Mechanoreception

mechanoreception, ability of an animal to detect and respond to certain kinds of stimuli—notably touch, sound, and changes in pressure or posture—in 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, and this in turn induces the appearance of impulses (“spikes”) in the associated nerve fibre. Various laboratory devices are used to record and observe these electrical events in the study of mechanoreceptors. In addition to electrophysiological studies, mechanoreceptive functions are also investigated more indirectly—i.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 humans) 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.

Humans experience pain as a result of stimulation of pain receptors (nociceptors), which are located in the skin and other tissues. Pain receptors respond to three different types of harmful (noxious, or nociceptive) stimuli: mechanical, thermal, and chemical. The pain sensation may be acute, involving a short-lived intense feeling of pain that subsides to dull throbbing, or chronic, involving long-lasting pain that often is associated with disease. The stimulation of pain receptors is characterized by a range of physiological and psychological responses, including an effort to withdraw from the stimulus. The reflex withdrawal of the hand from a flame, for example, may begin even before the person becomes conscious of the pain sensation.

Responses to painful stimuli also occur in nonhuman animals, and the question of how animals experience pain is of considerable interest to researchers. If a cat’s tail is accidentally stepped on, the animal’s cry 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, the person might again be inclined to suppose that the worm feels pain. Whether that is the case is inherently uncertain.

The following observations illustrate some of the difficulties in making judgments of the inner experiences of creatures other than humans. 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. The head does not respond. A more intense (“painful”) stimulus (for instance, pinching of the tail fin), however, makes the trunk perform “agonized” contortions, whereas the front part again remains calm. To attribute pain sensation to the writhing (but neurally isolated) rear end of a fish would contradict evidence that persons with similarly severed spinal cords report absolutely no feeling (e.g., pain or pressure) 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. 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.