Principles of Structural, Chemical, and Functional Organization of Sensory Receptors

The variety of structural, chemical, and func-tional features inherent in receptor cells of sense organs reflects their strict specialization. Each sense organ responds electively to an appropriate kind of chemical or physical energy from the environment: light, odorous or gustatory sub-stances, acoustic vibration, or currents of liquid. The response, usually termed reception, is initiated and accompanied by dynamic molecular trans-formations in specific and nonspecific biologically active chemical compounds located in particular structures--organoids of the sensory cells. At first sight, the occurrence of these specialized structures and of the chemical substances they contain would appear to preclude any comparison, as the rods and cones of photoreceptors with the peripheral processes of olfactory cells, and particularly with those of auditory receptors. However, modern biochemical, cytochemical, and electron micro-scopic methods provide convincing evidence show-ing that despite the occurrence of a number of distinctive features, the origin of structural and functional evolution of all the receptor cells in sense organs of vertebrates, and apparently in most invertebrates also, may be traced to a more or less modified cell, provided with motile cilia or flagella. In the more primitive forms, these are engaged in both locomotory and sensory functions (Fig. 1). Indeed, in vertebrates the peripheral processes of photoreceptors--rods and cones--represent mod-ified cilia or flagella. A similar cilium of flagellum has been shown to constitute the basic structure i