Müller cells, also known as retinal glial cells, have a crucial role in maintaining the internal structure of the retina. These cells are aligned radially, running parallel to the path of light through the retina. The outer ends of Müller cells form tight connections with the inner segments of the photoreceptors, creating a continuous layer called the outer limiting membrane (layer 3 of the retina).

In adults, the junction between layers 1 and 2 of the retina represents the contact surface between the anterior and posterior walls of the embryonic optic cup during development. This junction is structurally weak, and retinal detachment can occur at this point. Retinal detachment can lead to vision loss due to the displacement of the retina from the focal plane of the eye. It can also result in the death of photoreceptor cells, which rely on the blood supply from the choroid as the photoreceptor layer itself lacks vasculature. Deterioration of the pigmented epithelium can also lead to macular degeneration, a critical loss of high-acuity central and color vision that does not affect peripheral vision.

Within the external limiting membrane lies the outer nuclear layer (layer 4), housing the cell bodies and nuclei of the rods and cones. The outer plexiform layer (layer 5) contains synapses between the photoreceptors and retinal interneurons, such as bipolar cells and horizontal cells. These interneurons have their cell bodies in the inner nuclear layer (layer 6), which also accommodates the cell bodies of other retinal interneurons, including amacrine and interplexiform cells, as well as Müller cells.

The inner plexiform layer (layer 7) contains synapses between the retinal neurons of the inner nuclear layer, including bipolar and amacrine cells, and the ganglion cells. The ganglion cells' cell bodies reside in the ganglion cell layer (layer 8) and act as the output cells of the retina, transmitting visual information to the brain through their axons. These axons form the optic fiber layer (layer 9) and traverse the inner surface of the retina, avoiding the fovea, until they reach the optic disc. At the optic disc, the axons exit the eye as the optic nerve. The ganglion cell axons within the optic fiber layer are unmyelinated, but they acquire myelin once they reach the optic disc. The absence of myelin at the points where the axons cross the retina helps maintain minimal distortion of light passing through the inner retina.

The innermost layer of the retina is the inner limiting membrane (layer 10), which is formed by the end-feet of Müller cells.

Structure of Photoreceptors: Rods and Cones
Each rod or cone photoreceptor cell consists of a cell body (found in layer 4), an inner and an outer segment that extend into layer 2, and a set of synaptic terminals that connect with other retinal cells in layer 5 (Fig. 8.3). Cones have shorter outer segments compared to rods and contain stacked disc membranes formed by infoldings of the plasma membrane. Rods have longer outer segments that contain stacks of membrane discs floating freely in the outer segment, having detached completely from the plasma membrane during formation at the base. Both types of discs are abundant in visual pigment molecules, although rods have a higher visual pigment density, contributing to their greater sensitivity to light. A single photon can elicit a response in rods, whereas several hundred photons may be required for a response in cones.
Retinal glial cells, known as Müller cells, play an important role in maintaining the internal geometry of the retina. Müller cells are oriented radially, parallel to the light path through the retina. The outer ends of Müller cells form tight junctions with the inner segments of the photoreceptors, and these numerous connections have the appearance of a continuous layer, the outer limiting membrane (layer 3 of the retina).
Thejunctionbetweenlayers1and2oftheretinainadultsrepresentsthesurfaceofcontactbetweentheanteriorandposteriorwallsoftheembryonicopticcupduringdevelopmentandisstructurallyweak.Retinaldetachmentisseparationatthissurfaceandcancauselossofvisionbecauseofdisplacementoftheretinafromthefocalplaneoftheeye.Itcanalsoleadtothedeathofphotoreceptorcells,whicharemaintainedbythebloodsupplyofthechoroid(thephotoreceptorlayeritselfisavascular).Deteriorationofthepigmentedepitheliumcanalsoresultinmaculardegeneration,acriticallossofhigh-acuitycentralandcolorvisionthatdoesnotaffectperipheralvision.
Inside the external limiting membrane is the outer nuclear layer (layer 4) that contains the cell bodies and nuclei of the rods and cones. The outer plexiform layer (layer 5) contains synapses between the photoreceptors and retinal interneurons, including bipolar cells and horizontal cells, whose cell bodies are found in the inner nuclear layer (layer 6). This layer also contains the cell bodies of other retinal interneurons (the amacrine and interplexiform cells) and the Müller cells.
The inner plexiform layer (layer 7) contains synapses between the retinal neurons of the inner nuclear layer, including the bipolar and amacrine cells, and the ganglion cells, whose cell bodies lie in the ganglion cell layer (layer 8). As previously mentioned, the ganglion cells are the output cells of the retina; it is their axons that transmit visual information to the brain. These axons form the optic fiber layer (layer 9), pass along the inner surface of the retina while avoiding the fovea, and enter the optic disc, where they leave the eye as the optic nerve. The portions of the ganglion cell axons that are in the optic fiber layer remain unmyelinated, but they become myelinated after they reach the optic disc. The lack of myelin where the
Folding of membrane •Fig. 8.3 Rods and Cones. Thedrawingsatthebottom showthegeneralfeaturesofarodandacone.Theinsets showtheoutersegments.
axons cross the retina helps permit light to pass through the inner retina with minimal distortion.
The innermost layer of the retina is the inner limiting membrane (layer 10) formed by the end-feet of Müller cells.
Structure of Photoreceptors: Rods and Cones
Each rod or cone photoreceptor cell is composed of a cell body (in layer 4), an inner and an outer segment that extend into layer 2, and a set of synaptic terminals that synapse in layer 5 onto other retinal cells (Fig. 8.3 ). The outer segments of cones are not as long as those of rods, and they contain stacks of disc membranes formed by infoldings of the plasma membrane. The outer segments of rods are longer, and they contain stacks of membrane discs that float freely in the outer segment, having completely disconnected from the plasma membrane when formed at the base. Both sets of discs are rich in visual pigment molecules, but rods have a greater visual pigment density, which partly accounts for their greater sensitivity to light. A single photon can elicit a rod response, whereas several hundred photons may be required for a cone response.