The Visual System

Question 0

What determines the degree of refraction?

Answer 0

The difference in refractive index of the two substances and the angulation

Question 1

What is refractive index?

Answer 1

The ratio of the speed of light in air to the speed of light in a substance

I.e. 300,000/speed of light in substance

Question 2

What does a convex lens do?

Answer 2

Bends light inwards (focuses light)

Question 3

What does a concave lens do?

Answer 3

Diverges light

Question 4

What is refractive power measured in?

Answer 4

Dioptres

Dioptre = 1/focal length

Question 5

What is the refractive power or the lens?

Answer 5

20-34 dioptres

Question 6

What is accommodation?

Answer 6

The process of increasing the refractive power of the lens to focus on objects as they move closer

Question 7

What happens in the eye during accommodation?

Answer 7

Ciliary muscles contract, suspensory ligaments lose tension, lens relaxes (becomes more spherical)

Question 8

How does pupil size affect depth of focus?

Answer 8

Small pupil = increased depth of focus

Question 9

What are cataracts?

Answer 9

Cloudy/opaque area of the lens due to a coagulation of lens proteins

Question 10

What is an astigmatism?

Answer 10

Visual image in one plane focuses at a different distance to other planes or focus due to imperfections in the shape of the cornea

Question 11

What is myopia and how is it corrected?

Answer 11

Axial length of the eye is too long, corrected with concave lenses

Question 12

What is hyperopia and how is it corrected?

Answer 12

Axial length of the eye is too short, corrected with convex lenses or accommodation

Question 13

What are the layers of the retina?

Answer 13

Pigmented, outer nuclear, outer plexiform, inner nuclear, inner plexiform, ganglion cell, stratum opticum, inner limiting membrane

Question 14

What is the retina responsible for?

Answer 14

Contrast, movement and colour

Question 15

What is the fovea?

Answer 15

Centre of the retina, contains only comes, area where neuronal cells and blood vessels are displaced so that light strikes the comes directly, allowing for the highest visual acuity and discrimination

Question 16

What cones make up the foveal cone mosaic?

Answer 16

S cones: short wavelength (blue)
M cones: medium wavelength (green)
L cones: long wavelength (red)

Question 17

Describe the structure of photoreceptors

Answer 17

Outer segment containing membrane shelves lined with rhodopsin, inner segment containing mitochondria

Question 18

What is the function of rhodopsin?

Answer 18

Capture light and convert it to a neural signal

Question 19

What is rhodopsin made up of?

Answer 19

Retinal (derivative of vitamin A) in 11-cis formation and scotopsin

Question 20

What happens when rods are exposed to the dark?

Answer 20

Rods are depolarised

• inward current flows into outer segment carried by Na+ through CGMP channels
• current flows out through efflux of K+
• glutamate released from synaptic end feet due to opening of calcium channels

Question 21

What happens when rods are exposed to light?

Answer 21

• Light activated rhodopsin stimulates G-protein transducin and decomposed
• Transducin activates phosphodiesterase which breaks down CGMP
• CGMP gated channels close
• Efflux of Na+ causes hyperpolarosation
• Cell stops releasing glutamate

Question 22

What happens during light adaptation?

Answer 22

Photochemicals in rods and cones convert to retinal and opsin, remaining retinal converts to vitamin a

Results in decreased light sensitivity

Question 23

What happens during dark adaptation?

Answer 23

Retinal and opsin covert to light sensitive pigments, vitamin a converts to retinal

Results in increased light sensitivity

Question 24

What name is used to describe colour blindness involving lack of:
A) red cones
B) green cones
C) blue cones

Answer 24

A) protanope
B) deuteranope
C) tritanope

Question 25

What is the function of horizontal cells?

Answer 25

Connect laterally and provide lateral inhibitory feedback to allow for spatial resolution

Question 26

What is the layout of bipolar cells?

Answer 26

Connect vertically from photoreceptors and horizontal cells to ganglion cells, receiving info from rods or cones but never both

Question 27

What is the function of amacrine cells?

Answer 27

Aid the beginning of visual signal analysis

Question 28

What are the three types of ganglion cells and what are the responsible for?

Answer 28

1) Midget cells: Fine spatial discrimination, colour perception
2) Paracrine cells: Movement detection, spatial discrimination
3) Bistratified cells: Differences between S, M & L cones (perception of yellows and blues)

Question 29

How do ganglion cells communicate?

Answer 29

Action potentials

Question 30

How does the retina communicate?

Answer 30

Electronic conduction

Question 31

What is lateral inhibition?

Answer 31

The output of horizontal cells is always inhibitory, preventing lateral spread of light excitation (essential for transmitting contrast borders)

Question 32

What visual information remains on the same side and what visual information decussates?

Answer 32

Same side: Info from lateral/temporal side of the eye

Contralateral sure: Info from medial/nasal side of the eye

Question 33

What is the LGN and where is is located?

Answer 33

Primary relay centre for visual info, located in the thalamus

Question 34

Describe the two zones of the LGN

Answer 34

1) Magnocellular
- Layers 1-2
- Input from parasol ganglion cells
- Rapidly conducting
- Colour blind info only, general spatial info

2) Parvocellular
- Layers 3-6
- Input from midget ganglion cells
- Moderate conductance
- Conveys colour and accurate point-to-point spatial info

Question 35

Describe the sublayers of the LGN

Answer 35

Koniocellular sublayers

• Sublayers 1-6
• Receive input from Bistratified ganglion cells (layers 3&4)
• Moderate conductance
• Convey colour (yellows and blues)

Question 36

Describe the layers of the primary visual cortex

Answer 36

Layers 1-3: Transmit signals short distances laterally
Layer 4: Entry point for sensory info
Layers 5-6: Transmit signals greater distances