MECHANORECEPTION

ability of an animal to detect and respond to certain kinds of stimulinotably touch, sound, and changes in pressure or posturein its environment. Sensitivity to mechanical stimuli is a common endowment among animals. In addition to mediating the sense of touch, mechanoreception is the function of a number of specialized sense organs, some found only in particular groups of animals. Thus, some mechanoreceptors act to inform the animal of changes in bodily posture, others help detect painful stimuli, and still others serve the sense of hearing. Slight deformation of any mechanoreceptive nerve cell ending results in electrical changes, called receptor or generator potentials, at the outer surface of the cell; this, in turn, induces the appearance of impulses (spikes) in the associated nerve fibre. Laboratory devices such as the cathode-ray oscilloscope are used to record and to observe these electrical events in the study of mechanoreceptors. Beyond this electrophysiological approach, mechanoreceptive functions are also investigated more indirectlyi.e., on the basis of behavioral responses to mechanical stimuli. These responses include bodily movements (e.g., locomotion), changes in respiration or heartbeat, glandular activity, skin-colour changes, and (in the case of man) verbal reports of mechanoreceptive sensations. The behavioral method sometimes is combined with partial or total surgical elimination of the sense organs involved. Not all the electrophysiologically effective mechanical stimuli evoke a behavioral response; the central nervous system (brain and spinal cord) acts to screen or to select nerve impulses from receptor neurons. Man experiences sharp, localized pain as a result of stimulation of pain spots (probably free nerve endings) in the skin, and dull pain, usually difficult to localize, associated with inner organs. The sensory structures of pain spots in the skin differ from other receptors in that they respond to a wide range of harmful (noxious or nociceptive) stimuli. Excessive stimulation of any kind (e.g., mechanical, thermal, or chemical) may produce the human experience of pain. Apart from eliciting this subjective feeling of pain, stimulation of pain receptors in the human skin is objectively characterized by such signs of emotional expression as weeping and by efforts to withdraw from the stimulus. The reflex withdrawal of his hand from a burning stimulus may begin even before the person becomes conscious of the pain sensation. Judging from objective criteria, responses to painful stimuli also occur in nonhuman animals, but, of course, any subjective experience of pain sensation cannot be directly reported. Still, the question of painful experience among animals is of considerable interest because investigators (e.g., medical researchers) are often obliged to subject laboratory animals to treatments that would elicit complaints of pain from a man. If a cat's tail is accidentally stepped on, the pitiful screeching and efforts to withdraw are so strikingly similar to human reactions that the observer is led to attribute the experience of pain to the animal. If one treads accidentally on an earthworm and observes the animal's apparently desperate struggles to get free, he might again be inclined to suppose that the worm feels pain. This sort of mind reading, however, is inherently uncertain and may be grossly misleading. The following observations illustrate some of the difficulties in making judgments of the inner experiences of creatures other than man. After the spinal cord of a fish has been cut, the front part of the animal may respond to gentle touch with lively movements, whereas the trunk, the part behind the incision, remains motionless. A light touch to the back part elicits slight movements of the body or fins behind the cut, but the head does not respond. A more intense (painful) stimulus, however (for instance, pinching of the tail fin), makes the trunk perform agonized contortions, whereas the front part again remains calm. To attribute pain sensation to the painfully writhing (but neurally isolated) rear end of a fish would fly in the face of evidence that persons with similarly severed spinal cords report absolutely no feeling (pain, pressure, or whatever) below the point at which their cords were cut. Aversive responses to noxious stimuli nevertheless have a major adaptive role in avoiding bodily injury. Without them, the animal may even become a predator against itself; bats and rats, for instance, chew on their own feet when their limbs are made insensitive by nerve cutting. Some insects normally show no signs of painful experience at all. A dragonfly, for example, may eat much of its own abdomen if its tail end is brought into the mouthparts. Removal of part of the abdomen of a honeybee does not stop the animal's feeding. If the head of a blow-fly (Phormia) is cut off, it nevertheless stretches its tubular feeding organ (proboscis) and begins to suck if its chemoreceptors (labellae) are brought in touch with a sugar solution; the ingested solution simply flows out