Answer
A neuromuscular junction (NMJ) is the point where a neuron makes contact with its target muscle-- a synapse.. It comprises an axon terminal, a synapse, and a motor end plate.
Each of the terminal branches of a nerve fiber forms a separate synapse with the nerve fiber it serves. Where the terminal branch of the axon contacts the sarcolemma of the muscle fiber ( the motor end plate) it makes a slight indentation. The terminal branch of the axon--the synaptic knob-- is separated from the motor end plate by the synaptic cleft ( about 60 nm) . A Schwann cell( glia) surrounds the entire NMJ and separates it from the surrounding interstitial fluid (ISF). Within the synaptic knobs are synaptic vesicles containing the neurotransmitter acetylcholine(ACH).
Work Step by Step
An electrical signal travelling down the axon cannot cross the synaptic cleft. Instead, it causes the synaptic vesicles to release ACH into the cleft. The ACH serves as a chemical messenger (ligand) between the neuron and the muscle cell.
The response of the muscle fiber to the ACH is mediated by ACH receptors proteins (ACHR) . The nuclei just beneath the sarcolemma direct the synthesis of ACHR-- and there are about 50 million per muscle fiber. At this region the sarcolemma is folded to increase the the surface area of the motor end plate and enhance the sensitivity to ACH
The basal lamina (of glycoproteins and collagen) surrounds the entire NMJ --including the Schwann cell-
then passes into the synaptic cleft. The basal lamina and the sarcolemma contain the enzyme acetylcholinesterase (ACHe) that is able to break down or catabolize acetylcholine. After ACH has stimulated the muscle cell, ACHe is released ; it catabolizes ACH and ends its stimulation of the muscle fiber. This allows the muscle to relax.
