Nerve - Neuromuscular Junctions(NMJ The synapse between a...

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Nerve-Muscle Relationship Skeletal muscle must be stimulated by a nerve or it will not contract. Thus, skeletal muscle is considered voluntary (controlled by neurons in the brain or spinal cord). The neurons that control skeletal muscle are called somatic motor neurons. The cell bodies of somatic motor neurons are found in the brainstem or spinal cord; these neurons extend axons to the muscle cells. A single motor neuron may communicate with several (10s to 1000s) individual muscle cells, but each muscle cell is only contacted by one motor neuron. A motor neuron and all the muscle fibers it innervates is called a motor unit. The muscle fibers of a motor unit is dispersed throughout a whole muscle such that when signaled to contract by a neuron, there is a weak contraction over a wide area within the whole muscle. Several motor units must be recruited to contract the entire muscle. Fine control is accomplished by small motor units while large, strong movements require larger motor units.
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Unformatted text preview: Neuromuscular Junctions (NMJ) The synapse between a motor neuron and a muscle cell is a neuromuscular junction . At the NMJ, the motor neuron releases a neurotransmitter that stimulates a muscle cell. The neurotransmitter released by motor neurons is acetylcholine and it causes the skeletal muscle to contract. Before release, the ACh molecules are packaged in small vesicles in the synaptic terminal . When stimulated, the neuron releases the ACh by exocytosis into the synaptic cleft . The motor end plate is the specialized region of the muscle cell that has ACh receptors . The ACh released from the neuron diffuses across the synaptic cleft and binds to ACh-Rs; the receptors open and let Na + into the cell. Thus, ACh-Rs are both receptors and channels. Also found in the NMJ is the protein acetylcholinesterase . This enzyme breaks down ACh and ultimately causes relaxation of the muscle. Special glial cells called Schwann cells envelope and isolate the NMJ....
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