Sensory receptors can be divided in two types: general receptors and special receptors. The special receptors include the chemoreceptors in the mouth and nose, the photoreceptors in the eyes, and the mechanoreceptors in the ears.
General Receptors
General receptors are sensory receptors generally present throughout the body. These receptors are present in the skin, visceral organs, and muscles and joints.
Skin
The skin contains sensory receptors of touch, pressure, pain, and temperature. It is a mosaic of these tiny
receptors, as can be determined by passing a metal probe slowly over the skin. At certain points, there will be a feeling of pressure, and at others, a feeling of hot or cold (depending on the probe's temperature).
Three types of receptors are responsive to touch: the Meissner corpuscle, the Merkel disk, and the Krause end bulb. The Meissner corpuscle is concentrated in the fingertips, the palms, the lips, the tongue, the nipples, the penis, and the clitoris. Their prevalence provides these regions with special sensitivity. The Pacinian corpuscle and Ruffini ending are two different types of pressure receptors. Pacinian corpuscles are onion-shaped sense organs that lie deep inside the dermis. Ruffini endings are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers. Temperature and pain receptors are free nerve endings in the epidermis. Some free nerve endings are responsive to cold; others are responsive to heat. Cold receptors are far more numerous than heat receptors, but there are no known structural differences between the two.
Visceral Organs
The visceral (internal) organs have receptors that aid in maintaining homeostasis. For example, stretch receptors in the lungs respond to lung expansion, the aortic and carotid bodies are sensitive to pH and oxygen levels of the blood, and osmoreceptors in the hypothalamus detect blood osmolarity, which is dependent on the concentration of solutes in the blood.
Like the skin, many internal organs have pain receptors. Sometimes, this pain is felt as pain from the skin. This is called referred pain. Different internal organs have a referred pain relationship with certain areas of the skin. For example, pain arising from the intestine is located in the skin of the back, groin, and abdomen; pain from the heart is felt in the left shoulder and arm. This happens because nerve stimuli from the pain receptors of the internal organs travel to the spinal cord, where they make contact with neurons also receiving messages from the skin. The brain interprets this as pain in the skin.
Muscles and Joints
The sense of position and movement of limbs is dependent upon receptors termed proprioceptors (pro"pre-o-sep'torz). Proprioceptors are located in the joints and in associated ligaments and tendons that respond to stretching, pressure, and pain. Nerve impulses from these receptors are integrated within the cerebellum with those received from other types of receptors so that the person can monitor the position of body parts.
A muscle spindle consists of a bundle of modified muscle fibers with sensory nerve fibers wrapped around a short, specialized region somewhere near the middle of the spindle's length. Stretching of associated muscle fibers causes muscle spindles to increase the rate at which they fire, and for this reason, they are sometimes called stretch receptors. The knee-jerk reflex illustrates how muscle spindles act as stretch receptors. When the legs are crossed at the knee and the tendon at the knee is tapped, both the tendon and a muscle in the thigh are stretched.
Stimulated by the stretching, muscle spindles transmit impulses to the spinal cord, and thereafter, muscle contraction occurs, which causes the lower leg to jerk upward in a kicking motion. Normally, the sensory input from muscle spindles allows the central nervous system to regulate muscle contraction so that tone is maintained.
General receptors are located in the skin, the visceral organs. and the muscles and joints. General receptors in the skin respond to touch, pressure, pain, and temperature; others help to maintain homeostasis and allovv us to monitor the position of body parts.
