Visual System

Cornea

transparent tissue, first thing light hits when it enters your eye

Aqueous humor

produced by vascular part of ciliary body, replaced 12x a day, poor drainage results in glaucoma

Glaucoma

group of eye diseases results from increased pressure in eye

Lens

transparent tissue, protein clouding causes cataracts

adjustable light refraction for focusing at various distances

Vitreous Humor

gelatinous substance, contains phagocytes to clean up debris, aging can cause floaters

Cataracts

cloudy or blurry vision

Floaters

small specks, strands, etc that drift across your vision

Cornea - Air Interface

outermost boundary of eye where cornea touches air,

provides majority of light refraction

not adjustable for focusing due to large refractive index between air + cornea

Zonal Fibers

holds lens in place, connects to ciliary muscles

tense = flat lens = far

relaxed = round lens = near

Ciliary Muscle

shapes lens

relaxed = flat lens = far

contracted = round lens = near

Pupil

narrow light path, sympathetic and parasympathetic NS control

controlled by iris muscles

Optic Disc

point on retina where retinal ganglion cell axons exit eye to form optic nerve

where major retinal blood vessels enter and leave

no photoreceptors, creates natural “blind spot”

Macula Lutea

high acuity aka sharp, detailed central vision, color perception

filters UV light, site of macular degeneration

Fovea

center of macula, highest visual acuity

mostly cone photoreceptors

Retina

layer at back of eyeball where visual images are formed

Retinal Circuitry

convert photoreceptor activity into action potentials that travel to rest of CNS

inner wall = retinal neurons

outer wall = retinal pigment epithelium

Outer Wall (Retinal Pigment Epithelium)

prevents backscatter of light

removes + recycles photoreceptor discs

Photoreceptors

made up of cones + rods

absorb light

Rods

more numerous, sensitive to dim light

night vision

Cones

specialized for color + high visual acuity, responsive to bright light

color vision, ability to see fine detail

Interneurons

signal in very local areas (only make connections in retina)

bipolar cells, horizontal cells, amacrine cells

Projection Neurons

generate APs, send info to brain

only ganglion cells

Bipolar Cells

light ON and OFF signals in pathways

transmit visual information from photoreceptors to ganglion cells

Ganglion Cells

axons bundle together to form optic nerve, only cells that fire APs

Red-Green Deficiencies

most common type of color blindness

“Nopes”

1% of males missing either red cones or green cones

can’t distinguish between red and green

“Anomaly”

5% males have shifted green/red cones

can’t see different hues of red vs green

Agnosia

damage to WHAT pathway

inability to recognize objects/faces/etc until you interact with said object

Optic Ataxia

damage to WHERE pathway

difficulty reaching/other movements related to location

Direction of light entering eye travels from…

optic nerve towards photoreceptors

Visual information is process from…

photoreceptors to optic nerve

What happens in the outer segment of the retinal circuit?

photoreceptors

cAMP binds

Na+ and Ca2+ channels are opened so ions flow in

depolarization of graded potential + glutamate release towards bipolar cells

What happens in the inner segment of the retinal circuit?

bipolar cells

K+ leak channels are always opened so ions flow out

depolarization of graded potential + glutamate release towards bipolar cells

What happens to a photoreceptor disc in the dark?

cGMP constantly being made

depolarization

What happens to a photoreceptor disc in the light?

photon binds to rhodopsin GPCR

rhodopsin GPCR releases alpha-subunit of transducin

transducin binds to PDE

PDE releases phosphodiesterase

phosphodiesterase breaks down cGMP → GMP

hyperpolarization

What Pathway

dorsal → parietal lobe

Where Pathway

visceral → occipital lobe

Visual Pathway

visual stimulus → retina → optic nerve → optic chiasm → optic tract → thalamus → radiation → primary visual cortex

How is color determined?

relative activity of different classes of cones