Sensory receptors can be grouped according to the type of stimulus energy they transduce. These categories include (1) chemoreceptors, which sense chemical stimuli in the environment or the blood (e.g., the taste buds, olfactory epithelium, and the aortic and carotid bodies); (2) photoreceptors—the rods and cones in the retina of the eye; (3) thermoreceptors, which respond to heat and cold; and (4) mechanoreceptors, which are stimulated by mechanical deformation of the receptor cell membrane (e.g., touch and pressure receptors in the skin and hair cells within the inner ear).
Nociceptors—or pain receptors—have a higher threshold for activation than do the other cutaneous receptors; thus, a more intense stimulus is required for their activation. Their firing rate then increases with stimulus intensity. Receptors that subserve other sensations may also become involved in pain transmission when the stimulus is prolonged, particularly when tissue damage occurs.
Receptors also can be grouped according to the type of sensory information they deliver to the brain. Proprioceptors include the muscle spindles, Golgi tendon organs, and joint receptors. These provide a sense of body position and allow fine control of skeletal movements (as discussed in chapter 12). Cutaneous (skin) receptors include (1) touch and pressure receptors, (2) heat and cold receptors, and (3) pain receptors. The receptors that mediate sight, hearing, and equilibrium are grouped together as the special senses.
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