Retinal ganglion cell

Cell types in the layers of a mammalian retina -
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.)

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

In the visual system , retinal ganglion cells represent the third afferent neuron of the visual pathway : They receive processed information about the excitation of the light-sensing cells - rods or cones as the first afferent neuron - from intermediate neurons such as the bipolar cells as the second afferent neuron , process this information and then conduct different own signals to core areas of the lateral geniculate nucleus (CGL) in metathalamus of the midbrain on. From there, the visual radiation moves as a projection path to the primary visual cortex .

In the optic nerve, there are fewer axons of retinal ganglion cells whose signals are used for basic functions such as the day-night rhythm or for optical reflexes. These axons end at

Nuclei of the suprachiasmatic nucleus in the hypothalamus (light as a timer for circadian rhythms ),
Nuclei of the area pretectalis in the epithalamus (afferent reflex arc portion of the pupillary light reaction ),
Nuclei of the upper hill in the midbrain (afferents for reflex eye movements ).

morphology

The cell bodies of the ganglion cells are located in the innermost cell layer ( stratum ganglionare ) of the retina facing the vitreous body . This ganglion cell layer (GC) is only found in the optical part of the retina. It is most pronounced in the yellow spot ( macula lutea ) around the center of the retina around, is towards the middle thinner and missing in the central fovea ( fovea ). Because at this point the inner layers covering elsewhere are shifted sideways and so the sensory cells of the outer granular layer (ONL) are exposed to the incident light; only cones can be found here, and only M and L cones in the Foveola . The ganglion cells responsible for these central retinal regions are located in the surrounding marginal zones of the macula.

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

The axons of all ganglion cells, up to one and a half million in a human retina, converge internally as a nerve fiber layer ( stratum neurofibrarum ) to form the optic nerve ( optic nerve ) and leave the eyeball at the optic disc ( discus nervi optici ). In the field this point corresponds to the blind spot . From there, the axons run over the optic chiasm , sometimes crossing, mostly to the core areas of the lateral knee cusps ( Corpora geniculata lateralia , CGL) in the diencephalon, where they form synapses with downstream neurons .

Retinal ganglion cells are multipolar neurons with a long axon that can vary in size, structure, and branching pattern. Solely on morphological criteria - for example, after the appearance of their dendritic tree as "tiny ( midget )", "sparse ( sparse )", "spiny ( thorny )" or "two layers ( bistratified )" divorced - there are over a dozen types.

Functional assignment

In addition, ganglion cells are fanned out according to the target locations of the projection , classified on the basis of electrophysiological characteristics such as the conduction speed, and labeled with regard to the spectrum of light stimuli and the triggered signal patterns. Only then can several types be differentiated in such a way that they can be assigned different tasks in information processing.

The ganglion cells are the only cells in the retina that can develop action potentials ; in the other retinal neurons the conduction of excitation is electrotonic . These action potentials in the eye are passed on via the nerve fibers running as the innermost retinal layer . In primates such as humans, these are formed without mycelium , which only allows relatively slow transmission - but reduces the refraction of light. As they exit the eye, the individual axons of the ganglion cells are surrounded by a myelin sheath as optic nerve fibers, which enables the signals to be conducted more quickly . The optic nerve as a whole is encased by continuations of the meninges and can therefore be recognized anatomically as part of the brain.

Receptive fields

The area of the retina that can influence the state of excitation of a particular ganglion cell is called the receptive field assigned to it. A receptive field comprises a certain group of receptor cells - rods or cones - and is created by the convergence of several bipolar cells on one ganglion cell. Horizontal cells and amacrine cells contribute to the transmission of information, especially through lateral inhibition .

Usually receptive fields can be divided into a center and its periphery. These two areas have opposite effects on the ganglion cell. If the center is exciting and the periphery is inhibiting, then one speaks of an ON center ganglion cell, in the opposite case of an OFF center ganglion cell. With such a form of interconnection, among other things, the contrast of an optical perception can be increased. For example, the action potential frequency of an ON center ganglion cell is particularly high when the associated photoreceptors in the center are stimulated very strongly and those in the periphery are stimulated very little.

The size of the individual receptive fields is quite different. It depends on both the type of ganglion cell and its position in the retina. Within the macula lutea , the receptive fields are typically extremely small and contain only a few cones. For the fovea centralis as the area of sharpest vision - and here in particular for the foveola as the reference point "center" of the spatial relationships on the retina - there is finally a convergence of 1: 1. Outside the macula, the receptive fields contain significantly more sensory cells and increase in size towards the retinal periphery.

