Sensory receptors in the muscles, joints and tendons, other internal organs, and skin send nerve impulses to the spinal cord. From there, they travel up the spinal cord in tracts to the somatosensory areas of the cerebral cortex. These general sensory receptors can be categorized into three types: proprioceptors, cutaneous receptors, and pain receptors.
Proprioceptors are mechanoreceptors involved in reflex actions that maintain muscle tone and thereby the body’s equilibrium and posture. They help us know the position of our limbs in space by detecting the degree of muscle relaxation, the stretch of tendons, and the movement of ligaments. Muscle spindles act to increase the degree of muscle contraction, and Golgi tendon organs act to decrease it. The result is a muscle that has the proper length and tension, or muscle tone. Figure 9.1 illustrates the activity of a muscle spindle. In a muscle spindle, sensory nerve endings are wrapped around thin muscle cells within a connective tissue sheath. When the muscle relaxes and undue stretching of the muscle spindle occurs, nerve impulses are generated. The rapidity of the nerve impulses generated by the muscle spindle is proportional to the stretching of a muscle. A reflex action then occurs, which results in contraction of muscle fibers adjoining the muscle spindle. The knee-jerk reflex, which involves muscle spindles, offers an opportunity for physicians to test a reflex action. The information sent by muscle spindles to the CNS is used to maintain the body’s equilibrium and posture despite the force of gravity always acting upon the skeleton and muscles.
The skin is composed of two layers: the epidermis and the dermis. In Figure 9.2, the artist has dramatically indicated these two layers by separating the epidermis from the dermis in one location. The epidermis is stratified squamous epithelium in which cells become keratinized as they rise to the surface where they are sloughed off. The dermis is a thick connective tissue layer. The dermis contains cutaneous receptors, which make the skin sensitive to touch, pressure, pain, and temperature (warmth and cold). The dermis is a mosaic of these tiny sensory receptors, as you can determine by slowly passing a metal probe over your skin. At certain points, you will feel touch or pressure, and at others, you will feel heat or cold (depending on the probe’s temperature).
Three types of cutaneous receptors are sensitive to fine touch. Meissner corpuscles are concentrated in the fingertips, the palms, the lips, the tongue, the nipples, the penis, and the clitoris. Merkel disks are found where the epidermis meets the dermis. A free nerve ending called a root hair plexus winds around the base of a hair follicle and fires if the hair is touched. The three different types of cutaneous receptors that are sensitive to pressure are Pacinian corpuscles, Ruffini endings, and Krause end bulbs. Pacinian corpuscles are onion-shaped sensory receptors that lie deep inside the dermis. Ruffini endings and Krause end bulbs are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers. Temperature receptors are simply free nerve endings in the epidermis. Some free nerve endings are responsive to cold; others are responsive to warmth. Cold receptors are far more numerous than warmth receptors, but the two types have no known structural differences.
Like the skin, many internal organs have pain receptors, also called nociceptors, which are sensitive to chemicals released by damaged tissues. When inflammation occurs due to mechanical, thermal, electrical, or toxic substances, cells release chemicals that stimulate pain receptors. Aspirin and ibuprofen reduce pain by inhibiting the synthesis of one class of these chemicals.
Figure 9.1 Muscle spindle. When a muscle is stretched, a muscle spindle sends sensory nerve impulses to the spinal cord. Motor nerve impulses from the spinal cord result in muscle fiber contraction so that muscle tone ismaintained.
Figure 9.2 Sensory receptors in human skin. The classical view is that each sensory receptor has the main function shown here. However, investigators report thatmatters are not so clear-cut. For example, microscopic examination of the skin of the ear shows only free nerve endings (pain receptors), and yet the skin of the ear is sensitive to all sensations. Therefore, it appears that the receptors of the skin are somewhat, but not completely, specialized.
Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs. This is called referred pain. Some internal organs have a referred pain relationship with areas located in the skin of the back, groin, and abdomen; for example, pain from the heart is felt in the left shoulder and arm. This most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin.