Visual System

Question 1

Role of superior colliculus

Answer 1

1. Receives 10% of visual input
2. Integration with other sensory inputs
3. Controls saccadic eye movement
4. Descending motor output controlling neck muscles

Question 2

Role of pretectal nucleus in the midbrain

Answer 2

Bilateral input to Edinger-Westphal nucleus providing parasympathetic input to pupil- control of light level

Question 3

Where does the optic nerve synapse

Answer 3

1. The Lateral geniculate nucleus
2. Information from ipsilateral eye synapse in layers 2,3&5
3. Contralateral layers 1,4&6
4. Rods terminate in 1&2
5. Cones in 2,3&6

Question 4

Which portion of the retinas decussate

Answer 4

Nasal portion (at optic chiasma)

Question 5

Role of pupillary light reflex

Answer 5

Change size of pupil in response to changing light levels

Question 6

What muscles control the size of the pupil

Answer 6

Expand - dilator pupillae
(Sympathetic)

Contract- sphincter pupillae
(Parasympathetic)

Question 7

Anatomical pathway of pupillary light reflex

Answer 7

1. Photoreceptors
2. Optic nerve
3. Skips out LGN but synapses in the pretectal area in the midbrain
4. This info splits to both sides of the brain and synapses in the Edinger-Westphal nucleus
5. Info travels back via the occulo-motor nerve to the ciliary ganglion
6. Eye muscles

Question 8

What is the function of the accommodation reflex

Answer 8

To focus light onto the Fovea - making the image in focus

Question 9

What are the three processes that occur?

Answer 9

1. Ciliary muscles contract reflexively and lens becomes convex (+ refractive power)
2. Convergence of two eyes
3. Pupillary constriction improves focus by reducing aberration and increasing depth of focus

Question 10

Pathway for accommodation reflex

Answer 10

1. Visual cortex sends signal to pretectal area that image if out of focus
2. From pretectal to EW nucleus
3. From EW via O-M nerve to ciliary ganglion
4. Then to muscles in eye

Question 11

How do photoreceptors transducer light stimulus

Answer 11

Absorption of light protons by photopigments or photopsin (photo proteins)

Question 12

What substance do rods use to transducer light

Answer 12

1. Rhodopsin

2. 2- component protein that triggers signalling pathway

Question 13

What substance do cones use to transducer light

Answer 13

1. Several different photopigments with different absorption maxima
2. Therefore sensitive to different wavelengths of light (colour vision)

Question 14

Describe the structure of a photoreceptor

Answer 14

Outer segment

• Na+ channels gated by cGMP
• cGMP binds to cytoplasmic side of channel which allows Na+ influx

Inner segment

• non-gated K+ channel
• steady intracellular concentrations of Na+ and K+ maintained by Na/K pumps

In the dark potential sits at -30mV

Question 15

Describe a photoreceptor in the dark

Answer 15

1. cGMP gated Na+ channel is open
2. Na+ enters cell
3. Cell depolarised
4. Glutamate release
5. Hyperpoalrises ‘on’ bipolar cell
6. Depolarises ‘off’ bipolar cell

Question 16

Define a receptive field

Answer 16

Area of the retina that will evoke a response from a bipolar cell

Question 17

Describe the input to bipolar cells

Answer 17

1. Direct input from photoreceptors at centre of receptive field
2. Indirect input from surrounding photoreceptors via horizontal cell
3. Light hitting centre = opposite of light hitting surround