Vision 1

how a physical stimulus turns into an electrical stimulus

Light passes through the what?

pupil

What does the pupil/iris do?

iris constricts/dilates the pupil to allow less/more light

What is the purpose of the lens?

to focus the light on the retina

How do we adjust for far and near vision?

muscles along the side of the lens can change your focus point

Why is the layout of the retina strange?

light has to pass a bunch of cells before hitting the photoreceptors

What is the ganglion cell layer?

the output neurons of the retina

What is the inner nuclear layer?

bipolar cells

What is the path of light from ganglion cells to photoreceptors?

ganglion cells
amacrine cells
bipolar cells

horizontal cells

photoreceptors

Light goes _______ while the signal goes _______.

backward; forward
(ganglion to photoreceptor; photoreceptor to ganglion)

Where does the phototransduction occur in photoreceptors?

outer segment

What are the two types photoreceptors?

rods and cones

What is the difference between rods and cones?

the wavelengths of light they absorb in order to change the membrane potential

Where is the highest density of cones?

fovea

Where is the best visual acuity and why?

the fovea because that is where all the cones are

Where is the highest concentration of rods?

peripheral retina

Where do all the axons leave the retina?

in the blind spot of the eye

Why do we have a blindspot?

there are no photoreceptors there

What occurs in the outer segment of a photoreceptor?

phototransduction

Chemoreceptors are analogous to what?

metabotropic receptors

Mechanoreceptors are analogous to what?

ionotropic receptors

What is absorbing the photons? (converting light into electrical signal)

photopigment called opsin

What is the G protein called? (converting light into electrical signal)

transducin

What is the effector enzyme? (converting light into electrical signal)

phosphodiesterase (PDE)

What is the second messenger? (converting light into electrical signal)

cGMP

We will specifically be _______ an ion channel. (converting light into electrical signal)

closing

What does phosphodiesterase regulate?

cGMP levels

Photons strike the ______, which absorbs the photons, activating the G protein

opsin

Photoreceptors are depolarized in the ____.

dark

What does phosphodiesterase do?

breaks down cGMP into 5’ GMP

What happens when the cGMP concentration is reduced?

they unbind from the Na channels, closing them

What does closing the Na channels do?

decreases Na permeability, hyperpolarizing the membrane

Activating the photoreceptor __________ the membrane.

hyperpolarizes

What is the signaling cascade of phototransduction?

1) Photon strikes opsin

2) Opsin absorbs the photon

3) GDP is swapped for GTP

4) Phosphodiesterase is activated

5) PDE breaks down cGMP

6) Na channels close

7) cell membrane hyperpolarizes

Photoreceptors only respond to light within their what?

receptive field

When there’s something in the visual space that the cell responds to, that changes the what?

firing output of the cell

What is the vertical pathway of visual signaling in the retina?

1) photoreceptor

2) bipolar cell

3) ganglion cell

How did they map receptive fields?

by using a cat and record from its optic nerve

The optic nerve is the ____ of the retinal ganglion cells.

axons

What were they looking for in the cat experiment?

where on the screen you could change the firing rate of the cell that was being recorded from

When the ganglion cell fires a bunch of APs when the light is turned ON, that spot must be in the _________ _______ for that ganglion cell, making it a plus.

receptive field

When the light turns OFF, that spot must be _________ the receptive field for that ganglion cell, and marked with a minus.

outside

A _____ is used when the APs fire in s ganglion cell when the light turns on.

plus

A ______ is used when the APs fire in a ganglion cell when the light turns off.

minus

A neuron’s receptive field is all _______ ________ from which light can influence the neuron’s ability to evoke discharges

spatial locations

Light presented from any location outside of the receptive field has no _________ on the neuron.

influence

What is the layout of the plus/minus in off center-on surround?

minus center, plus surround

What is the layout of the plus/minus in on center-off surround

plus center, minus surround

How could you fire a stronger AP in the on surround-off center ganglion?

by filling the center with light

When the on center-off surround is completely full, the ganglion is what?

maximally firing

How do you get a big off response in a retinal ganglion cell? (on center-off surround)

by filling the surround with light

What happens when you uniformly shine light in the center and surround?

no response (cancel each other out)

What happens when you shine light in the center of a off center-on surround retinal ganglion?

the off response

What happens when you shine light in the surround of a off center-on surround retinal ganglion?

the on response

What happens when you uniformly fill the center and surround of an off center-on surround ganglion?

no response (cancel each other out)

If you have high _______ between the center and surround, you get lots of AP firing.

contrast

What is the key feature in the phototransduction pathway?

the sign of the synapse between the photoreceptor and bipolar cell

What does the sign of the synapse between the photoreceptor and bipolar cell dictate?

whether its inhibitory or excitatory

Although all photoreceptors release glutamate, glutamate can elicit either an excitatory or inhibitory response in the ________ _____.

bipolar cell

The synapse between bipolar and ganglion cells are always _________.

excitatory

What does glutamate bind to?

