The visual system

Question 1

What are the layers of the eye?

Answer 1

• Outermost sclera
• Uvea
• Retina

Question 2

What are the components of the uvea?

Answer 2

• Choroid sitting just deep to sclera
• Ciliary body and iris sit anteriorly

Question 3

What are the components of the retina?

Answer 3

• Retinal pigment epithelium
• Photoreceptor cells
• Bipolar cells
• Ganglion cell layer
• Nerve fibre layer

Question 4

What is the function of the retinal pigment epithelium?

Answer 4

• Prevents light from ‘bouncing around’ in the eyeball and causing glare

Question 5

What are bipolar cells?

Answer 5

• First order neurones receiving input from photoreceptors
• Bipolar cells are connected by horizontal cells
• Assist with enhancing edges through a process called lateral inhibition

Question 6

What is the ganglion cell layer?

Answer 6

• Receives input from bipolar cells
• Axons of ganglion form the nerve fibre layer

Question 7

What can examination of the retina by fundoscopy detect?

Answer 7

• Hypertensive retinopathy
• Diabetic retinopathy
• Macular degeneration

Question 8

What is the normal appearance of the fundus?

Answer 8

• Macula sits lateral to optic disc
• Branches of central retinal artery and vein are visible on macula

Question 9

What is the macula?

Answer 9

• Point of highest acuity

Question 10

What is the optic disc?

Answer 10

• Point of exit of ganglion cell axons

Question 11

What does occlusion of the central retinal artery cause?

Answer 11

• Sudden visual loss known as amaurosis fugax

Question 12

What is optical coherence tomography?

Answer 12

• A specialist technique that can be used to visualise the layers of the retina

Question 13

The eye is a pinhole camera. What is implied by this statement?

Answer 13

• Light from a lateral visual field is detected by the medial retina
• Light from an upper visual field is detected by the inferior retina

Question 14

What is the medial retina referred to as?

Answer 14

• Nasal
• Light from temporal field is detected by medial retina

Question 15

What is the lateral retina referred to as?

Answer 15

• Temporal
• Light from nasal field is detected by lateral retina

Question 16

Where do ganglion cell axons from the optic disc project to?

Answer 16

• Lateral geniculate nucleus via the optic tract

Question 17

Where does the lateral geniculate nucleus project to?

Answer 17

• Visual cortex via optic radiations

Question 18

Where do ganglion cells supplying the temporal retina project to?

Answer 18

• Ipsilateral cerebral hemisphere

Question 19

Where do ganglion cells supplying the nasal retina project to?

Answer 19

• Contralateral cerebral hemisphere
• Via optic chiasm
• I.e. these fibres decussate
• Left binocular visual field projects to the right hemisphere and vice versa

Question 20

Where do ganglion cells from the superior retina project to?

Answer 20

• Ganglion cells from superior retina = ganglion cells from inferior field
• Project through superior optic radiation
• Runs through parietal lobe

Question 21

Where do ganglion cells from the inferior retina project to?

Answer 21

• Ganglion cells from inferior retina = ganglion cells from superior field
• Project through inferior optic radiation
• Run through temporal lobe

Question 22

What does a localised defect in the retina cause?

Answer 22

• A small patch of visual loss called scotoma

Question 23

What does damage to the optic nerve lead to?

Answer 23

• Monocular blindness

Question 24

What does damage to the medial chiasm lead to?

Answer 24

• Bitemporal hemianopia

Question 25

What does damage to the optic tract cause?

Answer 25

• Contralateral homonymous hemianopia

Question 26

What does damage to the lateral geniculate nucleus cause?

Answer 26

• Contralateral homonymous hemianopia

Question 27

What does damage to both optic radiations cause?

Answer 27

• Contralateral homonymous hemianopia

Question 28

What does non-vascular damage to the occipital lobe cause?

Answer 28

• Contralateral homonymous hemianopia without macular sparing

Question 29

What does occlusion of the posterior cerebral artery cause?

Answer 29

• Contralateral homonymous hemianopia with macular sparing

Question 30

What conditions cause contralateral homonymous hemianopia?

Answer 30

• Damage to optic tract
• Damage to lateral geniculate nucleus
• Damage to both optic radiations
• Non vascular damage to occipital lobe (without macular sparing)
• Occlusion of posterior cerebral artery (with macular sparing)

Question 31

Why does occlusion of the PCA cause contralateral homonymous hemianopia with macular sparing?

Answer 31

• Area of the visual cortex that supplies the macula receives blood from the deep branch of the middle cerebral artery

Question 32

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

Answer 32

• Contralateral homonymous inferior quadrantanopia

Question 33

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

Answer 33

• Contralateral homonymous superior quadrantanopia

Question 34

Outline the pupillary light reflex

Answer 34

• Afferent arm is optic nerve
• Processing centres are pretectal nucleus which projects bilaterally to Edinger Westphal nuclei
• Efferent arm is oculomotor nerve

Question 35

What are the effects of the pupillary light reflex?

Answer 35

• Illumination of the eye leads to both direct and consensual pupillary restriction
• The consensual reflex is mediated by the bilateral projections from the pretectal nucleus

Question 36

Outline the accommodation reflex

Answer 36

• Afferent arm is optic nerve
• Processing centres = visual cortex (via LGN)
• Allows processing of visual image which then project to oculomotor and Edinger Westphal nuclei
• Efferent arm: oculomotor nerve

Question 37

What are the effects of the accommodation reflex?

Answer 37

• Focusing on a near object leads to pupillary constriction, convergence of the eyes (contraction of medial recti) and thickening of the lens