Dendritic Impulse Collisions and Shifting Sites of Action Potential Initiation Contract and Extend the Receptive Field of an Amacrine Cell

Abstract

Purpose:: We evaluated the contributions of somatic and dendritic impulses to the receptive field dimensions of amacrine cells in the amphibian retina.

Methods:: For this analysis we used the NEURON computer program with a multicompartmental, multichannel model of an On-Off amacrine cell with a three-dimensional structure obtained through computer tracing techniques. Simulated synaptic inputs were evenly spaced along the dendritic branches and organized into eight annuli of increasing radius. The first set of simulations activated each annulus individually, while a second approach activated each ring progressively to simulate an area summation experiment. Both sets of simulations were done with and without the presence of Na channels in the dendrites and soma.

Results:: The presence of dendritic Na channels increased the size of the receptive field beyond that achieved in their absence and allowed the physiological size of the receptive field to approximate the physical dimensions of the dendritic tree. This receptive field augmentation was the result of impulse generating ability in the dendrites which enhanced the signal observed at the soma. Unexpectedly, the receptive field dimensions observed in the area summation simulations were often smaller than that predicted from the summation of the annular simulations. Collisions of action potentials moving in opposite directions in the dendrites largely accounted for this contraction in receptive field size for the area summation studies.

Conclusions:: These results provide the mechanistic explanation for physiological recordings from amacrine cells that show similar phenomena.

Keywords: receptive fields • amacrine cells • computational modeling