Retina

The retina, retina (from the Latin rete, " net" ) or inner eye skin is the multi-layered, specialized nerve tissue that lines the inside of the eye of vertebrates, some squid and snails. In the retina, the incident light after it has passed through the cornea, the lens and the vitreous body, is converted into nerve impulses. The retinal pigment darkening is located inside the layer of light- sensitive photoreceptor cells ( photoreceptors ). The pulses in additional layers of nerve cells to follow inwardly processed and forwarded. In addition, the retina contains various supporting and supplying structures.

The retina is expressed simply, become a kind of screen for the image of our surroundings, similar to a canvas or a photosensitive film, and directs the light-induced excitations further stimuli to the brain.

Anatomy of the retina of the human

Directional and positional terms

The layers of the retina is known as the outside on the side of the retina, which is facing away from the light, so is closer to the outer wall of the eyeball. As inner, respectively corresponding to the light- facing side, which faces the Augapfelzentrum detail. The light passes through the retina that is from inside to outside. As the front ( anterior or distal) is called the retina shares, which are closest to the structures of the anterior segment of the eye, ie towards the cornea, iris and lens. Back (posterior or proximal) are therefore the retina shares on the wall of the eyeball, which is opposite to the cornea. For the indication of directions in the transverse axis of the expressions nose side ( nasal) or temporal side ( temporal) are needed.

Sections and limits

The largest part of the inner surface of the eyeball is of the pars optica retinae covered, which carries the photoreceptors. In front, about 3 mm behind the ciliary body ( ciliary body ), is the ora serrata, the transfer station and only solid connection between the non- pigmented neuroretina and the pigment content ( retinal pigment epithelium ). Prior to the ora serrata is the pars caeca retinae (Latin caecus "blind" ), the pars ciliaris retinae at the forefront as the ciliary body coats. The anterior retina sections can be viewed, for example, with a three- mirror contact lens on the intact eye in the contact lens examination.

The retina is located on the outside of the choroid ( choroidal ) which supplies the outer layers of the retina with nutrients by diffusion. Inside is the retina of the vitreous ( vitreous body ). In the optic nerve, the retina goes back into the optic nerve, which contains the nerve fibers of the ganglion cell layer.

Macroscopic image of the retina, fundus

Looking at the back of the eye ( fundus oculi or fundus ) through the pupil you are looking through the colorless and transparent neuroretina through to the darkly pigmented retinal pigment epithelium ( RPE) and the underlying, depending on type more or less darkly pigmented choroid. As a striking landmark is found about 15 ° nasally from the optic axis, the exit point of the optic nerve ( optic nerve head or papilla ). She stands out from her pink to light red color from the rest of the fundus Brown Orange. The papilla has an average diameter of about 1.5 mm and is individual usually shaped round or oval.

The blind spot

As a blind spot which will be identified in the field to which the optic nerve ( optic nerve ) projects ( about 15 ° temporal side of the fixation point ). Where the nerve fibers of the ganglion cell layer through all layers of the retina leave the eyeball, there are no light receptors are located ( see below). The area of ​​the optic nerve head is so truly "blind".

The fact that this "blind" spot is not perceived even in monocular vision, is the so-called supplementary effect. The visual system uses information that provides the receptors in the vicinity of the optic nerve, in order to complete the picture.

Vessels of the retina, macula lutea

Starting from the papilla one sees the large retinal vessels ( arteriole and Venola central retinal ), which are divided into an upper and lower clump and divide usually once in a nasal and temporal side branch. To their different shade of red and the caliber arterioles thereby can be differentiated from the venules.

The temporal range of vessel branches circle arc a poor vessel and vessel- free area in the center, the yellow spot ( the macula lutea ). It has a diameter of about 3 mm. The term " yellow spot " was introduced by Samuel Thomas von Soemmerring 1779, who had been watching the yellowish color of this retinal area. It is the only colored part of the retina, as it contains a yellow pigment, lutein.

