Motor End-Plates

C. Motor End-Plates: A motor end-plate, or myoneuraljunction, is a collection of specialized synapses of the terminal boutons of a motor neuron with the sarcolemma of a skeletal muscle fiber. It transmits nerve impulses to muscle cells, initiating contraction. Each myoneural junction has 3 major components:

1, The presynaptic (neural) component is the terminal bouton. Although extensions of Schwann cell cytoplasm cover the bouton, the myelin sheath ends before reaching it. The bouton contains mitochondria and acetylcholine-filled synaptic vesicles. The part of the bouton's plasma membrane directly facing the muscle fiber is the presynaptic membrane.

2. The synaptic cleft lies between the presynaptic membrane and the opposing postsynaptic membrane and contains a continuation of the muscle fiber's basal lamina, It also contains acetylcholinesterase , which breaks down the neurotransmitter so that when neural stimulation ends, contraction ends. The primary synaptic cleft lies directly beneath the presynaptic membrane and communicates directly with a series of secondary synaptic clefts created by infoldings of the postsynaptic membrane.

3. The postsynaptic (muscular) component includes the sarcolemma (postsynaptic mem brane) and the sarcoplasm directly under the synapse. The postsynaptic membrane contains receptors for acetylcholine and is thrown into numerous junctional folds. The sarcoplasm beneath the folds contains nuclei, mitochondria, ribosomes, and glycogen