Decsribe the two structural classes of ganglion cell?
M cells (magnocellular): large cell bodies, large dendritic trees, 10% of all ganglion cells
P cells (parvocellular): small cell bodies, small dendritic trees, >80% of all ganglion cells
What are M and P ganglion cells responsible for?
P: colour vision and visual acuity
What is the most important target of ganglion cell axons?
LGN of thalamus
Describe the optic chiasm, including its location?
Why is its location significant?
Fibres from right and left optic nerves combine to form optic chiasm
Lies at base of brain, anterior to pituitary, with internal carotid artery on either side
So, damage to the pituitary or internal carotid can affect the optic nerve and therefore vision
Describe the broad organisation of the visual pathway?
In each eye: temporal visual field encoded by nasal fibres, nasal visual field encoded by temporal fibres
Nasal fibres cross at optic chiasm
Temporal fibres go through chiasm but don't cross
Right visual field encoded by left hemisphere
Left visual field encoded by right hemisphere
Describe the organisation of the LGN?
Six layers (1-6)
Two cell types: Magnocellular (layers 1,2) and Parvocellular (layers 3-6)
Why do layesr 1 and 2 in the LGn appear darker than layers 1-6 in histological section?
Layers 1 and 2 are composed of magnocellular cells, which are much bigger and darker staining
Describe the inputs of the left and right eye to the LGN?
Right eye: layers 2,3,5
Left eye: layers 1,4,6
What happens after the ganglion cells have terminated in the LGN?
Cells that come out of LGN sned out axons that form large white matter tracts, called optic radiations
These optic radiations travel to the occipital lobe (V1)
Where is the primary visual cortex located?
Occipital lobe around the calcarine fissure
As far back as you can go in the occipital lobe, on the medial surface
Describe the retinotopic organisation of the primary visual cortex?
Neighbouring cells within the retina project to neighbouring cells in the LGN and visual cortex
Where is central vision represented in the primary visual cortex?
Describe the rules for localising lesions of the visual pathway?
1) Lesion that affects one eye only has to be beofre the chiasm, and a lesion that affects both eyes must be either at the chiasm or posterior to it
2) Lesion that affects bitemporal visual fields must be at the chiasm
3) Lesion that affects the same side in both eyes must be posterior to the chiasm
Why does macula sparing occur in some lesions of the visual pathway?
Posteriuor part of the occipital lobe, which represents the macula, is supplied by a different artery
What is the role of intrinsically photosensitive ganglion cells?
Tell brain how much light is in the environment
Therefore, provide information for: circadian rhythm, sleep regulation, pupil responses, light allodynia
How do ipGCs sense light?
Light activation of melanopsin leads to depolarisation of ipGCs
What is the pupil response dependent on?
1) Being able to detect light
2) Functioning iris (muscles for constriction and dilation)
Which muscles constrict and dilate the pupil?
What is their innervation?
Constrict: sphincter pupillae, innervated by parasympathetics
Dilate: dilator pupillae, innervated by sympathetics
Describe the pathway for the pupil response?
ipGCs detect light > project to the optical pretectal nucleus in the midbrain > send processes to Edinger-Westfal nucleus on each side > projections to each eye via ciliary ganglion
Which part of the brain is particularly important for driving circadian rhythm?
Suprachiasmatic nucleus in hypothalamus
How does the SCN receive information about light in order to regulate circadian rhythms?
Projections from ipGCs
Why do some blind people still experience photophobia?
If retina is still intact, ipGCs still detect light
Why is migraine associated with photophobia?
Pain pathway for migraine projects to the posterior nucleus of thalamus
ipGCs also project to the pisterior nucleus of thalamus
Therefore, exciting ipGCs can also cause the neurons from the pain pathway to become excited