The center of the " yellow spot " is a shallow depression called the fovea centralis, fovea or fovea shortly. Around the fovea centralis of the retina is thickened to a flat wall. This surface shape can be seen in young eye to light reflections. The fovea centralis as motor zero point represents the straight direction value, also called main Ehrich Tung. Each peripheral retinal point has a different sense of direction which leads to a corresponding number of secondary Ehrich directions.

The "yellow spot" is also the site of sharpest vision. Here, the retina has the highest resolution, which you use when reading about. The surrounding retina is the basis of the peripheral visual field and essentially serves the environment perception, the recognition of things " out of the corner of his eye ." In case of severe damage of the macula lutea, eg by age-related macular degeneration ( AMD), you can no longer read or drive a car.

Nerve cells of the retina

A retinal cross-section shows the light microscope a conspicuous stratification, which is formed by alternately nucleus- rich and - poor layers. The layers have distinctive cell types and subcellular compartments. The nerve cells of the retina can be divided into three groups:

• Photosensitive or photoreceptive cells which convert the incident light into nerve impulses. These include rods and cones and possibly other cell types.
• The intermediate cells or interneurons, which undergo the generated pulses of a first processing within the retina. These include the horizontal cells, bipolar cells and amacrine cells, the.
• The ganglion cells, which transmit the processed information to the next position outside the retina.

As a result of evolutionary development, after which the retina was formed from a protuberance of the diencephalon, it is the inverse construction, since the actual light-sensitive cells located in the outer layers, which are remote from the light farthest.

Photoreceptors

The photoreceptor cells occurs among the cells of the retina is particularly important, because they represent the actually light-sensitive cell type. The photoreceptor cells are highly polar cells, which consist of an outer segment, an inner segment, the cell body and axon of a specialized synapse end. A basic distinction in the retina of vertebrates between two photoreceptor cell types, rods and cones ( rods and cones in English ), which are distributed mosaic -like. The rods are to vision in low light specialized ( scotopic vision ), the cones are responsible for color vision ( photopic vision ). A third type of photoreceptor cells are the photosensitive ganglion cells that contribute to the synchronization of the internal clock with the day -night rhythm.

Trichromates people, i.e., they have three types of cones with different absorption maxima. Simplified one can say that there are red-sensitive, green-sensitive and blue-sensitive cones. The nervous system combines the signals from the three cone types to light of a certain spectral assign a color. The cones are less sensitive to light than the rods. Therefore, the perception of color changes at night, called the Purkinje effect. Therefore, the saying is true: At night all cats are gray ( to Remember: with the chopsticks you see black / white, with the pin colors).

Even a single light particle ( photon) can activate a stick. However, several sticks must be enabled to view the retina signals the presence of light. Upon impact of a photon to the embedded in the membrane stack photoreceptors Rhodopsin latter undergoes a conformational change. Thus an enzyme cascade is triggered, called the visual signal transduction cascade that ultimately altering the activity of the nerve cell leads ( pins as rods). For the elucidation of the importance of the retinal 1933-1958 American biochemist George Wald got the 1967 Nobel Prize for Medicine.

Perhaps one of the photoreceptors a third group of light-sensitive cells; these contain the pigment melanopsin. This cell type was discovered only recently and is still relatively unexplored. It has been demonstrated that the melanopsin cells act as photoreceptors and play in the function of the "internal clock " an important role. They send signals to the ( suprachiasmatic nucleus ), are generated in the circadian rhythms and passed as time information to the body ( see chronobiology ). According to new findings, these ganglion cells are also responsible for the pupillary reflex.

Horizontal cells, bipolar cells, amacrine cells

Two different types of horizontal cells interconnect the photoreceptors themselves. They are even with neighboring cells via gap junction in contact. Horizontal cells are used inter alia for contrast enhancement by lateral inhibition of neighboring photoreceptors.

The bipolar cells are innervated by the photoreceptors. In the mammalian retina are differentiated, depending on the species, eight to twelve types of bipolar cells, which contact pins ( pin - bipolar cells ), and a Bipolarzelltyp, the rods contacted ( rod bipolar cells ). A distinction is made between ON and OFF bipolar cells. ON- bipolar cells depolarize to a light stimulus, with OFF cells respond with a hyperpolarization.