AMPA receptors

What does glutamate binding to the AMPA receptors do? (ionotropic receptors)

depolarizes the cell (allows Na to flow through)

What happens if glutamate binds to metabotropic glutamate receptors?

hyperpolarization of the cell (closes Na channels)

Ionotropic receptors ______ the bipolar cells.

excite

Metabotropic receptors _______ the bipolar cells.

inhibit

The synapse between the photoreceptor and bipolar cell is _________ for on center cells and __________ for off center cells.

inhibitory; excitatory

What depolarizes on center-off surround bipolar cells?

light

What depolarizes off center-on surround bipolar cells?

darkness

If the off center center is depolarized in the dark, what is happening with the synaptic terminal?

releasing transmitter

In the off center receptive field, what happens when it is hyperpolarized? (in the light)

stops releasing excitatory transmitter onto the bipolar cell

What happens when we hyperpolarize the photoreceptor? (in off cells)

it hyperpolarizes the bipolar cell (no longer excited)

What happens when we hyperpolarize the bipolar cell? (in off cells)

the ganglion cell no longer receives excitatory transmitter, making more Na channels available by hyperpolarization

In off cells, ___________ of the photoreceptor causes it to release excitatory transmitter onto the bipolar cell.

depolarization

Why do off center receptive fields fire to light off?

because more Na channels are available and transmitter release causes a large depolarization

How are more Na channels made available?

via hyperpolarization

In on center cells, the synapse between the photoreceptor and the bipolar cell is __________.

inhibitory

In on center cells, the receptor is releasing __________ transmitter onto the bipolar cell and thus ___________ the bipolar cell.

inhibitory; hyperpolarizing

When the bipolar cell is hyperpolarized it does not release what onto the ganglion cell?

excitatory transmitter

On center receptive fields start as hyperpolarized in the ____, and depolarize when exposed to _____.

dark; light

In on center cells, when exposed to light, because the bipolar cell is no longer being inhibited, it what?

depolarizes

The depolarized bipolar cell releases _________ transmitter onto the ganglion cell.

excitatory

When the ganglion cell is excited it what?

fires APs

In on cells, when the light goes off, the photoreceptor _________ and releases __________ transmitter onto the bipolar cell.

depolarizes; inhibitory

In on cells, when the bipolar cell is _________, it causes the bipolar cell to stop releasing _________ transmitter onto the ganglion cell.

hyperpolarized; excitatory

When the ganglion cell is not excited it does what?

reverts to its resting potential

What do horizontal cells do?

receive excitatory input from photoreceptors and make inhibitory connections with photoreceptors

What is the order of steps when a photon strikes a on center-off surround photoreceptor? (no horizontal cell involved)

1) light hyperpolarizes the cone

2) reduces inhibition and depolarizes bipolar cell

3) increases excitation on retinal ganglion cell

What is the order of steps when a photon strikes an on center-off surround photoreceptor? (include horizontal cell)

1) light hyperpolarizes cone

2) decreases excitation onto horizontal cells

3) decreases inhibition on cone

4) increases glutamate output and inhibits bipolar cell

5) decreases output from retinal ganglion cell

What is the order of steps when a photon strikes an off center-on surround photoreceptor? (no horizontal cell involved)

1) light hyperpolarizes the cone

2) reduces excitation and hyperpolarizes bipolar cell

3) decreases excitation on retinal ganglion cell

What is the order of steps when a photon strikes an off center-on surround photoreceptor? (include horizontal cell)

1) light hyperpolarizes the cone

2) decreases excitation onto horizontal cells

3) decreases inhibition on cone

4) increases glutamate output and excites bipolar cell

5) increases output from retinal ganglion cell

Horizontal cells receive excitatory input from where?

their photoreceptors

Horizontal cells receive ________ input but they project ________ input onto the photoreceptor.

excitatory; inhibitory