Muscle spindle

Muscle spindles are specialized sensory organs embedded within skeletal muscles that function as mechanoreceptors for detecting changes in muscle length and the rate of stretch. Each spindle consists of several intrafusal fibers surrounded by a connective tissue capsule, with both sensory afferent and gamma motor efferent innervation. When a muscle is stretched, the spindle's intrafusal fibers are also stretched, activating primary (group Ia) and secondary (group II) afferents that send signals to the spinal cord and brain about the degree and speed of stretch. This information is critical for proprioception—our sense of body position—and for triggering protective reflexes such as the stretch reflex, which helps prevent overstretching or injury by causing rapid contraction in response to sudden elongation[1][2][3][5]. Unlike typical molecular drug targets such as receptors or enzymes, muscle spindles are multicellular structures rather than single molecules; thus they are not considered direct therapeutic targets but play an essential physiological role in neuromuscular function.

Other names

Muscle spindlesStretch receptor (in muscle)Intrafusal fiber organ