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SUMMARY OF KEY POINTS
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The Eye & Ear: Special Sense Organs SUMMARY OF KEY POINTS Eye
The eye has three tunics: the sclera and cornea form the outer fibrous tunic; the middle vascular layer (or uvea) consists of the choroid, ciliary body, and iris; and the retina forms the inner tunic.
The transparent cornea consists of an anterior stratified squamous epithelium on Bowman membrane, a thick avascular stroma, and an inner endothelium on Descemet membrane.
Aqueous humor is secreted by ciliary processes into the posterior chamber, flows through the pupil into the anterior chamber, and is drained by the scleral venous sinus in the limbus.
The iris stroma contains melanocytes and posteriorly has smooth muscle fibers of the sphincter pupillae muscle and the myoepithelial cells forming the dilator pupillae muscle.
The lens is a unique avascular tissue composed of long lens fibers, covered on its anterior side by cuboidal lens epithelium, and surrounded by a thick acellular layer called the lens capsule.
The lens is suspended behind the iris and its central pupil by the ciliary zonule of fibrillin fibers produced by epithelial cells covering the encircling ciliary body.
The retina has the two major parts derived from the embryonic optic cup: the pigmented epithelium next to the vascular choroid layer and the thicker neural retina.
Cells of the pigmented epithelium absorb scattered light, form part of a blood-retina barrier, regenerate 11-cis-retinal, phagocytose shed discs from rods, and support the rod and cone cells.
Rod cells are photoreceptors detecting light intensity with short rod-shaped outer segments; less numerous cone cells, with conical outer segments, are receptors for the primary colors (light of different wavelengths).
Rods have stacked membrane discs in which the membranes are densely packed with the protein rhodopsin with bound retinal.
Photons of light convert 11-cis-retinal to all-trans-retinal, causing rhodopsin to release the retinal (bleaching), and activate the adjacent G protein transducin, which causes a nerve impulse.
In the neural retina, the RCL is nearest to the retina pigmented epithelium and near the ONL that contains the cell bodies of these photoreceptors.
An OPL contains the photoreceptor’s axons connected in synapses with dendrites of various integrating neurons whose cell bodies form the INL.
Axons from cells in the INL form synapses in the IPL with neurons of GL, which send axons through the NFL to the optic nerve.
Eyelids are lined by conjunctiva, a stratified columnar epithelium with goblet cells, which also covers the anterior part of the sclera and is continuous with the corneal epithelium.
Lacrimal glands continuously produce the tear film, which drains into the nasal cavity via the ducts of the lacrimal apparatus.
Ear