CNS/Sensory V

Question 1

What is the sclera?

Answer 1

The white part of the eye

Question 2

What is the cornea?

Answer 2

It is what the sclera turns into at the center of the eye. It is clear.

Question 3

What is the iris?

Answer 3

It is the part of the eye that gives them their colour and that controls the dilation of the pupil.

Question 4

What is the vitreous humour?

Answer 4

It is the clear jelly-like substance that photons flow through after entering the eye.

Question 5

What is the retina?

Answer 5

It is where photos strike and where the neurons that line the eye are located.

Question 6

What is the fovea centralis?

Answer 6

The area with the highest visual acuity.

Question 7

What is the optic disk?

Answer 7

It is where the optic nerve leaves the retina. There are no photoreceptors and it is a blind spot.

Question 8

When light reaches the eye, it […]. Explain why.

Answer 8

It refracts. This is because it’s striking the boundary between air and the cornea, which have different properties.

Question 9

What is happening when your eye focuses?

Answer 9

Light is being bent to show up at a single point behind the lens, at the back of the eye.

Question 10

Which parts of the eye are responsible for refracting the incoming light? Which one is dominant?

Answer 10

The cornea and the lens. The cornea refracts light more than the lens does.

Question 11

What is accommodation in the eye? Which parts of the eye are responsible for it?

Answer 11

The lens of the eye can change its shape depending on the distance of the focal object from the eye, thus changing the amount of refraction. Notably, the cornea is fixed and cannot accommodate.

Question 12

Explain how accommodation in the eye works and what muscles are involved.

Answer 12

When an object moves out of your focal range, the lens will change shape to refract the light such that the object can get back into focus. This is controlled by the ciliary muscles, which can pull and release the lens.

Question 13

What does it mean to be myopic?

Answer 13

It means that the person is nearsighted.

Question 14

When a person is myopic, what is wrong with the eye?

Answer 14

The eyeball is too long, so the light entering the eye converges in front of the retina.

Question 15

What kind of lens is used to correct myopia?

Answer 15

A concave lens

Question 16

What does it mean to be hyperopic?

Answer 16

It means that the person is farsighted.

Question 17

When a person is hyperopic, what is wrong with the eye?

Answer 17

The eyeball is too short, so the light entering the eye converges behind the retina.

Question 18

What kind of lens is used to correct hyperopia?

Answer 18

A convex lens

Question 19

What is astigmatism?

Answer 19

When the lens or cornea are not spherical, causing refraction problems.

Question 20

What is presbyopia?

Answer 20

It is when the lens gets stiff over time and doesn’t accommodate for near vision as well.

Question 21

What is cataracts?

Answer 21

It is when the lens changes colour and becomes more opaque, making it harder for light to get in.

Question 22

Where does transduction occur?

Answer 22

It occurs at the photoreceptors at the back of the eye, closest to the retinal pigment epithelium.

Question 23

What are the major types of photoreceptors? What is their purpose?

Answer 23

Rods and cones. Rods are active in low light conditions, while cones are active in high light conditions and provide colour vision.

Question 24

Describe the order of major structures through which light passes in order to get to the retina.

Answer 24

Vitreous humour -> ganglion cells -> bipolar and amacrine cells -> horizontal cells -> rods and cones -> retinal pigment epithelium

Question 25

What is the role of ganglion cells?

Answer 25

They converge the many signals from photoreceptors and send through through the optic nerve.

Question 26

Explain the purpose of the fovea centralis.

Answer 26

When light passes through the eye towards the retinal pigment epithelium, it normally has to pass through all the circuitry, which can affect the signal. The fovea has its retinal circuitry shifted out of the way, preventing this problem and yielding high acuity.

Question 27

Describe the 4 steps in the processing of photons after they’ve hit the retina.

Answer 27

From retina to optic nerve:
1. Transduction
2. Change in neurotransmitter release
3. Processing and convergence
4. Becomes optic nerve

Question 28

Describe the structure of photoreceptors.

