optic nerve and visual fields

Question 1

phototransduction

Answer 1

• Photopigment lies in discs of outer segments
• Rods – rhodopsin
• Cones – cone opsin
• Opsins are transmembrane proteins which contain the light sensitive molecule retinal
• Different opsin structures mean retinal absorbs different wavelengths of light

Question 2

effect of a photon on a rhodopsin molecule

Answer 2

• Triggers conformation change in all-trans form
• This change triggers changes in the opsin structure
• This in turn triggers a cascade within the cell

Question 3

signals in the retina

Answer 3

• Photoreceptor → bipolar cell →retinal ganglion cell
• Horizontal and apocrine cells modulate signal
• Retinal ganglion cells not graded response – action potential

Question 4

blood supply

Answer 4

• Internal carotid artery: opthalmic artery, central retinal artery – which passes into the optic nerve, ciliary arteries, lacrimal artery, ethmoid & eyelid artery
• External carotid artery: facial artery – supplies medial lid and orbit

Question 5

neural visual pathway

Answer 5

• Information from the eyes is transmitted to the primary visual cortex
• The eye receives input from the temporal & nasal visual fields
• These inputs are received then transmitted to the primary cortex of the occipital lobe
• The primary visual cortex is located predominantly on the medial surface of the hemisphere in the region above and below the calcarine sulcus
• Surrounding this area the rest of the occipital lobe constitutes the visual association cortex – concerned with interpretation of visual images, recognition, depth perception & colour vision

Question 6

visual fields (up to lateral geniculate body)

Answer 6

• Optic nerve, carries the axons of the cells in the ganglionic layer of the retina
• They leave the orbit bilaterally via the optic canal to enter the cranial cavity
• Fibres from the nasal portion (closest to the nose) of the retina (carrying the temporal visual fields) cross at the optic chiasm which is located just anterior to the pituitary infundibulum
• The optic tracts carry the fibres posterolaterally around the cerebral peduncles to terminate at the lateral geniculate bodies of the thalamus
• Fibres from the temporal portion (furthest from the nose) of the retina (carrying the nasal visual fields) do the same as that mentioned above

Question 7

visual fields after the LGN

Answer 7

• After the lateral geniculate body, the optic radiations split into two
• The fibres carrying information from the inferior portions of the retina (and thus the superior visual fields) travel by looping laterally through the temporal lobe to the visual cortex – Meyer’s loop
• The fibres carrying information from the superior portions of the retina (and thus the inferior visual fields) travel by looping superiorly through the parietal lobe to the visual cortex – Baum’s loop

Question 8

unilateral field loss (left)

Answer 8

1) Damage to the left optic nerve = no vision through the left eye

Question 9

bitemporal hemianopia

Answer 9

2) Damage to the optic chiasma = loss of vision of the temporal visual fields

Question 10

homonymous hemianopia (left optic tract)

Answer 10

3) Damage to the left optic tract = loss of vision of the temporal field of the left eye and the loss of the nasal field of the right eye

Question 11

damage to Meyer’s loop (left)

Answer 11

loss of vision in the superior nasal field of the left eye and the superior temporal field on the right eye

Question 12

damage to Baum’s loop (left)

Answer 12

loss of vision in the inferior temporal field of the right eye and the inferior nasal field of the left eye