai


Post by lakshmidr »

The neuromuscular junction, as the name suggests is the site where the motor nerve fiber ends (called the axon terminal) on the muscle fiber of a skeletal muscle fiber. It is analogous to the synapse (or nerve-nerve junction) between two adjacent nerve fibers

 Each nerve fibre branches into several axon terminals at its lower end; each axon terminal lies close to an area in the muscle fiber termed motor end plate (see fig). The motor end plate is a specialized site on the membrane of the muscle fiber
 As shown in the figure, the axon terminal becomes wider at the end where it is close to the skeletal muscle fiber membrane that is thrown into numerous folds
 On one end of the neuromuscular junction, we have the axon terminal and the other end are the muscle fibers. The gap between these two is called the synaptic cleft. The axon terminal forms the presynaptic part while the muscle fiber is the post synaptic part
 The neuromuscular junction is therefore a site where transmission of nerve impulse occurs from a nerve to skeletal muscle
 The axon terminal consist of little pockets called synaptic vesicles that contain the chemical acetylcholine (abbreviated often to Ach), a neurotransmitter
 When a nerve impulse arrives at the neuromuscular junction, the acetylcholine in the synaptic vesicles is released into the gap or synaptic cleft by a process called exocytosis.


 The acetylcholine released from the synaptic vesicles attaches to nicotinic receptors on the motor end plate, which is a specialized area on the muscle fiber
 Following this, sodium channels in the muscle fiber open and there is influx of sodium ions into the muscle cell causing the membrane of the muscle to depolarize.
 Once the membrane is sufficiently depolarized, calcium also enters the muscle fiber referred to as “voltage-gated” calcium release
 When enough calcium has entered the muscle cell, more calcium is released into the muscle cell from a calcium reservoir present within the muscle cell called sarcoplasmic reticulum
 Since skeletal muscle fibers are interconnected to one another through gap junctions, the events happening in one muscle cell are transmitted to all the cells which begin to contract together making the entire muscle contract

Diseases of the neuromuscular junction may involve the presynaptic part (e.g., Lambert-Eaton syndrome, botulism) or postsynaptic part (e.g., myasthenia gravis)
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