Retina and Central Visual Pathway

Question 1

What is the function of the pigmented layer of the retina?

Answer 1

• Main site of light absoption- avoids excessive light rays causing photophobia
• Anchors photoreceptors

Question 2

What is the function of horizontal cells in the retina?

Answer 2

Regulate input from multiple photoreceptors

Lateral inhibition stops signals either side of the photoreceptor that is detecting light

Question 3

Where is central vision the sharpest and why?

Answer 3

At the macula as it has the highest density of cones (photoreceptors)

Question 4

What is amaurosis fugax?

Answer 4

A curtain over vision

Can be a symptom of a stoke due to occlusion of the central retinal artery

Question 5

What is macula degeneration?

Answer 5

Degeneration of the macula with age , affects the middle part of vision

Question 6

Identify the layers of the retina on this OCT (Optical Coherence Tomography)

Answer 6

Question 7

Describe the distribution of the temporal and nasal fibres of the optic nerve

Answer 7

Nasal fibres are medial

Temporal fibres are lateral

Question 8

Describe the route of the nasal and temporal fibres of the optic nerve as the reach the optic chiasm

Answer 8

Nasal fibres decussate ending up on the contralateral side

Temporal fibres remain ipsilateral

Question 9

What are the optic fibres known as once they exit the optic chiasm? Where do they run to?

Answer 9

Optic Tracts

Run from the optic chiasm to the lateral geniculate nucleus. In put from both eyes is carries in each tract

Question 10

Describe the route of the optic radiations as the exit from the lateral geniculate nucleus

Answer 10

• The superior radiation (from superior quadrant of optic nerve) travel to the parietal lobe forming ‘Baum’s loop’
• The inferior radiation (from the inferior quadrants of the optic nerve) travel to the temporal lobe forming ‘Meyer’s loop’
• Both radiations evenually end up at the Primary Visual Cortex

Question 11

What is the relationship between the visual fields and the optic nerve?

Answer 11

Nasal fibres responsible for temporal field of vision

Temporal fibres responsible for nasal field of vision

Question 12

What would be the effect of a lesion to the (left) optic nerve (CNII)?

Answer 12

(Left) Monocular Blindness

Question 13

What would be the effect of a lesion at the optic chiasm?

Answer 13

Optic Chiasm defects affect the nasal fibres as they decussate therefore, the temporal visual fields will be lost from both sides

Bilateral Hemianopia (tunnel vision)

Question 14

What would be the effect of a lesion of the (right) optic tract?

Answer 14

• Loss of the temporal fibres from the RHS therefore, loss of nasal visual field on the right
• Loss of nasal fibres from the LHS, therefore loss of temporal visual field on the left

Homonomous Hemianopia

Question 15

What is the relationship between optic radiations and fields of vision?

Answer 15

Superior radiations are responsible for the inferior quadrant field of vision

Inferior radiations are responsible for the superior quadrant field of vision

Question 16

What would be the effect of a lesion of the (right) superior optic radiation?

Answer 16

• Superior temporal fibre of ipsilateral side affected- affects the inferior nasal visual field on the right
• Superior nasal fibre from the contralateral side affected- affects the inferior temporal visual field of the left

Homonomous Inferior Quadrantanopia

Question 17

What would be the effect of a lesion to the right inferior optic radiation?

Answer 17

• Inferior temporal fibre affected on the ispilateral side- affects superior nasal visual field of the right
• Inferior nasal fibre of the contralateral side affected- affects superior temporal visual field of the left

Homonomous Superior Quadrantanopia

Question 18

What would happen if both the superior and inferior radiations were affected e.g. in stroke of middle cerebral artery?

Answer 18

• Superior and inferior temporal fibres are affected ipsilaterally- therefore affects nasal visual field ipsilaterally
• Superior and inferior nasal fibres are affected contralaterally - therefore affects the temporal visual field contralaterally

Homonomous hemianopia

Question 19

What is macula sparing?

Answer 19

Visual field loss that spares the centre of the visual field

Occurs as there is a dual blood supply to the macula; if the middle cerebral artery is occluded, the posterior cerebral artery can supply the macula

Question 20

What are the 3 aspects of the accommodation reflex?

Answer 20

• Convergence of eyes (medial rectus pulls eyes medially)
• Pupillary Constriction (constrictor pupillae focuses light)
• Convexity of the lens to increase refractive power (lens fatttens, done by ciliary muscle)

Question 21

Which centres in the midbrain respond to changes of the accommodation reflex?

Answer 21

Edinger Westphal nucleus causes pupil constriction and lens thickening

CN III (occulomotor) causes convergence by medial rectus