THE RETINA: RECEPTOR CELLS

The receptor cells are the RODS and CONES. Each consists of: * A process consisting of outer and inner segments. * An external fiber connecting the process with the soma. * A soma or body containing the nucleus. * An inner fiber that resembles an axon and synapses with a bipolar cell.

Rods are extremely sensitive to light and can react to a single photon. The light is absorbed by the pigment rhodopsin and the energy acquired results in decrease in cGMP which in turn causes the closing of Na+ channels. This hyperpolarizes the cell causing a release of synaptic vesicles.
Rhodopsin which can only detect variation in intensity of light resulting in gray level visio

Horizontal cells: When these cells are excited, they inhibit surrounding cells. This results in an image with sharp edges, enhancing resolution.
Amacrine cells: These cells are thought to be involved with detection of movement.

Directly posterior to the cornea and lens (along the visual axis) is an oval, yellowish area, the macula lutea, which has a central depression, the fovea centralis where resolution is the highest. This area contains only cones.

THE RETINA: FOVEA CENTRALIS
The image from the center of the visual field is projected onto the fovea centralis, an area which has no rods and in which the vision is the sharpest.
At the periphery of the retina, there is high convergence with many receptor cells synapsing with a single bipolar neuron. This results in large receptor fields and a low resolution image. In the fovea, there is a 1:1 correspondence between the receptors and bipolar cells resulting in a high resolution image. Note that the image is inverted and reversed.

THE RETINA: OPTIC DISK
Axons of the ganglion cells travel to the surface of the retina (adjacent to the vitreous body) where they form the nerve fiber layer. They run across the surface to the optic disc, a perforated area of the sclera. After crossing the sclera, they form the optic nerve. Because of the perforations, the optic disk is a weak spot of the retina and increased intraocular pressure can cause it to bulge outward (cupped disk).

The optic nerve travels posteriorly and medially through the orbit within the cone formed by the four rectus muscles. About ½ of the fibers of the optic nerve are from the fovea. The optic nerve passes through the optic canal in the root of the lesser wing of the sphenoid bone to enter the middle cranial fossa. Just anterior to the sella turcica, it joins with its counterpart to form the optic chiasm.
THE OPTIC CHIASM & TRACT
In the optic chiasm, fibers from the medial half of the retina (containing information of the lateral visual field) cross to the contralateral side. Fibers from the lateral half of the retina remain on the same side.
On each side, the optic tract projects backward to the lateral geniculate body of the thalamus. The left optic tract contains fibers from the left side of the visual field of each eye; the right optic tract contains fibers from the right side of the visual field of each eye.

THE OPTIC RADIATION

Axons from cells of the lateral geniculate body radiate through the temporal, parietal, and occipital lobes to the ipsilateral cortex on the banks of the calcarine fissure.
Fibers travel to the cortex in two bundles: * Fibers traveling to the superior bank of the fissure travel through the posterior limb of the internal capsule. Fibers traveling to the inferior bank of the fissure arch anteriorly passing through the white matter of the temporal lobe before passing posteriorly to the occipital lobe. This loop is fibers is called Meyer’s loop

THE PRIMARY VISUAL CORTEX
REMEMBER: Each optic radiation projects from the ipsilateral half of the retina. The information carried is from the contralateral half of the visual field, due to inversion & reversal of the image by the lens.
The portion of each radiation traveling to the superior bank of the calcarine fissure carries information from the superior portion of the retina, and therefore the inferior portion of the visual field. The Meyer’s loop fibers carry information from the lower retina, and thus from the upper part of the visual field to the lower bank of the calcarine fissure.

THE VISUAL CORTEX
The information received by the primary visual cortex is illlustrated in A below: It is inverted and reversed, and the center has high resolution which becomes progressively worse toward the periphery. In addition, there are fewer cones at the periphery, so the image contains less color at the periphery.
The visual cortex reconstructs the image with constant color and resolution.

INFLAMMATION OF THE OPTIC NERVE
Because nearly ½ of the fibers in the optic nerve are from the fovea, inflammation of the nerve will affect color perception and resolution in the central vision in the ipsilateral eye.
LESION OF THE OPTIC NERVE
A lesion that destroys the optic nerve will destroy all input from that retina, leading to monocular visual loss (blindness in that eye).

LESION OF THE OPTIC CHIASM
A lesion of the optic chiasm will destroy the fibers from the medial halves of both retinas where they cross. Remember that the medial half of the retina receives the image from the lateral half of the visual field. Thus a lesion of the optic chiasm will result in loss of the lateral halves of the visual fields, sometimes called “tunnel vision” (bitemporal hemianopia).

LESION OF THE OPTIC TRACT
A lesion of the optic tract will destroy the fibers from the ipsilateral sides of both retinas. Remember that these are carrying information from the contralateral side of the visual field. Thus a lesion of the optic tract between the optic chiasm and the lateral geniculate body will cause blindness in the contralateral visual field. Because the fibers destroyed are from the ipsilateral side of the retina, this is called homonymous hemianopia.

LESION OF MEYER’S LOOP
A lesion of Meyer’s loop will destroy the fibers traveling to the lower bank of the calcarine fissure. These fibers are carrying information from the upper contralateral visual quadrants. This is called homonymous quadrantanopia