the anatomical and physiological mechanisms that ensure the perception, analysis, and synthesis of mechanical, thermal, chemical, and other environmental stimuli that reach the skin and certain mucous membranes (for example, those covering the oral and nasal cavities and the genitalia). Like other analyzers, the skin analyzer consists of receptors, conducting pathways that transmit information to the central nervous system, and higher nerve centers in the cerebral cortex. The skin analyzer includes various kinds of skin sensibility: tactile (touch and pressure), temperature (heat and cold), and pain (nociceptive). There are more than 600,000 touch and pressure receptors (mechanoreceptors) in the human skin. The sensation of heat or cold arises in response to stimulation of the thermoreceptors, of which there are about 300,000, including about 30,000 heat receptors.
Scientists have not yet concluded whether there are independent receptors for pain. Some acknowledge the existence of four kinds of receptors (heat, cold, touch, and pain) with different systems of transmitting impulses. Others believe that if the stimulus is strong enough, the thermoreceptors or mechanoreceptors will cause a sensation of pain. Located among the skin receptors are free nerve endings that are usually regarded as pain receptors: for example, Meissner-Merkel tactile corpuscles, Golgi-Mazzoni and Vater-Pacini corpuscles (pressure receptors), Krause’s corpuscles (cold receptors), and Ruffini’s corpuscles (heat receptors). With the exception of the pain receptors, these receptors adapt readily to stimuli by a decrease in sensitivity.
Nerve fibers from the skin receptors in the central nervous system differ in structure and thickness and in the rate at which they conduct impulses. The thickest fibers transmit primarily tactile sensitivity at 50–140 m per sec. The fibers for thermal sensitivity are somewhat thinner and conduct impulses at a rate of 15–30 m per sec. Lacking a myelin sheath, thin fibers conduct impulses at a rate of 0.6–2 m per sec.
The sensory pathways of the skin analyzers pass through the spinal cord and the medulla oblongata into the thalamus opticus, which is connected to the posterior central convolution of the parietal cortex, where nervous excitation is converted into sensation. All the sensory pathways to the brain branch off into the reticular formation of the brainstem. Under normal conditions, skin stimuli are not perceived separately. Sensations arise in the form of complex integral reactions. The condition and interaction of the various divisions of the central and autonomic nervous systems, which take part in the integration of perceptions, determine the nature (modality) and emotional coloring of the sensations produced by the activity of the skin analyzers.
REFERENCESAndreev, L. A. Fiziologiia organov chuvstv. Moscow, 1941.
Granit, R. Elektrofiziologicheskoe issledovanie retseptsii. Moscow, 1957. (Translated from English.)
Kassil’, G. N. Nauka o boli. Moscow, 1969.
G. N. KASSIL’