When a nerve fibre approaches the muscle, it loses its myelin sheath and it branches which invaginate the sarcolemma of muscle fibres but does not merge with the muscle fibre membranes and thus form special structures—the neuromuscular junctions also called motor end plates.
The invagination membrane of nerve fibre is called presynaptic membrane, while the membrane of muscle which is in contact with the end plate is called postsynaptic membrane which is highly folded structure.
The gap between the pre-and postsynaptic membrane is called synaptic cleft which is always filled with a intracellular transmitter substance.
The endplate structures have numerous acetylcholine vesicles and mitochondria. The mitochondria produce either the acetylcholine—a chemical transmitter or supply energy for its synthesis.
When the motor impulse from the nerve is received on the end- plate a local depolarization occurs there and reaches the sancolemma with the accompanying secretion of acetylcholine resulting in the excitation of the muscle-fibres.
Difference between nerve impulse and electrical flow : Both these processes are entirely different.
An electric current flows at the speed of light and there is no chemical action involved in ft, only a physical action occurs.
But, in the nerves the impulse is an electrochemical action which proceeds from one spot to another and, relatively slow.
The nerve impulse can be compared with the fire proceeding in the fuse of a fire-work where the fire reaches the other end without diminution, and if the fuse was branched the fire would extend into each branch.