Cell types and information processing

According to the current state of knowledge, three main types of retinal ganglion cells are distinguished in visual information processing in primates :

Parasol ganglion cells - so-called diffuse bipolar cells converge on this cell type, which in turn receive their input from L- and M-cones, i.e. those sensory cells that are particularly sensitive to long- or medium-wave light. The receptive fields are comparatively large and the dendritic trees are well developed (parasol, English for 'parasol'). Since the parasol ganglion cells obviously do not differentiate between the two types of cones, they are "uncolored" and their output signal is achromatic. It probably serves mainly to distinguish between light and dark and is passed on to the magnocellular layers of the CGL . Parasol ganglion cells also process signals from rods, which with their high sensitivity are primarily used for twilight vision and do not differentiate between different spectral ranges.
Midget ganglion cells - Compared to the parasol cells, the midget ganglion cells have a small dendritic tree and tiny receptive fields, the center of which usually only comprises an M-cone or an L-cone (midget, English for 'tiny'). This center, assigned via midget bipolar cells, can be switched ON or OFF, the surrounding periphery then vice versa. Working together, they process the red / green contrast and offset the signals from L-cones and M-cones against each other as a difference (simplified: L - M or M - L). The axons of the midget ganglion cells project into the parvocellular layers of the CGL . This subsystem is the most recent in evolutionary terms; the different opsins of the M and L cones only emerged in primates through gene duplication. In the human retina, about 80% of ganglion cells are of the midget cell type.
bistratified ganglion cells - This cell type has very large receptive fields with an ON center of several S cones, i.e. the sensory cells that are particularly sensitive to short-wave light. Their signals are stimulated to dendrites of the ganglion cell via converging blue bipolar cells ("Blue-ON"). A periphery in the proper sense is missing here; Diffuse bipolar cells, on the other hand, collect signals from M and L cones and pass them on in an inhibitory manner (OFF) to a second dendrite layer of the bistratified ganglion cells (bistratified, for 'double layered'). In this way, the signal from the S spigot is compared and offset against a combined signal from L and M spigots (simplified: S - (M + L)). This way the blue / yellow contrast can be emphasized. The axons of the bistratified ganglion cells move into the coniocellular layers of the CGL , which are interlaminar and adjoin a large or small cell layer.

Named after the cell types of their respective target regions, parasol ganglion cells are also called M cells ("M" here for magnocellular ) and midget ganglion cells are also called P-cells ("P" here for parvocellular ); occasionally the bistratified ganglion cells are referred to as K cells ("K" for coniocellular ).

In addition, 1–3% of retinal ganglion cells are found in the mammalian eye, which do not contribute to visual image processing but are assigned to the non-visual system of ocular photosensitivity. These neurons contain a photopigment, melanopsin , that makes them sensitive to light themselves. They convert light into a longer-lasting and depolarized receptor potential , in contrast to the ciliary photoreceptors with contact to the retinal pigment epithelium (RPE).

(intrinsic) photosensitive retinal ganglion cells (ipRGCs) - so far, five main types (M1, M2, M3, M4, M5) with subtypes have been distinguished. However, their delimitations are still vague and in particular their respective physiological tasks are still unclear in detail. They receive signals from the retina, mainly from ON bipolar cells, and also pass signals on to neurons in the retina, especially amacrine cells. Their essential function, however, is carried out by their neurites running with the optic nerves

Nuclei of the suprachiasmatic nucleus in the hypothalamus (light as a timer for circadian rhythms ),
Nuclei of the area pretectalis in the epithalamus (afferent reflex arc portion of the pupillary light reaction ),
Nuclei of the upper hill in the midbrain (afferents for reflex eye movements ).

Notes and individual references

↑ The term "ganglion cell" goes back to the anatomical differentiation of the nervous system into a central part and the peripheral part, in which accumulations of nerve cell bodies are called ganglia . In this way, the cell bodies of neurons in the retina in the inner granular layer ( stratum nucleare internum ) are grouped together as the “ganglion retinae” and those in the ganglion cell layer ( stratum ganglionare ) as the “ganglion nervi optici”. Only the latter, the optic nerve ganglion cells, are usually called “retinal ganglion cells” (RGC).

↑ A brief schematic summary according to Dacey et al. (Washington, 2003) is reproduced in the Neuronbank Wiki under Retinal Ganglion Cell , Types of RGCs ( Memento of the original from July 25, 2009 in the Internet Archive ) Info: The archive link was automatically inserted and still Not checked. Please check the original and archive link according to the instructions and then remove this notice. . @1@ 2

↑ Eric R. Kandel, James H. Schwarz, Thomas M. Jessell (Eds.): Principles of neural science. 4th edition, international edition. McGraw-Hill, New York NY et al. 2000, ISBN 0-8385-7701-6 .

↑ Barry B. Lee, Paul R. Martin, Ulrike Grünert: Retinal connectivity and primate vision In: Progress in Retinal and Eye Research. Vol. 29, No. 6, November 2010, ISSN 1350-9462 , pp. 622-639, doi: 10.1016 / j.preteyeres.2010.08.004

↑ Gerald H. Jacobs, Jeremy Nathans : The Strange Color Sense of Primates. In: Spectrum of Science . 5/2010, pp. 44-51.

[1] The term "ganglion cell" goes back to the anatomical differentiation of the nervous system into a central part and the peripheral part, in which accumulations of nerve cell bodies are called ganglia . In this way, the cell bodies of neurons in the retina in the inner granular layer ( stratum nucleare internum ) are grouped together as the “ganglion retinae” and those in the ganglion cell layer ( stratum ganglionare ) as the “ganglion nervi optici”. Only the latter, the optic nerve ganglion cells, are usually called “retinal ganglion cells” (RGC).