5. Retina And Central Visual Pathways

Question 1

What is the function of the choroid?

Answer 1

Is the vascular supply to the retina.

Question 2

What two layers form the retina?

Answer 2

Pigmented layer (retinal pigment epithelium) and neural layer.

Question 3

What are the two functions of the retinal pigment epithelium?

Answer 3

Prevents excessive refraction of light rays as contains melanin which absorbs some light.
Anchors photoreceptor cells (rods and cones) which are responsible for picking up different types of vision and relaying it.

Question 4

What two types of vision are rods responsible for?

Answer 4

Black and which vision and low level lighting.

Question 5

What two types of vision are cones responsible form?

Answer 5

Colour vision and high definition vision eg central vision.

Question 6

How do signals from photoreceptor cells reach the optic nerve?

Answer 6

Photoreceptor cells synapse with bipolar cells, which synapse with axons of ganglia to the optic nerve.

Question 7

What is the function of horizontal cells?

Answer 7

Responsible for lateral inhibition, so prevent receptors next to the point of highest light intensity from sending impulses, so the light isn’t seen as more than one point.

Question 8

What causes the blind spot?

Answer 8

No photoreceptor cells at the optic disc.

Question 9

What type of vision is the macula lutea responsible for and why?

Answer 9

Central vision, has a thinner layer of retina, ganglion cells and axons of ganglion cells to the optic nerve, so distance light has to pass through is less, which gives more specific defined vision.

Question 10

What vision is the fovea centralis responsible for?

Answer 10

Highest concentration of cone cells, so is the area of the eye with the highest ability to see detailed vision.

Question 11

What changes can be seen on a fundoscopy with raised intracranial pressure?

Answer 11

Optic disc swelling and enlarged blind spot.

Question 12

What clinical sign can occlusion of the central retinal artery lead to?

Answer 12

Amaurosis fugax - black curtain descending over vision.

Question 13

What field of vision are the nasal fibres and temporal fibres responsible for?

Answer 13

Nasal fibres - temporal field of vision.

Temporal fibres - nasal field of vision.

Question 14

What visual loss if caused by a lesion occurring before the optic chiasm?

Answer 14

Signs are unilateral and ipsilateral

Question 15

What visual loss if caused by a lesion occurring at the optic chiasm?

Answer 15

Signs are bilateral.

Question 16

What visual loss if caused by a lesion occurring after the optic chiasm?

Answer 16

Signs are bilateral and contralateral.

Question 17

What is monocular blindness, and where is the lesion causing it?

Answer 17

Complete unilateral blindness.

Caused by a lesion of the optic nerve.

Question 18

What is bitemporal hemianopia, and where is the lesion causing it?

Answer 18

Tunnel vision - bilateral temporal blindness.

Caused by a lesion at optic chiasm affecting both nasal fibres.

Question 19

What is left homonomous hemianopia, and where is the lesion causing it?

Answer 19

Bilateral left sided blindness.

Caused by a lesion of the right optic tract affecting the right temporal and left nasal fibres.

Question 20

What is right homonomous hemianopia, and where is the lesion causing it?

Answer 20

Bilateral right sided blindness.

Caused by a lesion of the left optic tract affecting the left temporal and right nasal fibres.

Question 21

What happens to the blood supply in a macula sparing stroke?

Answer 21

Is a stoke affecting the posterior cerebral artery. Blood supply to most of the occipital lobe is lost, but the middle cerebral artery supplies the occipital pole, which represents the macula. Therefore this is spared, and so is macula function (central vision).

Question 22

Describe the light reflex.

Answer 22

Light enters the eye, stimulated the afferent nerve in the pathway, CNII. This synapses in the pretectal area, which gives rise to neurones supplying the Edinger Westphal nuclei bilaterally. Then both the CNIII are stimulated to cause direct and consensual pupillary constriction, by the parasympathetic fibres.

Question 23

What are the 3 aspects of the accommodation reflex and what causes each?

Answer 23

Convergence - medial rectus.
Pupillary constriction - constrictor pupillae.
Convexity of the lens to increase refractive power - ciliary muscle.

Question 24

What area of the brain must be involved in the accommodation reflex because it is relating to image analysis?

Answer 24

Cerebral cortex.

Question 25

Describe the pathway of the accommodation reflex.

Answer 25

Reflex follows the visual pathway via the lateral geniculate nucleus for the visual cortex, with the final aspect of the pathway being common with the light reflex via EWN.

Question 26

What is internuclear ophthalmoplegia and what causes it?

Answer 26

Disorder of conjugate lateral gaze.

Due to a lesion to the medial longitudinal fasciculations.