Horizontal Cells are neurons located in the retina that are common to all mammals.
Horizontal cells are located in the retina along with retinal bipolar cells and amacrine cells. They a part of the indirect pathway only, as opposed to bipolar cells which are part of the indirect and direct pathways in the eye. Much of the research on these cells has been done in cats, but rabbits and rhesus monkeys have been studied as well. These cells are also less numerous than bipolar cells. Horizontal cells receive stimuli from photo receptors in the eye and either return information to the receptors or forward that information on the bipolar cells in the eye.
Neuronal Type: Sensory Neuron
Horizontal cells are located in the outer plexiform layer of the retina. As shown in the figure, these cells synapse on bipolar cells and retinal photoreceptors. Some horizontal cells don't have axons in the conventional sense but have dendrites that function as axons. A-type and B-type horizontal cells in cats are characterized by bushy dendritic trees that cover an area of 75-250 micrometers depending on the type.
In primates three types of horizontal cells have been identified. The HI cell is similar to the B-type cell in cats. The HII cell was discovered in the rhesus monkey , and in humans the HIII. HI cells have contact with medium and short wavelength cones, HII cells contact short wavelength cones and HIII cells contact medium and long wavelength cones. Currently we are uncertain as to the flow of information through these cells in primates.
- Neurotransmitter: glutamate
Horizontal cells synapse between receptors and bipolar cells in the retina.
We know that these cells receive information from receptors through excitatory synapses because these cells are hyperpolarized, or inactivated, by light.
Receptors send information to the horizontal cells and in some species information is forwarded back to the receptors or on to bipolar cells. There is evidence that these cells are responsible for the receptive field surrounds of bipolar cells.