The Eye III (Beyond the Photoreceptors)

Question 1

Receptors release glutamate onto bipolar cells when they are ____________

Answer 1

Depolarized

Increased hyperpolarization = less glutamate released

Question 2

What are the two types of bipolar cells?

Answer 2

“OFF-pathway” bipolar cells and “ON-pathway” bipolar cells

Question 3

Describe “OFF-pathway” bipolar cells

Answer 3

Ionotropic Glu receprots: Glu acts as excitatory NT (depolarize)
Active when inputting photoreceptors are in the DARK

Question 4

Describe “ON-pathway” bipolar cells

Answer 4

Spontaneously depolarize in absence of input
Metabotropic Glu receptors: Glu acts as inhibitory NT
Inhibited when inputting photoreceptors are in the dark

Question 5

Both off and on pathway bipolar cells release what?

Answer 5

Excitatory NT onto ganglion cells

Question 6

Describe what happens with the photoreceptors and bipolar cells in the dark

Answer 6

Photoreceptors are depolarized, therefore Glu is released onto bipolar cell
OFF-pathway bipolar cell is excited –> release of excitatory NT –> depolarization of ganglion cell (enough depolarization results in AP in ganglion cell)
ON-pathway bipolar cell is inhibited

In the dark: OFF-pathway is excited; ON-pathway is inhibited

Question 7

Describe what happens with the photoreceptors and bipolar cells in the light

Answer 7

Photoreceptor is hyperpolarized therefore less Glu is released onto bipolar cell
OFF-pathway bipolar cell receives less excitation –> less release of excitatory NT onto ganglion cell –> less depolarization of ganglion cell (fewer AP)
ON-pathway bipolar cell receives less inhibition –> allows spontaneous depolarization of bipolar cell –> release of excitatory NT onto ganglion cell –> depolarization of ganglion cell (leads to AP)

Question 8

What do ganglion cells detect in the visual image?

Answer 8

Edges
Single ganglion cell receives input from group of neighbouring photoreceptors in a specific area of retina and respond to areas of contrast, or edges, in their receptive field

Question 9

What parts of the visual field do the nasal and temporal retina in each eye see?

Answer 9

The nasal retina sees the outer part of the visual field of that eye; the temporal retina sees the inner part of the visual field of that eye

Question 10

What is the visual pathway from the retina to the brain?

Answer 10

Ganglion cells form the optic nerve –> axons from the nasal retina cross over at the optic chiasm then join axons from the contralateral temporal retina to form the optic tract (partial decussation - optic tract has axons from both eyes) –> optic tract projects to thalamus LGN –> LGN neurons project to primary visual cortex

Question 11

How does the cortex extract features of an image?

Answer 11

Ganglion cells respond to edges
Neurons of primary visual cortex respond to bars of light in specific orientations
Neurons of secondary visual cortex respond to more complex shapes
Higher-level cortices: Combine all features into a perception (colour, movement, etc.)