Response tuning through intrinsic hyperactivation in ganglion cell photoreceptors


April 6, 2016 - 1:00pm
Northwest 243
About the Speaker
Elliott Milner (Do Lab)

Electrically excitable cells can enter a condition called depolarization block, in which they are hyperactivated and thus inoperative. Block is generally considered a component of pathologies or clinical treatments. We report that it supports the normal function of intrinsically photosensitive retinal ganglion cells (ipRGCs), mammalian photoreceptors that regulate fundamental processes such as the circadian rhythm, mood, and development. We found that mild activation of phototransduction drives the mechanism of spike generation while, in a subset of cells, stronger activation overdrives it into silence. Thus, hyperactivation causes these ipRGCs to have unimodal tuning curves. This unimodal tuning has a pronounced influence on their signaling repertoire, allowing firing in response to both brightening and darkening depending on light history. In addition, tuning varies across cells and exhibits light adaptation; thus, it provides the basis for flexible population coding of irradiance. A potential disadvantage of tuning through hyperactivation is the distortion/attenuation of overdriven spikes near block, which would cause undesirable variation in synaptic outflow. However, we report that spikes are rendered uniform during axonal propagation in ipRGCs. Thus, these neurons complement intrinsic hyperactivation with axonal physiology to represent particular stimulus values with fidelity.