1. Chapter 8 Special Senses – Eye, Ear, Taste Anatomy and Physiology

2. 5 senses: Touch, Taste, Smell, Sight, Hearing Special Sense Receptors Either Large and complex: eyes, ears Localized clusters of receptors: taste buds, olfactory epithelium

3. THE EYE AND VISION Vision studied the most. 70% of all sensory receptors in eyes. Optic tracts from eye to brain carry millions of nerve fibers. ANATOMY OF THE EYE 2.5 cm in diameter (1 inch) Only 1/6 th of eye surface is seen Remainder of eye cushioned and protected by fat and bony orbit.

4. Accessory structures of eye Extrinsic eye muscles Eyelids – protection, meets at medial/lateral corners of eye (medial/lateral canthus) Eyelashes – borders eyelids Meibomian glands (sebaceous glands) – eyelids edge – produces oily secretions that lubricate eye Ciliary glands – (sweat glands) between eyelashes

5. Lacrimal apparatus – contains lacrimal glands and many ducts that drain the lacrimal secretions into nasal cavity. Lacrimal glands found above lateral end of each eye. They release a dilute salt solutions (called tears) onto anterior surface of eyeball through several small ducts. Tears flush eyeball into lacrimal canals found medially, then into lacrimal sacs, and finally into nasolacrimal duct – which empties into the nasal cavity. Lacrimal secretions contain antibodies and lysozymes – bacteria destroying enzyme. When lacrimal secretions increase, tears spill over eyelids and fill nasal cavity causing congestion or “the sniffles”. This can happen when eye is irritated by foreign objects, chemicals, when emotionally upset. Since nasal cavity mucosa and the lacrimal duct system run together, colds or nasal inflammation cause the lacrimal mucosa to become inflamed and swell. This causes “watery eyes” due to lack of drainage.

6. Conjuctiva – membrane lining eyelids and covers outer surface of eyeball; secretes mucus to lubricate eyeball keeping it moist. Conjunctivitis – inflammation of conjunctiva; “pink eye”; caused by bacteria or virus and highly contagious.

7. Extrinsic (external) eye muscles 6 total attached to outer surface of eye allow eyes to move or follow moving objects. Inferior rectus muscle Inferior oblique muscle Lateral rectus muscle Superior rectus muscle Superior oblique muscle Medial rectus muscle

8. Internal structures: The eyeball Hollow sphere Wall made of 3 tunics (coats): sclera, choroid, retina Interior filled with fluids called humors: maintains shape of eyeball Lens – main focusing apparatus The 3 tunics: 1. Sclera – outer most tunic, thick and white, “white of the eye” Cornea – central anterior portion of sclera modified into a “window” for light to enter eye. Contains many pain fibers causing blinking and increased tearing when irritated. Has an amazing ability to repair itself. Only tissue in body that can take a transplant from another person and not have to worry about rejection. (no blood vessels keeps it beyond the reach of the immune system).

10. 2. Choroid – blood-rich nutritive tunic containing a dark pigment. (darkness prevents light from scattering inside eye). Choroid modified in front to form 2 smooth muscle structures: Ciliary body (lens attached to this) Iris – rounded opening of iris is pupil through which light passes. Circularly and radially arranged muscle fibers form the iris. (acts like diaphragm of camera) regulating amount of light that can enter the eye. Close vision/bright light – circular muscles contract, pupil constricts Distant vision/dim light - radial fibers contract, dilates pupil to allow in light

11. 3. Retina – sensory tunic – innermost layer. Retina extends anteriorly only to ciliary bodies. Contains millions of receptor cells called photoreceptors. 2 types: Rods –denser – allows us to see gray tones and gives us our peripheral vision. Few near center of retina. Cones – give us the details of what we are looking at in color under bright light conditions. Densest in center of retina. Fovea centralis – tiny pit containing only cones. Area of greatest visual acuity.

12. Photoreceptors are distributed entirely over the entire retina except where the optic nerve is located. Optic nerve – back of eye (called the optic disc) – composed of ganglion cell axons – “blind spot” When light from an object is focused on the optic disc, it disappears from view. Night blindness – caused by the rods not functioning properly (can be caused by lack of Vitamin A). Can be corrected if caught early. Color blindness – 3 types of cones detect colors. Lack of all 3 types of cones leads to total CB. Lack of one type leads to partial CB. Most common – lack of red cones or green cones. (red/green seen as same color).

13. Lens – light entering eye is focused on the retina by the lens. Flexible, biconvex crystal-like structure. Held upright by suspensory ligament attached to ciliary body. Aqueous humor – anterior chamber in front of lens; clear watery fluid. Similar to blood plasma Continually secreted by choroid Helps maintain intraocular pressure Provides nutrients for lens and cornea (due to no blood supply) Reabsorbed into blood through sclera venous sinus (canal of Schlemm) where the sclera and cornea meet. Vitreous humor – posterior chamber behind lens; gel-like substance. Keeps eyeball shape. Glaucoma – blocked drainage of aqueous humor causes increase pressure in eye, dramatically compresses the retina and optic nerve. Painful, possible blindness can occur. No real symptoms early on. (Elderly – common). Later, signs occur such as seeing halos around lights, headaches, blurred vision. Treated with eye drops which increase aqueous humor drainage, surgery also.

14. Cataracts – in youth, our lens is completely clear and like hardened jelly As we age, it becomes hard and cloudy. Vision becomes hazy which can eventually lead to blindness in affected eye. Surgery can correct – remove old lens, replace with new lens implant.

15. PATHWAY LIGHT TRAVELS Light passes through aqueous humor to vitreous humor – speed changes, rays bend (refracted) due to different densities of substances. Light also bends as it encounters the cornea, and lens. Cornea and humors bend light constantly at same rate; lens, however, changes bend as it changes shape. This helps focus object on retina. Larger bulge – more light bends Flattened – less light bends Resting eye set for distant vision. Over 20 feet, no change in lens. Light from closer objects scatters and spreads out causing bulge in lens.

16. VISUAL FIELDS AND VISUAL PATHWAYS TO THE BRAIN Axons carry impulses from retina in a bundle together at posterior aspect of eyeball. (optic nerve) Optic chiasma – where the fibers from the medial side of eye cross over to opposite side. These fiber tracts are called optic tracts. Binocular vision – our visual fields from each eye overlap giving us 3-D vision. Our visual cortex fuses the 2 slightly different images from each eye into one.

17. Image formed on retina due to light bending is reversed from left to right and upside down (inverted). Also smaller than actual object. Over converging (lens too strong) or under converging (lens too weak) of lens causes sight problems. Also bad vision occurs due to structural problems of eye.

18. Hyperopia – farsightedness - parallel light rays from distant objects focus behind retina – results from eyeball too short or “lazy” lens. Distant objects in focus, nearby objects blurry. Can cause eyestrain due to continually trying to contract ciliary muscles to see. Convex corrective lenses. Astigmatism – caused by unequal curvatures in different parts of the cornea or lens. Everything blurry. Special cylindrically ground lenses used to correct problem. Emmetropia – eye that focuses images correctly on retina. Myopia – nearsightedness – parallel light rays from distant objects fail to reach the retina and are focused in front of it. Distance blurry, nearby objects in focus. (can result from eyeball too long, lens too strong, cornea too curved). Concave corrective lenses. Farsightedness is focused behind. Corrected with convex lens.