4. Visual System

Question 1

what are the three layers of the eye?

Answer 1

outermost - sclera
middle - anterior uvea (pigmented vascular layer) - ciliary body and iris and posterior uvea - choroid layer
inner - retina

Question 2

what other layers are within the retinal layer?

Answer 2

superficial -> deep

• retinal pigment epithelium (prevents light from bouncing around in the eyeball)
• photoreceptor cells
• bipolar cells (first order neurones receiving input from photoreceptors. These bipolar cells are connected by horizontal cells which do ‘lateral inhibition’ to enhance edges)
• ganglion cell layer (receives input from bipolar cells) - and these axons form the nerve fibre layer below
• light has to travel from here to reach photoreceptor cells of retina

Question 3

how is the eye examined?

Answer 3

fundoscopy

Question 4

what is the normal appearance of the fundus?

Answer 4

the macula sits lateral to the optic disc

should see branches of central retinal artery and vein (vein is bigger)

Question 5

what is the occlusion of the central retinal artery called?

Answer 5

sudden visual loss - amaurosis fugax

Question 6

how can the layers of the retina be visualised?

Answer 6

optic coherence tomography (see pic)

Question 7

how is light visualised by the retina? (concepts)

Answer 7

• light from a lateral visual field is detected by the medial retina
  medial retina = nasal
  therefore, light from the temporal field is visualised by the nasal retina
• light from a medial visual field is detected by the lateral retina
  lateral retina = temporal
  therefore,light from the nasal field is visualised by the temporal retina

Question 8

how is light visualised by the retina? (full pathway - describing the diagram)

Answer 8

1. ganglion cell axons project to the lateral geniculate nucleus (part of thalamus) via the optic tract
2. which projects to the visual cortex through optic radiations
3. the ganglion cells supplying the temporal retina -> ipsilateral cerebral hemispheres vs the ganglion cells supplying the nasal retina -> decussate to the contralateral hemisphere via optic chiasm
4. ganglion cells from the superior retina (ie inferior field) project through superior optic radiation running through the parietal lobe and those from inferior retina (superior field) project through inferior optic radiations running through temporal lobe

Question 9

what is a localised defect in the retina called?

Answer 9

small patch of visual loss - scotoma

Question 10

what is damage to the optic nerve called?

Answer 10

monocular blindess

Question 11

what is damage to the medial chiasm called?

Answer 11

bitemporal hemianopia

Question 12

what is damage to the optic tract called?

Answer 12

contralateral homonymous hemianopia

Question 13

what is damage to the lateral geniculate called?

Answer 13

contralateral homonymous hemianopia

Question 14

what is damage to both of the optic radiations called?

Answer 14

contralateral homonymous hemianopia

Question 15

what can non vascular damage to the occipital lobe cause?

Answer 15

contralateral homonymous hemianopia without macular sparing

Question 16

what can occlusion of the posterior cerebral artery cause?

Answer 16

contralateral homonymous hemianopia with macular sparing ( as the area of the visual cortex supplying the macula receives blood from deep branch of MIDDLE cerebral artery)

Question 17

what can damage to the superior optic radiations(in parietal lobe) cause?

Answer 17

contralateral homonoymous inferior quadrantanopia

Question 18

what can damage to inferior optic radiations(in temporal lobe) cause?

Answer 18

contralateral homonoymous superior quadrantanopia

Question 19

what is involved in the pupillary light reflex?

Answer 19

afferent arm - optic nerve which is processed in the pretectal nucleus which projects bilaterally to edinger westphal nuclei (which contains parasympathetic preganglionic)
efferent arm - oculomotor nerve
due to the bilateral projections from the nucleus - consensual as well as direct pupillary constriction

Question 20

what is involved in the accommodation reflex?

Answer 20

afferent arm - optic nerve
the visual cortex via the lateral geniculate nucleus allows processing which can then be projected to teh oculomotor nerve via the edinger westphal nuclei
efferent arm - oculomotor nerve

Question 21

what are the effects of the accommodation reflex?

Answer 21

FOCUS ON NEARBY OBJECTS:

• pupillary constriction
• convergence of eyes (contraction of medial recti)
• thickening of lens