Vision

How do you have cones for S, M, L?

- You have genes for it

What kind of receptive fields do LGN have?

-Center surround receptive field - A little bigger than cells -Similar stimuli for oncenter/offcenter -LGN receptive fields are smaller in the foveal (center) region and larger in the periphery

What is the difference between rods & cones?

-Cones are less sensitive to light than rods, and they are responsible for DAYTIME (color) vision -Rods are responsible for night (monochrome) vision -Cone/Color

What do you call people who are colorblind?

-If they can only express 2 cone opsins -> dichromatic

Are there ionotropic light receptors?

-In humans and mammals.. no. -In algae, they have ionotropic light receptors

How do photons get into your eyes?

-Light sources bounce off objects and reflect photons into our eyes so we can see the object

How many kinds of cones are there in humans?

-Red, Blue, Green

How do images appear in the eye?

-The retinal image is flipped upside-down and right-to-left on the back of the eye

What do you call people who are NOT color blind?

-Trichromatic because they can express all 3 cone opsins

What happens in color blindness?

-Variation in genetic makeup -Colorblind people have mutated cone opsin protein (usually M cone opsin)

What is light?

-tiny particles called photons which travel at the speed of light: 186,000 miles/sec (faster than sound) -Photons = electromagnetic energy which are emitted from a radiant energy source -Photons have wave-like properties; free photons travel in bumpy ripples called wave packets -Distance between peaks and valleys of a photon's ripples is called a wavelength -Photon's wavelength determines color of light

What happens in the hermann grid illusion?

white black spots causes competition bt excitatory & inhibitory neurons

Where is the point of transduction in the eyes?

-Retina

What wavelength of light does Opsins respond best to?

-Rhodopsin -> Green (rods only experience one color) -each cone expresses specific cone opsin so respond best to different wavelengths (S, M, L)

How do axons that carry the same information from the medial hemifield to the brain?

-Right half of vision goes to right side of brain -Left half of vision goes to left side of the brain

What is another name for the photoreceptors?

-Rods & Cones

Where is density of rods highest in the retina?

-Surrounding in thefovea

What do you call someone who can perceive more colors than a normal person?

-Tetrachromatic -Female who inherits a cone opsin that responds to wave length of light slightly differently than normal cone opsin

What is optic radiation?

-Path from LGN to V1 -When LGN projects to visual cortex it looks like stripes (one line from each side of the eye) -Stripes are called ocular dominance column

What causes the change in receptor potential in photoreceptors?

- about -35 mV resting membrane potential -When very brief flash of light occurs, it is followed by a slow hyperpolarization lasting about 100-200 milliseconds -> receptor potential -Because photoreceptor is so depolarized in the dark, it constantly releases a lot of glutamate in the dark -Hyperpolarization causes the rod to release less glutamate in the light than in the dark

What is the order of cells in the retina?

-(from back to front): Rod/cones -> bipolar cells -> ganglion cell layer -> optic fiber nerve

How is the retina composed?

-5 layers of retina with photoreceptors (rods & cones) are near the outermost layer, so light must pass through the other layers to reach them

Where do photoreceptors release neurotransmitters onto?

-Bipolar cells & Horizontal cells

What happens after bipolar cells are inhibited of activated by photoreceptors?

-Bipolar cells release glutamate onto retinal ganglion cells which causes 2 effects on 2 different types of cell: 1. On-center ganglion cells: receive input from on-center bipolar cells so they become depolarized (excited) when light hits the photoreceptor 2. Off-center ganglion cells receive input from off-center bipolar cells so they become hyperpolarized (less excited) when light hits the photoreceptor

What is phototransduction?

-Broadly defined as the conversion of electromagnetic energy (such as light) into some other form of energy -In neuroscience: transduction of light energy into neural signals, which is the first step in visual perception -Phototransduction is performed by a specialized sensory organ at the back of the eye, called the retina, which translates visual images into the language of action potentials

What kind of receptive fields do ganglion cells have?

-Center surround 1. On center off surround cells: antipreferred stimulus is an annulus of light in the surround (inhibits cell) -preferred stimulus is a spot of light in the center *cell doesnt respond when you shine on surround and center bc it cancels out 2. off-center on-surround cells: preferred stimulus is an annulus of light in the surround -antipreferred stimulus is a spot of light in the center

How does color vision work?

-Depends on which combination of cones are activated to express certain colors -Light that activates L sees red -Combo of M & L = yellow color

What kind of receptor potential is it when light hits?

