Special Senses: The Eyeball

fibrous layer

• outermost layer, dense avascular connective tissue

• sclera and cornea

sclera

• opaque posterior region

• protects and shapes eyeball

• anchors extrinsic eye muscles

cornea

• transparent anterior layer

  • forms clear window that lets light enter and bends light as it enters eye

• numerous pain receptors contribute to blinking and tearing reflexes

vascular layer

• middle pigmented layer of the eye

• three regions: choroid, ciliary body, and iris

choroid region

• posterior portion

• supplies blood to all layers of eyeball

• brown pigment absorbs light to prevent scattering of light

ciliary body

• anteriorly, choroid becomes ciliary body

• consists of smooth muscles, ciliary muscles, that control shape of lens

iris

• colored part of eye, betwee cornea and lens

• contains pupil

pupil

central opening that regulates amount of light entering eye

• close vision and bright light cause constriction

• distant vision and dim light cause pupils to dilate; sympathetic control

muscles of the iris

• sphincter pupillae (causes constriction)

• dilator pupillae (causes dilation)

inner layer

• retina

  • millions of photoreceptor cells that transduce light energy, neurons, glial cells

• delicate two layered membrane

  • outer pigmented layer

  • inner neural layer

retina

• pigmented layer

  • absorbs light and prevents its scattering

  • stores vitamin a

• neural layer

  • transparent layer that is anterior

  • anterior end has serrated edges called ora serrata

neural layer of retina

• optic disc

  • site where optic nerve leaves eye

  • lacks photoreceptors, so referred to as blind spot

• has rods and cones

pathway of signal output

• transduced from posterior to anterior

• signal in response to light travels from photoreceptors to bipolar cells to ganglion cells

• AP generated in ganglion cells

• axons of ganglions leave posterior of eye and form the thick optic nerve

phototransduction

process by which pigment captures a photon of light energy, and converts it into action potentials (G-protein)

rod vision

• dim light, peripheral vision receptors

• more numerous, more sensitive to light

• no color vision or sharp images

• numbers greatest at periphery

• converging pathways

cone vision

• vision receptors for bright light

• high resolution color vision

• have straight pathways with “its own personal bipolar cell” to a ganglion cell

macula lutea

area at posterior pole lateral to blind spot

• contains mostly cones

fovea centralis

tiny pit in center of macula lutea that contains all cones, so is region with best visual activity

• eye movement allows us to focus in on object so that fovea can pick it up

retinal photoreceptor

key light absorbing molecule that combines with one of four proteins (opsins) to form visual pigments

• synthesized from vitamin A

rods

where is rhodopsin found

cones

where are green glue and red blue

rhodopsin

deep purple pigment of rods

steps of rhodopsin formation and breakdown

1) pigment synthesis

2) pigment bleaching

3) pigment regeneration

light adaptation and dark adaptation

activation of rods and cones depend on

light adaptation

when moving from darkness into bright light we see glare because:

• both rods and cones are strongly stimulated

• large amounts of pigments are broken down instantaneously, producing glare

visual actuity improves over 5-10 mins as rod system turns off

dark adaptation

when moving from bright light into darkness, we see blackness because:

• cones stop functioning in low intensity light

• bright light bleached rod pigments, so they are still turned off

rhodopsin accumulates in the dark, so retinal sensitivity increases in 20-30 mins