Vision

Question 1

phototransduction cascade in rods

Answer 1

photon excites rhodopsin, causes configuration change in retinal molecule from cis to trans => activates transducin => activates phosphodiesterase (PDE) => hydrolyzes cGMP => cGMP concentration drops => cGMP-gates channels close => less Na+ and Ca2+ enter the cell => cell is hyperpolarized => graded decrease in glutamate release

Question 2

mechanisms that shut off the phototransduction cascade

Answer 2

1) rhodopsin kinase phosphorylates the activated rhodopsin
2) arrestin binds to the activated rhodopsin and blocks its interaction with transducin

Question 3

retinal cell layers from back to front

Answer 3

• pigment epithelium
• outer nuclear layer - rods/cones
• outer plexiform layer - synapse between photoreceptors and bipolar cells
• inner nuclear layer - bipolar/horizontal/amacrine cells
• inner plexiform layer - synapse between bipolar cells and ganglion/amacrine cells
• ganglion cell layer

Question 4

Explain the mechanisms facilitating the ON/OFF center-surround receptive fields of bipolar cells

Answer 4

ON-center bipolar preferentially respond to a bright stimulus in the center and a dark stimulus in the surround

OFF-center bipolar cells preferentially respond to a dark stimulus in the center and a bright stimulus in the surround

this is because glutamate has opposite effects on ON-center and OFF-center bipolar cells:

– glutamate is excitatory for OFF bipolar cells –> because OFF bipolar cells have ionotropic (AMPA) receptors that depolarize the cell in response to glutamate ==> dark –> more glutamate –> happy OFF bipolar cell/ light –> less glutamate –> sad OFF bipolar cell

– glutamate is inhibitory to ON bipolar cells –> because ON bipolar cells have a metabotropic receptor that hyperpolarize the cell in response to glutamate: light –> less glutamate –> less inhibition –> happy ON bipolar cell/ dark –> more glutamate –> more inhibition –> sad ON bipolar cell

Question 5

Explain how horizontal cells contribute to the center surround receptive field, providing a concrete example

Answer 5

Horizontal cells provide opponency between the center and the surround of a bipolar cell’s receptive field through lateral inhibition.

For example, when light is shined on the surround of an ON center bipolar cell’s receptive field, photoreceptors in the surround are hyperpolarized by the light and release less glutamate on nearby horizontal cells –> nearby horizontal cells therefore release less GABA onto the center photoreceptors –> so the depolarized center photoreceptor in the dark will release glutamate onto the ON-bipolar cell which hyperpolarizes it.

Question 6

What are the similarities and differences between rods and cones?

Answer 6