The term "retina" is almost always taken to mean the portion of the eye's innermost tunic that's sensitive to light. And transduction of light into nervous impulses is the chief function of the retina, without doubt. In a sense, the retina is the eye: everything else is there simply to ensure that the sensitive part of the retina is properly maintained and that an image is brought to it in focus.
But in addition to this function, the retina has an insensitive region, a portion whose elements do not respond to light rays and have other functions in the normal eye. The line of demarcation between the sensitive and insensitive portions of the retinal tunic is the ora serrata, a radial zone about three-quarters of the way towards the anterior aspect of the globe. It's indicated in the image at left by the arrow. The insensitive region, about 25% of the retinal tunic, is devoted to participating in the production of aqueous humor. The remaining 75% is dedicated to the eye's job of transducing electromagnetic energy into nervous signals, and it's described in more detail below.
The business of the eye takes place in the sensitive region, covering three-quarters of the innermost tunic. It is here that the image is formed, in exactly the same way that a camera's lenses cast an image onto a sheet of film. But the retina does much more than any film, because the initial signal processing and interpretation takes place here. Consequently it is a very complex structure.
The vertebrate retina has nine layers whose origins are from the inner wall of the primitive optic cup. A tenth layer, the pigment cell layer, lies just outside these nine, and is so closely associated with it in structure and function that it can be considered a part of the retina.
In vertebrates the light-sensitive elements are in the outermost of the nine layers, and rays of light forming the image must pass through the other eight layers before the nervous signal can be generated. In other words, the "raw" image is actually formed at the back (the outer part) and the "processed" signal that results is routed towards to the inner portion of the globe inward by converging axons from a series of integrator cells located in the inner portions of the retinal tunic.
Perhaps the most logical way to understand the progression of events in the transduction of vision is to consider the layers of the retina in sequential order, beginning at the outermost, and following the signal back through the tunic to the point of exit via the optic nerve.
This layer has vital functions in increasing acuity of vision and in ensuring that the light sensitive portions of the rods and cones are maintained in working order. It is a single layer of cuboidal cells.
These cells are impregnated with melanin, but they also contain lipofuscin as a product of their metabolic activity. The light-sensitive parts of the retina are continually "turned over" to maintain optimum function. The pigment layer cells phagocytose the ends of the rods and cones as they are renewed: hence the accumulation of lipofuscin. The lipofuscin and melanin complement the pigmentation of the choroid portion of the uveal tunic. Together these two layers increase the contrast of the visual image by absorbing light that would otherwise be reflected back inwards towards the rods and cones. The pigment epithelium forms part of the blood-retina barrier and in some "lower" vertebrates may be able to regenerate the retina proper.
Another and even more important function of the pigment epithelium is the storage and synthesis of trans-retinal, or vitamin A. This vitamin is reversibly convertible into trans-retinal, and serves as a raw material for the cyclic formation and breakdown of rhodopsin, the visual pigment.
The outermost layer of the retina "proper" (i.e., the nine that are formed from the inner part of the optic cup) is the bacillary layer or the layer of rods and cones. These are the actual light-sensitive elements. Rods and cones are a variant form of neuronal cell, whose light sensitive portions represent one of the many examples of the modification of cilia for sensory purposes.
The so-called outer limiting "membrane" isn't really a membrane at all. Instead, it's the site of numerous occluding junctions. These seal off the "disposable" parts of the rods and cones, the actual light-sensitive portions of these cells, from the rest. The junctions are between the plasma membranes of the rod segments and a glial element, the Müller cell.
This outer limiting layer has the spurious appearance of a "membrane" in light microscopic preparations, because there are so many junctions, all closely packed together. The electron microscope reveals their true nature. They serve to isolate the inner layers of the retina from potentially harmful material in the blood circulation, forming a blood-retina barrier.
This layer is the location for the nuclei and cell bodies of the rod and cone cells (whose sensitive elements project outwards as the bacillary layer). Collectively the nuclei of these cells constitute the outer nuclear layer.
Inwards of the outer nuclear layer is a relatively clear zone. This is the site of numerous synapses between the rod and cone cells and the processes of various integrator neurons, the outer plexiform layer.
Once the initial neural signal is generated in the rods and cones, it must be passed to other neural elements for processing and further handling. The dendrites of these cells and the synaptic portions of the rod and cone cells constitute the outer plexiform layer, a region of synapses.
The integrating elements are the horizontal, bipolar, and amacrine cells. The dendrites of the bipolar cells, in particular, comprise most of the outer plexiform layer.
The cell bodies and nuclei of the integrator neurons, particularly the bipolar and horizontal cells, are located in the inner nuclear layer, next inwards from the outer plexiform layer.
The inner plexiform layer, like the outer one, is a region of synapses. Here the bipolar cell processes synapse with the dendritic processes of ganglion cells, the final neuronal element of the eye itself.
While the ganglion cells have synapses with bipolar cells in the inner plexiform layer, their cell bodies are located in the next retinal layer, the ganglion cell layer, which has far fewer nuclei than the inner or outer nuclear layers. Like other neurons, these ganglion cells have axons, which carry the generated signal. These axons are bundled into tracts that run radially, forming the next layer of the retina.
The axonal fibers from the ganglion cells, bundled together, run radially around the inner surface of the retina, converging at the site of origin of the optic nerve. At that point the nerve penetrates the retina and the sclera. At this point there are no light sensitive elements; hence this is the "blind spot" found in all vertebrate eyes.
The innermost layer, again, is not really a membrane, but is instead a place where the foot processes of the Müller cells come together. The body of the elongated Müller cell runs from one side of the retina to the other, and "fills in the spaces," so to speak, between the other elements.
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