The bipolar cells innervate the amacrine cells. Amacrine cells provide - as before, the horizontal cells - for both lateral and vertical interconnection of the neural network in this layer of the retina and also contribute to the modulation of the signal processing at. There are over 30 different Amakrinzelltypen. One type, the so-called A2- Amakrinzelle, passes signals of rod bipolar cells in the cone bipolar cells and thus links these two signal paths. Another type of amacrine cells, the so-called starburst cell is involved in the perception of movement.

Ganglion cells

Downstream of the bipolar and amacrine cells are the ganglion cells. They are the output neurons of the retina and transmit the visual information via the optic nerve ( optic nerve ) to the next switching station in the brain, the lateral geniculate body. Overall, about 20 ganglion cell types can be distinguished.

Layers of the retina

The light penetrates the entire retina and is only detected by the photoreceptor cells. From the vitreous toward the choroid, one can distinguish the following layer structure in the healthy retina. ( The pictures should be noted that the light is always directed from bottom to top. At this light vector is then also orient the labels for inside and outside, namely inside the vitreous cavity and the outside of the direction on the limiting choroid, ie not in the senses of the body after body appearance affairs, but, conversely, from the interior of the eyeball outward. )

Epiretinal or internal limiting membrane ( membrana limitans interna, ERM / ILM epiretinal / internal limiting membrane)

The inner boundary layer of the retina is the basal and plasma membrane of Müller's cells and possibly other glial cells. It also consists of collagen fibers and Proteoglykosiden.

This appears to be homogeneous in the light microscope layer forms the inner boundary of the retina from the vitreous body. It covers the entire retina and goes forward with a slightly coarsened structure over the Zonulalamelle.

The older histologist were of the view that the brush-like Endfüße of Mueller's supporting fibers are firmly anchored in the limiting membrane, the latter is therefore to be regarded as gliöses product. However, the electron microscope showed that the inner limiting membrane has the character of a basement membrane and can be clearly distinguished from the base points of Müller's cells. The clinically detectable strong adhesion of the membrane to the retina probably comes about through the filled with cement substance intermediate layer which connects the Gliafortsätze of Müller's cells with the limiting membrane.

Gaertner summarizes the internal limiting membrane as a peripheral compression of the vitreous tissue, since their fiber lamella layers stand out from the retina and pass into the vitreous membrane. The thickness of the boundary membrane is approximately 2-3 microns.

Nerve fiber layer (NFL - nerve fiber layer)

It consists of clusters of nerve fibers ( axons ) of the ganglion cells. These carry the processed information of all photoreceptors to the outside. The nerve fiber layer contains - in humans - about 1 million Ganglienzellaxone. These nerve fibers are unmyelinated and receive their myelin sheaths after the exit from the eyeball. Destruction of the nerve fiber layer has an irreparable blindness result ( for example, when glaucoma ) for the affected retinal area.

In the fovea ( the fovea ), the fiber density is lowest. The axons run from here in a star shape, starting in a more or less pronounced arch to the exit point of the optic nerve ( optic disk ). Between the fovea and optic disc they run together, forming the so-called makulopapilläre bundle. All other Ganglienzellaxone run around this bundle either up or down around an arc. Here they never exceed the horizontal centerline ( Rhaphe ).

The nerve fibers of the anterior retina, the ganglion cell bodies lie furthest away from the disc, extending in the outermost layer of the fiber. This will keep them when they move to the optic nerve head outside and therefore also occupy a more peripheral position in the optic nerve. The axons of the ganglion cells, which are closer to the optic disc, run shallower in the NFL, thus they are also more likely to be located centrally in the optic nerve.

Also centrifugal fibers - from the brain to the retina - have been variously described in the nerve fiber layer. However, their function and significance is uncertain. Some authors assign them in the act of seeing Hemmfunktionen, others bring in connection with the vascular innervation of the retinal vasculature.

