S4) The Retina and Central Visual Pathways

Question 1

Identify the structures observed in the retina in a cross-section of the eyeball

Answer 1

Question 2

What are rod cells and what do they do?

Answer 2

Rod cells are photoreceptor cells in the retina which function in low intensity light and are responsible for black and white vision

Question 3

What are cone cells and what do they do?

Answer 3

Cone cells are photoreceptor cells in the retina which function in high intensity light and are responsible for high acuity vision (detects colours)

Question 4

What are bipolar cells and what do they do?

Answer 4

Bipolar cells are cells which exist between photoreceptors in the retina and act indirectly/directly to transmit signals from the photoreceptors to the ganglion cells

Question 5

What are horizontal cells and what do they do?

Answer 5

- Horizontal cells are the laterally interconnecting neurons which help integrate and regulate the input from multiple photoreceptor cells

• They also allow the eyes to adjust to see well in both bright & dim light conditions

Question 6

What are amacrine cells and what do they do?

Answer 6

Amacrine cells are inhibitory neurons and project their dendrites to the inner plexiform layer to interact with retinal ganglion cells and/or bipolar cells

Question 7

What are ganglion cells and what do they do?

Answer 7

A retinal ganglion cell is a type of neuron in the retina which receives visual information from photoreceptors via bipolar cells and amacrine cells

Question 8

Identify the seven structures observed in a fundoscopy of the eye

Answer 8

Question 9

Illustrate the layout of the visual pathway

Answer 9

• Temporal fibres run ipsilateral
• Nasal fibres decussate at the optic chiasm
• Optic tract runs to the lateral geniculate nucleus
• Optic radiations split into superior and inferior and runs to the primary visual cortex

Question 10

Briefly, describe the layout of the visual fields

Answer 10

• Nasal fibres are responsible for our temporal field of vision
• Temporal fibres are responsible for our nasal field of vision

Lesions at any point in the pathway will correspond to a pattern of visual loss

Question 11

How are visual field defects named?

Answer 11

These are named based on the area of visual loss rather than the site of the lesion

Question 12

Describe the pattern of visual field loss at the following locations:

• Before the optic chiasm
• At/after the optic chiasm
• After the optic chiasm

Answer 12

• Before the optic chiasm: signs are unilateral and ipsilateral
• At/after the optic chiasm: signs are bilateral
• After the optic chiasm: signs will be bilateral and contralateral

Question 13

What is the general cause of monocular blindness?

Answer 13

Monocular blindness is caused by a lesion of the optic nerve (right)

Question 14

Identify some of the underlying pathological causes of monocular blindness in children and adults

Answer 14

• Optic nerve glioma or retinoblastoma (children)
• Optic sheath meningiomas (middle aged)

Question 15

What is the general cause of bitemporal hemianopia?

Answer 15

- Bitemporal hemianopia is caused by a lesion at the optic chiasm

• It affects both nasal fibres, and thus, both temporal fields lost

Question 16

What is the general cause of left homonymous hemianopia?

Answer 16

• Left homonymous hemianopia is caused by a lesion of the right optic tract
• It affects the right temporal and left nasal fibres

Question 17

What is the general cause of right homonymous hemianopia?

Answer 17

• Right homonymous hemianopia is caused by a lesion of the left optic tract
• It affects the left temporal and right nasal fibres

Question 18

Identify some of the underlying pathological causes of right/left homonomous hemianopia

Answer 18

• Vascular causes (stroke – common)
• Neoplasia
• Trauma

Question 19

What is a quadrantanopia?

Answer 19

A quadrantanopia is an anopia affecting a quarter of the field of vision, associated with a lesion of an optic radiation

Question 20

Localise a quadrantopia observed in the following visual fields:

• Left field
• Right field
• Superior field
• Inferior field

Answer 20

• Left field → Right hemisphere
• Right field → Left hemisphere
• Superior field → Inferior radiations
• Inferior field → Superior radiations

Question 21

The occipital lobe has dual blood supply.

Identify these arteries

Answer 21

• Posterior cerebral artery
• Middle cerebral artery (occipital pole)

Question 22

In four steps, explain the concept of macular sparing after a stroke

Answer 22

⇒ Stroke can affect the posterior cerebral artery

⇒ Most of occipital lobe will be lost

⇒ Middle cerebral artery supplies the occipital pole (represents the macula)

⇒ Macular function (central vision) will be spared

Question 23

In four steps, describe the nervous pathways involved in the light reflex

Answer 23

⇒ Light stimulates the afferent nerve in the pathway (CN II)

⇒ Afferent nerve synapses in pretectal area

⇒ Gives rise to neurones supplying Edinger Westphal nuclei bilaterally

⇒ Both CN III are stimulated to cause direct and consensual pupillary constriction (parasympathetic fibres)

Question 24

Describe the three aspects of the accommodation reflex as well as the structures involved

Answer 24

• Convergence (medial rectus)
• Pupillary constriction (constrictor pupillae)
• Convexity of the lens to increase refractive power (ciliary muscle)

Question 25

Briefly, explain how the cerebral cortex is involved in the accommodation reflex (image analysis)

Answer 25

The reflex follows the visual pathway via the lateral geniculate nucleus to the visual cortex