L7 - Special Transductory Mechanisms

Question 1

Light passes through all layers to reach the photoreceptors, except ____ in primates

Answer 1

Fovea

Question 2

2 types of photoreceptors? Compare.

Answer 2

Rods – more numerous than cones (20:1)

• Low light, B&W
• Duration of response is long
• Rhodopsin (pigment)
• Many rods converge on one bipolar cell, low light specialization, low acuity
• On centre bipolar cells

Cones – bright light colour vision

• Duration of response is short
• 3 types (different colours with different opsins)
• In highest acuity part of retina (fovea), each cone connects to 1 or 2 bipolar cells
• Have both on & off centre bipolar cells

Question 3

In order of increasing wavelengths of light for red, blue, green cones and rods.

Answer 3

Blue, Rods, Green, Red

Question 4

Why can’t primates see as well in low light?

Answer 4

High density of cones (bright light) in fovea compared to rods (low light).

Question 5

Prey animals (sheep, cows, horses) have a visual streak, what is that?

Answer 5

Horizontal line of high density detectors that pick up the horizon, looking out for disturbances so they can run away from predators.

Question 6

Are photoreceptors de or hyperpolarised by light?

Answer 6

Hyper

Question 7

How does this phototransduction occur in dark vs in light

Answer 7

IN DARK

1) Na+ influx
2) K+ efflux
3) Depolarisation

IN LIGHT

1) Light/opsin interaction activates phosphodiesterase to breakdown cGMP
2) Low cGMP close cGMP-dependent Na+ channels
3) Closing Na+ channels hyperpolarizes membrane (Reduced Na+ influx, K+ efflux)
* Same process in rods & cones, but opsins differ
* Despite being 2nd messenger driven, process is very fast

Question 8

In recording processes at the level of cell membranes, what happens to membrane potential & current?

Answer 8

• Membrane potential gets smaller (less hyperpolarised) as you go up
• Current is recorded as larger as you go up

Question 9

How does photoreceptor hyperpolarization influence bipolar cells?

Answer 9

• Photoreceptor terminals continuously release glutamate in the dark.
• Hyperpolarization at the level of the outer segments leads to hyper polarization at the terminals of the photoreceptor cells – passive conduction
• Hyperpolarization of the terminals lead to reduced influx of Ca2+ ions.
• Reduced Ca2+ influx leads to reduced glutamate release.

Question 10

Off-centre Bipolar cells

Answer 10

-When light hits the center, reduced glutamate release from photoreceptors causes off-centre bipolars to hyperpolarize

Question 11

On-centre Bipolar cells

Answer 11

-When light hits the center, reduced glutamate release from photoreceptors causes on-centre bipolars to depolarizes

Question 12

What’s special about ganglion cells?

Answer 12

Only neurons in retina with axons that leave the eye and fire AP when depolarized by glutamate.
-Membrane potential of ganglion cells follows that of input bipolar cells

Question 13

Olfactory transduction - how does it work?

Answer 13

• > Involves chemicals dissolving in the mucous layer of olfactory epithelium
• > If mucous layer is too thick (from a cold), odourants can’t penetrate the layer and you lose your sense of smell
• All activate G olf (trimer), alpha subunit interacts with & activates adenylate cyclase
• Cyclic AMP activates cAMP dependent cation channel to depolarize membrane & produce a receptor potential

Question 14

Where are the olfactory receptors found?

Answer 14

On the cillia

Question 15

Taste receptor cells

Answer 15

• Salt & acid detected via passage through ion channels in taste cell membrane leading to depol.
• Sweet, bitter & umami have specific G-protein coupled receptors leading to membrane depol.
• Depol. leads to transmitter (glutamate) onto primary afferent terminals