Retina - Chapter 21,26

Question 1

Define what is meant by sensory coding:

Answer 1

Changing some external stimulus into a pattern of neural impulses.

Question 2

What are the mechanisms for adaptation in the retina?

Answer 2

1) Slow Voltage-gated Na+ channel Inactivation in the photoreceptor adapts to darkness, membrane potential can be reduced to levels between -70 and -40 mV and light sensitivity is restored.
2) Calcium-gated K+ current in the photoreceptor in response to sudden intense light. Sudden light closes all depolarizing channels (Na+, Ca2+) so the only current is hyperpolarizing. As intracellular [Ca2+] decreases Calcium gated K+ channels begin to close bringing the intracellular voltage to levels between -40 and -70.

Question 3

What is the mechanism for bursting in the retina?

Answer 3

Voltage gated Ca2+ channels deactivate in ~10 ms.

Question 4

Give the phototransduction mechanism in photoreceptors:

Answer 4

Photon strikes rhodopsin in disks of outer segment, this activates transducin. Transducin binds GTP (instead of GDP) and activates cGMP phosphodiesterase. cGMP phosphodiesterase breaks down cGMP. Reduced concentration of cGMP cause cGMP-gated (Na+) channels to close. Cell is hyperpolarized and glutamate release from the photoreceptor is stopped. Once transducin hydrolyzes GTP into GDP this process is reversed.

Question 5

Describe how reduced glutamate release from photoreceptors creates neural signals downstream:

Answer 5

Glutamate has an inhibitory effect on “on-center” bipolar cells and an excitatory effect on “off-center” bipolar cells. In the dark, inhibitory connections are active on ganglion cells reducing firing. In the light inhibition is removed and ganglion cells can fire.