Intrafusal

Intrafusal refers to the specialized muscle fibers contained within the muscle spindle, a sensory structure embedded in skeletal muscle that detects changes in muscle length. Intrafusal fibers are distinct from the more numerous extrafusal fibers, which are the primary contractile elements of a muscle.

Within each spindle, intrafusal fibers are categorized mainly as nuclear bag fibers and nuclear chain fibers. The central region of these fibers is noncontractile and houses sensory endings, while the polar regions are contractile and receive innervation from gamma motor neurons. The spindle is enclosed by a connective tissue capsule and lies in parallel with extrafusal muscle fibers.

Sensory innervation includes Ia (group I) and II (group II) afferent endings. Ia afferents generally wrap around the central region of intrafusal fibers and are highly responsive to dynamic changes in length, providing rapid feedback about stretch. II afferents terminate on the ends or along the static portions of intrafusal fibers and convey information about maintained, or static, muscle length. Gamma motor neurons innervate the polar regions of intrafusal fibers, adjusting their tension and thereby modulating spindle sensitivity. Dynamic gamma neurons primarily influence dynamic bag fibers, while static gamma neurons influence static bag and chain fibers.

Functionally, intrafusal fibers serve as the sensor apparatus for proprioception, enabling the nervous system to monitor and regulate muscle length and movement. They contribute to the stretch reflex and to broader motor control by maintaining spindle responsiveness during muscle contraction through gamma coactivation, which prevents the spindle from going slack.