Physiology of the neuromuscular junction

Abstract

The neuromuscular junction consists of a motor neurone and a muscle cell separated by a narrow synaptic cleft. The nerve and muscle do not come into direct contact. Transmission of the action potential from the nerve to the muscle occurs by release of acetylcholine from the presynaptic nerve terminal. Motor neurone The motor neurones which control skeletal mus-cle are long cells which have their origin in the ventral horn of the spinal cord. They extend to muscle cells in the periphery over a distance of up to 1 m. The metabolic centre of the nerve cell is its cell body, which lies near its origin. Information from the cell body is transmitted down a long cylindrical structure known as the axon. Axons are typically 10–20 µm in diameter and are surrounded by a myelin sheath. This sheath serves to increase the speed of transmis-sion of the action potential to the neuromuscular junction. It consists of many layers of cell mem-brane tightly wrapped around each other acting as an insulator. The myelin sheath is interrupted by gaps (nodes of Ranvier) which participate in the propagation of the action potential along the nerve, speeding up nerve conduction. Before the nerve reaches the neuromuscular junction, the axon branches into several terminals to innervate many muscle cells. A muscle cell has only one neuromuscular junction and is innervat-ed by only one nerve. A nerve, and the muscle cells which it innervates, comprise a motor unit. The number of muscle cells per motor unit varies from a few to several thousand, depending on the function of the muscle. The largest number is seen in strong bulky muscles concerned with coarse movement; the smallest number are in muscles which perform delicate movements (e.g. the eye). The synapse is the area of the nerve lying closest to the muscle cell; it is situated opposite a specialised area of the muscle cell called the end plate. The synapse and the end plate are separated by a gap (approximately 20 nm) called the synaptic or junctional cleft which is filled with extracellular fluid. It is at the synapse that the action potential, which has travelled along the nerve, causes the release of the neuro-transmitter, acetylcholine. Acetylcholine travels across the synaptic cleft and binds selectively to the post-synaptic motor end plate of the muscle causing an action potential to travel through it.