VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 



51 



Fig. 16. Inhibition of discharge of a Limulus photoreceptor unit by antidromic 

 stimulation of the nerve fibers leading from neighboring units. The electrical stimuli 

 were applied at constant frequency. The records are arranged in order of increasing 

 strength of the antidromic stimulus from bottom to top. The stimulus artifacts can 

 be seen as small spikes in the top record. (Figure courtesy of T. Tomita.) 



cell body in this preparation. Secondly, when the pipette is just outside the 

 receptor cell the small spikes that are recorded are positive as are the large 

 spikes recorded from within the cell. If the spikes actively depolarized the cell 

 we should expect the spikes recorded from just outside to be negative as they 

 are when electrodes are just outside spinal motor neurons. Thirdly, Hartline 

 (1952) has recorded potentials with an electrode placed in various positions on 

 the surface of an isolated but unopened ommatidium. At a critical location on 

 the neck of the ommatidium the spikes reversed in sign becoming positive 

 when the electrode was moved toward the cornea and negative as it was moved 

 toward the nerve. Unfortunately we have not been able to repeat this experi- 

 ment with an opened ommatidium in order to see where this reversal occurred 

 with reference to the internal structures. We have thus far been able to obtain 

 only positive spikes from electrodes inserted into ommatidia. 



We have also looked for slow potentials that might be associated with the 

 inhibitory effect. There is some indication that they exist but are usually 

 masked by larger potentials generated by stray illumination reaching the active 

 ommatidium. They would be expected to be small if they resulted from an 

 electrotonic spread into the cell body from the region of the axon under the 

 cross connections. 



This is as far as we have gotten at present. We badly need some way of 

 determining the location of the tip of the microelectrode within an ommatidium 

 with respect to the histologically identifiable structures. It may be that micro- 



