Lecture 11 - Superior colliculus, pulvinar (LA) Flashcards
Pupil constriction is mediated by light via symp or para?
Para
Dilation is mediated by the absence of light via symp or para?
Symp
What is the accessory optic system?
The primary visual system in fish and birds
Superior colliculus
A key area for the orienting response, directing gaze quickly to a novel stimulus (touch, sound, etc.)
Pulvinar (part of thalamus)
May be involved in allocation of attention, role of visual stimulation, sensitive to emotion or relay between different cortical areas
Not all axons leaving retina go to LGN - some also branch earlier to? Para or symp?
Pretectal nucleus in the midbrain -> projects bilaterally to Edinger-Westphal nuclei -> motor innervation to the pupillary sphincter (part of para - chollinergic)
Are the pretechtal and Edinger-Westphal nucleus in or near the 3rd nerve nuclei?
Near - this means that damage to the 3rd nerve nuclei will not affect pupillary constriction
What is important about the bilaterality of the pretectal nucleus projection to the Edinger-Westphal nuclei?
It means that shining a light into either eye causes both pupils to constrict
What is the “near triad”?
When looking at things near us, there is a synergistic interrelationship between: 1) Pupillary constriction (better focus from “pinhole” effect of smaller pupil) 2) Lens accommodation (thicker lens) 3) Convergence of the two eyes (to keep image on corresponding points of both retina) Of the fibres reaching the cillary ganglion, most (by a 30:1 ratio) go to the cillary muscles of the lens, so that when the pupil constricts, the lens accommodates.
Does “near triad” occur in blind people?
Yes - using finger as a target to give yourself a sense of “near
What makes the pupil dilate?
-Innervation to the sphincter is inhibited so this muscle relaxes -Dilator muscles are excited via symp NS (hypothalamus)
What is Horner’s syndrome?
Congenital or acquired condition where there is disruption to the pupillary circuit responsible for dilation of the pupils -Lose dilation (so results in contraction) -Lid droops -Sweating disrupted (dry face in hot room) -Blood vessels dilate in the region innervated by this nerve chain (flushed face)
How does lung cancer result in Horner’s syndrome?
As the neuron for the dilator pupillary circuit goes above the apex of the lung - tumours on the apex would disrupt this circuit
In congenital Horner’s syndrome, why does the affected eye have less pigment (is lighter) than the other?
During development, symp NS is important for the distribution of pigment in the iris - disrupt this and one eye will have less pigment. However, this is innocuous.
3 things pupil is driven by?
Light, sense of near, psychological arousal
Two areas in accessory optic system that is like to mammals?
Visual Wuist (analogous to PVC) Pretectum
What stimuli do the directionally-selective neurones of the AOS prefer?
Large ans slow-moving - it is part of the circuitry for optokinetic nystagmus and visual-vestibular interaction, both of which involve motion of much or all of the visual field
*Nystagmus refers to rapid involuntary movements that may cause one or both eyes to move from side to side, up and down or around in circles.
Differences b/n Optokinetic and Vestibular system?
OPTO
- Visually driven
- Responds to low freq
- Sustained responses
VEST
- Driven by acceleration of semicircular canals
- Responds to high freq
- Transient responses
How separate are the geniculocortical and AOS pathways? How do responses vary across mammalian species then? (Cats, monkeys, humans)
In mammals- they’re not, they converge. This means that OKN (optokinetics) is generated from both direct input to the AOS as well as input coming back from visual cortical areas.
CATS- NOT (nucleus of the optic tract) gets both retinal and cortical inputs BUT retina dominates
MONKEYS- NOT gets both, cortex dominates
HUMANS- NOT gets both, cortex GREATLY dominates
Moral? Old CNS don’t disappear- they just get more deeply embedded in new structures
Superior colliculus is bigger/smaller in humans compared to birds?
Smaller
In birds, the colliculus has projections that resemble?
Primary dorsal and ventral systems- so evolution moved these functions from midbrain to cortex in primates
SC role in mammals - rather than for conscious vision, its more focused on?
Gaze control- moving our eye
Why do we move our eyes?
Take snapshots and our brain integrates this so we think we see clearly- other animals like birds can just move their heads hence gaze control in not important
Where does SC receive inhibitory input from?
Basal ganglia (striatum- sub nigra) from the frontal cortex
How are SC motor cells organised?
Retinotopically
What gets the eyes moving/orient gaze quickly?
Retina -> SC -> Brainstem
(motor centres)
This is for short latency responses (fast responses) - BUT still need longer term responses mediated by genicular cortical pathways (the ones we use for conscious vision) as the direct, faster pathway doesn’t say anything about WHAT the stimulus is.
What receptive fields overlap in SC?
3 senses (auditory, visual, somatosensory)- makes integration across the sense modalities possible
Allows for tactile responses as these nonvisual inputs to the SC make possible for accurate saccades to nonvisual stimuli and allow for the non-visual enhancement of response to a multisensory stimulus
eg. something lands on my arm, I can look at it
T/F: There is redundancy in mammals as when SC is damaged, FEF can take over
T
How does the FEF (Frontal eye fields) get the eyes to move on its own if the SC is damaged?
Through the direct FEF -> reticular formation pathway
* Only makes long responses but not the short express saccades that the colliculus is involved with
When inhibitory input from basal ganglia is affected, what happens?
People lose the ability to inhibit unwanted gaze
Where does the pulvinar receive input from? Where does it project to?
Both layers of SC and projects widely to the various visual cortical areas and parietal cortex
Where is the pulvinar located?
On top of the SC
T/F: Pulvinar only has retinotopic areas
F- has both non-ret and ret
Lesions of the pulvinar (esp. medial) can impair?
Facial emotion processing
Pulvinar nucles of the thalamus is able to?
1) Integrate visual activity across cortical areas
2) help develop a salience map across the visual field
3) Aid in processing of emotionally sig stimuli
