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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.

  • The acoustic meatus of the external ear ends at the tympanic membrane and its mucosa ...

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