27 - Visual System (Targets of Ganglion Cells)

Question 1

Name for loss of half of visual field in both eyes, temporal sides

Answer 1

Bi-temporal hemianopia

Question 2

M ganglion cells

Answer 2

Parasol ganglion cells.
Detect motion.
Can be either on or off

Question 3

P ganglion cells

Answer 3

Midget ganglion cells.
Colour vision, visual acuity.
More common
Can be either on or off

Question 4

Part of thalamus that visual information goes to

Answer 4

Lateral geniculate nucleus

Question 5

Targets of ganglion cell axons 
1
2
3
4
5

Answer 5

1. Lateral geniculate nucleus (thalamus)
   - Major target of most GCs
   - Visual pathway
2. Pretectum (midbrain, Edinger-Westphal nucleus)
   - Pupil responses
3. Suprachiasmatic nucleus (hypothalamus)
   - Circadian rhythm
4. Superior colliculus
   - Eye movements
5. Other: various nuclei of thalamus
   - Photophobia

Question 6

Location of optic chiasm

Answer 6

Base of brain, superior to pituitary gland (which is in sella turcica).
Lies between internal carotids
Above body of sphenoid

Question 7

Optic fibres that cross at optic chiasm

Answer 7

Nasal fibres (those closest to nose)

Question 8

Hemisphere of eye that views left hand side of visual field

Answer 8

Right hemisphere (because light from the left enters eye, hits the right part - eye is spherical).

Question 9

Visual information processed by right visual cortex

Answer 9

That from the left visual field of both eyes.

Question 10

What has happened in a bitemporal hemianopia?

Answer 10

Lesion at the optic chiasm (EG: like in a pituitary tumour).

Impinges on optic nerve tracts that are decussating.

Question 11

Location of lateral geniculate nucleus

Answer 11

Posterior, lateral part of the thalamus, beside medial geniculate nucleus

Question 12

Is there mixing of information from different eyes and cell types at the LGN?

Answer 12

No. Segregation of information from M and P cells,eyes, into discrete layers (six layers).

Question 13

Structure of LGN

Answer 13

Six layers of neurons.
Two types of cells:
Layers 1 and 2 are magnocellular layers (from M cells)
Layers 3-6 are parvocellular layers (from P cells)

Question 14

Optic radiations

Answer 14

Very large white matter tracts from LGN to the occipital lobe (visual cortex).

Question 15

Primary visual cortex V1 location

Answer 15

Occipital love about the calcarine fissure.

Question 16

Mapping of information from central vision (on retina) on the visual cortex

Answer 16

Most posterior part of V1.

More peripheral parts of visual field are more anteriorly located

Question 17

Appearance of lesion before optic chiasm

Answer 17

Has to be unilateral.

Question 18

Appearance of lesion at optic chiasm

Answer 18

Lateral parts of vision of each eye affected (bitemporal hemianopia)

Question 19

Appearance of a lesion at optic tract between optic chiasm and LGN

Answer 19

Bilateral defect in visual field on same side (EG on left side of both eyes, if right optic tract is affected).

Question 20

Appearance of lesion of striate cortex (occipital lobe) from vascular problems

Answer 20

Lose vision bilaterally in both eyes on one side, but macula is spared (‘macular sparing’), because central vision has different blood supply within visual cortex.

Question 21

Why can blind people respond to light, maintain a circadian rhythm?

Answer 21

Some ganglion cells (very small population) can detect light, contain a protein similar to opsins (melanopsin, similar to opsins of invertebrates).

Question 22

Roles of photosensitive ganglion cells
1
2
3

Answer 22

1) Circadian rhythm
2) Pupil responses
3) Photophobia (response to injury, disease of eye)

Question 23

What does light stimulation of melanopsin in ganglion cells lead to?

Answer 23

Depolarisation (NOT hyperpolarisation) of ganglion cells.

Question 24

Direct response of pupil

Answer 24

Shine light in one eye, pupil should constrict.

Question 25

Consensual response

Answer 25

Shine light in one eye, pupil of other eye should constrict

Question 26

Pupil response pathway
1
2
3
4

Answer 26

1) Ganglion cells detect light with melanopsin
2) Signal travels down optic tract, goes to pretectal nucleus in midbrain
3) Projection to Edinger-Westphal nuclei on both sides, which send a parasympathetic fibre to ciliary ganglion
4) Ciliary ganglion sends fibre, leads to constriction of pupil

Question 27

Part of brain important in pupil response

Answer 27

Pretectal nucleus in midbrain

Question 28

When do babies begin sleeping and waking in a particular cycle?

Answer 28

~12 weeks

Question 29

Pathway of circadian rhythm
1
2

Answer 29

1) Intrinsically-photosensitive ganglion cells detect light with melanopsin
2) Project to supra-chiasmatic nucleus (nucleus in hypothalamus).

Question 30

Photophobia

Answer 30

Pain on looking at light

Question 31

Pain pathway for migraine 
1
2
3
4
5

Answer 31

1) Migraine pain is thought to be due to nerves signalling from the Dura (blood vessels in the meninges).
2) Pain from the dura is carried by the trigeminal nerve.
3) Pain pathway: dura-trigeminal nerve-brainstem-thalamus
4) Pain signal is carried to posterior nucleus of the thalamus (same nucleus as melanopsin ganglion cells synapse with).
5) This stimulates greater pain, as neurons in the posterior thalamus will be firing more upon light stimulation