Classification of Receptors
The old subjective classification of the sense organs into touch, smell, taste, sight, and hearing is no longer sufficiently inclusive, for there are nearer twenty-five than five different kinds of receptors that can be distinguished in man alone, not to mention additional ones which characterize animals other than man.
By considering receptors in relation to their adequacy for particular stimuli rather than the sensations produced through them, an objective rather than a subjective basis is found for their classification, making possible not only more accurate description but also the inclusion of certain known receptors for which no corresponding sensation is recognized.
Classifying receptors first in accordance with the source of the stimuli for which they are adequate, there are three major groups, designated by Sherrington as exteroceptors, whose stimuli come from the outside; and proprioceptors, and interoceptors, whose stimuli arise within the organism itself.
Exteroceptors receive mechanical, chemical, and radiant stimuli. The more deeply seated proprioceptors have to do with the control of the musculature and the working of parts of the body upon each other, while interoceptors are associated primarily with the digestive tract and things like food within but not a part of the body.
Exteroceptors are environmental organs proper, dealing with the objective outside world, while proprioceptors and interoceptors are more subjective and personal, leading to sensations more difficult to define, such as hunger, thirst, fatigue, muscle tonus, sexual excitation, visceral pain, nausea, and the general sense of well-being.
Although it is possible for the physicist by means of such gadgets as dynamos, thermopiles, telephones, chemical solutions, photoelectric cells and the like, to convert all the various stimuli arising in the external environment to the common denominator of electric energy, man and animals are not provided with any specific receptors for the electric current itself. If they were, universal electric sense organs might replace the diversity of receptors which have been evolved.
Table XIII gives an arrangement of receptors with the stimuli for which they are adequate and, so far as possible, the sensations which in each case result.