The motor end plate is a flat branching structure, which is mainly composed of neuron axon terminal endings. It marks the site of the neuromuscular junction through which a lower motor neuron innervate several muscle fibers of a skeletal muscle. It functions as a synapse for electrochemical transmission from the efferent neuron to the muscle fiber. The motor end plate lies over the sarcolemma, which is the cell membrane of the muscle fiber, but it does not touch it directly. There is a space between them; it is the synaptic cleft.
The nerve endings at the neuromuscular junction is surrounded by a Schwann's cell; hence, it is myelinated. The part of the sarcolemma under the motor end plate forms deep grooves, with folds. Mitochondria are plentiful in the neuron axon terminals and also in the sarcoplasm beneath them. Remember, the sarcoplasm is the muscle fiber cytoplasm contained in the sarcolemma. During muscle relaxation, the sarcolemma is electrically polarized. Contraction occurs when efferent nerve impulses start a wave of depolarization which jumps and spreads quickly over the sarcolemma from the motor end plate.
In addition, many synaptic vesicles, which contain the neurotransmitter acetylcholine, are present within the axon terminals. The motor end plate is a directed terminal synapse where depolarizing efferent impulses from the lower motor neuron cause the synaptic vesicles to fuse with specific sites on the presynaptic terminal. The resulting exocytosis of acetylcholine into the synaptic cleft leads to depolarization of the juxtaposed area of the sarcolemma. (Exocytosis is the ejection of materials from within a cell).
Depolarization in efferent excitation leads to an influx of calcium ions which cause synaptic vesicles to make contact with the axolemma (the axon membrane at the terminal end) and discharge their content into the synaptic cleft. The acetylcholine receptors in the muscle fiber sarcolemma lie mostly along the opening of the junctional folds.
Below, schematic picture of motor end plate on sarcolemma of muscle fiber.
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