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Special Senses: Vision

Special Senses: Vision. 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

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Special Senses: Vision

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  1. Special Senses: Vision

  2. The accessory structures of the Eye • Eyebrows • Shade eyes from sunlight • Prevent sweat in eyes • Eyelids • Lacrimalcaruncle– 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 lacrimalsac, ultimately draining into the nasal cavity via the nasal lacrimal duct • Tears – mucus, antibodies, and lysozyme (bacteria destroying enzyme!)

  3. 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 • Cilaryzonule – 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

  4. 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)

  5. 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)

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

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