annulospiral

An annulospiral ending is a type of sensory nerve ending found in skeletal muscle spindles. It consists of a single primary afferent fiber that winds in a helical, spiral fashion around the equatorial region of intrafusal muscle fibers, particularly around the nuclear bag fibers. This arrangement gives the ending its name and role in sensing muscle stretch.

These endings are associated with group Ia afferent fibers and provide rapid information about changes in muscle length and the velocity of those changes. When the muscle stretches, the spiral wrapping is deformed, generating action potentials that travel to the spinal cord. Ia afferents project monosynaptically to homonymous motor neurons, contributing to the stretch reflex, and they also participate in broader proprioceptive pathways. The annulospiral ending is dynamic in nature, responding strongly to the rate of length change, while contributing to static length information as part of the broader spindle signaling. Gamma motor neurons can adjust the sensitivity of intrafusal fibers, thereby modulating the responsiveness of annulospiral endings during movement.

Annullospiral endings reside alongside secondary sensory endings within muscle spindles, which primarily convey static length information. Together, these endings provide the central nervous system with essential proprioceptive input for motor control, posture, and coordinated movement. Damage or dysfunction of these endings can impair stretch reflexes and proprioception, affecting muscle tone and fine motor performance.