The Retina and Central Visual Pathways

Question 1

Why does the position of the optic nerve form a ‘blind spot’ of vision?

Answer 1

there are no photoreceptors located in the area of the optic nerve leaving the retina/eye

Question 2

What is the purpose of the pigmented layer of the reint, and in which population does this pose a potential problem?

Answer 2

prevent excess light reflections within the retina which would negatively affect vision

problem for albino population as they lack melanin, which allows excessive scattering of light on their reitna in bright conditions

Question 3

Which type of photoreceptor in the eye are responsible for black and white/low level lighting vision?

Answer 3

Rods

(this is the reason we can only detect monochrome colours at night, as the cones are not active)

Question 4

What is the function of the cone photoreceptors in the eye?

Answer 4

Colour Vision

Detailed Vision in Bright Light

Question 5

Which cells of the reitna converge to form the optic nerve?

Answer 5

Ganglion Cells

Question 6

What is the function of horizontal cells in the retina?

Answer 6

lateral inhibition

allows the photoreceptors detecting the brightest light to inhibit neighbouring cells to allow the contract/sharpness of the image to be enhanced

Question 7

What can be found in the region of the optic disc on the back of the retina?

Answer 7

Optic Nerve

Central Vessels of Optic Nerve
(central retinal artery & vein)

Question 8

What is the functional importance of the fovea of the retina?

Answer 8

highest density of photoreceptors

higest area of visual acuity

Question 9

Does the fovea contain a higher density of rod or cone shaped photoreceptors?

Answer 9

Cones

(better for high quality images)

Question 10

Other than a high density of photoreceptors, how else if the fovea adapted to perform it role of vision?

Answer 10

thinner retinal layer due to absence of retinal ganglion cell axons

Question 11

What is the underlying pathological process in a detached retina?

Answer 11

detachment of the photoreceptors from the underlying pigmented layer of the retina

Question 12

Identify the following structures seen on fundoscopy

Answer 12

Question 13

What symptom do patients commonly present with when suffering from Amaurosis Fugax and what is the underlying pathology?

Answer 13

curtain coming down over vision

transient monocular blindness believed to be due to microemboli or vasospasm

Question 14

Why can the optic disc appear to be oedematous with raised intracranial pressure?

Answer 14

Optic nerve is connected to the brain and is surrounded by the meningeal layers, which allows any pressure to transmit from the brain along the optic nerve

Question 15

Identify the structures indicated by the red arrows

Answer 15

Question 16

Which fibres of the optic nerve decussate at the optic chiasm?

Answer 16

Nasal Fibres

Question 17

What are the two branches of the optic radiations?

Answer 17

Superior Radiation

Inferior Radiation

Question 18

Outline which fibres of the optic nerve are responsible for the temporal and nasal field of vision

Answer 18

Nasal Fibres = Temporal Field of Vision

Temporal Fibres = Nasal Field of Vision

Question 19

How are visual field defects recorded in the medial notes?

Answer 19

named based on the area of visual loss

Question 20

What defect in the visual pathway would lead to a right sided monocular blindness?

Answer 20

lesion of the right optic nerve (1)

e.g. optic sheath meningiomas

Question 21

What defect in the visual pathway would lead to bitemporal hemianopia?

Answer 21

lesion at optic chiasm (2)

affects both NASAL fibres

e.g. pituitary/anterior communicating artery disease

Question 22

What defect in the visual pathway would lead to left sided homonomous hemianopia?

Answer 22

lesion of right optic tract (3)

affected right temporal and left nasal fibres

e.g. vascular stroke/neoplasia

Question 23

Answer 23

Question 24

Defect in which area of the visual pathway causes quadrantanopias?

Answer 24

Superior and Inferior Radiations

Question 25

Why do patients suffering a stroke in the occipital lobe (caused by occlusion of the posterior cerebral artery), sometimes present with macula sparing?

Answer 25

Occipital lobe has a dual blood supply from:

• posterior cerebral artery
• branch of middle cerebral artery

thus, the blood flow can be preserved to the occipital pole which provides central vision

Question 26

Outline the pathway of the pupillary light reflex

Answer 26

Retinal Sensory Afferent (via CN II) > pretectal area & lateral geniculate nucleus

pretectal synapses with Edinger-Westphal Nucleus (midbrain) > parasympathetic fibres on CN III leave Edinger-Westphal Nucleus > pass via cillary ganglion > synapse at sphincter pupillae

direct and consensual pupilly constriction

Question 27

What 3 process are involved in the accommodation reflex of the eye?

(think 3 C’s)

Answer 27

• *C**onvergence
• (medial rectus muscle)*

Pupillary Constriction
(constrictor pupillae)

• *C**onvexity of Lens
• (cillary muscle - lens thickening)*

Question 28

Outline the nervous pathway involved in the accommodation reflex

Answer 28

retina > optic nerve > lateral geniculus >

communicates with Edinger-Westphal nucleus

efferents travel via occulomotor nerve (parasmpathetics) > sphinctor pupillae and medial rectus muscle

Question 29

Why does the accommodation reflex require the additional involvement of the primary visual cortex and the pupillary light reflex does not?

Answer 29

accommodation refelx requires the eye to focus on a image that is moving towards the face, whereas the light reflex does not involve any image processing in the primary visual cortex