Horizontal cell
R: rods, C: cones,
H: horizontal
cell, Bi: bipolar cell, A: amacrine cell, G: ganglion cell,
GC: ganglion cell layer.
(The light comes in from below.)
Horizontal cells are special nerve cells in the retina of the eye , which interconnect the information flows from the photoreceptors ( rods and cones ) to the bipolar cells of the retina laterally.
Put simply, a light stimulus incident on the retina is registered by the photoreceptors. Transferring their excitation to bipolar nerve cells , and these signals are derived in the vertical direction to ganglion cells of the retina continue to which the information on the optic nerve direction of the central nervous system to pass. The horizontal cells - whose cell bodies are in the inner granular layer (INL) on the side facing the outer plexiform layer (OPL) - intervene with their extensions in the synapses between photoreceptors and downstream bipolar cells and thus already ensure mutual influence at this level different - regionally neighboring - local individual signals.
Physiological function
The main function of the horizontal cells is to form lateral connections between the vertical connections of the photoreceptors to their downstream bipolar cells and thus to modulate the signal flows there by lateral inhibition . They thus help the eye to adapt to light and to increase the image contrast .
Horizontal cells are nerve cells that transmit information (like other neurons) through changes in the transmembrane tension and the resulting change in their release of neurotransmitters . The neurotransmitter they release to downstream neurons is gamma-aminobutyric acid (GABA) - an inhibitory neurotransmitter . If GABA acts on other neurons via the synaptic gap, it has a hyperpolarizing effect on them - it increases the transmembrane tension of the downstream cells and thus reduces the likelihood that these in turn will transmit signals. The effect is an inhibition of the signal flow.
Biological benefits
A horizontal cell receives information from many photoreceptors. At the same time, it also sends its information back to many photoreceptors. In simplified terms, the transmission of excitation can be imagined as follows: Light is registered by the photoreceptors. They send the information to the horizontal cells. The horizontal cells in turn send their signal back to the synapses between the photoreceptors and the bipolar cells and can thus influence the strength of the information that is ultimately transmitted from the photoreceptors to the bipolar cells and thus further towards the brain.
This mechanism is very important for light adaptation . Let us imagine that we are sitting in a shady, gloomy room and reading light text on a darker sheet of paper. If we now go out into the sun, we can still read the writing without any problems, although the bright letters now have a much higher light intensity due to the sunlight. To this adaptation contribute u. a. the horizontal cells.