Proprioception means “sense of self”. In the limbs, the proprioceptors are sensors that provide information about joint angle, muscle length, and muscle tension, which is integrated to give information about the position of the limb in space. The muscle spindle is one type of proprioceptor that provides information about changes in muscle length. The Golgi tendon organ is another type of proprioceptor that provides information about changes in muscle tension.
The figure schematizes the different components in a muscle spindle. Muscle spindles are small sensory organs that are enclosed within a capsule. They are found throughout the body of a muscle, in parallel with extrafusal fibers (typical muscle fibers). Within a muscle spindle, there are several small, specialized muscle fibers known as intrafusal fibers (only one is drawn due to artistic limitations). Intrafusal fibers have contractile proteins (thick and thin filaments) at either end, with a central region that is devoid of contractile proteins. The central region is wrapped by the sensory dendrites of the muscle spindle afferent. When the muscle lengthens and the muscle spindle is stretched, this opens mechanically-gated ion channels in the sensory dendrites, leading to a receptor potential that triggers action potentials in the muscle spindle afferent.
The intrafusal fibers are innervated by an efferent neuron known as the gamma motor neuron (MN). (The efferents that innervate extrafusal fibers are known as alpha motor neurons). The role of the gamma MN is to maintain muscle spindle sensitivity, regardless of muscle length. When the extrafusal fibers have been stimulated to contract by alpha MN activation, the gamma MN is simultaneously excited. This is known as alpha-gamma coactivation. The gamma MN stimulates contraction in the two ends of the intrafusal fiber, readjusting its length and keeping the central region of the intrafusal fiber taut, which is necessary to keep the muscle spindle afferent responsive.