Anatomy, Histology, and Pathology of the Eye and Ear. Eye
Accessory Structures: o Eyebrow catch foreign bodies and perspiration o Eyelid and eyelashes Eyelashes also catch foreign material + perspiration, and provide shade. A stye is an infection of sebaceous glands in eyelids o Lacrimal gland – parasympathetic causes gland to release tears dry eye = ↓ lacrimal gland secretion Tears: salt, H2O, mucus, lubricants. All tears will enter nasal cavity. Dry eye may occur due to some medications ie. Antihistamine, antidepressants, or birth control. Dry eye syndrome decline in quality or quantity of tears. o Symptoms: pain, burning sensation, or gritty sensation. Affects mostly >65, pregnant and menopausal women. External Features of the Eye o Sclera – white of the eye. o Cornea – focusing light. o Iris – color of the eye. o Pupil – Middle black part of eye. Conjunctiva – red eye: can be conjunctivitis an inflammation of the conjunctiva – superficial blood vessels dilate causing edema and discharge Mucous membrane that covers cornea and schlera Reflected on inner aspect of eyelids Bulbar conjunctiva covers eyeball Palpebral conjuctiva reflected onto the lid from the Bulbar and secretes fluid over the eye. Anterior and Posterior Aqueous Chambers and Posterior Vitreous Body o Cranial nerves 3,4,6 control eye muscles. o Cornea o Anterior aqueous chamber (in front of iris, behind cornea) 2 chambers: 1st chamber btw the cornea & iris, 2nd chamber btw lens & iris. Fluid circulates in chambers o Posterior aqueous chamber(behind lens and iris) 1 chamber filled with vitreous humor(jelly-like), helps maintain shape of eye Fluid consists of H2O, collagen fibers, hyaluronic acid. Helps keep retina back against the eye maintains shape. Non-circulating o Drainage angle trabecular meshwork where fluid is absorbed. o Iris smooth muscle helps constrict and dilate iris. o Scleral venous sinus (Canal of Schlemm): fluid collects in veins from trabecular meshwork. o Ciliary body: diffusion of fluid.
o Sclara o Glaucoma: increased intraocular pressure due to failure of aqueous humor to drain into the canal of Schlemm. Pressure causes the optic nerve to degenerate lead to blindness. Three Layers of the Eyeball o External (fibrous tunic) layer Sclera and cornea o Middle (Vascular tunic) layer Choroid – mainly blood vessels has melanocytes that contain melanin that absorbs light and prevents reflection. Ciliary body Diffusion of plasma and fluid from capillaries Iris Circular and radial muscles of the iris Pupil Consticts as circular muscles of iris contract (parasympathetic) Dilates as radial muscles of iris contract (sympathetic) o Internal (Inner) layer Retina Retinal arteries and veins: important to bring blood to the cones + rods. Pigment epithelium: at the backside of the retina. Outer, non-visual layer. Contains melanin-containing pigment cells btw choroids & retina. Melanin acts to absorb stray light rays which prevent reflection and scattering of light within eyeball. Neural portion Rods and Cones and other bipolar cells Rods important for night vision, 100x more sensitive that cones but not as sensitive to color vision. Cones fxn in bright light, they are concentrated in the fovea, are important for color vision. o Central fovea of macula lutea: yellow spot that has the most ocularity. Fovea contains cons at high density o Optic disc: blind spot b/c there is not retina to this spot. o Optic (II) nerve leaves the retina. Nerve impulses propagate along optic nerve fibers toward optic disc. Lens o Avascular structure made of protein (crystallins) arranged like layers of an onion. Fxn to focus light rays correctly on the retina. Cataract – loss of transparency of lens with age/injury, UV rays, steroids, or complications of disease (diabetes). Can replace with artificial lens. Pathology of Vision
Has capillaries which release fluid into posterior (2nd) chamber of the anterior chamber Fluid if reabsorbed in the Scleral venous sinus.
o Age-related macular degeneration: progressive disease of retina where light sensitive cells in central area (the macula) stop working & die. Causes central vision (reading) impairment. Get formation of drusen (fat) Rods More rods than cons Rods are important for night vision Rods are 100x more sensitive than cons Not sensitive to color visual, but sensitive to generate vision
Cons Concentrated in the fovea Less sensitive than rods Important for color perception Ear External Ear o Auricle: ear lobe o External auditory canal: meatus o Tympanic membrane: ear drum Middle Ear o Connected with the Eustachian tube which leads to the throat. o Auditory ossicles: malleus, incus, stapes: attached to oval window o Tensor tympani and stapedius muscles – dampen load sounds by preventing ossicles from over vibrating. o **Sound causes eardrum to vibrate ossicles vibrate stapes in oval window virates. Internal Ear: Balance & Hearing o Bony & membranous labyrinth with lymph fluid o Vestibule containing the various sacs and ducts o Bony membranous semicircular canals and attached ampulla o Cochlea – receives sound waves form stapes in oval window. Fluid vibrates in cochlea to carry the sound.
Disorders of Hearing Deafness: significant to severe hearing loss. There are different forms of deafness. o Sensorineural deafness: caused by impairment of the cochlea or cochlear branch of VIII. o Conduction deafness: caused by impairment of the middle and external ear structures. Atherosclerosis reduces blood supply to ear Otosclerosis – deposition of new bone around oval window. Repeated loud noise destroys receptor hair cells in cochlea Drugs (aspirin & streptomycin) impacted ear wax, eardrum injury, stiffening of the ossicle joints.
Middles Ear Infection (Otitis Media) o Bacterial infection that passes from the throat to the middle ear cavity via Eustachian tube. o Common in children where tubes are shorter, wider and horizontal→ don’t drain properly → causes decreased drainage, pain, fever
Mechanisms of Equilibrium Vestibular system: important for the perception of movement and orientation & maintain equilibrium or balance. Disorders of Balance and Equilibrium Motion sickness: excess stimulation of the vestibular apparatus. May cause vomiting and nausea. Meniere’s Syndrome: increase in the amount of lymph fluid which enlarges membranous labyrinth causing fluctuating hearing loss, attacks of vertigo (dizziness), and roaring tinnitus (ringing in ears).