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Simple Anatomy of the Retina by Helga Kolb – Webvision. Helga Kolb. 1. Overview. When an ophthalmologist uses an ophthalmoscope to look into your eye he sees the following view of the retina (Fig. In the center of the retina is the optic nerve, a circular to oval white area measuring about 2 x 1. From the center of the optic nerve radiates the major blood vessels of the retina.

Approximately 1. 7 degrees (4. A circular field of approximately 6 mm around the fovea is considered the central retina while beyond this is peripheral retina stretching to the ora serrata, 2. The total retina is a circular disc of between 3. Polyak, 1. 94. 1; Van Buren, 1.

Kolb, 1. 99. 1). The retina is approximately 0. The optic nerve contains the ganglion cell axons running to the brain and, additionally, incoming blood vessels that open into the retina to vascularize the retinal layers and neurons (Fig.

A radial section of a portion of the retina reveals that the ganglion cells (the output neurons of the retina) lie innermost in the retina closest to the lens and front of the eye, and the photosensors (the rods and cones) lie outermost in the retina against the pigment epithelium and choroid. Light must, therefore, travel through the thickness of the retina before striking and activating the rods and cones (Fig. Subsequently the absorbtion of photons by the visual pigment of the photoreceptors is translated into first a biochemical message and then an electrical message that can stimulate all the succeeding neurons of the retina. The retinal message concerning the photic input and some preliminary organization of the visual image into several forms of sensation are transmitted to the brain from the spiking discharge pattern of the ganglion cells.

A simplistic wiring diagram of the retina emphasizes only the sensory photoreceptors and the ganglion cells with a few interneurons connecting the two cell types such as seen in Figure 2. Fig. 2. Simple organization of the retina. When an anatomist takes a vertical section of the retina and processes it for microscopic examination it becomes obvious that the retina is much more complex and contains many more nerve cell types than the simplistic scheme (above) had indicated. It is immediately obvious that there are many interneurons packed into the central part of the section of retina intervening between the photoreceptors and the ganglion cells (Fig 3). All vertebrate retinas are composed of three layers of nerve cell bodies and two layers of synapses (Fig. The outer nuclear layer contains cell bodies of the rods and cones, the inner nuclear layer contains cell bodies of the bipolar, horizontal and amacrine cells and the ganglion cell layer contains cell bodies of ganglion cells and displaced amacrine cells. Dividing these nerve cell layers are two neuropils where synaptic contacts occur (Fig.

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The first area of neuropil is the outer plexiform layer (OPL) where connections between rod and cones, and vertically running bipolar cells and horizontally oriented horizontal cells occur (Figs. The second neuropil of the retina, is the inner plexiform layer (IPL), and it functions as a relay station for the vertical- information- carrying nerve cells, the bipolar cells, to connect to ganglion cells (Figs. In addition, different varieties of horizontally- and vertically- directed amacrine cells, somehow interact in further networks to influence and integrate the ganglion cell signals. It is at the culmination of all this neural processing in the inner plexiform layer that the message concerning the visual image is transmitted to the brain along the optic nerve. Central and peripheral retina compared.

Central retina close to the fovea is considerably thicker than peripheral retina (compare Figs. This is due to the increased packing density of photoreceptors, particularly the cones, and their associated bipolar and ganglion cells in central retina compared with peripheral retina. Central retina is cone- dominated retina whereas peripheral retina is rod- dominated. Thus in central retina the cones are closely spaced and the rods fewer in number between the cones (Figs. The outer nuclear layer (ONL), composed of the cell bodies of the rods and cones is about the same thickness in central and peripheral retina. However in the peripheral the rod cell bodies outnumber the cone cell bodies while the reverse is true for central retina.

In central retina, the cones have oblique axons displacing their cell bodies from their synaptic pedicles in the outer plexiform layer (OPL). These oblique axons with accompanying Muller cell processes form a pale- staining fibrous- looking area known as the Henle fibre layer. The latter layer is absent in peripheral retina. Watch A Long Way Down Online Free HD there.

The inner nuclear layer (INL) is thicker in the central area of the retina compared with peripheral retina, due to a greater density of cone- connecting second- order neurons (cone bipolar cells) and smaller- field and more closely- spaced horizontal cells and amacrine cells concerned with the cone pathways (Fig. As we shall see later, cone- connected circuits of neurons are less convergent in that fewer cones impinge on second order neurons, than rods do in rod- connected pathways. A remarkable difference between central and peripheral retina can be seen in the relative thicknesses of the inner plexiform layers (IPL), ganglion cell layers (GCL) and nerve fibre layer (NFL) (Figs. This is again due to the greater numbers and increased packing- density of ganglion cells needed for the cone pathways in the cone- dominant foveal retina as compared the rod- dominant peripheral retina. The greater number of ganglion cells means more synaptic interaction in a thicker IPL and greater numbers of ganglion cell axons coursing to the optic nerve in the nerve fibre layer (Fig.

Muller glial cells. Muller cells are the radial glial cells of the retina (Fig. The outer limiting membrane (OLM) of the retina is formed from adherens junctions between Muller cells and photoreceptor cell inner segments.

The inner limiting membrane (ILM) of the retina is likewise composed of laterally contacting Muller cell end feet and associated basement membrane constituents. The OLM forms a barrier between the subretinal space, into which the inner and outer segments of the photoreceptors project to be in close association with the pigment epithelial layer behind the retina, and the neural retina proper. The ILM is the inner surface of the retina bordering the vitreous humor and thereby forming a diffusion barrier between neural retina and vitreous humor (Fig. Throughout the retina the major blood vessels of the retinal vasculature supply the capillaries that run into the neural tissue.

Capillaries are found running through all parts of the retina from the nerve fibre layer to the outer plexiform layer and even occasionally as high as in the outer nuclear layer. Nutrients from the vasculature of the choriocapillaris (cc) behind the pigment epithelium layer supply the delicate photoreceptor layer. Foveal structure.

The center of the fovea is known as the foveal pit (Polyak, 1. Radial sections of this small circular region of retina measuring less than a quarter of a millimeter (2. Fig. 1. 2a) and for monkey (Fig. The fovea lies in the middle of the macula area of the retina to the temporal side of the optic nerve head (Fig. FR-EE Ill Manors Full Movie.