Visual Pathway Flashcards
What kind of glutamate receptors can bipolar cells express?
Metabotropic
Ionotropic
Why are there different types of ganglion cells?
Each type provides info for different stream of vision
What are two types of ganglion cells?
M/parasol
P/midget
Are M and P ganglion cells ON or OFF ganglion cells?
Each type has ON and OFF type
- M ON
- M OFF
- P ON
- P OFF
Describe M ganglion cells
Motion Large cell bodies Long processes Covers large area of retina 10-15% of ganglion cells Especially prone to disease
Describe P ganglion cells
Colour vision Visual acuity Small cell bodies Short processes 80% of ganglion cells
What are the targets of ganglion cell axons?
Lateral geniculate nucleus (LGN) in thalams Pretectum in midbrain Suprachiasmatic nucleus in hypothalamus Superior colliculus Various other nuclei of thalamus
Why do ganglion cell axons project to the LGN?
Major target of most ganglion cells
Visual pathway
Why do ganglion cell axons project to the pretectum?
Pupil responses
Why do ganglion cell axons project to the suprachiasmatic nucleus?
Circadian rhythm
Why do ganglion cell axons project to the superior colliculus?
Eye movements
Why do ganglion cell axons project to the nuclei in the thalamus other than the LGN?
Photophobia
What is the optic chiasm?
Fibres from right and left optic nerves combine
How many ganglion fibres cross at the optic chiasm?
Half
Where is the optic chiasm?
Base of brain anterior to pituitary
From which hemifield does the nasal retina receive light information?
Temporal hemifield
From which hemifield does the temporal retina receive light information?
Nasal hemifield
Which fibres cross at the optic chiasm?
Nasal fibres
Which hemisphere views each hemifield?
Right visual hemifield viewed by left hemisphere
Left visual hemifield viewed by right hemisphere
What part of the visual field does a stroke affect when it’s in the visual cortex?
Half of each field in each eye, rather than one entire eye
What is the name of the tract after the optic chiasm?
Optic tract
Where do ganglion cell axons in the optic tract synapse?
LGN
How many neurons are there in the visual pathway?
2
Why is the anatomical position of the optic chiasm relevant to pathology?
Pituitary gland just below
Condition affecting it can affect chiasm; eg: in pituitary macroadenoma
What is the classic presentation of a person’s visual field when they have a pituitary macroadenoma?
Bitemporal hemianopia
Which hemifields are affected in bitemporal hemianopia?
Temporal hemifields
Where do second order neurons in the visual pathway arise from?
LGN
How is information in the LGN segregated?
By eye and ganglion cell type
How many layers are there in the LGN?
6
What are the types of cells in the LGN?
Magnocellular
Parvocellular
Where do M ganglion cells synapse?
Magnocellular layers: 1 and 2
Where do P ganglion cells synapse?
Parvocellular layers: 3-6
Which layers receive input from the right eye?
2, 3, 5
Which layers receive input from the left eye?
1, 4, 6
Where do LGN neurons project to?
Primary visual cortex (V1)
What is the Broddman’s area of V1?
Area 17
Where in the brain is the primary visual cortex?
Occipital lobe around calcarine fissure
- On medial aspect
- Upper and lower banks around fissure
How is information organised in the primary visual cortex?
Retinotopic organisation = neighbouring cells within retina project to neighbouring cells in LGN and visual cortex
Which part of the visual cortex receives information from the macula?
Most posterior part = macular cortex
Which part of the visual cortex receives information from the peripheral retina?
Anterior parts
What visual field is affected when there’s a lesion in the right optic nerve?
Right eye visual field
What visual field is affected when there’s a lesion at the optic chiasm?
Temporal hemifields
What visual field is affected when there’s a lesion in the right optic tract?
Left visual hemifields
What visual field is affected when there’s a lesion in the right optic radiation?
Upper left quadrant of visual fields of both eyes
How can the visual field of the macula be spared when there’s a lesion in the right visual cortex?
Only happens with vascular event
Macular cortex supplied by different blood supply than rest of occipital lobe
If infarct in vessel supplying rest of occipital lobe, then macular vision spared
What is the tract from the LGN to the primary visual cortex called?
Optic radiation
What are intrinsically photosensitive ganglion cells?
Small population of ganglion cells containing visual pigment = melanopsin
Light activation of melanopsin > intrinsically photosensitive ganglion cells depolarise
What are the functions of intrinsically photosensitive ganglion cells?
Circadian rhythm Sleep regulation Pupil responses General info about light levels Light allodynia
What is light allodynia?
Photophobia associated with
- Migraine
- Ocular injury
- Infection
What do pupil responses depend on?
Being able to detect light
Functioning iris
What are the two muscles of the iris?
Dilator pupillae
Sphincter pupillae
Where is one projection of melanopsin ganglion cells?
Optical pretectal nucleus (OPN)
Where do ganglion fibres heading to the OPN diverge from the visual pathway?
After optic chiasm
Before thalamus
Where do neurons from the OPN synapse with to control pupil responses?
Edinger-Westfal nucleus
Which side of the brain does the OPN send fibres to in order to provide information for pupil motor responses?
Ipsilateral
Contralateral
At what age do babies start to develop circadian rhythms?
6-12 weeks
Where do intrinsically photosensitive ganglion cells project to in order to provide information for the circadian rhythm?
Suprachiasmatic nucleus (SCN) in hypothalamus
What is the effect of light in migraines?
Worsens pain
What is the pain pathway for migraines?
Thought to be due to nerves signalling from dura
Pain from dura carried by trigeminal nerve
Dura > trigeminal nerve > brainstem > posterior nucleus of thalamus
Why does light cause pain to worsen in migraines?
Intrinsically photosensitive ganglion cells synapse in posterior nucleus of thalamus > influence pain fibres
