430 



Comparative Animal Physiology 



and the impulse frequency is lower. At a still lower intensity the latency is 

 further prolonged and the frequency is still lower. In all of these records 

 the impulses stop with cessation of illumination. Impulses in single fibers of 

 the Limulus optic nerve have been recorded for periods of illumination lasting 

 30 minutes.^" These impulses occur at a low frequency, and probably con- 

 tinue as long as the eye is illuminated. 



The frequency patterns of nerve impulse discharge in all fibers of the 

 Limulus optic nerve are similar. The condition in the vertebrate optic nerve 

 is quite different.^** Three functionally different types of fibers have been 

 discovered by Hartline, using the difficult technique of recording from sin- 

 gle intraocular fibers in the frog (Fig. 134). Fiber A responds with a volley 

 shortly after illumination begins ("on" burst). The impulse frequency de- 

 clines, at first rapidly, then more slowly, and is interrupted by a quiet pe- 

 riod. The impulse frequency decreases until the end of illumination, at 

 which time the fiber responds with another burst of impulses ("off" burst). 

 These fibers comprise about 20 per cent of the total fiber population. Fiber 

 B exhibits an "on" burst and an "off" burst of impulses and does not re- 

 spond to continued illumination. These fibers comprise about 50 per cent of 



Fig. 134. Trains of spike potentials initiated in three types of frog optic nerve fibers 

 upon onset and cessation of illumination of the eye. The absence of the white line indi- 

 cates illumination of the eye. A, Fiber responds with initial "on" burst and maintained 

 discharge during period of illumination; B, fiber responds with "on" burst and a burst 

 on cessation of illumination ("off" burst); C, fiber responds only with "off" burst. Time 

 marked in 1/5 sec. From Hartline.*® 



the total fiber population. Fiber C exhibits only an "off" burst of impulses. 

 This type of fiber comprises about 30 per cent of the total fiber population. 

 The optic nerve discharge patterns in invertebrates are of considerable 

 interest because these observations cast some light on the complexities of fre- 

 quency patterns observed in the vertebrate eye. In the segmental nerve 

 of an anterior segment of the earthworm^-^-* and in the ventral nerve cord 

 anterior to the sixth abdominal ganglion of the crayfish Prosser^^* recorded 

 a discharge appearing shortly after illumination which builds up slowly to a 

 maximum. The receptors involved, probably the unicellular receptors of 

 earthworms (page 383) and the sixth abdominal ganglion of the crayfish, 

 are believed to be primitive photoreceptors. Prosser^^^ also recorded a nerve 

 discharge pattern in the pharyngeal commissures of the crayfish on illumina- 

 tion of the stalked compound eyes. The discharge consisted of an "on" burst, 

 a maintained discharge of declining frequency, and an "off" burst. Hartline 

 recorded from single optic nerve fibers of the distal and proximal retinae of 



