Extrafusale

Extrafusale, usually written as extrafusal in English, refers to the primary contractile fibers of skeletal muscle. These fibers lie outside the muscle spindles and are the workhorses that generate force and produce movement when the muscle shortens. They are long, multinucleated, striated cells organized into sarcomeres, containing the molecular machinery (actin, myosin) for contraction.

The extrafusal fibers are typically categorized by muscle fiber type: slow-twitch type I fibers, fast-twitch type IIa fibers, and fast-twitch type IIx (or IIb in some species). Type I fibers resist fatigue and support sustained activity, while type II fibers generate rapid, powerful contractions. The distribution of fiber types varies by muscle and by species, contributing to differences in strength and endurance.

Neural control of extrafusal fibers occurs through alpha motor neurons in the spinal cord and brainstem. Activation by these neurons releases acetylcholine at the neuromuscular junction, causing contraction. The size principle explains recruitment order, with smaller motor units activated before larger ones as force requirements increase. In parallel, gamma motor neurons adjust the sensitivity of muscle spindles by altering intrafusal fiber tension, ensuring accurate proprioceptive feedback during movement.

Relation to intrafusal fibers: extrafusal fibers are the main effectors of force production, while intrafusal fibers within muscle spindles provide sensory input about length and rate of stretch. Together, they coordinate movement and posture. Clinically, disorders affecting motor neurons or myofibers predominantly impact extrafusal function, leading to weakness or impaired motor control.