CLASSIFICATION OF SENSORY RECEPTORS
1. How do exteroceptors and interoceptors differ from each other?
2. What are proprioceptors?
3. Review muscle spindle function.
4. What is the relationship of myelination to speed of transmission of nerve impulses?
Sensory receptors are end organs of afferent nerves and belong to one of two main physiological groups: (1) exteroceptors, which detect stimuli that arise external to the body, and (2) interoceptors, which detect stimuli that originate within the body.
The exteroceptors detect stimuli near the outer surface of the body and include those from the skin that respond to cold, warmth, touch, and pressure. The special receptor organs for hearing and vision are also classified as exteroceptors. The interoceptors detect stimuli from inside the body and include receptors for taste, smell, and those within the viscera that respond to pH, distention and spasm (as in the bowel), and flow (as in the urethra) and equilibrium sensors in the inner ear. The proprioceptors are a special class of interoceptors that signal conditions deep within the body to the central nervous system. Proprioceptors are located in skeletal muscles, tendons, ligaments, and joint capsules. Examples of proprioceptors are muscle spindles, Golgi tendon organs, and joint receptors. Just as muscle spindles prevent undue stretch of muscles (see Chapter 4), Golgi tendon organs in tendons and ligaments respond to stretch by countering muscle tension that caused the stretch. Muscle spindles and Golgi tendon organs are sensitive to stretching and reflexively prevent undue stretch of muscles, tendons, and ligaments. Muscle spindles also provide for muscle tone so that purposeful contraction is more effective and helps to prevent collapse of standing animals as a result of the force of gravity. Joint receptors are sensitive to the position or angle of joints and provide for a sense of body position. Because of the need for rapid transmission of proprioceptive impulses, the proprioceptive fibers have the heaviest myelination of all peripheral nerve fibers.
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