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Lecture 4

NROC64H3 Lecture Notes - Lecture 4: Internuclear Ophthalmoplegia, Saccade, Reticular Formation


Department
Neuroscience
Course Code
NROC64H3
Professor
Matthias Niemeier
Lecture
4

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Lecture 4 - Oculomotor Systems 2
The Brainstem
- BS sends out a pulse signal required to drive saccade onset comes
from two distinct areas in the brainstem
o 1. Pontine reticular formation: horizontal saccades
Contains excitatory burst neurons (EBNs) source
of the high frequency burst of discharges
responsible for driving the lateral and medial
rectus muscle to move the eye look left/right
o 2. Mesencephalic reticular formation- vertical saccades
Contains vertical burst neurons (VBNs) source of
the high frequency burst of discharges
responsible for driving the inferior and superior
recti in charger of lateral movements of eye
(up/down)
- The extraocular muscles are innervated by three cranial -nerves:
o the abducens (CN VI) lateral rectus to pons (abducens
nucleus PPRF horizontal saccades)
o the trochlear (CN IV) from superior oblique to the
trochlear nucleus
o the oculomotor (CN III) combines a whole bunch of nerves (coming from the superior,
medial and inferior rectus as well as the inferior oblique) and feeds it to the midbrain
- Inhibitory burst neurons (IBNs) in the caudal pontine reticular formation work with the
Excitatory burst neurons (EBNs) to silence the antagonist motoneurons
o If we want to look a etai a, e dot at all the usles to otat all at oe
because that stops you from moving your eyes need some relaxed and others
contracted. IBN and EBN together in order to look in different directions.
o IBN + EBN = SBN (saccadic burst neurons)
- Main characteristic of SBN:
o duration is tightly correlated to the duration of the corresponding saccade if it is firing
longer, the saades duration is longer
o the number of spikes is related to saccade amplitude more spikes = higher amplitude
o peak burst discharge is correlated with peak saccade velocity
- Omnipause neurons (OPNs): act as an inhibitory gate for saccades
o discharge at a constant rate when gaze is steady and pause their firing during saccades
in all directions keep eyes still and able to concentrate/fixate without eyes darting all
over (OPNs fire at a constant rate when gaze is steady) when eyes want to move to
another target, burst neurons fire to allow the movement
o duration of the pause correlated to saccade duration
o duration of firing correlated to fixation
- There is a fast (pulse) which is a result of a burst of activity in the PPRF that quickly shifts the
eyes to the moving object
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- There is a slow (step) phase when tonic neurons in the nucleus prepositus hypoglossi (PPH)
maintain a steady signal related to eye position
- Disorders of horizontal eye movements caused by brainstem lesions are classified into three
groups:
o 1. lateral gaze palsy: inability to produce horizontal, conjugate eye movements in one or
both directions
caused by lesion involving PPRF or abducens nucleus area of brain that
controls horizontal movements or area with nerves that controls it
unable to move both eyes in one or both direction (left/right or left and right)
o 2. internuclear ophthalmoplegia: ipsilateral eye will not adduct and the contralateral
eye demonstrates horizontal nystagmus (when the eyes move back and forth really
quickly)
caused by lesion involving MLF
only one eye unable to move in a certain direction
o 3. one-and-a-half syndrome: conjugate horizontal gaze palsy in one direction plus an
internuclear ophthalmoplegia in the other
caused by lesion to both of above
The Cerebellum
- Cerebellar vermis plays large role in saccade metrics
o electric stimulation evokes saccadic eye movements
- Cerebellar vermis projects to the caudal fastigial nucleus which in turn contacts
the brainstem centres for saccades
o Used for automatic saccades eause dot eed highe ai egios – like the
frontal lobe when we are automatically looking to something in our environment
- Cerebellum monitors motor commands via a feedback model in efference copy the brainstem
is going to send out information of what it told the eyes to do, it will get an efference copy that
says what it is supposed to do then it does what it was supposed to be doing makes sure
commands are getting towards their destined target can do on-line correction during eye
movement
- Lesions to vermis causes several eye movement deficits:
o Transient hypometria undershooting the saccade- not getting to where they need to
go on the saccade
Omnipause fire constantly when eyes are held at fixation
Then we decide we want to look left burst neurons fire to
make a saccade to look to the left
Abducens internuclear neuron will feed through the medial
rectus motoneuron for one side (indirect pathway), or straight
to lateral rectus to move eyes
These will contract and opposite muscles will relax, allowing us
to look to a direction
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o Permanent loss of short-term saccadic adaptation at oet thei ee oeets
(on-line-correction)- shoot off at very inaccurate angle ad at oet
o Increase in endpoint variability - random endpoints more variation
o Iaility to opesate saadi fatigue when asked to make long sequence of
saccades, eyes will fatigue and drop in velocity but in a normal person would have an
increased duration when this fatigue occurs, heeas ou dot see this i soeoe
with a lesion in a lesion they do go slower (drop in velocity) but they are not
compensating in duration
Why do we see this in a normal person? Because we get tired and loose
attention/bored decrease in velocity and take longer tom make eye
movements
o Loss of pursuit gain eyes slowly getting behind in following target
o Impairment in adapting initial pursuit response to changes of target velocity
- Caudal fastigial nucleus contains saccade-related neurons for both saccades and smooth
pursuit
- Timing of the bursts of saccade-related neurons depends on the direction and amplitude of the
saccade executed
- Unilateral inactivation leads to ipsiversive saccadic hypermetria and controversive saccadic
hypometria if you inactivate one side of the brain, that same side of the brain will get
hypermetric (look past target) and the opposite side will be hypometric (Look short of the
target)
- Caudal fastigial nucleus also involved in internal feedback of a motor command (efference
copy) and it will corrects for anticipated errors by rapid modifications of saccade duration
- Copies of premotor commands to ocular motoneurons are sent to paramedian tract, which
then project mossy-fibre input to the floccular region of the cerebellum
o Damage to this relay impairs gaze-holding (wont be able to hold gaze and concentrate
on that) and retinal slip (eyes will move and slip)
Superior Colliculus
- SC, Brainstem and Cerebellum autoati saades ad dot euie highe ai aeas
- The SC integrates multiple sources of sensory, motor, and cognitive signals
- SC sends motor commands directly to the brainstem circuitry
- Two largely independent structures in the SC with functionally distinct roles:
o 1. Salience map: the sensory qualities that make a stimulus distinctive from its
surroundings bars on contrast sensitivity, SM helps distinguish the black and white
thing that is most relevant in the environment will be more salient
o 2. Priority map: the integration of visual salience and behavioural relevance Yes, when
you look at something new, you will be more drawn to the thing that is more colourful
in that image, what stands out; you are directly drawn to it but you can override that
with behavioural relevance of finding something specific; if you are shown a picture with
lots of distinguishable things, you can draw attention to one of the things Whees
Waldo for example
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