Given that the action potential output of retinal ganglion cells
(RGCs) determines the nature of the visual information that is transmitted
from the retina, an understanding of their intrinsic impulse firing
characteristics is critical for an appreciation of the overall processing
of visual information. Recordings from RGCs within an isolated whole-mount
retina preparation showed that their normal impulse firing from the
resting membrane potential (RMP) was linearly correlated in its frequency
with the stimulus intensity. In addition to describing the relationship
between the magnitude of the current injection and the resulting impulse
frequency (F/I relationship), we have characterized the properties of
individual action potentials when they are elicited from the RMP. In
contrast, hyperpolarizing below the RMP revealed that RGCs displayed a
time dependent anomalous rectification, manifested by the appearance of a
depolarizing sag in their voltage response. When an adequate period of
hyperpolarization was terminated, a fast phasic period of “rebound
excitation” was observed, characterized by a brief phasic burst of
impulse activity. When compared to equivalent action potential firing
evoked by depolarizing from the RMP, rebound spiking was associated with a
lower threshold and shorter latency for impulse activation as well as a
prominent, phasic, burst-like doublet, or triplet of impulses. The rebound
action potential had a more positive voltage overshoot and displayed a
higher peak rate of rise in its upstroke than those correspondingly
generated by depolarizing current pulses from the RMP. Blocking sodium
spikes with TTX confirmed that the preceding hyperpolarization led to the
recruitment and subsequent generation of a transient depolarizing voltage
overshoot, which we have termed the net depolarizing overshoot (NDO). We
propose that the NDO boosts the generation of sodium spikes by triggering
rebound spikes on its upstroke and crest, thus accounting for the observed
voltage dependent change in the firing pattern of RGCs.
