Abstract
Abstract: :
Purpose: On–Off direction selective ganglion cells (DSGCs) fire when a stimulus moves in the preferred direction, and are silent when the stimulus moves in the opposite direction. Current–clamp recordings show that the directional–difference of the excitatory postsynaptic potentials (EPSPs) seen at the soma is often not large. Moreover, stimuli that cover small a fraction or ‘subunit’ of the receptive field are able to elicit directional spikes. The experiments described here aimed to determine how the spike threshold in DSGCs contributes to generating directional responses. Methods: Spikes recorded in current–clamp were elicited during preferred direction stimulation of DSGCs in whole–mount retinas. TTX was applied locally to the somas of the DSGCs by pressure ejection from a second electrode. In some experiments, we used Oregon–Green BAPTA–1 or 2 in the recording electrode to measure changes in dendritic calcium concentration using a two–photon microscope. Because the dye was excited using 930nm light, the DSGC could also be stimulated with moving stimuli during the imaging experiments. Results: Local application of TTX to the soma blocked somatic spikes and unmasked another class of smaller spikes about 10mV in amplitude. During TTX application, as somatic spikes disappeared, small spikes appeared. Bath application of TTX blocked all spiking. The small spikes encoded the same preferred direction as the somatic spikes. Hyperpolarization of the soma also suppressed somatic spikes and unmasked small spikes. Somatic spikes displayed an absolute refractory period of about 2ms, while the small spikes were not refractory and could superimpose. Dendritic calcium transients were observed even for a single somatic spike, and the total calcium fluorescence was strongly correlated with the number of somatic spikes. Dendritic calcium transients were not elicited by EPSPs, were abolished by TTX, but were unaffected by NMDA receptor antagonists. Conclusions: We propose that the small spikes originate in the dendrites, and initiate somatic spikes. Somatic spikes back–propagate and generate calcium transients in the dendrites. Spike thresholding in the dendrites might allow directional discrimination to occur even for stimuli that cover a small fraction of the receptive field, and produce strong local depolarization, but weak somatic depolarization.
Keywords: electrophysiology: non–clinical • ganglion cells • receptive fields