The skin contains receptors that respond to touch, pressure, and temperature. The relationships between receptors and the cutaneous sensations are not completely understood. Meissner's corpuscles are sensitive to touch and Pacinian corpuscles to deep pressure. Ruffini endings transmit information about warmth and Krause's bulbs about cold. Information is transmitted from the receptors to nerve fibers that are routed through the spinal cord to the brainstem. From there they are transmitted to an area of cortex in the parietal lobe. Skin senses also undergo various kinds of sensory adaptation. For example, a hot tub can be initially so hot that it is intolerable, but after awhile one can sit in it without discomfort.
The Cutaneous Skin Senses
Pain. Pain receptors are mostly free nerve endings in the skin. Information is transmitted by two types of pathways to the brain by way of the thalamus.
The fast pathway (myelinated) detects localized pain and sends that information rapidly to the cortex.
The slow pathway (unmyelinated) carries less‐localized, longer‐acting pain information (such as that concerning chronic aches).
Many of the neural circuits that deliver pain signals to the brain and the spinal cord use substance P as a neurotransmitter. In addition, chemicals in the body called endorphins (chemicals with actions similar to those of morphine) increase in concentration when the body is responding to pain by serving as neuromodulators (chemicals that increase or decrease—modulate—the activity of specific neurotransmitters).
The gate control theory of pain, proposed by Ronald Melzack, proposes that a “neurological gate” in the spinal cord controls the transmission of pain impulses to the brain. Determination of whether the gate is open or closed depends upon a complex competition between different types of nerve fibers.
Acupuncture is a procedure developed by the Chinese for controlling pain by the insertion of long needles in various parts of the body. Although it is not known precisely how acupuncture alleviates pain, one theory suggests that the needles activate large nerve fibers and close the pain gate, while another suggests that the needles cause the release of endorphins, which serve as analgesics.