Extrafusal

Extrafusal muscle fibers are the primary contractile elements of skeletal muscles. They lie outside the muscle spindles and generate the force used in voluntary movement and reflexive actions. Together with connective tissue, these fibers form the bulk of the muscle, arranged in fascicles and surrounded by endomysium, perimysium, and epimysium. When extrafusal fibers contract, they shorten the muscle and transmit force to tendons via the muscle's connective tissue matrix.

These fibers are innervated by alpha motor neurons. A single alpha motor neuron and all the extrafusal fibers it controls constitute a motor unit. Activation of such a unit produces a coordinated contraction of its fibers.

Extrafusal fibers are classified by metabolic and contractile properties into slow-twitch Type I and fast-twitch Type II (including IIa and IIx in humans). Type I fibers are fatigue-resistant and primarily oxidative; Type II fibers generate force rapidly and rely more on glycolytic metabolism. The proportions of fiber types vary by muscle, activity, and training, and can adapt with use.

In contrast to extrafusal fibers, intrafusal fibers lie within muscle spindles and are innervated by gamma motor neurons. The spindle senses stretch and provides sensory feedback that regulates muscle tone and reflexes, while extrafusal fibers execute movement.

Clinically, loss or impairment of alpha motor neuron input to extrafusal fibers can reduce force generation and cause weakness or paralysis, whereas changes in spindle function affect proprioception and reflex sensitivity.