are the laterally interconnecting neurons in the outer plexiform layer
of the retina
eyes. They help integrate and regulate the input from multiple photoreceptor cells
. Among their functions, horizontal cells are responsible for allowing eyes to adjust to see well under both bright and dim light conditions.
There are three basic types of horizontal cells, designated HI, HII and HIII. The selectivity of these three horizontal cells, towards one of the three cone types
, is a matter of debate. According to studies conducted by Boycott and Wassle neither HI cells nor HII cells were selective towards S,M, or L cones. By contrast, Anhelt and Kolb claim that in their observations HI cells connected to all three cone types indiscriminantly, however, HII cells tended to contact S cones the most. They also identified a third type of horizontal cell, HIII, which was identical to HI but did not make contact with S cones.
The HII cells also make connections with rods, but do so far enough away from the horizontal cell's soma such that they do not interfere with the activities of the cones.
They span across cones and summate inputs from them all to control the amount of GABA
released back onto the photoreceptor cells, which hyperpolarizes them. Their arrangement together with the on-center and off-center bipolar cells
that receive input from the photoreceptors constitutes a form of lateral inhibition
, increasing spatial resolution at the... Read More