Session 4 The retina and central visual pathways

Question 1

What are the three layers of the eye?

Answer 1

Outermost SCLERA which is tough and is continuous with the dural sheath of the optic nerve

UVEA which is the pigmented vascular layer

• choroid sits deep to sclera
• ciliary body and iris sit anteriorly

Retina = the neural layer

Question 2

Describe the layers of the retina from superficial to deep and a bit about them

Answer 2

Retinal pigment epithelium = prevents light from ‘bouncing around’ the eyeball which would cause glare

Photoreceptors cells = convert light into signals

Bipolar cells = first order neurones receiving input from photoreceptors
Horizontal cells = connect the bipolar cells and assist with enhancing edges through ‘Lateral Inhibition’

Ganglion cell layer = receives input from bipolar cells
Axons of the ganglion cell’s form the NERVE fibre layer

Again the list:
Retinal pigment epithelium layer > photoreceptor cells > bipolar and horizontal cells > ganglion cell layer > nerve fibre layer (formed by axons of ganglion cells)

Question 3

How can you examine the retina and what could it detect?

Answer 3

Fundoscopy which can detect signs of diseases such as hypertensive retinopathy, diabetic neuropathy and macular degeneration

Question 4

Describe the normal appearance of the fundus

Answer 4

The macula = dark area with the fovea in the middle

Macula is lateral to the optic disk
(Can tell which eye it is because macula will be on the outside)
Left eye = macula on right and optic disk on left
Right eye = macula on left and optic disk on right

Question 5

What is the optic disk

Answer 5

Point of exit of ganglion cell axons

Question 6

What are visible on the macula?

Answer 6

Branches of the central retinal artery and vein

Question 7

What is and what causes amaurosis fugax?

Answer 7

Occlusion of the central retinal artery (branch of the opthalmic artery= ICA) causes sudden visual loss = amaurosis fugax

Question 8

What is the name of the specialist technique used to visualise the layers of the retina?

Answer 8

Optical coherence tomography (OCT)

Question 9

What does OCT stand for and what does it visualise?

Answer 9

Optical coherence tomography

Used to visualise the layers of the retina

Question 10

What is light from a lateral visual field detected by?

What is light from an upper visual field detected by?

Answer 10

Lateral visual field = detected by MEDIAL retina (referred to as nasal)

Upper visual field = detected by INFERIOR retina

Question 11

What is the medial retina referred to and what does it detect?

Answer 11

= nasal

Detects light from a lateral visual field and light from the temporal field

Question 12

What is the lateral retina referred to and what does it detect?

Answer 12

Referred to as ‘temporal’

Light from the temporal field is detected by the nasal retina

Question 13

What is the name of the nucleus that ganglion cell axons project into?

Answer 13

A part of the thalamus called the lateral geniculate nucleus

Via the optic tract

Question 14

What does the lateral geniculate nucleus protect to? Through what?

Answer 14

Projects to the visual cortex through the optic radiations

Question 15

A) Ganglion cells supplying the temporal retina project to …

B) Ganglion cells from the nasal retina project to …

What does this imply?

Answer 15

A) The ipsilateral cerebral hemisphere

B) The contralateral hemisphere via the optic chiasm (i.e. they decussate)

This implies that the left binocular visual field projects to the right hemisphere an the right binocular visual field projects to the left hemisphere

Question 16

What do the ganglion cells from the superior retina (i.e. the inferior field) project through? Which lobe?

What do the ganglion cells from the inferior retina (i.e. the superior field) project through? Which lobe?

Answer 16

Superior retina (inferior field) = superior optic radiation running through the parietal lobe

Inferior retina (superior field) = inferior optic radiation running through the temporal lobe

Question 17

What is a scotoma and what causes it?

Answer 17

A localised defect in the retina causes a small patch of visual field loss = Scotoma

Question 18

What can damage to the optic nerve lead to?

Answer 18

Monocular blindness

Question 19

what can damage to the medial chiasm cause?

Answer 19

Bitemporal hemianopia

Question 20

What can damage to the optic tract cause?

Answer 20

A contralateral homonymous hemianopia

Question 21

What can cause a contralateral homonymous hemianopia?

Answer 21

• damage to optic tract
• damage to lateral geniculate
• damage to both optic radiations
• non-vascular damage to the occipital lobe = CTT without macular sparing
• occlusion of the posterior cerebral artery = CTT with macular sparing

Question 22

Why does occlusion of the posterior cerebral artery cause contralateral homonymous hemianopia WITH macular sparing?

Answer 22

Due to the fact that the area of visual cortex that supplies the macular receives blood from the deep branch of the middle cerebral artery

Question 23

What causes contralateral homonymous inferior quadrantanopia?

Answer 23

Damage to the superior optic radiations (in the parietal lobe)

Question 24

What causes contralateral homonymous superior quadrantanopia?

Answer 24

Damage to the inferior optic radiations in the temporal lobe

Question 25

What does damage to the superior optic radiations in the parietal lobe cause?

Answer 25

Contralateral homonymous inferior quadrantanopia

Question 26

What does damage to the inferior optic radiations in the temporal lobe cause?

Answer 26

Contralateral homonymous superior quadrantanopia

Question 27

Describe the pupillary light reflex

Answer 27

Afferent arm: optic nerve (CN 2)
Processing centres: pretectal nucleus which projects bilaterally to Edinger Westphal nuclei (which contains parasympathetic preganglionics)
Efferent arm: Oculomotor nerve (CN 3)
Effect = illumination fo the eye leads to both direct and consensual pupillary constriction
The consensual reflex is mediated by the bilateral projections of the pretectal nucleus

Question 28

Describe the accommodation reflex

Answer 28

Afferent arm: optic nerve (CN 2)
Processing centres: visual cortex (via lateral geniculate nuclei) allowing processing of visual image which then project to oculomotor and Edinger Westphal nuclei
Efferent arm: oculomotor nerve (CN 3)
Effect: focusing on a near object leads to pupillary constriction, convergence of the eyes (contraction of medial recti) and thickening of the lens