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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
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!)
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
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: Pigmented layer
Absorb light, prevent scatteringPhagocytes – remove damaged photoreceptorsStore vitamin A
Neural layer3 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 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 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
Emmetropic vision– normal (20ft)Ciliary muscles are relaxedLens is at its thinnest (lowest refraction)“Real Image” is projected upside down and
backwards Accomadation – for objects less than 20ft
Ciliary muscles contractLens 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 visionConvergence – medial rotation of eye ballsProlonged periods cause eye strain
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 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