Horizontal Cells

Horizontal cells, also known as H-cells, are specialized neurons found in the retina of the vertebrate eye. They are named so due to their distinctive horizontal orientation within the retinal neural circuitry. Horizontal cells mediate lateral interactions between photoreceptor cells and bipolar cells, allowing for modulation of signals and enhancing spatial contrast and visual information processing.

Function

Horizontal cells play an essential role in visual processing by serving as an intermediary connection between photoreceptor cells (rods and cones) and bipolar cells within the retina. They receive input from neighboring photoreceptor cells and modulate the signals before transmitting them to bipolar cells, thereby influencing the output signals generated by bipolar cells. This lateral interaction between horizontal cells and photoreceptor cells helps in adjusting visual responses to varying levels of illumination and increasing sensitivity to edges and contrast in the visual scene.

Structure

Horizontal cells exhibit a characteristic morphology, characterized by their horizontally oriented dendritic and axonal processes. They consist of two main subtypes: the A-type horizontal cells and the B-type horizontal cells, distinguished by their different connectivity patterns within the retinal circuitry. A-type horizontal cells typically receive input from a single cone photoreceptor, while B-type horizontal cells receive input from multiple cones.

Types

There are primarily two types of horizontal cells found in the retina: A-type horizontal cells and B-type horizontal cells. These subtypes differ in their connectivity and functional properties. A-type horizontal cells provide feedforward inhibition to bipolar cells, helping in contrast enhancement and center-surround antagonism, while B-type horizontal cells provide feedback inhibition to cones, contributing to gain control and tuning receptive fields of cone photoreceptors.

Role in Visual Perception

The intricate interplay between horizontal cells, photoreceptor cells, and bipolar cells within the retina influences various aspects of visual perception. By modulating signals at the photoreceptor-bipolar cell synapse, horizontal cells contribute to the perception of edges, contrast, and spatial information. Additionally, horizontal cells assist in regulating the dynamic range of the retina, allowing for adaptation to different levels of illumination and facilitating optimal visual processing.