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