Muscle Spindle

A muscle spindle is a specialized sensory receptor located within skeletal muscle that detects muscle length and changes in length, including the speed of stretch. Muscle spindles provide the nervous system with information used for proprioception, posture, and the control of movement. They are found in most skeletal muscles and are enclosed within a connective tissue capsule .^1,2

Inside each spindle are specialized small muscle fibers called intrafusal fibers, which lie approximately parallel to the ordinary force-producing, or extrafusal, muscle fibers. Sensory nerve endings wrap around regions of the intrafusal fibers. Primary, or group Ia, sensory endings respond to muscle length and changes in length and are particularly sensitive to dynamic stretch, while secondary, or group II, endings provide substantial information about maintained muscle length .^1,2

Muscle spindles also receive motor innervation from gamma motor neurons. These neurons activate contractile regions of the intrafusal fibers and adjust the sensitivity of the spindle, helping it continue to provide useful sensory information as the surrounding muscle changes length during movement .^1,2

Signals from muscle spindles contribute to the stretch reflex, in which stretching a muscle can increase activation of that same muscle through spinal neural circuits. Muscle spindles should not be confused with Golgi tendon organs. Muscle spindles primarily signal muscle length and changes in length, whereas Golgi tendon organs are associated with muscle-tendon structures and primarily signal muscle force or tension .¹