(Transcribed from Dr. Kalliecharan’s lecture, 6 April 2000 by Brian Buschman)
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The eye is made of three concentric layers:
1) Tunica fibrosa, the outer layer composed of the sclera and the cornea.
2) Tunica vasculosa, the middle layer composed of the chorid, the ciliary body and the iris. Tunica vasculosa is the vascular supply of the eye.
3) The refractive media is the inner layer made of the retina and lens.
The eye has extrinsic muscles to point the eye towards what you want to look at and intrinsic muscles to adjust the aperture to focus and adjust the amount of light that is allowed in. Light in the eye is detected by the retina and transmitted via the optic nerve/tract to the lateral geniculate nucleus.
The outer layer is made of two parts, the sclera which is a thick outer connective tissue layer and the cornea. The cornea is a transparent avascular structure on the anterior surface. It is made of five layers.
a) The corneal epithelia is the anterior layer.
b) Deep to that is Bowman’s membrane which is the basal lamina of the corneal epithelia.
c) The middle contains a stroma.
d) Descement’s membranes is a basal lamina of the corneal endothelium.
e) The deepest layer is the corneal endothelium.
The cornea must remain moist at all times or it will begin to become opaque (i.e. you will go blind).
The middle contains the choroid which is the vascular supply located deep to the retina. It also includes the ciliary body, which holds the lens, and the iris which controls the amount of light allowed into the eye. The ciliary body is a wedge shaped structure that had two functions by two parts:
1) The ciliary process produces aqueous humor from the inner ciliary non-pigmented columnar cells. The aqueous humor is to moisten the lens and posterior cornea. It is secreted into the posterior chamber and then passes into the anterior chamber.
2) The ciliary muscle stretches the sclera which causes the lens to relax. The ciliary muscle receives parasympathetic innervation.
The iris has a pigmented layer whose color determines the eye color. The opening in the center is the pupil which size is controlled by the dilatory and sphincter pupillae muscles. The sphincter receives parasympathetic innervation and the dilator sympathetic.
The inner layer is made of two major parts, the lens and retina. The lens is located anteriorly between the ciliary bodies with aqueous humor on either side that is made by the ciliary process made to keep it moist. The lens is focused by suspensory ligaments that hold it to the ciliary body. Posterior to the lens is the vitreous humor which is a ball of gel that supports the eye.
The lens is made of a lens capsule, externally, and lens fibers, internally. The lens fibers are elongated cells that have lost their organelles. It also has a subcapsular epithelium anteriorly.
The retina is the photoreceptor which has two layers. I have only considered some of them to be important for us to know and discussed them starting from the outer (most posterior) to the interior layers.
1) Retinal pigmented epithelium (RPE) is the most posterior layer made of columnar cells that produce the visual pigments and melanin. It supplies the second layer with the visual pigments and the melanin is used to absorb light that is not picked up by the photoreceptors.
2) Photoreceptor layer contain the cones and rods which are the photoreceptor ends of the neurons of the eye.
a. Rods are low light photoreceptors which use rhodopsin as their pigment. Multiple rod cells synapse at a single bipolar neuron which produces summation as the signals are added together. This leads to a lesser lever of visual acuity. When stimulated by light they use Ca2+ to hyperpolarize which causes the closure of voltage gated Na2+ channels. This change is passed on to the mentioned bipolar neurons.
b. Cones are the bright light photoreceptors that have a high visual acuity. They have a similar function to rods except they use iodopsin as their visual pigment and that one rod cell synapses at one bipolar neuron.
3) Layer three in the external limiting membrane.
6) Layer six is the cell bodies of the bipolar neurons which are the relay neurons in the retina.
8) Layer eight include the ganglia that link the bipolar neurons to the fibers of the optic nerve.
9) Layer nine is made of the fibers of the optic nerve which are unmyelinated at this point. When they cross through the optic disk at the posterior point in the eye, they becomes the optic nerve/tract and acquire myelin sheathes. Note that the term optic tract is more accurate then optic nerve because it is part of the CNS and carried the senses directly to the lateral geniculate ganglia. It does not have PNS pseudounipolar neurons that synapse to interneurons in the CNS which take it to the lateral geniculate ganglia.
10) The inner limiting membrane of the retina.
The conjunctiva is the stratified columnar epithelia with goblet cells that lines the inside of the inside of the eyelid. It keeps the outer side of the cornea moist.
The layer of the skin and conjunctiva that cover the cornea. It has three types of glands:
1) Gland of Zeis which are sebaceous glands that secrete into the base of the eyelashes.
2) Meibornian glands which are modified sebaceous glands that secrete directly to the surface of the skin.
3) Glands of Moll that are sweat glands of the skin.
Tears are created in tubuloalveolar glands and are secreted into the fornix of the conjunctiva. They contain lysosomes as an antibacterial agent. They drain via the lacrimal canaliculi to the lacrimal sac and into the nasolacrimal duct that was discussed a while back by Dr. Tabor.
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