In the nerve fiber layer and the adjacent ganglion cell layer also run the larger blood vessels of the retina. In addition, this layer also neuroglia cells and scattered ganglion cells are present. The thickness of the nerve fiber layer is about 20-30 microns.

Ganglion cell layer ( GCL or GC - ganglion cell layer)

Contains the cell bodies of the ganglion cells.

The dendrites of the ganglion cells move into the adjacent inner plexiform layer ( IPL) where they branch and receive the signals from the bipolar cells and amacrine cells the. Forwarded to the visual centers in the brain, the signals exclusively via the axons of the ganglion cells, which run in the nerve fiber layer and the optic nerve bundle.

The size of the cell body is different, the arrangement of the cells in the peripheral retina generally single-layered, multi-layered in the middle and central zones. In addition to the ganglion cells are found in the GCL some types of amacrine cells ( ' displaced amacrine cells ') and glial cells. The thickness of the GCL is about 20-30 microns.

Inner plexiform layer (IPL - inner plexiform layer)

Last pre-processing before the information is passed to the visual centers of the brain.

The IPL consists of a dense network that is formed by the axon terminals of bipolar cells and the dendrites of amacrine and ganglion cells. The bipolar cells provide the signals from the photoreceptors of the outer retinal layers. These signals are modulated by synaptic interactions with amacrine cells and finally to the ganglion cells.

Occasionally found in the IPL and cell body of " misplaced " ( misplaced) neurons, as well as neuroglia. The thickness of this layer is about 50-70 microns.

Inner nuclear layer (INL - inner nuclear layer)

The inner nuclear layer contains the cell bodies of functionally very different cells.

INL in the cell bodies of bipolar, amacrine and horizontal cells are located on the pre-processing of signals, as well as the neuronal cell bodies of the support fabric, the Muller cells. If the outermost horizontal cells, followed by the cell body of the bipolar cells and Muller cells are at the inner perimeter, the amacrine cells. Overall, we can distinguish 10 to 12 cell layers. The thickness of the inner nuclear layer is 30 microns.

Outer plexiform layer (OPL - outer plexiform layer)

The outer plexiform layer establishes the connection between photoreceptors and downstream cells.

Dendrites of bipolar and horizontal cells are interconnected with the ends of the photoreceptor synaptic and thus form the first stage of the information processing intraretinal. This layer also includes the processes of the Mullerian supporting fibers. In the transition zone to the next layer is the lower supply network of the retinal capillaries, the descendants of the central retinal artery are. These vessels run very constant in a plane and come across hardly found in other layers. The thickness of this layer is about 20 microns.

Outer nuclear layer ( ONL - outer nuclear layer)

Outer nuclear layer, the layer of the cell bodies of the photoreceptor.

The cell bodies of the rods and cones in the ONL grouped parallel to each other and extend their thickened light sensitive appendages, the outer segments toward the RPE. The cores of the pins are in a single layer near the interface membrane, the form of the rod 4 to 6 layers. An exception to this arrangement is the people in the Fovearegion, are also stored several layers in the journal.

Is to be noted particularly that significantly more cell nuclei are found in this layer, as in the layer of bipolar and ganglion cells. The thickness of this layer is about 40 microns.

Outer limiting membrane (ELM - external limiting membrane)

The outer limiting membrane is the final impermeable membrane.

It is carried foothills of the Müller 's cells and cell-adhesion compounds ( Adhering junction) formed with the photoreceptor cells. This fibrillar network can freely circular openings for passage of the receptors. In the region of the ora serrata, the outer limiting membrane continues into the substance that connects the two layers of the ciliary epithelium.

Inner segment ( IS - intra- segment)

Metabolic rich compartments of photoreceptors.

The IS is the area of the photoreceptor cells, the mitochondria and endoplasmic reticulum (ER) includes. Here, inter alia, protein synthesis, and other metabolic activities. Separately, the inner from the outer segments by a narrow Verbindungscilium through which all the materials for the outer segment must be actively transported.

