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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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the accessory structures of the eye
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!)
internal eye structures
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
vision physiology
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)

retina 3 layers
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)
photoreceptors
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
photoreception
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
focusing of light on the retina
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
common vision problems
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
visual pathway
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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