at the severed neck. At any rate, responsiveness to mechanical deformation is a basic property of living matter; even a one-celled organism such as an amoeba shows withdrawal responses to touch. The evolutionary course of mechanoreception in the development of such complex functions as gravity detection and sound-wave reception leaves much room for speculation and scholarly disagreement. the ability of an animal to detect and respond to certain kinds of stimulinotably touch, sound, and changes in pressure or posturein its environment. Mechanoreception depends upon a slight deformation of a mechanoreceptive nerve cell, which causes an electrical charge at the outer surface of the cell, thus activating the nerve fibre to an appropriate response. The mechanoreceptors in humans are located in pain spots, or pressure points, in the skin, which are probably clusters of free nerve endings. The receptors in these pain spots respond to a wide range of stimuli, sometimes with reflexive speed, e.g., a pricked finger may be withdrawn before the brain has even registered the pain. Depending on location, the pain spots may have a greater or lesser ability to distinguish among stimuli. For example, the separate stimuli of a two-pronged prick on a fingertip can be distinguished if the two prongs are only two millimetres apart. On the back of the hand the same stimuli would be felt as a single prong. At the tip of the tongue, where there are about 200 pressure points per square centimetre, the double prong can be detected as such if the two are only 1 mm apart. The lateral-line organs of many fishes are an example of highly specialized mechanoreceptors. From head to tail along part of the spine and in certain areas of the face, fish have visible mechanoreceptor organs along a lateral line. These organs can detect very slight, local displacements of the water such as are produced by movements of other animals. The receptors, therefore, can anticipate touch at a distance. Each of these receptor organs, called neuromasts or sense-hillocks, is made up of a cluster of sensory cells surrounded by long, slender, supporting cells, topped with hairs that project into a jellylike substance called the cupula which bends in response to currents in the water. The cupula projects into the water and is replaced continuously from below to compensate for the wearing away of its outer surface by water; thus it maintains its high degree of sensitivity. Frogs and other amphibians have a lateral-line system in the embryonic and tadpole stage; as they metamorphose into adult, largely terrestrial animals, the lateral-line system disappears. In most fishes the lateral-line system begins a series of grooves, forming a canal filled with a watery fluid. Disturbances in the outside water are transmitted into the canal system through pores. The neuromasts react, registering the disturbances and passing on the information via electrical nerve impulses. There are other types of mechanoreceptors. Some animals living on or near the water's surface use ripples to locate prey nearby; still others use bottom currents or even air currents to orient themselves or to detect the movement of other animals. The structures in the body that respond to vibrations in the surrounding airsoundare mechanoreceptors. Other structures, often associated with sound receptors, allow the organism to sense its orientation with respect to gravity. Still others inform the brain of the extension and translation of limbs or the state of various muscles. Additional reading John Field (ed.), Handbook of Physiology, section 1, Neurophysiology, 3 vol. (195960), chapters on nonphotic receptors, posture and locomotion, vestibular mechanisms, initiation of impulses at receptors, touch and kinesis, and pain; J.D. Carthy and G.E. Newell (eds.), Invertebrate Receptors (1968), chapters on mollusk statocysts, invertebrate proprioceptors, chordotonal organs, and mechanoreceptive transduction; P.H. Cahn (ed.), Lateral Line Detectors (1967), contributions of 35 investigators; M.J. Cohen and S. Dijkgraaf, Mechanoreception, in T.H. Waterman (ed.), The Physiology of Crustacea, vol. 2 (1961); S. Dijkgraaf, The Functioning and Significance of the Lateral-Line Organs, Biol. Rev., 38:51105 (1963); E. von Holst, Die Arbeitsweise des Statolithenapparates bei Fischen, Z. Vergl. Physiol., 32:60120 (1950), a classical study on statoreception in the labyrinth; I.P. Howard and W.B. Templeton, Human Spatial Orientation (1966), extensive coverage of the regulation of body posture; O. Lowenstein, Labyrinth and Equilibrium, pp. 6082 in Physiological Mechanisms in Animal Behaviour, in Symp. Soc. Exp. Biol., no. 4 (1950); D. Mellon, The Physiology of Sense Organs (1968); C.L. Prosser and F.A. Brown, Comparative Animal Physiology, 2nd ed. (1961), a textbook survey of mechanoreception and equilibrium; A.V.S. de Reuck and J. Knight (eds.), Myotatic, Kinesthetic, and Vestibular Mechanisms (1967), deals primarily with mammals; J. Schwartzkopff, Mechanoreception, in M. Rockstein (ed.), The Physiology of Insecta, vol. 1 (1964). Sven Dijkgraaf