eye overview

Question 1

what are the key brain areas involved in visual information processing?

Answer 1

retina - lateral geniculate nucleus (in thalamus) - optic radiation (geniculocalcarine tract) – primary visual cortex

Question 2

name and describe 5 features of the eye

Answer 2

Pupil - lets light in, appear black because all light that enters is absorbed on the retina

Iris - muscles controlling amount of light entering eye

Cornea - transparent covering of pupil and iris, refracts light onto retina

Lens = behind the pupil, second part of the refractive system, but the lens can adjust how much it refracts light unlike the cornea

Sclera - outer layer (minus cornea), tough and protective, maintains shape

Question 3

where is the aqueous humor and what does it do?

Answer 3

Behind the cornea and in front of the lens = aqueous humor - supports cells without the need for blood vessels that would interfere with corneal function

Question 4

what is the fluid-filled main body of the eye?

Answer 4

vitreous humor = fluid filled, maintains shape
Retina goes all around the sides and back of the eye

Question 5

what nerves and muscles are in the eye?

Answer 5

Extraocular muscles, controlled by oculomotor nerve (CN III)

Optic nerve -carries axons from retina to brain (CN II)

To flatten and weaken the lens, suspensory ligaments are tight, ciliary muscles relax, and vice versa

Question 6

how is light refracted in the eye? include the definition of focal length, and why distant objects require less refraction

Answer 6

Cornea refracts 80% of light, occurs due to cornea having a higher density than air

Focal length = distance from refractive surface to point of convergence of parallel light rays

Parallel light rays don’t need as much refraction to focus on the retina. Distant objects have more parallel rays, so the lens is made thinner for distant objects, because the cornea provides adequate refraction

The opposite can be said for nearer objects

Question 7

describe the features of the retina

Answer 7

The macula is the centre portion of the retina that produces even sharper vision with its rods and cones

The fovea is the pit inside the macula with only cones, so has highest visual acuity

Rest of retina = mostly rods
Light travels to the back of the retina where the photoreceptors are, via muller cells (glia cells of the retina)

Question 8

macula…

Answer 8

only cones = highest visual acuity

Question 9

from front to back, what are the layers of the retina?

Answer 9

The ganglion cell layer, closest to vitreous humor, outputs info from retina to brain

Inner plexiform layer - SYNAPSES between ganglion and amacrine cells receiving input from bipolar cells

Inner nuclear layer - amacrine, horizontal and bipolar cell bodies

Outer plexiform layer - (closer to the back of the eye, the sclera) - synapses between bipolar and horizontal cells receiving input from photoreceptor cells

Outer nuclear layer - cell bodies of the photoreceptors

Photoreceptor outer segments

Pigment epithelium - absorbs any light the photoreceptor cells doesn’t

Question 10

the inner plexiform layer - what can be found there?

Answer 10

its all synapses

Bipolar to amacrine
Amacrine to bipolar

Bipolar to ganglion cells
Amacrine to ganglion cells

Question 11

the inner nuclear layer - what are the roles of the cells present?

Answer 11

amacrine cells modulate the information between GCs and BCs

horizontal cells modulate information between PRs and BCs

Question 12

where do you get graded potentials and where do you get action potentials in the eye?

Answer 12

Photoreceptors transduce light to electrical signal, sending it back out to the ganglion cells, whose axons go to the optic nerve

The photoreceptors and bipolar cells produce graded potentials, action potentials only occur at the ganglion cells

Question 13

function of the pigmented epithelium?

Answer 13

absorbs any light the photoreceptor cells don’t, this increases visual acuity because it ensures photons are not reflected back onto photoreceptors and distort image

Question 14

give an explanation of the phototransduction cascade in photoreceptors

Answer 14

ligand gated ion channels with ligand binding site on the intracellular side of the ‘outer segment’ (the one furthest back) of PRs

The ligand is cGMP (broken down by a phosphodiesterase activated by the opsin GPCR, which is on in light due to retinal going from cis to trans)

dark = inactive opsin = high levels of cGMP so LGIC is open, allowing an influx of Na+ (and other cations), depolarising the cell in dark

When its light, levels of cGMP drop, these gated channels close (and K+ channels remain open) so there’s no influx of Na+ and the photoreceptors are hyperpolarised in light

Question 15

rods vs cones?

