Amacrine

Amacrine cells are a diverse set of interneurons in the retina that participate in the processing of visual information. They are situated in the inner nuclear layer and the inner plexiform layer, where they form synapses with bipolar cells, ganglion cells, and other amacrine cells. Unlike many neurons in the retina, many amacrine cells are anaxonic, and signals are transmitted through their expansive dendritic networks rather than along long axons.

Amacrine cells comprise numerous subtypes with varying morphology and neurotransmitters. The majority are inhibitory, using gamma-aminobutyric acid (GABA) or glycine to suppress activity in their targets; a minority use acetylcholine, dopamine, or other transmitters. Their dendritic arbors span the inner plexiform layer, allowing them to modulate synapses between bipolar cells and ganglion cells at multiple depths of retinal layering.

Their primary function is to modulate and refine visual signals before they reach the brain. Through lateral and feedback inhibition they shape receptive field size, temporal dynamics, and contrast gain. Some circuits contribute to motion detection and direction selectivity, notably starburst amacrine cells that interact with certain direction-selective ganglion cells. The rod pathway also relies on amacrine intermediaries such as the AII amacrine cell, which channels rod signals to both ON and OFF pathways via glycinergic synapses.

Beyond general inhibitory roles, amacrine cells participate in light adaptation and circadian modulation, with dopaminergic amacrine cells releasing dopamine in response to ambient illumination. Overall, amacrine cells form a diverse and critical network that enables complex processing of visual information within the retina.