Overview of the Eye
The eye is highly specialized in converting light energy into nerve action potentials in order to transmit visual information to the brain. The key photoreceptors, known as the rod cells and cone cells, and downstream neurons lie within the retina. The rest of the structures in the eye are important for focusing images onto the retina and to support the retina.
The wall of the eye is composed of three layers:
the outer corneo-scleral layer
the intermediate uveal layer
the inner retinal layer
The corneo-scleral layer supports the structure of the eye with its tough fibroelastic composition. This layer has two components:
The cornea is located in the anterior portion of this layer. It is transparent and functions to roughly focus images onto the retina.
The sclera is located in the posterior portion of this layer. The sclera is opaque and it is the site of insertion for the extraocular muscles.
The junction between cornea and sclera is called the limbus. The surface of the eye is covered by conjunctiva, which reflect into the eyelids.
The uveal layer is highly vascular. This layer has three components:
The choroid is located in the posterior portion. It lies between the sclera and the retina and is heavily pigmented. It serves two main functions: it supports the retina and absorbs light that has passed through the retina. This prevents it from reflecting and decreasing the integrity of the image formed on the retina.
The ciliary body is located in the anterior portion of the uveal layer, right behind the limbus. It is attached to the lens via the suspensory ligament (zonule). The ciliary body contains smooth muscle that controls the shape of the lens .
The iris extends from the ciliary body to cover the front of the lens. It regulates the amount of light reaching the retina. The aperture of the iris is known as the pupil.
The retinal layer contains photoreceptors (rod cells and cone cells) as well as neurons that transmit the information to the optic nerve. This layer is divided into two sections by a line near the ciliary body known as the ora serrata. Posterior to the ora serrata, the layer is photosensitive. Anterior to the ora serrata, the retinal layer becomes a non-photosensitive epithelium. The retinal layer has ten sub-layers, which will be discussed later. For now, it is important to realize that the photosensitive cells are actually located at the most posterior portion in this layer; light must actually traverse downstream neurons and support cells before hitting the photoreceptors.
Chambers of the Eye
There are three chambers within the eyeball:
The anterior chamber lies between the iris and the cornea.
The posterior chamber lies between iris and lens.
The vitreal cavity occupies the bulk of the eye between the lens and posterior wall.
Both anterior and posterior chambers are filled with a clear watery aqueous humor . The aqueous humor is secreted into the posterior chamber by the ciliary body. It then circulates through the pupil and finally drains in to the canal of Schlemm at the angle of the anterior chamber. The vitreal cavity is filled with a gel-like, transparent substance called the vitreous body.
Transmission of Visual Information
When light hits the eye, it is first roughly focused on the retina by the curvature of the cornea. The lens then provides fine focus of the corneal image upon the retina. Note that the curvature of the lens can be adjusted, depending on the tone of the ciliary body. The visual axis of the eye then passes through a region of retina called fovea. The fovea can be easily identified by a depression in the retina in the cross section. This region is the area of highest visual acuity. Surrounding the fovea is a yellow-pigmented zone called the macula lutea. As the light reaches the retina, the rod cells and cone cells transmit the information to neurons. The afferent nerve fibers eventually converge into the optic nerve and exit the eye. The location where optic nerve converges is known as the optic disc. Since there are not photoreceptors in this region, the area constitutes a blind spot.
Sub-layers of Retina
Layers of the Retina
The different cell types of the retina is categorized as follows:
Photoreceptor cells: rod cells and cone cells ("the 1st neuron" in the visual information transmission path)
Intermediate neurons: bipolar cells, horizontal cells, and amacrine cells . ("the 2nd neuron")
Retinal ganglion cells : these are cells of the optic tract neurons ("the 3rd neuron")
Pigmented epithelial cells
Neuron support cells
Histologically, the retina is divided into ten layers. For this lab, simply understand the general organization of retina. You will study these layers in greater detail in the Neuroanatomy course. The layers are as follows (starting from the outermost layer):
The pigmented epithelium is in contact with Bruch's membrane, which separates retina from the choroid. The pigmented epithelium is a light-absorbing layer that reduces random reflections of unabsorbed light.
The photoreceptor layer contains photosensitive outer segments of rods and cones.
The outer limiting membrane is not a true membrane, but simply represents the junction between Muller cells, which provide structural and nutritional support for the retinal neuron, and the photoreceptor cells.
The outer nuclear layer contains cell bodies of the rods and cones.
The outer plexiform layer contains synapses between axons of photoreceptors and dendrites of intermediate neurons.
The inner nuclear layer contains cell bodies of intermediate neurons and Muller cells.
The inner plexiform layer contains synapses between intermediate neurons and ganglion cells of the optic tract.
The ganglion cell layer contains cell bodies of ganglion cells.
The optic nerve fiber layer contains axons of ganglion cells.
The inner limiting membrane separates the retina from the vitreous body.