2. The accessory structures of the Eye  Eyebrows Shade eyes from sunlight Prevent sweat in eyes  Eyelids Lacrimal caruncle – sebecous and sweat glands “sandman” Reflex blinking every 3-7 seconds to prevent drying Eyelashes – nerve endings to trigger blink reflex  Conjunctiva Transparent mucous membrane (lubricates eye) Lines eyelids and white of eye Very thin, lots of bv  Lacrimal Apparatus Lacrimal gland – continually releases tears thru excretatory ducts, blinking spreads tears downward Tears drain through paired openings called punctum, through the canaliculi into the lacrimal sac, ultimately draining into the nasal cavity via the nasal lacrimal duct Tears – mucus, antibodies, and lysozyme (bacteria destroying enzyme!)

4. Internal Eye Structures  Sclera tendon like “white” of eye shapes eye and attaches to muscles avascular  Cornea Avascular Window allows light into eye Pain receptors Can be transplanted w/no rejection!!  Aqueous Humor Clear fluid, similar to blood plasma Forms & drains continuously  Choroid Vascular, dark brown (prevent light from scattering)  Ciliary body Ciliary muscles – smooth muscle control lens shape Cilary zonule – ligamenrts hold lens in place  Iris Smooth muscle possibly containing brown pigments Pupil – round central opening  Lens Biconvex, transparent, flexible structure avascular  Vitreous Humor Transmits light & maintains interocular pressure Lasts a lifetime  Retina

5. Vision Physiology  Objects have color b/c they absorb some wavelengths of light and reflects other w/in the visible spectrum  Refraction of light occurs when it meets the surface of a different medium at an oblique angle (the greater the angle, the greater the bending)  A convex lens will bend light so it converges at a focal point creating a “real image” – upside down and reversed (concave lenses diverge light)

6. Retina – 3 layers:  Pigmented layer Absorb light, prevent scattering Phagocytes – remove damaged photoreceptors Store vitamin A  Neural layer 3 main types of neurons ○ Photoreceptors (millions) Rods – dim light & peripheral vision receptors Cones – color vision & high acuity -Macula –oval to blind spot (mostly cones)w/fovea centralis (only cones) in center ○ Bipolar cells ○ Ganglion cells – generate action potentials (and horizontal cells and amacrine cells help w/ visual processing) Optic Nerve – ganglion cell axons ○ Optic disc – blind spot (lacks photoreceptors)

8. Photoreceptors  Detect photons between 700-400nm  Rods – presence or absence of photons and cannot discriminate wavelength (very sensitive)  Cones only function in bright light ○ Blue – pigments sensitive to blue light ○ Green ○ Red Stimulation in various combinations creates perception of variety of colors  Colorblindness – 1 + cones absent or nonfunctional

9. Photoreception  Names rods and cones refer to shape of cell  Arrival of a photon alters membrane potential changing rate of neurotransmitter release  Discs contain visual pigments that absorb photons derived from rhodopsin (opsin protein varies w/type of photoreceptor + retinal made from Vit A)  Night Blindness due to insufficient vitamin A (converted from carotene) or functional photoreceptors

10. Focusing of Light on the Retina  Emmetropic vision– normal (20ft) Ciliary muscles are relaxed Lens is at its thinnest (lowest refraction) “Real Image” is projected upside down and backwards  Accomadation – for objects less than 20ft Ciliary muscles contract Lens bulges, increases refraction of lens Near point of vision 4 inches (closer in children and increases w/age) Pupils constrict, preventing divergent light from blurring vision Convergence – medial rotation of eye balls Prolonged periods cause eye strain

11. Common Vision Problems  Myopia (nearsighted) Object focused in front of retina (due to long eyeball) Corrected w/concave lens or LASIK to flatten cornea  Hyperopia (farsighted) Object focused behind retina (due to short eyeball) Corrected w/convex lens  Astigmatism Unequal curvature of lens and/or cornea causing blurred images

12. Visual Pathway  Photoreceptor  bipolar cell  ganglion cell (converge to form optic nerve)… half nerve fibers cross at optic chiasma to opposite thalamic nucleus  brain stem (midbrain pupillary reflex centers) and cerebral cortex of occipital lobe  Collateral branches from optic tract to hypothalamus for circadian rhythms