Vison II: Retinal physiology and receptive fields

Neuro 330

What are the different types of retinal cells

• photoreceptors (rods and cones)

• Bipolar cells

• Ganglion cells

• Horizontal cells

• Amacrine cells

What do photoreceptors do?

Hyperpolarize in response to light

1st step in visual processing

Converts light into neural signals via phototransduction

Release glutamate into bipolar cells

What do bipolar cells do?

Relay neuron between photoreceptors and ganglion cells

Two types:

• On bipolar → depolarize in response to light

• OFF bipolar → hyperpolarize in response to light

Create the On/ OFF parallel pathways

What do Ganglion cells do?

• The output neuron of the retina

• Their axons form the optic nerve

Two major types

• M cell— motion, contrast, coarse detail

• P cells— fine detail, color

What do Horizontal cells do?

• Interneurons that run laterally across the retina

• Located in the outer plexiform layer

• Connect photoreceptors to each other and to bipolar cells

• Have AMPARs— are depolarized by glut and release GABA (inhibitory) onto neighboring PRs

What do Amacrine cells do?

• Located at the inner plexiform layer

• Connect bipolar and ganglion cells laterally

• Help process motion and timing

What is the off pathway transmission?

Light hits which hyperpolarizes photoreceptors, so they release less glutamate →

OFF bipolar cells are also hyperpolarized so they turn off and release less glutamate →

All together this decreases OFF ganaglion cell APs ( less glutamate= less APs)

What is the ON pathway transmission?

Light hits which hyperpolarizes photoreceptors so less glutamate is released →

ON bipolar cells depolarize (turn on), releasing more glutamate →

All together this increases ON ganglion cells APs (more glutamate= more APs)

ON bipolar vs On galglion cells

On bipolar cells are activated by light

ON ganglion cells increase firing with light

What do ON bipolar cells do

• Uses mGluR6 (metabotropic receptor)

• mGluR6 is an inhibitory receptor

• in the dark → lots of glutamate → mGluR6 activated → cell hyperpolarized

• in the light → less glutamate →mGluR6 inactive → cell depolarizes

Inverts the photoreceptor signal directly

What do OFF bipolar cells do?

• Use ionotropic glutamate receptors (AMPA/kainate)

• These are excitatory receptors

• In the dark → lots of glutamate → cell depolarizes

• In the light → less glutamate → cell hyperpolarizes

Follows the photoreceptor signal directly

What is a receptive field?

The region of the visual field that when stimulated, changes the firing rate on a neuron

Visual fields vs Receptive fields

Visual fields is that area you're able to see

Receptive filed is the portion of the retinal that is activated to see an object— when light hits a portion of the retina, there is a change in firing rate of the neuron in that area

Do Rods have a larger or smaller receptive fields than cones?

Larger receptive fields → blurrier vision

smaller receptive fields = more acuity (see clearer)

What does center On/ surround OFF mean?

Th Center ON bipolar cells:

1. Light in center → photoreceptor hyperpolarizes → releases less glutamate

2. Less glutamate → mGluR6 inactive (inhibitory receptor no longer activated)

3. ON bipolar cell depolarizes

4. ON-center ganglion cell fires

The Surround (horizontal cells):

1. Light in surround → neighboring photoreceptors hyperpolarize

2. Horizontal cells receive less glutamate → become more active (release more GABA) (they are inhibitory)

3. Horizontal cells inhibit center bipolar cell

4. Ganglion cell inhibited

How do Horizontal cells work

1. Photoreceptor receives light → releases glutamate

2. Glutamate activates horizontal cell

3. Horizontal cell inhibits neighboring photoreceptors and bipolar cells

4. This suppresses activity in the surround region (OFF surround)

What does OFF center/ On surround mean?

OFF center:

1. Dark in center → photoreceptor stays depolarized → releases more glutamate

2. More glutamate → OFF bipolar depolarizes (excitatory receptor)

3. OFF bipolar excites OFF-center ganglion cell → FIRES

On surround":

1. Light in surround → neighboring photoreceptors hyperpolarize → release less glutamate

2. Less glutamate → horizontal cells less active (release less GABA)

3. Less lateral inhibition onto center bipolar cell → center is released from inhibition

4. Ganglion cell fires ​

What happens in different OFF center/ On surround mean scenarios?

Stimulus	Response
Dark spot in center	Strong firing ✅
Light in center	Inhibited ❌
Light in surround	Fires ✅
Large diffuse light (covers both)	Weak response (cancel out)

On center vs OFF center?

	ON-Center	OFF-Center
Prefers	Bright spot in center	Dark spot in center
Center response to light	Fires	Inhibited
Surround response to light	Inhibited	Fires
Best stimulus	Small bright spot	Small dark spot

On bipolar vs On ganglion cells in response to light?

What are the 3 types of cones and their pigments:

Cone Type	Peak Wavelength	Color
S cones (blue)	~420 nm	Blue
M cones (green)	~530 nm	Green
L cones (red)	~560 nm	Red

L + M cones (yellow)

What is the opponent color theory and what are the opponent channels?

• Color is not detected by single cones — it's determined by comparing activity across cone types

Three Opponent Channels:

• Red vs. Green (L cone vs. M cone)

• Blue vs. Yellow (S cone vs. L+M cones)

• Light vs. Dark (luminance channel)

Excited by one inhibited by the other

Which colors are RGS excited and inhibited by?

Channel	Excited By	Inhibited By
Red/Green	L cones (red)	M cones (green)
Green/Red​	M cones (green)	L cones (red)
Blue/Yellow	S cones (blue)	L+M cones (yellow)

What happens if you stare at red then look at white?

1. L cones (red) are heavily stimulated → become fatigued/adapted

2. The +L/-M opponent channel is continuously activated

3. After prolonged stimulation → L cones become less responsive

4. White light stimulates ALL cone types equally

5. But your L cones are fatigued → respond weakly

6. Your M cones are fresh → respond normally

7. Result → M cone activity dominates → you see GREEN​

What is the trichromatic theory

Every combo of color received by the brain is a combo of blue, green, and red

Describe the properties of synapses between photoreceptors and on-center bipolar neurons in the retina that produce the on-center responding

1. Light → center PRs hyperpolarize → release less glutamate

2. Less glutamate → mGluR6 on on-center bipolar cells becomes inactive

3. mGluR6 inactivation → cation channel opens → cation influx → bipolar cell depolarizes → "on" response

Surround pathway:

1. Light hits center PRs → less glutamate released onto horizontal cells

2. Horizontal cells become less active → release less GABA

3. Less GABA onto surround PRs → surround PRs depolarize → release more glutamate

4. More glutamate onto on-center bipolar cells → mGluR6 more active → cation channel closes → bipolar cell hyperpolarized → surround "off" response

If you have an L+M- retinal ganglion cells, what would happen to it’s firing if you have

a) Red light in the center?

b) Red light in the surround?

c) Yellow light in the center?

a) High excitatory response, L cones are activated by red

b) Below bassline response, surround is being driven and green inhibits red

c) Baseline response; yellow activates red and green cones equally