Answer 28

They consist of an outer segment, which captures light. It has many stacked membranes below it, which contain opsin molecules that catch photons. There is also cGMP in the outer segment when it’s dark. They also have an inner segment, which can send transmitters if a photon is caught.

Question 29

How many types of opsins do we have? Give one example and the type of photoreceptor is is found in.

Answer 29

4. For example, rhodopsin is found in rods (not cones).

Question 30

How do opsin molecules capture photons?

Answer 30

They contain chromophores called retinene, which contain a protein that allows the photons to be captured.

Question 31

Describe the state of photoreceptors in the dark.

Answer 31

When it’s dark, there is a lot of cGMP in the outer segment. There are also cGMP-gated sodium channels, which are open in the dark because of the abundance of cGMP. These open sodium channels depolarize the membrane and thus allow photoreceptors to continually fire off neurotransmitters.

Question 32

Describe what happens when a photoreceptor gets exposed to light conditions after being in the dark.

Answer 32

When light enters the eye, a photon that enters and hits an opsin molecule that has a chromophore. This triggers a G protein cascade, which converts cGMP to GMP. When that occurs, the cGMP starts to disappear in the outer segment, so the cGMP-gated channels start to close. When they close, it hyperpolarizes the membrane and neurotransmitter release is reduced.

Question 33

Light causes photoreceptors to […]

Answer 33

Hyperpolarize

Question 34

Compare rods and cones in terms of:
a) Sensitivity and day/night vision
b) Amount of opsin
c) Amplification
d) Response time
e) Type of light sensitivity

Answer 34

a) R: high sensitivity, night vision. C: low sensitivity, day vision
b) R: more rhodopsin, captures more light. C: less opsin
c) R: High amplification (single photon closes many ion channels). C: lower amplification
d) R: slow response time. C: faster response time.
e) R: more sensitivity to scattered light. C: most sensitive to direct axial rays.

Question 35

How do the rod and cone system compare in terms of acuity?

Answer 35

The rod system has lower acuity because it is not present in the central fovea. The cone system has high acuity because it is concentration in the fovea.

Question 36

How do the rod and cone systems compare in terms of opsins?

Answer 36

Rod system: achromatic with only one type of opsin
Cone system: chromatic with three types of opsin

Question 37

How does dark adaptation work?

Answer 37

In light, the rods are inactive and the cones are active. When you go in the dark, temporary blindness occurs until the rods re-activate and take over.

Question 38

How does light adaptation work?

Answer 38

In dark, the cones are inactive and the rods are active. When you go into bright light, the rods initially get saturated and you experience temporary blindness until the rods inactivate and the cones take over.

Question 39

Explain what happens chemically during phototransduction in rods when a photon hits an opsin molecule

Answer 39

When a photon hits an opsin molecule, it breaks the bond between the opsin and the chromophore. This prevents the opsin molecule from capturing any more photons until it’s put back together.

Question 40

Explain the cycle of opsin+chromophore in dark adaptation in rods.

Answer 40

In the dark, rods have rhodopsin (opsin+chromophore). When exposed to light, it hyperpolarizes the photoreceptors and causes the bond between the opsin and the chromophore to break. To reassemble, the chromophore must interact with the retinal pigment epithelium, at which point it can be put back together if dark conditions return. The time taken for the reassembly of the rhodopsin accounts for the dark adaptation.

Question 41

Explain the cycle of opsin+chromophore in light adaptation in rods.

Answer 41

When you go into bright light, the rods all have their opsin+chromophore put together (rhodopsin). They all respond to light and it takes time for these bonds to break and for the rods to deplete and cones to take over. This period accounts for the blinding experience of light.

Question 42

What cells in the retina have receptive fields?

Answer 42

Retinal ganglion cells.

Question 43

What are the two types of receptive fields on ganglion cells?

Answer 43

Some look for light around the outside (excitatory) and dark on the inside (inhibitory), and others look for light on the inside and dark around the outside.

Question 44

Retinal ganglion cells signal […] across their receptive fields.

