Vision Physiology

Question 1

Which part of the eye does the visual defect “glaucoma” affect?

Answer 1

Aqueous humor anterior chamber
- increased fluid which increases pressure inside the eye

Question 2

Which part of the eye is known as the blind spot?

Answer 2

Optic disk

Question 3

Which part of the eye does the visual defect “cataracts” affect?

Answer 3

It affects the lens
- cataracts is the breakdown of crystallin proteins in the lens, cannot be corrected with glasses

Question 4

List the visual defects related to the shape of the cornea/eyeball?

Answer 4

• myopia (near sightedness)
• hyperopia (far sightedness)
• presbyopia (age related loss of near vision)

Question 5

Describe what the changes to the lens are during visual accommodation?

Answer 5

Unaccommodated:
- lens is thinner and flatter thus there is less refraction of light, this is good for far vision

Accommodated:
- les is rounder this is more refraction of light, good for new vision

Question 6

What are the 5 types of neural cells found in the retina?

Answer 6

Ganglion cells, amacrine cells, bipolar cells, horizontal cells, photoreceptors cells

Question 7

What are the two kinds of photoreceptors?

Answer 7

Rods and cons
- rods = extremely sensitive to light, allows us to see in very dim light conditions
- cons = less sensitive to light, allows us to see in colour, allows for high visual acuity

Question 8

What is retinitis pigmentosa?

Answer 8

When the photoreceptors gradually begin to die (genetic cause)

Question 9

What are the visual defects related to the macula?

Answer 9

Age related macular degeneration:
- wet = abnormal blood vessels grow behind macula, these leak and lead to scarring of macula
- dry = macula thins over time

Macula is the centre of the back of the eyeball (yellow spot) - corresponds with the central area in our visual field + point of maximum visual acuity

Question 10

Describe the characteristics of the retinal pigment epithelium and list its functions.

Answer 10

It is a single layer of epithelial cells which are pigmented (melanin) and ciliated.

• they are involved in the phagocytosis of photoreceptor membrane discs
• convert trans retinol to 11-cis retinal
• transports waste and nutrients
• releases growth factors

Question 11

What are main types of glial cells contain the retina?

Answer 11

1. Microglia
   - phagocytic cell (they envelop dead neurons and other debris)
   - release growth factors
   -involved in synaptic pruning
   - innate immune cells
2. Astrocytes
   - help form the retina blood barrier
   - help maintain homeostasis
   - involved in synaptic pruning
   - act as biochemical scaffolding
3. Muller cells
   - helps form the retina blood barrier
   - acts as biochemical scaffolding
   - involved in neurotransmitter and K+ homeostasis
   - provides neurons with glucose
   - may serve as neuronal progenitor cells
   - acts as living optical; fibres

Question 12

Explain the currents and membrane potentials of photoreceptors cells in the dark and in the light.

Answer 12

In the perspective of rod cells - allows us to perceive light in the dark

• there is a continual release of glutamate by the photoreceptors
• photoreceptors are depolarised in the dark
• this is due to the facts that the dark allows for many cGMP cation channels to remain open, allowing for influx of Na+ and efflux of K+
• in light, the cGMP gated cation channels close, allowing for reduced Na+ influx and K+ efflux
• photoreceptors are hyperpolarised in the light and
• less glutamate is then released

Question 13

Describe the response of bipolar and ganglion cells to the glutamate release from cone cells in the presence of no light

Answer 13

-photoreceptors is depolarised and therefore releases glutamate
- ON centre bipolar cells have mGluR6 receptors while OFF centre bipolar cells have AMPA/kainate receptors
- ON centre receptor cells will receive less glutamate, resulting in it hyperpolarising and inhibiting the release of glutamate to the ganglion cell (no excitation, less AP generated)
- OFF centre receptors cells will receive more glutamate resulting in it depolarising and excitation, therefore releasing excitatory neurotransmitters on the ganglion cells (more APs generated)

Question 14

What is the role of horizontal cells?

Answer 14

They regulate the amount of neurotransmitter released by photoreceptor

Question 15

Outline the central pathway for vision.

Answer 15

• optic nerve is formed by the convergence of axons from the retinal ganglion cells which receive impulses from photoreceptors of the eye
• after nerve formation, the nerve leaves the bony orbit via the optic canal (sphenoid bone) and enters cranial cavity running along the surface of middle cranial fossa
• within the fossa, optic nerves from each eye unite to form the optic chiasma
• here, the fibres from nasal (medial) half of each eye cross over to the contralateral optic tract while fibbers from temporal (lateral) halves remain ipsilateral
• each optic tract travels to its corresponding cerebral hemisphere to reach the lateral geniculate nucleus in the thalamus where fibres synapse
• axons from LGN then carry visual info via a pathway called the optic radiation which can be divided into 2
• upper optic radiation = carried fibres form superior retinal quadrants, travels through parietal lobe to reach visual cortex
• lower optic radiation = carries fibres from inferior retinal quadrants, tells though temporal lobe to reach visual cortex