(2) Receptors can be classified according to their location. Exteroceptors
are near the surface of the body. Viscereceptors or enteroceptors are located in the
blood vessels and viscera. Proprioceptors are receptors located in muscles, tendons,
joints, and the external ear. Exteroceptors, located near the surface of the body,
provide information about the external environment. These receptors, sensitive to
stimuli outside the body, transmit sensations of hearing, sight, smell, taste, touch,
pressure, temperature, and pain. The exteroceptors located in the skin provide the
sensations of pain, touch, temperature, and pressure.
(a) Pain. Receptors for pain are not only found in the skin but in
practically every tissue of the body. These receptors may be stimulated by stimuli for
other sensations. When the receptors for touch, pressure, heat, and cold reach a
certain threshold, they stimulate the sensation of pain also. Since pain receptors are
sensitive to all stimuli, these receptors perform a protective function by telling the body
of changes that may be a danger to the body. There are two types of pain receptors:
somatic pain receptors and visceral pain receptors. Somatic pain comes from the
stimulus of receptors in the skin and receptors in skeletal muscles, joints, tendons, and
fascia. Visceral pain comes from stimulation of receptors in the viscera.
(b) Touch. Touch sensations generally result from stimulation of tactile
receptors in the skin or the tissues immediately beneath the skin. Light touch refers to
the ability to recognize exactly what point of the body is touched. Crude touch refers to
the ability to perceive that something has touched the skin although its exact location,
shape, size, or texture cannot be determined. Receptors for touch include root hair
plexuses, free nerve endings, Merkel's discs, Meissner's corpuscles, and end organs of
Ruffini. Root hair plexuses are dendrites arranged in networks around the roots of
hairs. Free nerve endings are found everywhere in the skin. Merkel's discs are disc-
like formations of dendrites attached to deeper layers of epidermal cells. Meissner's
corpuscles, located in the dermal papillae of the skin, are egg-shaped receptors
containing a mass of dendrites enclosed by connective tissue.
(c) Temperature. These receptors are sensitive to heat and cold. It is
thought that temperature receptors are free nerve endings.
(d) Pressure. Pressure receptors generally come from the stimulation
of touch receptors in deeper tissues. Sensation from these receptors lasts longer and is
spread over a greater area than the sensation from touch receptors.
b. Protection. The function of skin is to protect the body's underlying structures
from bacterial invasion, drying out, and harmful light rays. The acid mantle (pH 4.2 to
5.6) on the skin surface protects the body from bacteria and irritants. Skin keeps the
body from excessive water and electrolyte loss.