Table 1.
Values of the standard set of biophysical parameters for regions of the ganglion cell.
Figure 1.
A) Scaled light and uncaging EPSPs (traces are averages of 5 trials). B) Confocal image of ganglion cell dendrites. C) A plot of XY resolution for the cell in B. * = photolysis.
Figure 2.
Dendritic summation is supralinear in retinal ganglion cells.
A) A confocal image of an On ganglion cell filled with Oregon green-BAPTA (100 µM). Trains of 3 uncagings were delivered to an individual location (arrow) at varying intervals (1–125 ms). Lower left trace is the light response, bar indicates light duration. B) Summed depolarization uncaging responses (traces are averages of 8 trials). C) A plot of the total charge for summed depolarizations/sum of 3 individual depolarization charges for locations <50 µm from the soma. D) Locations >125 µm from the soma. E) Pooled data (ANOVA p<0.001; N = 11 On cells).
Figure 3.
Dendritic summation has a directional component.
A) A single confocal plane of an On cell showing uncaging locations over 3 dendrites. B) Individual EPSPs moving either away (black) or toward (grey) the soma for the dendrite in A (400 ms delay between locations; averages of 8 trials). C) Summed EPSPs moving away (black) or toward the soma (50 ms delay between locations; averages of 8 trials). D) A plot of the amplitude ratio (summed EPSP/3x single EPSP). 0.88±0.05 away, 0.73±0.04 toward; paired t-test p<0.005; N = 14 On cells; P14–18). E) A plot of the charge ratio for summed depolarizations moving away from or toward the soma. 1.39±0.08 away, 1.10±0.05 toward; paired t-test; p<0.005; N = 15 On cells.
Figure 4.
Cortical neurons favor centripetal summation.
A) A hippocampal oriens lacunosum moleculare cell with uncaging locations (+) and EPSPs (average of 5 trials) for each location moving toward or away from the soma. B) Expected (calculated) charge for multiple activation (grey) vs. actual summed EPSP. C) Summation ratio for peak amplitude. D) Summation ratio for total charge. (* paired t-test; p<0.05 N = 6 cells; ** p<0.025; N = 5 cells).
Figure 5.
Direction selectivity is maintained in culture.
A) Summed EPSPs from cultured ganglion cells. B) Normalized ratio of the peak EPSP amplitude for both directions. C) Normalized ratio of charge ratio in both directions.
Figure 6.
Directional summation increases excitability for centrifugal sequences.
A) 5 consecutive traces showing suprathreshold responses for the away activation sequence. B) Matched traces for activation moving toward the soma. C) A plot of the average spike probability in both directions for all experiments showing suprathreshold activity.
Figure 7.
NMDA receptors increase summation but do not confer directionality.
A) Summed traces (average of 8 trials) for activation of 3 locations along a single dendrite under control conditions. Lower: A plot of the charge ratio for all experiments. Summation was larger in the away direction (** paired t-test; p<0.01; N = 9 cells). B) Traces from the same 3 locations after application of APV (50 µm). Lower: A plot of the charge ratio for all experiments. Summation was still larger in the away direction (* paired t-test; p<0.05; N = 9 cells).
Figure 8.
Blockade of HCN channels increases distal EPSP size.
A) Traces (average of 8 trials) for proximal and distal locations before and after application of ZD7288 (100 µM) to block HCN channels. B) A plot of the increase in charge in both directions after ZD7288 application. C) Summation ratio for three uncaging pulses to the locations in A, with ZD7288 in the bath (*ANOVA; p<.05; N = 8 cells).
Figure 9.
Retinal ganglion cells express HCN1 and HCN 4 channels.
A) Traces obtained by subtracting responses to voltage steps (−45–100 mV) before and after application of ZD7288. B) A plot of normalized tail current amplitude at each voltage. C) Images of cultured ganglion cells showing MAP2 (dendritic marker), and HCN1 or HCN4 staining. D) Pixel intensity for proximal and distal regions along the dendrites (paired t-test; p>0.05; N = 115 dendrites; 50 cells).
Figure 10.
HCN channels confer centrifugal directionality to summation.
A) Summed EPSPs (average of 8 trials) before and after ZD7288 application. B) Plots of the charge ratio (summed depolarization/sum of individual depolarization charges) after application of ZD7288. There was no significant difference in the charge ratio after blockade of HCN (paired t-test, p>0.05; N = 8 cells). C) A plot of the change in the charge ratio after channel block.
Figure 11.
Directional summation in a simple model system.
A) A cell with an unbranched dendrite was activated at multiple locations to elicit summed EPSPs. B) EPSPs for a model without dendritic voltage-dependent channels as measured at the soma. C) EPSPs after dendritic HCN channels have been added. D) EPSPs after Na/K channels were added to the model. E) EPSPS from a model with Na/K/HCN channels. F) A plot of the % direction selectivity (DS; [away-toward]/toward*100) for amplitude and charge in each condition from B–E.
Figure 12.
Variations in the parameter set show that DS is robust. A)
Varying Na conductance shows its effect in amplifying the directional difference. Left, a 15% increase in Na conductance (solid gray trace) amplified both peaks but more so for the centrifugal direction. A 15% decrease (dashed gray trace) reduced the amplification more for the centrifugal direction. B) Summary of charge (black bars) and amplitude (gray bars) differences plotted against % DS. The increase in Na conductance magnified both charge and amplitude differences. The decrease in Na conductance diminished both. C) For a 25% change in HCN conductance the DS slightly increased or decreased, due to the shift in resting membrane potential. D) Summary of charge and amplitude in response to changes in HCN conductance. E) A 25% change in K conductance had the inverse effect of a change in Na conductance, because a larger K conductance canceled the regenerative effects of the Na channels. F) Charge and amplitude summary for changes in K conductance.
Figure 13.
HCN modulates directional summation.
A) A model of the cell in Figure 10. Circles indicate input sites. Traces are for activation of the middle (M) 3 locations moving away from the soma (color) or toward the soma (black). B) Plots of directional summation (DS) ([away-toward]/toward*100) for amplitude and charge. P = 3 most proximal locations, M = middle 3 locations, D = 3 most distal locations, A = alternating locations (distal, middle, proximal). C) A model of another cell and activation of two dendrites. D) Plots of DS for both dendrites.