Outer segment (OS - outer segment)

The outer segment is the light- sensitive compartment of photoreceptors. Here, the outer segments of the photoreceptors extend from Verbindungscilium to the RPE. On Verbindungscilium, new disc membranes, Membranabschnürungen packed with rhodopsin. Rhodopsin is stored in the discs, and initiates the visual signal transduction. By the de novo synthesis, these discs move the RPE and are phagocytosed there.

Retinal pigment epithelium ( RPE - retinal pigment epithelium )

Light filter and mass transfer with photoreceptors. Very outside the retina from the RPE, a hexagonally structured, layered epithelium is limited, which forms the interface between the retina and the choroid ( choroid ). The cells of the RPE contain melanin black colored melanosomes, which represent functional light filter. Apical finger-like embrace, mikrovilläre processes of the RPE, photoreceptor cells, which serve to nourish the photoreceptors, the recycling of the old disc membranes of the photoreceptor outer segments and the regeneration of the bleached retinal from rhodopsin is activated. Basal these cells show deep furrows in the interest of better material exchange with the blood vessels of the choroid. RPE cells prevent entry of blood from the highly vascularized choriocapillaris ( the strong blood boundary layer of the choroid ). In the pars optica retinae the connection between the RPE and the neural retina is maintained only by an actively generated by the RPE pull, a strong connection exists only at the ora serrata.

Diseases of the retina

Examples of diseases of the retina are:

• Diabetic retinopathy as a result of state of a ( poorly controlled ) diabetes
• Macular Degeneration
• Retinal detachment
• Netzhautforamen
• Vascular occlusion (ischemia, infarction)
• Central serous retinopathy ( swelling -induced retinal detachment )
• Retinitis pigmentosa ( hereditary genetic disease of the retina, and retinitis pigmentosa (RP ) called )
• Hypertensive retinopathy ( caused by high blood pressure disease of the retina )
• Eclamptic retinopathy ( occurring from eclampsia in pregnant women in the context of retinal disease )
• Retinopathy of prematurity ( retinopathy of prematurity )
• Acute zonal occult outer retinopathy Azoor
• Retinoschisis retinal cleavage
• Peripheral retinal degeneration ( thinning of the retina, " glitters clouds' )

Aspirin

A U.S. longitudinal study from Wisconsin suggests that regular use of aspirin may be could lead to retinal damage and significantly increases the risk of age-related macular degeneration ( AMD). However, a causal relationship with the development of disease could not be detected.

Investigation procedure

A standard method for the examination of the retina is the direct or indirect ophthalmoscopy or fundoscopy ( ophthalmoscopy ); they do is the illumination of the fundus and the observation of the reflected image with a magnifying glass. Since the invention of the ophthalmoscope in 1851 by Hermann von Helmholtz, this method is well established in the ophthalmoscopic diagnosis.

In the last decades have been developed for other procedures retina diagnosis.

• A relatively new, technically superior and more expensive method is optical coherence tomography ( OCT), which extends the mapping of the study area to the third dimension. It enables the production of high-resolution cross-sectional images or 3D tomography with a quality comparable to the histological picture ( resolution down to 3 microns compared to 0.3 microns at the light microscope). Here the individual retinal layers can be resolved and measured in thickness. This very minor differences can determine which might be relevant for the adequate treatment of certain diseases of the retina or in the testing of drugs.
• Other imaging techniques offer novel scanning laser ophthalmoscopes, such as the Heidelberg Retina Tomograph (HRT ), which can generate high-resolution three-dimensional layer or reliefs by a point or zeilenweisem retinal scanning and confocal aperture and lighting technology.

Further investigation and assessment opportunities exist on electrophysiological level by

• The electroretinogram ( ERG)
• The electrooculogram ( EOG),

As well as for the representation of retinal blood flow by

• Fluorescein angiography ( FLA also, FAG or angio abbreviated )
• The indocyanine green angiography
• The retinal vessel analysis.