Muscle and Joint Receptors

JOINT RECEPTORS

Four types of receptors have been described in the joint capsule and ligaments. Golgitype endings are located in ligaments but not in the capsule, and are innervated by large-diameter (Aα) fibers; they are slowly-adapting receptors that respond to joint position with changes in their tonic discharge rates. Ruffini terminals and paciniform corpuscles, which resemble pacinian corpuscles but are smaller, are found in the joint capsule and are innervated by medium-diameter (Aβ) fibers. Ruffini terminals respond to both movement and position, whereas paciniform corpuscles respond only to movement. Free nerve endings, supplied by small group III (Aδ) fibers and unmyelinated C fibers, are found in both ligaments and joint capsules; they are thought to respond to extreme, painful movement of the joint. The part played by these four receptor types in signaling joint position is not well understood. A particular difficulty arises from the fact that the most receptors respond only at maximum joint extension or flexion, whereas position sense is sensed throughout the entire range of a movement.

MUSCLE RECEPTORS

Muscles also contain four types of receptors, two of which—Golgi tendon organs and muscle spindles—are specific to muscle and contribute to the proprioceptive control of reflexes.

Golgi tendon organs are encapsulated receptors located in a tendon, close to the junction of the tendon and the corresponding muscle. The tendon organ capsule surrounds a bundle of tendon fascicles, which are connected to 3 to 25 muscle fibers. Each tendon organ is innervated by a single group Ib (Aα) fiber that enters the capsule and forms spraylike endings in contact with the tendon fascicles. Because it is connected in series with the muscle fibers, the tendon organ is stretched and thereby excited when muscle tension increases. Tension produced by active muscle contraction has been shown to be more effective in exciting tendon organs than tension produced by passive muscle stretch.

The muscle spindle is a complex receptor consisting of intrafusal fibers, a bundle of small muscle fibers encased in a sheath. The fibers typically do not run the entire length of the muscle; instead, they insert into one or both ends of the sheath of a large extrafusal muscle fiber. The intrafusal fibers are of two types: smaller nuclear chain fibers, in which the cell nuclei lie in a line along the middle portion of the fiber, and larger nuclear bag fibers, in which the nuclei are more clustered. Both nuclear bag and nuclear chain fibers are innervated by small-diameter gamma motor fibers, which increase the sensitivity of the spindle by causing a contraction of the intrafusal muscle fibers. Each spindle receives afferent innervation from a single, large group Ia (Aα) fiber, which forms large annulospiral (primary) endings around both nuclear chain and nuclear bag fibers, and from one to five medium group II (Aα) fibers, which form flower spray (secondary) endings chiefly on nuclear chain fibers. Because these spindles lie parallel to the extrafusal muscle fibers, they are stretched when the muscle lengthens. The range of muscle stretch encountered during normal movement excites both kinds of afferent fibers but in somewhat different fashions. The group II fibers respond to lengthening with an increase in their tonic discharge rate, which remains constant as long as the muscle is stretched, whereas the group la fibers respond especially vigorously to the dynamic phase of muscle lengthening and, more weakly, to maintained stretch.

The remaining two classes of muscle receptors include pacinian corpuscles, which are innervated by group II (Aβ) fibers and respond to vibratory stimuli, and free nerve endings, which are innervated by group III (Aδ) or IV (C) fibers and respond to strong, noxious stimuli. Thus they resemble corresponding types of receptors found in other tissues.

Only gold members can continue reading. Log In or Register to continue