440 Comparative Animal Physiology 



numerous behavioral adjustments. Light in excess is damaging, and many 

 animals are protected by pigments in outer body layers. 



In order that light energy be effective biologically it must be absorbed and 

 a photochemical reaction occur. Light-absorbing compounds of many types 

 have evolved— chlorophylls in plant chloroplasts, melanins and others in 

 chroma tophores--but carotenoids seem to be universal in photoreceptors which 

 initiate behavorial responses. From a kinetic analysis of behavior in molluscs, 

 from analysis of pigment extracted from retinas of vertebrates and cephalopods, 

 from action spectra of various visual functions in molluscs, arthropods, and 

 vertebrates, it appears that the carotenoid rhodopsin, or compounds very 

 similar to it, is the most widespread and perhaps the most primitive animal 

 photoreceptor carotenoid. Fresh-water fish have used a different substance, 

 porphyropsin, but reversion to rhodopsin occurred on migration to sea and 

 to land. The photosensitive pigment is broken down under the influence of 

 light according to a first order reaction and then is reformed from intermediates 

 from several sources by more complex reactions. 



The photochemical breakdown is followed by a series of events which lead 

 ultimately to the reaction of the organism. One such event is the retinal 

 potential which, in simple eyes at least, represents electrical activity of the 

 sensory cells. Next appear impulses in the optic nerve; these may build up 

 gradually, may start at a high frequency and then decline, may show various 

 patterns depending on the synaptic organization of the retina. The events con- 

 necting photochemical breakdown with retinal potential and this potential with 

 initiation of nerve impulses are uncertain. The excitatory signal, whether it be 

 transmitted in a nerve or from one part of a cell to another as in a flagellate, 

 elicits a response which depends in part on the complexity of integrative and 

 motor organization. 



The nature of the behavioral response is also influenced by the photo- 

 receptor, whether it forms pattern images or merely signals light, whether it 

 permits fine resolution of objects in a pattern, and its manner of response to 

 flicker. To make maximum use of stimulating light a variety of mechanisms 

 have evolved which function in focusing, in varying the range of accom- 

 modation, in regulating the sensitivity of the sense cells and the amount of 

 light reaching them in different states of light and dark adaptation. Some 

 animals merely show increased activity on illumination, others orient toward 

 or away from a light by indirect or direct paths and the orientation may vary 

 according to the physiological state of the organism. Some animals go between 

 two sources of light, others go directly to one of two sources. In more complex 

 behavior the resix)nse is influenced by past experiences, competing stimuli, and 

 other factors, but even here are direct responses as in pupillary constriction 

 and some eye movements of mammals. The underlying principle in photo- 

 reception is the quantitative relation between photochemical breakdown, 

 action potentials, and behavior, even though the type of behavior may vary 

 with type of eye and motor organization. 



REFERENCES 



1. Adrian, E. D., /. Physiol. 104:84-104 (1945). Electric response of eye: human. 



2. Adrian, E. D., ). Physiol. 105:24-38 (1947). Electrical response of eyes: verte- 

 brate. 



