Physiology of photoreceptors

Question 1

What is the direct pathway for signal transmission?

Answer 1

photoreceptors deduce light into electrical current ->
bipolar cells ->
ganglion cells (axons comprise the optic nerve)

Question 2

What is the role of horizontal cells?

Answer 2

recieve input from photoreceptors and project to other photoreceptors and bipolar cells

Question 3

What is the role of amacrine cells?

Answer 3

recieve input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells

Question 4

What is the role of photoreceptors?

Answer 4

convert electromagnetic radiation to neural signals

Question 5

What happens to vertebrate photoreceptor cells membrane potentials when they are exposed to light?

Answer 5

membrane potential hyper polarises

Question 6

Why is a vertebrate photoreceptor cells membrane potential positive?

Answer 6

the dark current

Question 7

What is the dark current?

Answer 7

a cGMP gated Na channel that is open in the dark due to the release of neurotransmittors (glutamate) and closed during light

Question 8

How does the dark current allow the brain to percieve objects?

Answer 8

the change of Na due to light signals to the brain

Question 9

What cells can cause action potentials?

Answer 9

ganglion cells

Question 10

What neurotransmittor is released to open the Na channels?

Answer 10

glutamate

Question 11

What is the membrane potential equation in the dark?

Answer 11

Pna = Pk
Vm = between End and Ek

Question 12

What is rhodopsin (rods) made up from?

Answer 12

retinal - vit A derivative

opsin - G protein coupled receptor

Question 13

Where are rods present?

Answer 13

membrane folds - called discs in outer segment

Question 14

What does light convert 11-cis-retinal to?

Answer 14

all-trans-retinal

Question 15

What is the role of all-trans-retinal?

Answer 15

activates transducin -> molecular cascade -> decrease cGMP which leads to the closure of cGMP gated Na channels

Question 16

What does decreased Na entry lead to for cGMP gated Na channels?

Answer 16

hyperpolarisation

Question 17

What is transducin?

Answer 17

a G protein that activates phosphodiesterase (PDE) which hydrolyses cGMP leading to decreased cGMP levels

Question 18

What are the benefits of rods?

Answer 18

good for seeing in dim light - high convergence, low visual acuity

Question 19

What are the benefits of cones?

Answer 19

good for seeing in normal daylight with high visual acuity - high density of cones, high visual acuity

Question 20

How do we see shades?

Answer 20

cones of different wavelengths overlap

Question 21

What type of cone sees blue?

Answer 21

short wave cone

Question 22

What type of cone sees red?

Answer 22

long wave cone

Question 23

What type of cone sees green?

Answer 23

middle wave cone

Question 24

Where are rods found in the retina?

Answer 24

peripheral retina

Question 25

Where are cones found in the retina?

Answer 25

center of the retina - fovea

Question 26

What is the monocular visual field?

Answer 26

what each eye sees individually

Question 27

What is the binocular visual field?

Answer 27

what both eyes overlap to see

Question 28

Where would the right visual field go to?

Answer 28

left cortex

Question 29

Where would the superior visual field go to?

Answer 29

lower cortex

Question 30

What cells recieve mononoclar vision?

Answer 30

4c layer

Question 31

What cells recieve binocular vision?

Answer 31

all cells outside the 4c layer

Question 32

What is ambylopia/cortical blindness?

Answer 32

no problem with the eye, but one eye has better vision than the other

Question 33

What is ambylopia caused by?

Answer 33

strabismus - wandering eye in infancy - must be corrected by eye patch over good eye as signals from eyes need to compete equally

Question 34

What is Hebbs postulate?

Answer 34

basis for the loss of binocularity basically: if axon A is near enough to touch axon B and takes part in exciting it and firing it, a growth process takes place in one or both cells allowing As efficiency to fire cell B to increase - long term potination

Question 35

What can Hebbs postulate lead to?

Answer 35

one eye being deprived of axons

Question 36

Where is a colloid cyst found?

Answer 36

intravertebral foramen

Question 37

What does an ependymanos arise from?

Answer 37

ependymal cells

Question 38

Where is a epidural haematoma found?

Answer 38

between skull and dura - arterial bleed

Question 39

Where is a subdural haematoma found?

Answer 39

between dura and arachnoid - venous bleed

Question 40

What is hydrocephalus?

Answer 40

accumulation of CSF in the ventricular system or around the brain, enlargement of the ventricles increase CSF pressure

Question 41

What is hydrocephalus due to?

Answer 41

too much CSF production | obstruction of flow