Absorption of Stimulus Energy 95 



membrane affected, are similar to those drawn for the two other 

 examples given in this chapter. It also follows that a maximum 

 response would necessarily include all available sites of the mem- 

 brane where photoelectric conversion can occur. 



Additional evidence of the independence of adjacent stimulus 

 sites was obtained from experiments employing selective light 

 adaptation. Hagins found experimentally that a region of squid 

 photoreceptor cell which has been subjected to sufficient light to 

 bleach 5 per cent, of its visual pigment undergoes a ten-fold 

 increase in photic threshold (as measured by the size of the 

 receptor potential). However, adjacent regions of the cell which 

 has been partially light-adapted in this manner were found to 

 have retained their control values of sensitivity. Thus, whatever 

 steps are involved in the conversion of photic energy into increases 

 in membrane conductance, each part of the membrane appears 

 to be able to effect them independently of the rest. There is, in 

 fact, no interference between adjacent parts by chemical products 

 or alterations of membrane structure. 



A critical question which remains concerns the precise 

 mechanisms by which the breakdown of pigment in a photo- 

 receptor cell is coupled to changes in the electrical properties of 

 the membrane. Perhaps the most plausible hypothesis is that a 

 chemical released by the photolytic reaction diffuses to nearby 

 membrane regions and there induces the standard increases in 

 ionic conductance.^^ According to this interpretation photo- 

 receptors are one type of chemoreceptor, with the provision that 

 the adequate stimulus not only affects the membrane from the 

 inside surface, but is endogenously produced ! Arvanitaki and 

 Chalazonitis have recently shown' that certain dyes injected into 

 living axons can couple light energy to the cell membrane, and 

 depolarization and spiking occur throughout the period of 

 illumination. Perhaps a more intensive research with model 

 photoreceptor systems like this will lead to a greater understanding 

 of those which have evolved their own specific pigment com- 

 plement. 



Very little more can be said at present about the actual physical 

 changes in the sensory membrane which result from energy 

 absorption. There can be no doubt that some of the remaining 

 problems in sensory physiology, such as the control of impulse 



