[1][2] They detect the amount and rate of change in length of a muscle.

[1] They constitute the muscle spindle,[2] and are innervated by both sensory (afferent) and motor (efferent) fibers.

[3] Intrafusal muscle fibers are not to be confused with extrafusal muscle fibers, which contract, generating skeletal movement and are innervated by alpha motor neurons.

Intrafusal muscle fibers are walled off from the rest of the muscle by an outer connective tissue sheath consisting of flattened fibroblasts and collagen.

[3] Gamma efferents from small multipolar neurons from anterior gray column innervate it.