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a sensory nerve structure that perceives and transforms stimuli from an organism’s external or internal environment and transmits information about the agent of the stimulus to the nervous system. Receptors vary in structure and function. They may be free nerve endings, endings covered with a special capsule, or specialized cells in such complex structures as the retina or Corti’s organ, which consist of many receptors.
Receptors may be external—exteroceptors—or internal—interoceptors. Exteroceptors are located on the external surface of the body of an animal or man, where they receive such stimuli from the external world as light, sound, and heat. Interoceptors are found in such tissues and internal organs as the heart, lymphatics, blood vessels, and lungs. They receive stimuli that give information about the condition of internal organs (visceroceptors) and the position of the body or part of it in space (vestibuloceptors). The proprioceptors, a type of interoceptor, are located in muscles, tendons, and ligaments. They transmit information about the static condition and dynamics of the muscles.
Mechanoreceptors, photoreceptors, chemoreceptors, and thermoreceptors respond to different types of stimuli. Dolphins, bats, and moths have receptors sensitive to ultrasound. The receptors of some fishes are sensitive to electric fields. The question of the existence of receptors sensitive to magnetic fields in certain birds and fishes awaits further study.
Monomodal receptors respond to stimuli of only one kind, either mechanical, photic, or chemical. They include receptors differing in level of sensitivity and reaction to stimuli. For example, the photoreceptors of vertebrates are subdivided into the more sensitive rod cells, which function as receptors of twilight vision, and the less sensitive cone cells, which enable man and some animals to see in the daylight and to perceive different colors. The mechanoreceptors of the skin are subdivided into the more sensitive phase receptors, which react only to the dynamic phase of deformation, and static receptors, which also react to constant deformation. Such specialization permits detection of the most significant properties of a stimulus and refined analysis of the stimuli received.
Multimodal receptors react to stimuli of more than one kind, such as chemical and mechanical or mechanical and temperature. The specific information coded in the molecules is transmitted to the central nervous system along the same nerve fibers in the form of impulses, which during their course receive repeated energy reinforcement.
The historic distinction between distance receptors (visual, auditory, and olfactory), which receive signals from a source of stimulation some distance away from the organism, and contact receptors, those that come into direct contact with a source of stimulation, is still retained. A distinction is also made between primary and secondary receptors. In primary receptors, the substrate that reacts to an external influence is embedded in the sensory neuron itself, which is directly (primarily) excited by the stimulus. In secondary receptors, additional specialized (receptive) cells are situated between the acting agent and the sensory neuron. The energy of external stimuli is transformed into nerve impulses in these cells.
All receptors have a number of properties in common. They are specialized to receive certain types of stimuli. During the action of a stimulus, a change occurs in the variation of the bioelectric potential on the cell membrane. This process, called receptor potential, either generates rhythmic impulses in the receptor cell directly or causes them to appear in another neuron, connected to the receptor by a synapse. The frequency of impulses increases with increasing intensity of stimulation. If stimulation is prolonged, the frequency of impulses in the fiber branching from the receptor decreases. This reaction, called physiological adaptation, varies in duration from receptor to receptor.
The high sensitivity of receptors to adequate stimulation is measured by the absolute threshold or minimum intensity of stimulation capable of exciting the receptors. Thus, five to seven quanta of light striking the eye’s receptors cause a sensation of light, whereas a single quantum is sufficient to excite an individual photoreceptor. Receptors may also be excited by inadequate stimulation: an electric current can cause a sensation of light or sound by acting on the eye or ear. Sensations are related to the specific sensitivity of receptors that came into being during the evolution of organic nature. Vivid perception of the world is caused chiefly by information coming from the exteroceptors. Information from the interoceptors does not produce distinct sensations.
The functions of the various receptors are interrelated. The interaction of vestibular receptors and of the cutaneous receptors and proprioceptors with the visual receptors is effected by the central nervous system. This interaction causes perception of the size and shape of objects and their position in space. Receptors may also interact among themselves without the involvement of the central nervous system by virtue of their direct contact with one another. Such interaction among visual, tactile, and other receptors is an essential element in the mechanism of spatial and temporal contrast.
Receptors are controlled by the central nervous system, which adjusts them according to the needs of the organism. These adjustments, whose mechanism has been insufficiently studied, are effected by means of special efferent fibers located close to some receptor structures.
Receptor functions are investigated by recording bioelectric potentials directly from receptors or associated nerve fibers and also by recording the reflexes elicited by stimulating receptors.
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Vinnikov, Ia. A. Tsitologicheskie i molekuliarnye osnovy retseptsii: Evoliutsiia organov chuvstv. Leningrad, 1971.
Fiziologiia cheloveka. Edited by E. B. Babskii. Moscow, 1972. Pages 436–98.
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A. I. ESAKOV
Pharmacologic receptors (also cell or tissue receptors). Pharmacologic receptors are situated on the membrane of effector cells. They receive regulatory and trigger signals from the nervous and endocrine systems and are exposed to many pharmacologic agents that selectively act on such cells. The receptors transform this action into the cell’s specific biochemical or physiological reactions. The pharmacologic receptors that carry out the activity of the nervous system are the ones that have been studied in greatest detail.
Two types of pharmacologic receptors transmit the influence of the parasympathetic and motor divisions of the nervous system (through the mediator acetylcholine): The N-cholinergic receptors transmit nerve impulses to the skeletal muscles and from neuron to neuron within the nerve ganglia, while the M-cholinergic receptors help regulate cardiac activity and the tone of smooth muscles. The influence of the sympathetic nervous system (through the mediator norepinephrine) and of the hormone secreted by the adrenal medulla (epinephrine) is transmitted by the α- and β-adrenergic receptors. Stimulation of the α-adrenergic receptors constricts blood vessels, raises blood pressure, dilates the pupils, and causes some smooth muscles to contract. Stimulation of the β-adrenergic receptors raises blood sugar levels, activates enzymes, dilates blood vessels, causes smooth muscles to relax, and increases the frequency and intensity of cardiac contractions. Thus, the functional effects are realized through both types of adrenergic receptors, while the metabolic effects are realized mainly through the β-adrenergic receptors.
Some pharmacologic receptors are sensitive to dopamine, serotonin, histamine, polypeptides, and other endogenous biologically active substances, as well as to the pharmacologic antagonists of some of these substances. The therapeutic action of a number of pharmacologic agents results from their specific effect on specific receptors.
REFERENCESTurpaev, T. M. Mediatornaia funktsiia atsetilkholina i priroda kholinoretseptora. Moscow, 1962.
Manukhin, B. N. Fiziologiia adrenoretseptorov. Moscow, 1968.
Mikhel’son, M. Ia., and E. V. Zeimal’. Atsetilkholin. Leningrad, 1970.
B. N. MANUKHIN