Retina horizontal cell

Horizontal cells are special nerve cells in the retina of the eye, which (side) interconnect the photoreceptors ( rods and cones ) and the bipolar cells of the retina laterally.

An incident on the retina light stimulus is shown in simplified form registered by the photoreceptors. They transfer their excitation to bipolar nerve cells and this route signals in the vertical direction of ganglion cells of the retina on which to pass on the information via the optic nerve toward the central nervous system. The horizontal cells now - whose cell bodies in the inner nuclear layer (INL ), outer plexiform layer of the next (OPL ) are - engage with their projections into the synapses between photoreceptors and bipolar cells downstream and influence on the horizontal plane.

Function

The essential function of horizontal cells is to form lateral interconnections between photoreceptors and bipolar cells and to modulate the signal flow on lateral inhibition. They thus contribute to light adaptation of the eye as well as to an increase in image contrast.

Horizontal cells are nerve cells, which forward information to other neurons by intracellular voltage change and the effected thereby changing their neurotransmitter release. The distributed of them on downstream neurons neurotransmitter gamma -aminobutyric acid ( GABA) - an inhibitory neurotransmitter. Does GABA across the synaptic cleft to other neurons, then it acts hyperpolarizing - so it lowers the voltage of the cell and thus makes them more negatively charged.

Photoreceptors, which derive their information inter alia, to the horizontal cells, are a particular type of neuron. To transfer an excitation not ( such as " ordinary " neurons ) on the frequency of action potentials. While most other neurons are negatively charged at rest ( about -70 mV ), ( positively charged) suddenly depolarize with excitement over a certain threshold and thus distribute a large amount of neurotransmitters that nerve conduction takes place in photoreceptors differently are in rest position they loaded less strongly negative (about -40 mV), slightly depolarized. Now arrives a light stimulus decreases its voltage and they are even more negative charged ( up to max. -65 MV). The intensity of the light stimulus is encoded by the strength of the negativity ( hyperpolarization ). This hyperpolarization causes a reduction of the amount of undistributed neurotransmitters ( glutamate in this case ). Light irritation thus ultimately leads to less the Plasma glutamate by the photoreceptors.

So Does a light stimulus, the less glutamate of the photoreceptors acts on the horizontal cells. This reduction of the neurotransmitter into the horizontal cell to reduce their activity. Consequently, it pours itself out also less neurotransmitters ( GABA).

Task

A horizontal cell receives information from many photoreceptors. At the same time it sends its information back too many photoreceptors. The excitation forwarding You can simplifies as follows imagine: light is registered by the photoreceptors. The information send email to the horizontal cells further. The horizontal cells in turn send their signal back to the synapses between the photoreceptors and the bipolar cells and thus the strength of the information that is passed ultimately from the photoreceptors to the bipolar cells and thus further towards the brain that influence.

This mechanism is very important for the light adaptation. Let us imagine that we are sitting in a shady, gloomy room and read a light font on a dark 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 the sunlight a much higher light intensity. At this adaptation contribute, inter alia, the horizontal cells.

Let us first consider the situation in the gloomy room, where we see a bright spot on a dark sheet of paper. Very simplistic, we suppose that this lighter point falls on a photoreceptor. The neighborhood of the point is so dark in the dim room that all other photoreceptors register no light.

Suppose now that the light intensity of the bright point leads to the activated photoreceptor is hyperpolarized to -65 mV exactly. This corresponds exactly to its maximum hyperpolarization threshold. This hyperpolarization of the receptor makes this now less glutamate at the horizontal cell synapses on him, pour out. However, a horizontal cell receives from many photoreceptors information - therefore causes a reduction of a single neurotransmitter receptor as in our case, hardly a change in activity of the horizontal cell. Their influence on the conduction can be neglected in this case. Because the horizontal cell therefore has almost no influence on the conduction of photoreceptor to bipolar cell, the light stimulus intensity can be transferred one- to-one to the bipolar cell.

Imagine now the situation in which we consider ourselves the same bright spot on a dark paper in the sunlight. Let's say, the light intensity, which is now reflected by the dark paper is just as strong as the intensity of the bright spot in the gloomy room. The bright spot, however, is now in the sunlight still much brighter than before.

Now, not only the individual photoreceptor, the seems to be the bright spot, active, but also many other receptors, since now the dark paper is as bright as the light reflection, as the point before in the dim room. Let 's first discuss the excitation line before, if there is no influence by the horizontal cells would be: All photo receptors located in the vicinity of bright point are up to their maximum threshold hyperpolarized from -65 mV, since the ambient light spot now as bright is, as before, the bright spot even in the dim room. But what about the photoreceptor to the seems of the now much brighter point? Already in the gloomy room this receptor was stimulated to its maximum threshold of - 65mV, ie even if the stimulus is even brighter (like now in the sunlight ), the receptor can be hyperpolarized to -65 mV only. In this case, would be the activity of the photoreceptor on which appears even brighter the point not different from the activity of the surrounding receptors, as all are hyperpolarized to its limit of - 65mV. This meant that we actually could not even perceive the brighter point, because the entire field of view would be perceived as " equal light ".

However, here comes the influence of horizontal cells to the fore: Because even now many photoreceptors are hyperpolarized by light stimulation, obtain individual horizontal cells of many receptors information, which means that they get an important impact. As described, the light stimulus causes a reduction of the glutamate receptors of the emissions to the horizontal cells. They will thus become less active and pour themselves less GABA in photoreceptors from. GABA is an inhibitory as described ( inhibitory ) neurotransmitter, so the results in the photoreceptors that these hyperpolarize even stronger. If, as in this case by reducing the activity of many horizontal cells GABA is much less redistributed back to the photoreceptor, this leads to a depolarization at the receptors. These are thus again a bit less negatively charged - that all of the voltage level of all active PRs is thus offset to some extent upward ( positive direction ).

Suppose in our case at once, the GABA - reduction by the horizontal cells causes a depolarization in the photoreceptors to 20 mV. Those photoreceptors which the receptor as falling within the bright point surrounded, are now held at their border at -65 mV only at -45 mV. The same effect also acts on the receptor as falling within the bright point. He is now so again significantly more hyperpolarized (possibly to the limit of -65 mV) to signal that occurs to an even brighter light stimulus than on the surrounding receptors on him.

Through this mechanism, we can thus also in the harsh sunlight still perceive the brighter spot in front of a darker background.