Absorption of Stimulus Energy 93 



respect. It is possible, of course, that the dendrite tip is bathed 

 in a medium which is unhke the normal extracellular fluid, or, 

 alternatively, that the properties of the dendritic membrane are 

 unique with regard to their permeability characteristics. In 

 either case, the fact that absorption of a stimulating substance had 

 occurred would have to be transmitted to the more conventional 

 impulse-supporting membrane regions of the dendrite. Any 

 modifications in the permeability properties, as suggested above, 

 would also require rather unconventional mechanisms for the 

 generation of receptor currents, and these have yet to be proposed, 

 or even seriously considered. 



Experiments performed in recent years by Hagins and his 

 colleagues*^' *^ upon primary photoreceptor cells of the squid have 

 amply demonstrated that multiple independent sites for the 

 absorption and transduction of stimulus energy occur in these 

 structures. To do this, extracellular electrodes were positioned 

 at different depths within a slice of squid retina, so as to record 

 potentials at different points along the rather long outer segments 

 of the primary visual cells. The experimental situation and some 

 results can be seen in figure 43. In the unexcited state, there 

 was no potential difference between the electrodes, A and B, 

 since both were extracellular, and Vq — V^ = o. Now, a stimula- 

 ting light would be expected to produce an inward current (i.e. 

 a current sink) in the receptor cell membrane. This does, in fact, 

 happen, but the locus of this sink is strictly confined to the region 

 of the sense organ upon which the light impinges, no matter how 

 bright it is, and this locus moves when the stimulus moves. In 

 the diagram shown in figure 43, the stimulus spot at A produces 

 a maximum current sink at this electrode, so that Vb — V^ 

 approaches large positive values. As the spot is moved towards 

 B, the current sink passes through a region equidistant between 

 the two electrodes (i.e. the external resistances between the spot 

 and the two electrodes are roughly equal) and the potential 

 between them falls to zero. As the spot is moved past this point, 

 the sign of the potential reverses, and at B, maximum negative 

 values for Vg — V^ are recorded. Under normal conditions of 

 stimulation, the entire outer segment of each sensory cell would be 

 exposed to the incident light, so that the current sink would 

 encompass its whole length. The bulk of the outward current 



