Physiology 

Motor neurons, whose axons constitute the somatic nervous system, supply skeletal muscles and bring about movement (motor means “movement”). • The cell bodies of almost all motor neurons are within the ventral horn of the spinal cord. • The only exception is that the cell bodies of motor neurons supplying muscles in the head are in the brain stem. • Unlike the two-neuron chain of autonomic nerve fibers, the axon of a motor neuron is continuous from its origin in the CNS to its ending on skeletal muscle.

• • Motor-neuron axon terminals release ACh, which brings about excitation and contraction of the innervated muscle cells. Motor neurons can only stimulate skeletal muscles. Areas of the brain that exert control over skeletal muscle movements include the motor regions of the cortex, the basal nuclei, the cerebellum, and the brain stem

An action potential in a motor neuron is rapidly propagated from the cell body within the CNS to the skeletal muscle along the large myelinated axon (efferent fiber) of the neuron. • As the axon approaches a muscle, it divides and loses its myelin sheath. Each of these axon terminals forms a special junction, a neuromuscular junction.

• Each branch innervates only one muscle cell; therefore, each muscle cell has only one neuromuscular junction. • A single muscle cell, called a muscle fiber, is long and cylindrical. • Within a neuromuscular junction, the axon terminal splits into multiple fine branches, each of which ends in an enlarged knoblike structure called the terminal button, or bouton. • This specialized underlying portion of the muscle cell membrane is called the motor end plate.

ACh is the neuromuscular junction neurotransmitter.

1. An action potential in a motor neuron is propagated to the terminal button.
2. This local action potential triggers the opening of voltage-gated Ca2+ channels and the subsequent entry of Ca2+ into the terminal button.
3. Ca2+ triggers the release of acetylcholine (ACh) by exocytosis from a portion of the vesicles.
4. ACh diffuses across the space separating the nerve and muscle cells and binds with receptor-channels specific for it on the motor end plate of the muscle cell membrane.
5. This binding brings about the opening of these nonspecific cation channels, leading to a relatively large movement of Na+ into the muscle cell compared to a smaller movement of K+ outward.
6. The result is an end-plate potential. Local current flow occurs between the depolarized end plate and the adjacent membrane.
7. This local current flow opens voltage-gated Na+ channels in the adjacent membrane.
8. The resultant Na+ entry reduces the potential to threshold, initiating an action potential, which is propagated throughout the muscle fiber.
9. ACh is subsequently destroyed by acetylcholinesterase, an enzyme located on the motor end-plate membrane, terminating the muscle cell’s response.

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