The Eye II (The Retina and Phototransduction)

Question 1

What part of the retina has the highest visual acuity?

Answer 1

The fovea, because light from centre of field of vision focuses here

Question 2

What are the components of the retina?

Answer 2

Photoreceptors (rods and cones)
Ganglion cells
Bipolar, amacrine, horizontal cells
Pigment epithelium

Question 3

What type of potential do photoreceptors use?

Answer 3

Graded potentials

Question 4

What are ganglion cells? How do they work?

Answer 4

Output cells with axons from the optic nerve
Work by action potentials (only cell in visual system to use AP) - all other cells use graded potentials as a rapid way to detect change in stimulus intensity

Question 5

What is the pigment epithelium?

Answer 5

Back layer of the retina that absorbs light to prevent it from reflecting back onto photoreceptors
Non-neuronal

Question 6

What is the order of retina components, from the back of the retina forward?

Answer 6

Retinal pigment epithelium –> photoreceptors –> horizontal cells –> bipolar cells –> amacrine cells –> ganglion cells –> (blood vessels) (light strikes here)

Question 7

What is the fovea centralis?

Answer 7

Part of the retina where the cell layers are pushed back from the photoreceptors
Has the highest visual acuity with no blood vessels = more clear image

Question 8

What is the macula lutea?

Answer 8

Part of the retina with few blood vessels and high acuity

Contains the fovea

Question 9

What is the optic disc?

Answer 9

Site where axons of ganglion cells leave the eye

Entry and exit of blood vessels occurs here as well, can’t detect light (blind spot)

Question 10

Describe the density of photoreceptors and degree of convergence at the fovea

Answer 10

High density of photoreceptors
Low convergence (few neurons synapse onto post-synaptic neuron)

Question 11

Describe the density of photoreceptors and degree of convergence at the peripheral retina

Answer 11

Low density of photoreceptors
High convergence
Integrates information from a larger area of the retina = lower acuity (coarse resolution of detail)

Question 12

What type of photoreceptors does the fovea and periphery contain?

Answer 12

Fovea = only cones (see in the daylight - not sensitive to light)
Periphery = only rods (see in the dark - very sensitive to light)

Question 13

What is macular degeneration?

Answer 13

Bluriness and distortion of centre of visual field that is slowly progressive with no cure

Question 14

What happens with a retinal detachment?

Answer 14

The retina pulls away from the wall of the eye causing small tears in the retina
Tears allow fluid behind the retina, which worsens detachment
Causes abnormal perception of shadows and flashes of light

Question 15

What is the difference between the inner and outer segment of photoreceptors?

Answer 15

Inner segment contains the nucleus and organelles

Outer segment contains the photopigments

Question 16

What is the function of photopigment? What membrane proteins do they contain? What is the significance of the membrane protein?

Answer 16

Absorbs light
Intrinsic membrane protein (opsin) and chromophore (retinal - derivative of vitamin A)
The type of membrane protein (opsin) determines the wavelength to which the photopigment is most sensitive

Question 17

What photopigment do rods contain?

Answer 17

Rhodopsin (very sensitive to light - activated by one photon)

Question 18

What photopigment do cones contain?

Answer 18

Three different types each sensitive to a different wavelength for red, blue, and green light

Question 19

How does phototransduction occur in the dark?

Answer 19

Guanylate cyclase converts GTP to cGMP –> cGMP binds to sodium/calcium channels, keeping them open –> membrane is DEPOLARIZED at rest (dark)

Question 20

How does phototransduction occur in the light?

Answer 20

Light causes a conformational change in retinal –> this results in a conformational change in opsin and stimulation of transducin by the opsin –> transducin activates phosphodiesterase –> phosphodiesterase converts cGMP to GMP –> concentration of cGMP in cell is reduced –> cGMP dissociates from sodium/calcium channels –> sodium and calcium channels close –> hyperpolarization of membrane