22 The Physiology of Sense Organs 



change across part of the membrane of the neuron. The evidence 

 for this kind of activity was still largely circumstantial, and 

 arguments were taken, for example, from observations on the 

 effect of injury upon nervous activity. Thus, in 1932 Adrian^ 

 had stated, * Whether or not depolarization explains the discharge 

 from sense organs, there is no doubt that injury, which involves 

 permanent depolarization, can set up a rhythmic discharge in a 

 nerve fiber. These injury discharges are abnormal effects, but 

 the injured and active states are so closely allied that it is worth 

 spending a little time over them. Both give the same potential 

 change, for both involve a breakdown in the polarized surface 

 membrane, though in the intact fiber the breakdown is promptly 

 repaired.' Another argument utilized the fact that repetitive 

 responses can be evoked in axons by prolonged pulses of depolariz- 

 ing current.^' ^^ It was found that impulse trains thus generated 

 could be rather precisely controlled, both in frequency and 

 duration, by variations in the amplitude and time-course of the 

 stimulus current. Thus, it was argued that if^some region^of_jhe 

 sense cell membrane were able to generate similar prolonged 

 currents under the influence of a natural stimulus, repetitive 

 firing of the neuron could be presumed to result, the impulse 

 frequency being a direct function of stimulus intensity. ^^ Graded 

 long-lasting variations in potential were, in fact, observed by 

 Hartline and Graham** in their original (and now classic) 

 observations on the Limulus compound eye. What made these 

 findings so remarkable, in the face of several earlier observations**' 

 on slow potential changes in visual systems, was their appearance 

 in what were then believed to be trueprimary sensory neurons, 

 rather than non-neural sensory cells, /it is now realized that the 

 interpretation of these records was oversimplified; the investigators 

 were undoubtedly recording impulse activity in second-order 

 neurons — the eccentric cells — ^which are driven to activity by 

 strictly graded electrical changes in the primary sensory, or 

 retinula, cells. i"» i°") There is still some controversy regarding the 

 ability of the latter to support propagated regenerative electrical 

 activity; in either case, the slow potential change in the baseline 

 of the records obtained by Hartline and Graham can be inter- 

 preted as a summation of the graded response of both the retinula 

 cells and the eccentric cell itself.*'' ^°' 



