retinal physiology including phototransduction

Question 1

What are the two main structures of the retina?

Answer 1

The two main structures are the Retinal Pigment Epithelium (RPE) and the Neurosensory Retina

Question 2

What are the functions of the Retinal Pigment Epithelium (RPE)?

Answer 2

The functions of the RPE include visual pigment regeneration, phagocytosis of shed photoreceptor outer segment discs, maintenance of the outer blood-retinal barrier, absorption of light to reduce scatter, and regeneration and repair after injury

Question 3

What are the roles of rods and cones in vision?

Answer 3

Rods are responsible for vision at low light levels, do not mediate color vision, and have low spatial acuity. Cones are active at higher light levels, are capable of color vision, and are responsible for high spatial acuity

Question 4

What is the primary visual pigment in rods?

Answer 4

The primary visual pigment in rods is rhodopsin​

Question 5

What happens during rhodopsin bleaching?

Answer 5

Rhodopsin bleaching involves the separation of all-trans retinal (formed from isomerization of 11-cis retinal) from opsin, known as photodecomposition. The rhodopsin is said to be bleached by the action of light within photoreceptors

Question 6

Describe the process of rhodopsin regeneration.

Answer 6

All-trans retinal enters the chromophore pool in photoreceptor outer segments and RPE cells, isomerizes to 11-cis retinal, and binds to opsin to form rhodopsin. This process is independent of light and is known as rhodopsin regeneration

Question 7

What is the visual cycle?

Answer 7

The visual cycle is the equilibrium between the photodecomposition and regeneration of visual pigments

Question 8

What is phototransduction?

Answer 8

Phototransduction is the process of translating the information content of a light stimulus into electrical signals, occurring in the photoreceptors

Question 9

What occurs in rods during phototransduction?

Answer 9

In rods, rhodopsin absorbs light, breaking the 11-cis bond of retinal, and the opsin molecule undergoes conformational changes to an activated state called Meta-rhodopsin II, initiating a reaction that controls the inflow of cations into rod outer segments​

Question 10

How does light affect cGMP levels and Na+ channels in photoreceptors?

Answer 10

Light decreases cGMP levels, closing Na+ channels, reducing the dark current, and hyperpolarizing the photoreceptor. This stops the release of glutamate from the synaptic terminal​

Question 11

How does cone phototransduction differ from rod phototransduction?

Answer 11

Cone phototransduction is comparatively insensitive but faster and capable of adapting enormously to ambient light levels, with visual acuity increasing with increased illumination