Answer
The nerve cells that carry efferent messages to muscle are known as motor neurons. Each motor neuron has a muscle target, but before the axon gets to the muscle it divides up into several terminal branches. each of which innervates a muscle fiber. So while one nerve fiber has branches that go to several muscle fibers, each muscle fiber is supplied by only one motor neuron. When a nerve stimulus approaches the end of a neuron, it spreads out over all the branches of the neuron and stimulates all the muscle fibers it supplies. As a consequence , when the nerve fiber is stimulated all these muscle fibers contract together.
The place where the nerve cell meets or synapses with the muscle cell is called the neuromuscular junction (NMJ) or the motor end plate. Each terminal branch of the axon within the NMJ forms a separate synapse with a muscle fiber. making shallow indentations in the nerve cell membrane or sarcolemma. Therefore one neuron is able to stimulate a muscle fiber at several points within the NMJ.
The terminal ends of a neuron do not touch the sarcolemma directly. There is a narrow space between each end of an axon and the sarcolemma of the muscle fiber. This is the synaptic cleft ( 80 nm) . Small knobs or vesicles develop in the nerves endings immediately opposite to the synaptic cleft . These vesicles or synaptic knobs are filled with the neurotransmitter acetylcholine (ACH) which is later released by exocytosis.
Work Step by Step
Acetyl (ACU) is a neurotransmitter. It is contained in the synaptic vesicles and is released by the Action potential which travels down the anon. ACH is released into the synaptic cleft
Acetylcholine Receptors (ACHR).
These are proteins in the part of the plasma membrane of muscle cells facing synaptic knobs across synaptic clefts. ACHR proteins are able to bind or take up ACH Their are millions of ACHR molecules ( >50 mill) on the sarcolemma which is folded to increase reaction surface area, and to make available more ACHR molecules for reaction with the ligand (ACH).
The binding of ACH by ACHR is necessary to trigger muscle contraction . Therefore, the nuclei under the sarcolemma in this area of the muscle, are specially dedicated to the synthesis of ACH receptors . A deficiency in the synthesis of ACHR molecules --as in myasthenia gravis --results in a kind of paralysis.
Acetylcholinesterase (ACHe)
Acetylcholinesterase is the enzyme that breaks down ( catabolizes) acetylcholine.
A basal lamina -- composed of collagen and glycoproteins--separates the NMJ and the associated glia ( Schwann cells ) from the surrounding connective tissues. Acetylcholinesterase is secreted by the sarcolemma
and the basal lamina. It breaks down ACH after the latter has stimulated a muscle. This one of the important mechanisms for turning off the action of ACH and allowing muscles to relax.