Answer 15

Fovea = cones only
Rods = rhodopsin
Cones = three different opsins, S, M and L for short wavelength etc…

RODS = many bipolar neurons synapsing with one retinal ganglion cell = convergence of multiple summations mean high sensitivity, but lower resolution/acuity

CONES = one bipolar neuron to one retinal ganglion cell = lower sensitivity as there’s only one neuron trying to trigger an action potential, but higher visual acuity

Question 16

explain the difference between ON (centre) and OFF (centre) bipolar cells

Answer 16

when dark, photoreceptors are depolarised, so release more glutamate, when it is light they become hyperpolarised, releasing less glutamate

Bipolar cells are named based on how they respond to light - which means how they respond in to less glutamate

ON bipolar cells become depolarised as a result of more light/less glutamate, so they must use metabotropic glutamate receptors (the GPCR) as this results in an inhibitory cascade
light = less glutamate = less inhibition = depolarisation

Off bipolar cells become hyperpolarised in light due to less glutamate, so they use the excitatory ionotropic glutamate receptor AMPA
light = less glutamate = less excitation = fewer positive ions in = hyperpolarisation

Question 17

knockout experiments of two proteins showed bipolar cells no longer responding to light, what proteins were they?

Answer 17

TRPM1 (ON bipolar cells)

Nyctalopin

Question 18

photoreceptors — bipolar cells —
then what? include a detail difference between ON and OFF bipolar cells

Answer 18

Bipolar cells release glutamate, synapsing with ganglion and amacrine cells in the inner plexiform layer

Within the IPL the OFF cells only go a little ways into the layer, while the ON cells go all the way into the IPL (closer to ganglion cell layer)

Question 19

explain the receptive field organisation of bipolar cells

Answer 19

A bipolar cell has a ‘receptive field centre’ where it synapses directly with several photoreceptors
and then a ‘receptive field surround’ where it receives info from the surrounding photoreceptors via a middle man - a horizontal cell

However
The horizontal cell is inhibitory (releases glycine, an inhibitory NT) and so, when the surround is light, PRs of the surround hyperpolarise, so this horizontal cell releases less inhibitory glycine onto the PRs in the ‘centre’ of the receptive field, so these PRs release more glutamate
(for ON cells) = more glutamate = more inhibition = less activation.

light on surround (for ON cells) = hyperpolarisation

basically, the surround being lit up or being dark, causes the surround PRs to release less or more glutamate, causing mess or more inhibition of the central PRs from the horizontal cell, so the surround has the opposite effect to the centre (whether the centre is ON or OFF)

Question 20

explain the receptive field of retinal ganglion cells

Answer 20

you’ve got ON vs OFF-centre RGCs

looking at ON-centre cells:
When there is no light at all, the on-centre ganglion cell will fire at a baseline rate…

When light is shined on the centre of the RF, firing rate is at max during the stimulus

If light is only on the surround, baseline firing is suppressed

Diffuse light - on the entire receptive field - firing rate is similar to baseline - there is no DIFFERENCE to detect

off-centre RGCs are the opposite way around

Question 21

why do we end up with ON-centre and OFF-centre retinal ganglion cells?

Answer 21

the RGCs inherit this organisation from the bipolar cells with their whole centre-surround receptive fields

an ON-centre RGC is simply receiving input from an ON-bipolar cell (and vice versa)

Question 22

what are the features of magnocellular ganglion cells? include the function

Answer 22

larger dendritic tree and therefore larger receptive field

Cannot respond to colour

Transient response - stops responding if the stimulus is prolonged

Faster conduction

High sensitivity

Function = motion detection (low resolution), so good that they have a transient response, and a fast response

Question 23

what are the features of parvocellular ganglion cells? include the function

Answer 23

smaller dendritic tree and therefore smaller receptive field (so higher resolution)

Most ganglion cells are parvocellular - 80%

Respond to difference in colour

Sustained response - until stimulus is removed

Much slower in conduction

Low sensitivity

Function = form and colour

Question 24

which kind of ganglion cell sees form and colour? which detects movement?

Answer 24

colour = parvocellular

motion = magnocellular

Question 25

which ganglion cell is slower in conduction and lower in sensitivity?

Answer 25

parvocellular