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Motor Unit

Muscle fibers are arranged into discrete entities known as motor units. The muscle fiber motor unit is the most basic neuromuscular contractile unit, and it is composed of a single α-motor neuron and all the muscle fibers that it innervates. A single motor neuron can innervate as few as 10, or as many as several thousand, muscle fibers.

As a motor neuron enters its muscle at the motor point, the nerve axons branch many times, and each muscle fiber in the motor unit is contacted by a single nerve ending at the motor end plate (typically at the fiber midsection). Adjacent muscle fibers do not typically belong to the same motor unit, because fibers from a single unit usually are distributed throughout the muscle.

Also, fibers belonging to the same motor unit share similar fiber type and contractile characteristics. As few as 10 fibers may be seen per motor unit in areas such as ocular muscles, where fine motor control is necessary. In large, force-generating muscles such as the quadriceps, however, there may be thousands of fibers per motor unit.

Contraction of a motor unit is initiated by an action potential arising from the nerve cell body that lies in the anterior horn of the spinal cord. The action potential reaches the muscle at a synapse known as the motor end plate or neuromuscular junction.

Nerve action potentials pass efficiently and rapidly from the spinal cord to peripheral muscle because of a myelin sheath surrounding the axon that serves as an electrical insulator. The myelin sheath surrounds peripheral nerves until they reach their destination with the motor end plate.

At the motor end plate, the axon loses its myelin sheath and expands into a Schwann cell–enveloped synaptic terminal. The nerve terminal of the presynaptic axon forms many terminal folds that include thousands of synaptic vesicles containing the neurotransmitter acetylcholine (ACh). The ACh-containing vesicles congregate in specialized release sites called active zones, which are  adjacent to the postsynaptic folds of the muscle membrane known as the sole plate.

The sole plate is a specialized region of the muscle sarcolemma that is characterized by numerous junctional folds containing ACh receptors and chemically gated ion channels. An action potential reaching the terminal axon will stimulate an influx of calcium ions through voltage-gated calcium channels into the presynaptic cell.

The increased intracellular calcium concentration mediates the fusion of synaptic vesicles with the axon membrane and the release of ACh into the neuromuscular cleft. ACh diffuses across the cleft, then binds to the ACh receptors of the postsynaptic muscle membrane that mediate depolarization of the muscle sarcolemma, thereby initiating an action potential in the muscle.

This action potential rapidly spreads along the muscle fiber to trigger calcium release and subsequent excitation–contraction coupling and, thereby, muscle contraction. ACh released by the axon terminal and bound to the muscular ACh receptors is rapidly deactivated by acetylcholinesterase enzymes in the synaptic cleft bound to the postsynaptic membrane.

One byproduct of this enzymatic inactivation, choline, is then reabsorbed by the terminal axon and resynthesized to ACh for future transmitter release.

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