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Proprioceptive Receptors 

Proprioceptors are located in muscles, tendons, joint ligaments and in joint capsules. There are no specialized sensory receptor cells for body proprioception⁴. In skeletal (striated) muscle, there are two types of encapsulated proprioceptors, muscle spindles and Golgi tendon organs (Figure 2.22), as well as numerous free nerve endings. Within the joints, there are encapsulated endings similar to those in skin, as well as numerous free nerve endings.

Figure 2.22 A muscle spindle receptor and Golgi tendon organ in the bicep muscle.

Muscle Spindles. Muscle spindles are found in nearly all striated muscles. A muscle spindle is encapsulated and consists of small muscle fibers, called intrafusal muscle fibers, and afferent and efferent nerve terminals (Figure 2.23).

Figure 2.23 A muscle spindle with its sensory and motor innervation. The primary muscle spindle afferent terminates as annulospiral endings in the central area of the intrafusal muscles whereas the secondary muscle spindle afferent terminates as flower spray endings in more polar regions of intrafusal muscles. The motor endplates of gamma motor neurons are located in the polar regions. The muscle spindle is attached to the surrounding extrastriate muscles and lays with its long axis in parallel with the long axes of the surrounding muscle.

Intrafusal muscles are found exclusively in muscle spindle receptors and are distributed throughout the body among the ordinary extrafusal muscle fibers of skeletal muscles. The intrafusal fibers are attached to the larger, surrounding extrafusal muscle fibers. They are oriented in parallel with the extrafusal fibers but do not contribute directly to muscle strength when they contract because of their small size. 

There are two types of afferent terminals in the muscle spindle (Figure 2.23). The annulospiral endings wrap around the central region of the intrafusal fibers, whereas the flower-spray endings terminate predominantly in more polar regions (away from the central area) of the intrafusal fibers. The 1° afferents forming the annulospiral endings are called the primary muscle spindle afferents, whereas those forming the flower-spray endings are called the secondary muscle spindle afferents.

In addition to afferent terminals, the terminals (motor endplates) of gamma motor neurons end on intrafusal muscle fibers. They will be described in detail in the chapters covering motor systems.

In summary, the muscle spindles are proprioceptors specialized to monitor muscle length (stretch) and signal the rate of change in muscle length by changing the discharge rate of afferent action potentials. Muscle spindles are most numerous in muscles that carry out fine movements, such as the extraocular muscles and the intrinsic muscles of the hand. There are fewer spindles in large muscles that control gross movements of the body (e.g., the muscles of the back). 

Figure 2.24 The Golgi tendon organ is located at the junction of muscle and tendon. The Golgi tendon organs resemble the Ruffini corpuscles. That is, the 1° afferent terminal fibers are intertwined with collagenous fibers of the tendon and the entire organ is encapsulated in a fibrous sheath.

Golgi Tendon Organs. Golgi tendon organs are found in the tendons of striated extrafusal muscles near the muscle-tendon junction (Figure 2.22). Golgi tendon organs resemble Ruffini corpuscles. For example, they are encapsulated and contain intertwining collagen bundles, which are continuous with the muscle tendon, and fine branches of afferent fibers that weave between the collagen bundles (Figure 2.24). They are functionally "in series" with striated muscle.

The Golgi tendon organ collagen fibers are continuous with the extrafusal muscle at one end and with the muscle tendon at its opposite end. Consequently, the mechanical force on the organ is maximal when the extrafusal muscles contract, shorten, and increase the tension on the tendon. When the muscles contract, the 1° afferent terminals are compressed and remain compressed as long as the muscle remains contracted. The Golgi tendon organ 1° afferent response to sustained isometric muscle contraction is slowly adapting, and the 1° afferent generates action potentials as long as the tension is maintained. The responses of the Golgi tendon organ 1° afferent axon is maximal when the contracted muscle bears a load, e.g., when lifting a heavy object.

The Golgi tendon organ is a proprioceptor that monitors and signals muscle contraction against a force (muscle tension), whereas the muscle spindle is a proprioceptor that monitors and signals muscle stretch (muscle length). 

Joint Receptors. Joint receptors are found within the connective tissue, capsule and ligaments of joints (Figure 2.25). The encapsulated endings resemble the Ruffini and Pacinian corpuscles and the Golgi tendon organs.

Figure 2.25 The joint receptors are free nerve endings and encapsulated endings in the joint capsule and joint ligaments. The encapsulated receptors in the joint capsule resemble Pacinian and Ruffini endings whereas those in the ligaments resemble Golgi tendon organs.

The joint 1° afferents respond to changes in the angle, direction, and velocity of movement in a joint. The responses are predominantly rapidly adapting with few joint 1° afferents signaling the resting (static) position of the joint. It has been suggested that information from muscles, tendons, skin and joints are combined to provide estimates of joint position and movement. For example, when the hip joint is replaced — removing all joint receptors — the ability to detect the position of the thigh relative to the pelvis is not lost. 

Free Nerve Endings. As mentioned above, free nerve endings of 1° afferents are abundant in muscles, tendons, joints, and ligaments. These free nerve endings are considered to be the somatosensory receptors for pain resulting from muscle, tendon, joint, or ligament damage and are not considered to be part of the proprioceptive system.

Table III
Receptor	Type	Sensation	Signals	Adaptation
Muscle Spindle	Encapsulated annulospiral and flower spray endings	Muscle  stretch	Muscle length & velocity	Rapid initial transient and slow sustained
Muscle: Golgi Tendon Organ	Encapsulated collagen	Muscle tension	Muscle contraction	Slow
Joint: Pacinian	Encapsulated & layered	Joint Movement	Direction & velocity	Rapid
Joint: Ruffini	Encapsulated collagen	Joint pressure	Pressure & Angle	Slow
Joint: Golgi Organ	Encapsulated collagen	Joint torque	Twisting force	Slow

Summary

In this chapter, you have learned about somatosensory stimuli and the receptors of three components of the somatosensory systems. These three components provide accurate information about the location, shape, texture, and movement of tactile stimuli, (discriminative touch), the position and movement of body parts (proprioception) and the application and location of painful stimuli (nociception). Tactile and proprioceptive stimuli are the mechanical forces produced when skin contacts external objects (discriminative touch), limbs oppose the force of gravity (body position) and muscles contract and body parts move. Painful stimuli are tissue-damaging forces. The sensations produced are those of touch, pressure, flutter, and vibration/movement (discriminative touch), body position and movement (proprioception), and sharp cutting pain. The discriminative touch receptors are encapsulated 1° afferent terminals (Meissner, Pacinian and Ruffini corpuscles), hair follicle endings and Merkel complexes in skin. The proprioceptive receptors in muscle are also encapsulated and include the muscle spindle and Golgi tendon organ. The joint receptors are similar to the encapsulated endings in skin and tendon and are found in the joint capsule and ligaments. The sharp cutting nociceptors are free nerve endings. 

Although it is convenient to subdivide somatosensory receptors and pathways for didactic, clinical and research purposes, it is important to keep in mind that most somatosensory stimuli act simultaneously and in varying degrees on all somatosensory receptors in the body part stimulated. For example, placing a heavy, cold object in an outstretched hand produces tactile, thermal, and proprioceptive sensations that allow us to appreciate the presence (touch, pressure), temperature, and weight of the object and provide proprioceptive information for finger, wrist and arm adjustments so we do not drop the object.

Test Your Knowledge

Make the best match between the receptor type and the sensation elicited when the receptor is stimulated.