Visual System I

Liquid sphere is surround by what three layers

the retina layer, uveal layer, sclera layer

what is the retina

the light sensitive layer

what does the uveal layer contain?

• choroid (capillaries and melanin, the light absorbing pigment)

• cilliary body (muscles to adjust lens)

• iris: colored portion with muscle that regulate the size of the pupil

What is the sclera

white fibrous tissue, with light-transparent cornea at the front

why is the human eye considered to be “mis-engineered”?

other species have…

• better lenses

• better muscle control for focus

• nicitating membranes (a third eyelid = provides more protecting and moisture)

• more photodetectors

• better arranged blood vessels

• two foveas — binocular and monocular

• better acuity

how are we able to see space

when the retina is fixated on whatever is straight ahead. normal (monocular) human visual field extends to approximately 60 degrees nasally

what are the two things needed to make images in the retina

refraction and accommodation

how do refraction and accommodation do this

refraction is when the cornea and the lens bend the light to achieve a focused image

accommodation is when there are changes in refractive power of the lens

when/why does the lens become flat? when does it become round? what causes it to do this?

flat = viewing distant objects — zonule fibers

round: near objects — cillary muscles change refactory power

what are catatracts

opacities in the transparent lens. they account of 50% of blindness

what are vertebrate eyes

the concept that our eyes are like cameras when capturing images. the image is originally upside down and curved but our eye flips it

what is myopia. where is the focus point?

nearsightedness: when the cornea is too curved OR the eyeball is too long. where is the focus point: before the retina

(50% of the population has this)

what is hyperopia. where is the focus point?

farsightedness. the refractory muscles are too weak. the focus point is after the retina

What is notable about the Macula lutea and the fovea. what does it look like?

the macula lutea and the fovea have the highest visual acuity. the macula lutea is the ring AROUND the fovea and the fovea is the red center.

what is notable about the optic disk

theres no photoreceptors in the optic disk. blood vessels enter and optic fibers leave the eye, which causes the eyes blind spot.

what fills in the missing info the optic disc causes?

the cortical mechanisms

what is macular degeneration

• a loss of vision in the center of the visual filed (the macula) because the retina sustains damage

• it causes people to have trouble recognizing faces BUT peripheral vision remains

• it affects 6 million people in the US

what is the “dry” form of macular degeneration

when debris lies between the retina and the choroid. the retinal pigment epithilium disappears and theres a loss of photoreceptors

90% of cases

what is the “wet” form of macular degeneration

the more “severe” version. blood vessels grow from the choroid and the retina detaches

how is macular degeneration treated

laser and medication to stop the growth of blood vessels

what is the three neuron chain for vision.

photoreceptor (rod or cone) — bipolar cell — ganglion cell

what is the role of photoreceptors in the three neuron chain

photoreceptors convert light energy into changes into neural signals

what is the role of ganglion cells in the three neuron chain

ganglion cells fire action potentials that are relayed to the brain via the optic nerves. they are the only source of output from the retina

watch a video explaining retinal circuitry

what are the five classes of neurons

• photoreceptors (rods and cones)

• bipolar cells

• horizontal cells

• amacrine cells

• ganglion cells

what is the role of bipolar cells

transmit signals from the photoreceptor cells to the ganglion cells (kinda like interneurons!)

what is the role of horizontal cells

modify the responses of the bipolar cells

what is the role of amacrine cells

modify the responses of the gnalgion cells

how are rods and cones distinguished

• shape — rod = rectangle, cone = triangle

• light sensitivity

• photopigment they use

• distribution across the retina

• pattern of synaptic connection

rods

• extremely sensitive to light (only one photon is needed for a response)

• has very low spatial resolution

cones

• low light sensitivity (needs more than 100 for a response)

• has high spatial resolution

• responsible for the perception of color

where are the cones the most concentrated? where are they the least concentrated?

most concentrated — the center of the fovea — foveola, which has the highest visual acuity

least concentrated — throughout the retina

where are the rods the most concentrated? where are they the least concentrated?

most concentrated — throughout the retina

least concentrated — the fovea

what happens to photoreceptors when we are in the dark

they become depolarized; when theyre depolarized, more neurotransmitters release

what happens to photoreceptors when we are in the light

photoreceptors become hyperpolarized; when that happens, less neurotransmitters release

receptor potentials are ____

graded

watch video explaining the dark current

how does photo transduction work in the light (watch video)

receptor protein rhodospin activates the G protein transducin, which is a effector enzyme. phosphodiesterase is then released which then decreases the 2nd messanger (cGMP) — the Na+ channels close — the photoreceptor hyperpolarizes

there are three kinds of cones for vision. what type of vision does this cause and what does each subtype represent

we have trichromatic vision

• L = long wavelengths that peak at red

• M = medium, responds to medium wavelengths that peak at green

• S = short wavelengths that peak at blue — 5-10% are absent from the fovea

protanopia

loss of long (red) wavelength perception

deuteranopia

loss of medium (green) wavelength perception

tritanopia

loss of short wavelengths resulting in blue-yellow blindness, extremely rare

what do photoreceptors do when exposed to light

hyperpolarize— less glutamate

what are the functional differences between on and off center cells when HIT WITH LIGHT

on center: metabotropic glutamate receptors — they receive less glutamate so they DEPOLARIZE to release MORE glutamate

off center: inotropic glutamate receptors — they receive more glutamate so they HYPERPOLARIZE to release LESS glutamate

what do photoreceptors do when in the dark

photoreceptors depolarize — more glutamate

what are the functional differences between on and off center cells when IN THE DARK

on center: inotropic — receives MORE glutamate which makes it hypoerpolarize — less glutamate is released

off center: metobatropic: receives LESS glutamate which makes it depolarized — more glutamate is released

center vs surround properties of ganglion cells are shaped by ____

lateral inhibition from interneurons

how is this possible

antagonism of the surroudning area is casued by lateral connections made from the horizontal cells. horizontal cells are depolarized by glutamate from the photo receptors, and release GABA to hyper polarize receptors

essentially, the horizontal causes changes in the photoreceptor that are induced by light in the surrounding area

what is the visible light spectrum detected by human photoreceptors

400 nm to 700 nm