Neurotransmitters and retinal circuits.

The retinal circuits and neurotransmitters of the mammalian retina have been described in great depth over the past century. The retina is often regarded as a "simple" circuit that conveys light information detected in the photoreceptors through second-order intermediate cells to retinal ganglion cells that output this information to the rest of the brain. However, considerable signal processing and feature extraction occur in this simple circuit before this signal is passed down the optic nerve. The mechanisms underlying this signal processing include (i) the transformation of analog graded potentials (generated in photoreceptors and bipolar cells) to the digital spike output of retinal ganglion cells, (ii) rectification of light inputs into ON and OFF channels to aid perception of light increments and decrements, (iii) extraction/amplification of spatial and temporal features such as direction selectivity via an interplay of excitatory and inhibitory inputs, and (iv) an adaptational mechanism to change the physiology of the retina to allow it to function over>10log units of illumination. Understanding the physiology of this tissue provides important mechanistic insights into general neuronal function and may provide a "window to the brain" to understand central neuronal disorders.