Muscle spindle

Muscle spindles are specialized sensory receptors located within skeletal muscles, primarily embedded in parallel with extrafusal muscle fibers, rather than at the tendon junction. They are especially abundant in muscles involved in fine motor control, such as those of the hands and neck.

Each muscle spindle contains several small, specialized skeletal muscle fibers known as intrafusal fibers, which are enclosed within a connective tissue capsule. Intrafusal fibers differ structurally and functionally from the larger force-producing extrafusal fibers.

Near the central region of each intrafusal fiber is a noncontractile (non-striated) area, where sensory nerve endings are concentrated. Two main types of sensory endings are present:

• Primary endings (Ia afferents), which respond rapidly to muscle stretch

• Secondary endings (II afferents), which detect sustained muscle length

The polar (striated) ends of intrafusal fibers are contractile and are innervated by gamma motor neurons, which adjust spindle sensitivity and maintain tension during muscle contraction.

When a skeletal muscle is stretched beyond its resting length, the muscle spindle detects the change in length and the rate of stretch. Sensory impulses travel through afferent fibers to the spinal cord, where they synapse directly with motor neurons that innervate the same muscle. This results in contraction of the stretched muscle.

This reflex response is called the stretch reflex (myotatic reflex). It functions to resist sudden muscle lengthening, maintain posture, stabilize joints, and help preserve limb position against gravitational or external forces.