Answer 44

The relative differences of light (contrast)

Question 45

What happens to retinal ganglion cells if you shine light with a bright center and a dark surround?

Answer 45

The ones with receptive fields that have an excitatory center and an inhibitory surround will fire off many signals, while the ones with inhibitory centers and excitatory surrounds will not fire off a signal.

Question 46

What happens to retinal ganglion cells if you expose the eye to a dark center with a bright surround?

Answer 46

The ones with receptive fields that have an excitatory center and an inhibitory surround will not send off a signal, while the ones with inhibitory centers and excitatory surrounds will fire off many signals.

Question 47

What determines the chromatic sensitivity of the photoreceptor?

Answer 47

The opsin molecule.

Question 48

Why is it harder for us to differentiate between colours at night?

Answer 48

Because in the day, we have 3 different cones that span a larger electromagnetic spectrum. At night, we only have 1 rod, so the spectrum of colours we can see is more limited.

Question 49

Retinal ganglion cells differentiate between colours using […]

Answer 49

Colour-opponent receptive fields

Question 50

What are the 2 types of colour-opponent receptive fields?

Answer 50

Red/green and blue/yellow

Question 51

What is the cause of colour blindness?

Answer 51

The lack of a correctly functioning cone, which limits the colour spectrum that you can see.

Question 52

Describe the flow of information from the visual field to the brain.

Answer 52

Both the right and left visual fields have a temporal and a nasal side. These are flipped on the retina, with the nasal side being flipped to the outside and vice versa with the temporal side. The information from each eye travels down the optic nerve to the optic chiasm. At the optic chiasm, the axons from the inner (originally emporal) neurons cross over. The axons from both right sides of the visual fields and from both left sides of the visual fields (contralateral visual fields) respectively continue along the optic tract until they reach the lateral geniculate nucleus (thalamus). They then split off into optic radiations that head to the visual cortex.

Question 53

Where is the visual cortex located?

Answer 53

In the occipital lobe of the brain.

Question 54

If someone has a lesion on their optic nerve on the left side, what is the consequence on their vision?

Answer 54

They will completely lose their vision in their ipsilateral (left) eye

Question 55

If someone has a lesion on their optic tract on the left side, what is the consequence on their vision?

Answer 55

They will lose vision in the contralateral (right) side of the visual field in both eyes.

Question 56

If someone has a lesion on the optic chiasm right at the center, what is the consequence on their vision?

Answer 56

Since this is where the temporal axons of both visual fields cross over, there will be bilateral loss of the temporal half of the visual field.

Question 57

If someone has a lesion on the visual cortex on the left side, what is the consequence on their vision?

Answer 57

Since this is where the information arrives for the contralateral side of both visual fields, there will be a contralateral loss of vision in both eyes.

Question 58

At the primary visual cortex, describe the receptive fields and the types of images recognized.

Answer 58

The receptive fields are small and it recognizes simple image features such as oriented line segments.

Question 59

Once the visual information reaches the primary visual cortex, what are the possible pathways it can travel along?

Answer 59

The parietal visual stream (where) or the temporal visual stream (what).

Question 60

Describe the parietal visual stream in terms of the receptive fields and the type of information recognized.

Answer 60

The receptive field are large and it recognizes spatial features and motion. it also eventually encodes visual information with other sensory modalities (polymodal).

Question 61

Describe the temporal visual stream in terms of the receptive fields and the type of information recognized.

Answer 61

The receptive fields are large and it recognizes complex image features, such as faces.

Question 62

Explain how the model of V1 orientation elective responses works.

Answer 62

The primary visual cortex can differentiate between the orientation of the light signal across a line of retinal ganglion cells. If the light is horizontal, it will send a weak signal because it’s only hitting the center of a few center-surround receptive fields. But a vertical signal spanning across many retinal ganglion cells will send a much stronger signal.

Question 63

What is the pupillary reflex and what is it used for?

Answer 63

Light is shone in one eye and both pupils should contract. This is often used to asses damage to the brain, as if this does not happen, it indicates that something might be pushing on one of the cranial nerves that mediate this reflex.