Retinal ganglion cell

As ganglion cells of the retina or retinal ganglion cells ( RGC ) are different in the ganglion cell layer of the retina located stratum ganglionare nerve cells of the eye called, along whose axons form the optic nerve.

In the visual system represent retinal ganglion cells, the third afferent neurons of the visual pathway represents: Receive from intermediary neurons such as bipolar cells as the second afferent neuron they assembled information on the excitation of the light-sensitive cells - rods or cones as the first afferent neuron -, process this information and then initiate different own signals to core areas of the lateral geniculate nucleus (CGL ) in metathalamus of the diencephalon. From there, the optic radiation moves as a projection path to the primary visual cortex.

In the optic nerve run less numerous and axons of retinal ganglion cells whose signals for basic functions such as the day -night rhythm or optical reflexes are needed. These axons terminate at

• Nuclei of the nucleus in the hypothalamus suprachiamaticus ( light as a timer circadian rhythms )
• Nuclei of the pretectal area in the epithalamus ( afferent reflex arc portion of the pupillary light reaction ),
• Nuclei of the superior colliculus in the midbrain ( afferents for reflex eye movements ).

Morphology

The cell bodies of the ganglion cells are located in the innermost of the glass body facing layer of cells (stratum ganglionare ) the retina. Only the optical part of the retina to find these ganglion cell layer (GC). It is most pronounced in the yellow spot ( macula lutea) at the center of the retina around the middle is thinner now and is missing in the central fovea ( fovea centralis ). Because at this point elsewhere covering inner layers are laterally shifted and so the sensory cells of the outer nuclear layer ( ONL ) is exposed to the incident light; here are found exclusively cones, and in the foveola only M and L cones. The responsible for this central retinal regions ganglion cells are located in the surrounding edges of the macula.

The dendrites of retinal ganglion cells branch out depending on the cell type and different rich form in the inner plexifomen layer ( IPL) synapses with the bipolar cells and amacrine cells of the inner nuclear layer located further out (INL ).

The axons of all the ganglion cells in a human retina to a million and half run, within, as the nerve fiber layer (stratum neurofibrarum ) to the optic nerve ( optic nerve ) together and leave at the papilla ( optic disc ) the eyeball. In the field of this point corresponds to the blind spot. From there, pull the axons via the optic chiasm, partially crossing, mainly to core areas of the lateral geniculate body ( corpora geniculata lateralia, CGL) in the hypothalamus, where they form synapses with neurons downstream.

Retinal ganglion cells are multipolar neurons with long axons, which may be different in size, construction and branching pattern. Alone by morphological criteria - for example, after the appearance of their dendritic tree as a " tiny ( midget ) ", " sparse ( sparse ) ", " spiny ( thorny )" or " two-layer ( bistratified ) " divorced - there are over a dozen types.

In addition, ganglion cells are fanned by the target sites of projection, classified on the basis of electrophysiological characteristics as the line speed, as well as in relation to the range of light stimuli and disengaged signal pattern. Thus only can then several types differentiate so that they can be attributed to the different information processing tasks.

The ganglion cells are the only cells of the retina, which can form action potentials; in the other retinal neurons, the conduction is elektrotonisch. These action potentials are headed in Fort eye on the running as the innermost layer of the retina nerve fibers. In primates such as humans, these are formed markarm what only a relatively slow forwarding allows - but reduces the refraction of light. With discharge from the eye, the individual axons of the ganglion cells are then surrounded the optic nerve fibers mark rich by a myelin sheath, thereby allowing a faster line of the signals is possible. The optic nerve is surrounded by a total of continuations of the meninges and thus anatomically recognizable as a part of the brain.

Receptive fields

The area of ​​the retina, the impact can take on the state of arousal of a particular ganglion cell is called its associated receptive field. A receptive field consists of a specific group of receptor cells - rods or cones - and results from the convergence of several bipolar cells to ganglion cell. Horizontal cells and amacrine cells are involved in the dissemination of information, especially through lateral inhibition.

In most cases, receptive fields in a center and its periphery can be subdivided. These two areas have an opposite effect on the ganglion cell. Is the center exciting and peripheral inflammatory, one speaks of an ON - center ganglion cell in the opposite case of an OFF - center ganglion cell. With such a form of interconnection can be strengthened, among other things, the contrast of an optical perception. For example, the action potential frequency of an ON- center ganglion cell is particularly high when the associated photoreceptors of the center and are very strongly excited in the periphery rather little.

The size of the receptive fields is quite different. It depends both on the type of the ganglion cell, as well as their positioning in the retina. Within the macula lutea, the receptive fields are typically very small and comprise only a few cones. For the fovea than the area of sharpest vision - and especially for the foveola as the reference point "center" of spatial relations on the retina - is finally also a convergence of 1:1. Outside the macula clearly include the receptive fields more sensory cells and take to the retina periphery in size.

Cell types and information processing

In the present state of knowledge three main types of retinal ganglion cells can be distinguished in visual information processing:

• Parasol ganglion cells - In this cell type converge so-called diffuse bipolar cells, which in turn receive their input from L- and M-cones, so those sensory cells that are particularly sensitive to long - or medium -wavelength light. The receptive fields are relatively large and strongly developed dendritic trees ( parasol, english for, umbrella '). Since the parasol ganglion cells apparently do not distinguish between the two types of cones, they are " No color ", its output signal is achromatic. It probably serves mainly the light-dark distinction and is passed to the magnocellular layers of the CGL. Parasol ganglion cells process signals from adjacent rods, which are used with its high sensitivity, especially the night vision and do not distinguish between different spectral regions.
• Midget ganglion cells - Compared to the parasol cells, the midget ganglion cells have a small dendritic tree and tiny receptive fields, usually only covers the center only one M-cones or L cones ( midget, english for, tiny tot '). This over - midget bipolar cells associated center can be switched ON or OFF, the surrounding Peripheral then each vice versa. In common interaction they afford the processing of the red / green contrast and set off the signals from L cones and M-cones against each other as the difference ( simplified: L - M or M - L). The axons of the midget ganglion cells project to the parvocellular layers of the CGL. In this subsystem is the evolutionarily recent; the different opsins of the M and L cones are developed only in primates by a gene duplication. In the human retina, about 80 % of the ganglion cells of the midget cell type.
• Bistratified ganglion cell - This cell type has very large receptive fields with an ON - center of several S-cones, that are particularly sensitive for short-wavelength light-sensitive cells. Their signals are passed through converging blue- bipolar exciting to dendrites of the ganglion cell ( "Blue- ON"). A peripheral in the proper sense is missing here; diffuse bipolar cells, however, collect signals from M and L cones and give them inhibitory (OFF ) to a second Dendritenschicht of bistratified ganglion cells further ( bistratified, for layered double '). Thus, the signal from the S-cones is a combined signal from L- and M-cones compared and offset ( simplified: S - (M L)). In this way, the blue / yellow contrast can be highlighted. The axons of bistratified ganglion cells move to the koniozellulären layers of the CGL, which bear as sub-layers each interlaminar a large - or small-cell layer.

According to their respective target regions named, hot parasol ganglion cells and M cells ( M here for " magnozellulär " ) and midget ganglion cells and P - cells (P here for " parvozellulär "); occasionally the bistratified ganglion cells are referred to as K- cells (K for " koniozellulär ").

Some special ganglion cells contain a pigment, melanopsin, which makes them sensitive to light itself to ( intrinsic) photosensitive retinal ganglion cells.