. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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. Fr
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