VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 



43 



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Fig. 9. Elevation in potential above the resting level in a Limulus photoreceptor 

 cell as a function of the logarithm of illumination. (From the same records as Fig. 8.) 



First, a large electrode would sometimes injure a receptor cell so that a very 

 rapid spike discharge was produced after which spike activity could no longer 

 be elicited. The slow potential still appeared in response to light. 8 



Secondly, slow potentials have been found associated with the spike dis- 

 charge in many other receptors and have been interpreted as generator poten- 

 tials which remain after abolition of spike activity by means of local anesthetics. 

 Examples are the experiments on the muscle spindles of frogs (Katz, 1950) and 

 on the stretch receptor in the abdominal segments of the crayfish (Eyzaguirre 

 and Kuffler, 1955). In both of these cases it was possible to abolish spike ac- 

 tivity with local anesthetics and still obtain the slow potentials. 



Thirdly, we have made some experiments in which polarizing current was 

 passed through the recording microelectrode (MacNichol et al., 1953). A ten 

 thousand megohm resistor was attached at one end to the micropipette elec- 

 trode and the other end to a source of potential that could be varied with re- 

 spect to the reference electrode which was connected to the return lead of the 

 amplifier. Thus we could record and stimulate through the same pipette simul- 

 taneously. 



When the electrode was placed intracellularly a positive potential applied to 

 the resistor caused a current to flow in such a direction as to increase the 

 polarization of the cell membrane. Potentials applied in the other direction 

 caused current to flow in such a direction as to depolarize the membrane. 

 Fig. 10 shows oscillograms obtained with such a preparation. 



Initially, the cell was discharging spontaneously due to injury by the 

 electrode. Positive potentials speeded the discharge and negative potentials 

 slowed it down. Fig. 11 shows the nearly linear relationship between the 

 change in frequency and the applied current. 



8 This phenomenon was observed much earlier by Hartline using external electrodes 

 (Hartline et al, 1952). 



