250 



H. K. HARTLINE, F. RATLIFF AND W. H. MILLER 



L . ■ liiiiii i III i ill 111 iti It/ I lu i i I i 1 , . J I ;.i I iii 



Fig. 8. Inhibition of impulse discharge in a single optic nerve fiber by illumination 

 of nearby ommatidia and by antidromic volleys in other optic nerve fibers. (Top 

 record) — During steady state of impulse discharge in fiber, elicited by illumination 

 of its ommatidium, nearby ommatidia were illuminated for a period marked by 

 black line. {Middle record) — Inhibition of activity of same fiber by antidromic 

 volleys at 42/sec in the other optic nerve fibers. Small spikes in record were due to 

 physical spread of action potentials of antidromic impulses in nerve trunk to lead- 

 off electrodes ; they indicate duration of antidromic stimulation. {Bottom record) — 

 Antidromic volleys of 20/sec. Time in 1 sec. (From Tomita, 1958.) 



The observed slowing of the discharge of an omtnatidium is not the resuU 

 of interference with the access of hght to its receptor mechanism, such as 

 might resuh if, for example, there were some rapid migration of pigment, 

 or other retino-motor response. This is proved by the fact that the after- 

 discharge following intense illumination of a receptor can also be inhibited, 

 and to the same degree as the discharge elicited during illumination (Fig. 9). 

 Likewise the spontaneous activity that is sometimes observed in deteriorating 

 preparations is usually inhibited in the same manner, and so is any dark dis- 

 charge brought about by alteration in the ionic balance of the solutions bath- 

 ing the eye. The frequency of discharge of impulses elicited by passing 

 electric current into an eccentric cell impaled by a micropipette can also be 

 slowed by illuminating neighboring regions of the eye. 



The mechanism of the inhibition in the eye of Limulus is, of course, a 

 subject of great interest. However, it has not yet been analyzed very thoroughly 

 and we will not treat it in detail here. We have noted that the inhibitory action 

 depends on the integrity of the nervous interconnections in the plexus and 

 have speculated that the synaptic regions in the clumps of neuropile are sites 

 of its mediation. It is evidently exerted at, or ahead of, the site of impulse 

 generation within the receptor unit, for a microelectrode inserted in an 

 ommatidium records the same slowing of the discharge as is observed more 