-Inhibitory receptor potential which results in less glutamate

What happens after light is transduced in a graded hyperpolarization of photoreceptors?

-It causes less glutamate to be released by rod or cones which causes 2 effects 1. on-center bipolar cell: INHIBITED by glutamate (via metabotropic glutamate receptor) so when there isn't any glutamate they become DEPOLARIZED (disinhibited) when light hits photoreceptor (increased firing rate) 2. Off-center bipolar cell: EXCITED by glutamate so when there isnt glutamate they become hyperpolarized (less excited when light hits the photoreceptor) (decreased firing rate)

What is the Lateral Geniculate Nucleus of the thalamus?

-LGN is the thalamic relay nucleus for vision -Receives input from retinal ganglion cells via optic tract -sends output to primary visual cortex (v1) via optic radiation -composed of 6 layers -ipsilateral eye: 2,3,5 -contralteral eye:1,4,6 Layer 1 & 2 are called MAGNOcellular due to large cell bodies layer 3-6 are called parvocellular due to small cell bodies

What kind of ion channels are photoreceptors?

-Ligand-gated sodium channels that are open in the dark -Channels are held open by a ligand called cyclic guanosine monophosphate (cGMP) that binds on the inside of the sodium channel protein -Light hyperpolarizes the phtoreceptor by preventing cGMP from binding to the sodium channel

What is the structure of the eye?

-Light enters front of eye by passing through cornea and then though pupil -> the iris (which is the colored part of the eye) controls how much light enters by adjusting pupil size -> lens focuses light on the retina at the back of the eye -Ciliary muscles distort the shape of the lens to focus objects at different viewing distances onto the retina

What causes the ion channels to be closed when light hits?

-Llamellae membrane has lots of opsins -In rods, called rhodopsin, in cones, called cone opsins -Normally, opsins bind to retinal (a form of vitamin A) -When light hits, retinal breaks away and allows the opsin to be binded to a g protein which is called transducin -This g protein unit that acts like a metabotropic ion channel -Part of the subunit breaks out and binds to a subtrate, cGMP phosphodiesterase, and activates it -PDE starts breaking down cGMP (which binds to the sodium channels) by converting it to 5-GMP -5'-GMP can't bind to sodium channels like cGMP so the sodium channels close

Why are men more likely to be colorblind?

-M&L opsin genes are on the X chromosome: women have 2 chances to inherit whereas men only have 1 so they are more likely to be colorblind

Do bipolar cells fire action potentials?

-NO.

Do rods & cones fire action potentials?

-NO. ROD & CONE photoreceptors DO NOT fire action potentials!!) -They fire graded potential, it is not all or nothing... as more light hits them they produce less glutamate

Which cell is an exception to glutamate excitatoriness?

-On-center bipolar cells because they are inhibited by glutamate

WHere do the optic nerves decussate?

-Optic chiasm before they head to LGN (lateral geniculate nucleus)

Where is the blind spot?

-Optic disk - Nasal side (back of retina) of retina with NO photoreceptors, which is where optic nerve (axons of ganglion cells) leave the retina to the brain

What is the structure of the rod cells?

-Outer segment of the rod cell contains thin layers of membrane called lamellae which contain light sensors -Base of the photoreceptor contains vesicles full of glutamate that can be released onto other neurons -Nucleus of the cell is located between the outer segment and the base -Just beneath the outer segment is an area filled with mitochondria and other cellular machinery

What is the purpose of horizontal cells?

-They receive information from several photoreceptors and put all of that information together to laterally inhibit groups of photoreceptors to make edges of light and dark areas more prominent (When one cell activates in response to light, its activity impairs or prevents neighboring cells from activating. This causes the edges between light and dark areas to appear more prominent than they would be otherwise)

What is the visuotopic map?

-Topographic mapping -Each hemi of V1 contains visuotopic map of contralateral hemifield -Neurons physically closer together tend to have receptive fields that are nearby in the visual field

Do ganglion cells fire action potentials?

-YES, they are the first in phototransduction to fire action potentials!!!!

What is an analogy for retina?

-a TV screen (or computer screen) is made up of thousands of tiny dots called pixels

How does ganglion cells work for color vision?

-preference for color cells instead of light inside center -same as center surrounf receptive field

WHere is density of cones highest in the retina?

-the Center of the retina (optic disk) -thats why center of vision is most acute