BIO448 - Vision I

anterior chamber

aqueous humor/fluid filled chamber in front of the lens and the pupil

vitreous humor

chamber beyond the lens

retina found along the back and sides of this chamber

function of lens

refract protons so that they focus on the retina, and more specifically the fovea

zonule fibers

tiny muscle? fibers connecting the ciliary muscles to the lens so that the latter can thicken or become thinner as needed

contracted

when the ciliary muscles are ___ they let go of the zonule fibers, thus the lens is thicker

relaxed

when the ciliary muscles are ___ they pull on the zonule fibers are ___ the lens is thinner

choroid

thin layer lining the entire vitreous humor whose function is blood supply

retina

neural part of the eyeball

on the inside, above ethe choroid

nervous system, convert electrical activity from photoreceptors into action potentials

sclera

white, fibrous cornea

outermost layer of eyeball

translucent

fovea

depression in the retina

area where all the photons should be focused on

aka light coming from what you focus on in your visual field should focus there

optic disk

junction of retina and optic nerve

optic nerve

area where axons go out of the eyeball

NO photoreceptor there, aka blind spot

refractive index

Changing the ___ of the lens to help focus light on the surface of the retina

adjustable

the cornea does most of the refraction of light, but is not ___

distance viewing

ciliary muscles relaxed, pulling on fibers and thus making lens thinner

near vision

ciliary muscles contracted thus letting fibers go, making lens thinker

myopia

light/photons focus in chamber before reaching retina

hyperopia

light/photons focus past the retina

development of human eyes

• nervous system develops as a single layer of cells arranged in a tube (neural tube) which is filled with fluid

• the optic vesicle in neural tube start folding on itself to form the optic cup, which will end up being the eyeball

pigment retinal wpithelium

melanin containing structure that reduces backscattering of light that enters the eyes

aka reason why photons not bouncing all around eyeball

photoreceptor outer segment

layer of neurons under pigment retinal epithelium

contains photoreceptors (cones and rods)

outer nuclear layer

layer under photoreceptor outer segment

contains cell somas of photoreceptors

outer plexiform layer

under outer nuclear layer

where photoreceptor cells synapse on next group of cells (bipolar cells, horizontal cells)

inner nuclear layer

under outer plexiform layer

where cell somas of bipolar and horizontal cells are

inner plexiform layer

under inner nuclear layer

where bipolar cells synapse on ganglion cells

amacrine cells

take info from bipolar cells and transfer it to many different ganglion cells

lateral information transfer

ganglion cells

output cell of retina

axons go through retina and run through optic nerve to thalamus and cortex

3 neurons circuit

photoreceptors —> bipolar cells —> ganglion cells

horizontal cells

spread info from one photoreceptor to others

lateral information transfer

photoreceptor disks

part of photoreceptor membrane filled with light-sensitive photopigment, and other proteins involved in phototransduction

capture photon of light

limited lifespan of 12 days

make up outer segment of photoreceptors, which is embedded in pigment retinal epithelium

inner

___segment of photoreceptors contains the nucleus and other organelles

connected to outer segment by cilium

make new membrane and proteins and send them to outer segment as vesicles to renew disks

cilium

part of photoreceptors connecting the inner and outer segment of the eye

glutaminergic

rods and cones are ___ neurons

phagocytosed

disks shed after their 12 days lifespan and are ___ by the pigment retinal epithelium

the tip

discs are continuously added at the base and move progressively toward ___

removal of photoreceptor discs

• disc curls

• tip becomes spherical

• tip separate from rod

• tip is engulfed by pigment retinal epithelium

luminance

change in light intensity = change in __ = change in neurotransmitter release

aka change in the number of photons

action potentials

photoreceptors do not have ___ they rely solely on graded changes in membrane potential

they do have voltage gated Ca2+ channels though

-40

photoreceptors are at rest at around ___ mV

hyperpolaried

the more light, the more __ the photoreceptors become, thus Ca2+ channels stay closed, thus less neurotransmitter released

dark

in the ___

• the Ca2+ and Na+ channels are gated by cyclic nucleotide cGMP which binds and open them which depolarizes the membrane

• lots of cGMP

  • leaky K+ channel in inner segment gets potassium out

light

in the ___

• reduced Na+ influx due to light and reduced amount of cGMP, meaning the Ca2+ and Na+ channels stay closed

• hyperpolarization of the membrane

• enzyme breaks ligand (cGMP) down

open

in the dark, voltage gated calcium channel ___

close

in the light, voltage gated channels __

retinal

light absorbing chromophore sitting in groove of opsin protein

rodopsin

opsin protein containing light absorbing chromophore in rods

conformational

light causes a __ change in retinal, lading to changes in the opsins

light causes the 11-cis-retinal to become all trans retinal

conopsins

opsins proteins containing light absorbing chromophore in cones

phosphodiesterase

enzyme breaking down cGMP into GMP

turned on under high luminance conditions

overall mechanism

• photon refracted to hit retina

• changes in retinal and opsins

• transducin activated

• phosphodiesterase activated

• break down of cGMP

• channels closing

• cells hyperpolarizing

amplification

1 photon activates 800 transducin (G-protein) molecules and 8000 PDE, which then leads to the breakdown of 6 cGMP per PDE aka 4800 total

saturation

exposure to too many photons leads to __ of the photoreceptor (rods more sensitive to light than cones) because cGMP and retinal are reduced/all used up

retinal cycle

critically important for maintaining light sensitivity

all trans retinal binds to IRBP (internal retinal binding proteins) which then go through the pigment retinal epithelium where it is made back into 11-cis-retinal, binds to IRBP again and is delivered to photoreceptors