27 - Visual System (Targets of Ganglion Cells) Flashcards
Name for loss of half of visual field in both eyes, temporal sides
Bi-temporal hemianopia
M ganglion cells
Parasol ganglion cells.
Detect motion.
Can be either on or off
P ganglion cells
Midget ganglion cells.
Colour vision, visual acuity.
More common
Can be either on or off
Part of thalamus that visual information goes to
Lateral geniculate nucleus
Targets of ganglion cell axons 1 2 3 4 5
- Lateral geniculate nucleus (thalamus)
- Major target of most GCs
- Visual pathway - Pretectum (midbrain, Edinger-Westphal nucleus)
- Pupil responses - Suprachiasmatic nucleus (hypothalamus)
- Circadian rhythm - Superior colliculus
- Eye movements - Other: various nuclei of thalamus
- Photophobia
Location of optic chiasm
Base of brain, superior to pituitary gland (which is in sella turcica).
Lies between internal carotids
Above body of sphenoid
Optic fibres that cross at optic chiasm
Nasal fibres (those closest to nose)
Hemisphere of eye that views left hand side of visual field
Right hemisphere (because light from the left enters eye, hits the right part - eye is spherical).
Visual information processed by right visual cortex
That from the left visual field of both eyes.
What has happened in a bitemporal hemianopia?
Lesion at the optic chiasm (EG: like in a pituitary tumour).
Impinges on optic nerve tracts that are decussating.
Location of lateral geniculate nucleus
Posterior, lateral part of the thalamus, beside medial geniculate nucleus
Is there mixing of information from different eyes and cell types at the LGN?
No. Segregation of information from M and P cells,eyes, into discrete layers (six layers).
Structure of LGN
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)
Optic radiations
Very large white matter tracts from LGN to the occipital lobe (visual cortex).
Primary visual cortex V1 location
Occipital love about the calcarine fissure.
Mapping of information from central vision (on retina) on the visual cortex
Most posterior part of V1.
More peripheral parts of visual field are more anteriorly located
Appearance of lesion before optic chiasm
Has to be unilateral.
Appearance of lesion at optic chiasm
Lateral parts of vision of each eye affected (bitemporal hemianopia)
Appearance of a lesion at optic tract between optic chiasm and LGN
Bilateral defect in visual field on same side (EG on left side of both eyes, if right optic tract is affected).
Appearance of lesion of striate cortex (occipital lobe) from vascular problems
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.
Why can blind people respond to light, maintain a circadian rhythm?
Some ganglion cells (very small population) can detect light, contain a protein similar to opsins (melanopsin, similar to opsins of invertebrates).
Roles of photosensitive ganglion cells
1
2
3
1) Circadian rhythm
2) Pupil responses
3) Photophobia (response to injury, disease of eye)
What does light stimulation of melanopsin in ganglion cells lead to?
Depolarisation (NOT hyperpolarisation) of ganglion cells.
Direct response of pupil
Shine light in one eye, pupil should constrict.
Consensual response
Shine light in one eye, pupil of other eye should constrict
Pupil response pathway 1 2 3 4
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
Part of brain important in pupil response
Pretectal nucleus in midbrain
When do babies begin sleeping and waking in a particular cycle?
~12 weeks
Pathway of circadian rhythm
1
2
1) Intrinsically-photosensitive ganglion cells detect light with melanopsin
2) Project to supra-chiasmatic nucleus (nucleus in hypothalamus).
Photophobia
Pain on looking at light
Pain pathway for migraine 1 2 3 4 5
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
