Motor unit

A motor unit consists of one motor neuron and every muscle fiber it innervates. Its activation produces coordinated contraction in those fibers.

Motor unit recruitment: The process of activating additional motor units to increase muscle force. Recruiting more units adds their fibers’ force to the muscle’s output.
Skeletal muscle: Voluntary, striated muscle attached to bones or other structures. Motor units organize the neural control of this muscle tissue.
Motor unit size principle: The pattern in which smaller, lower-threshold motor neurons are generally recruited before larger ones. It predicts the order in which units join as force demand rises.
Slow-twitch muscle fiber: A fatigue-resistant muscle fiber type specialized for sustained, relatively low-force contraction. Units containing these fibers differ from fast units in speed, force, and endurance.
Rate coding: The control of force by changing how frequently a neuron sends action potentials. A motor neuron can increase its unit’s force by firing more rapidly.
Muscle fiber: A single elongated, contractile cell within skeletal muscle. It is the individual muscle cell included in a motor unit.
Muscle tone: The ongoing partial contraction and resistance to stretch present in resting muscle. Low-level motor-unit activity contributes to maintaining tone.
Fast-twitch muscle fiber: A muscle fiber type that contracts rapidly and produces relatively high force, but often fatigues sooner. Its properties distinguish powerful, fast motor units from fatigue-resistant ones.
Neuromuscular junction: The synapse where a motor neuron communicates with a muscle fiber. Each fiber in a unit receives the neuron’s signal through this specialized synapse.
Motor end plate: The specialized region of a muscle fiber membrane beneath a neuromuscular junction. It receives the motor neuron’s signal at each innervated fiber.

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