Brainstem Generators Flashcards
What does it mean for an eye movement to be conjugate?
The movement of both eyes are coordinated in the same direction
What are examples of conjugate eye movements?
- Vestibular-Ocular Reflex (VOR)
- Optokinetic Reflex (OKR)
- Smooth pursuit
- Saccades
What is a dysjunctive eye movement?
When the eyes move in opposite directions e.g. vergence
What is an example of dysjunctive eye movements?
Vergence movements e.g. convergence or divergence
What is the purpose of slow eye movements?
To Maintain Gaze
To Compensate for self- or object-motion
To Reduce image blur & improve acuity
What are examples of slow eye movements?
- VOR- Vestibular ocular reflex (reflex that accounts for head motion) , OKR - ocular kinetic reflex
- Smooth Pursuit (tracking a target)
- Vergence (tracking a target moving in depth)
What is the purpose of fast eye movements?
To Change fixation
What is an example of a fast eye movement?
•Saccades - these are rapid eye movements
(The purpose of saccades is to looking at new stimuli; scanning objects of interest; searching for information)
Which brainste nuclei controls slow conjugate movements (e.g. VOR, OKR, Smooth Pursuit)?
Medial Vestibular Nucleus (MVN)
What is an example of a slow dysjunctive movement?
Any vergence movement - e.g. convergence, diveregnce
Which brainstem nuclei controls slow dysjunctive eye movements?
Mesencephalic Reticular Formation (MRF)
Fast conjugate movements (saccades) are controlled by two different brainstem nuclei- how is this control dictated?
Horizontal saccades are controlled by Paramedian Pontine Reticular Formation (PPRF)
[Para median - because it runs close to the mid line, ‘Pon-‘ bc it is in the pons]
Vertical saccades are controlled by the : rostral interstitial nucleus of Medial Longitudinal Fasciculus (riMLF)
Why is the MLF (Medial Longtitudinal Fasciculus) important in conjugate eye movements?
It is the means through which we activate synergists when trying to get both eyes to look in the same direction. - Fnacy way of saying this is:
It Coordinates the activity of Ocular Motor Neurons that innervate binocularly synergistic muscles
[E.g. If we were trying to look to the right with both eyes, dextroversion, the Right lateral rectus would be innervated from the abducens nerve/orginating from abducens nuclei, - 60% of the axons in the abducens nuclei would go toward innervating the right lateral rectus and 40% would travel through the MLF to activate the left medial rectus (the synergist so both eyes could look to the right together)- the activation of the synergists happens via travelling through the MLF]
What is the MLF (Medial Longtitudinal Fasciculus) made up off?
The MLF contains numerous, separate tracts of heavily myelinated (fast conducting) axons forming direct connections between the Oculomotor, Trochlear & Abducens Nuclei & the Brainstem Generators Nuclei
What does Fasciculus mean?
Bundles
What can we tell about the structure and location of the MLF from its name?
MLF - Medial Longititidduinal Fasciculus
Medial - close to the midline
Longtitudinal - travels up and downwards
Fasciculus - made up of bundles
What does unilateral damage to the MLF cause?
Inter-Nuclear Ophthalmoplegia (INO) - when the synergist muscle is failing to fully be innervated.
Px may also display nystagmus - unsteady fixation - that wondering of the eye.
What are the causes of Inter-Nuclear Ophthalmoplegia (INO)?
Causes include: vascular infarcts, brainstem tumours, demyelinating diseases (e.g., multiple sclerosis), drug toxicity, viral infection, trauma
What is gaze palsy?
A disorder in which pxs cannot make saccades , rapid eye movements - ‘an inability to shift bi-foveal fixation’.
This can occur both horizontally and vertically
Is both horizontal and vertical gaze palsy within the same px common, and if so why?
No it is not common.
Rarely, both directions affected because the Generator Nuclei are in different brainstem locations.
What cells does the PPRF (Paramedian Pontine Reticular Formation) contain?
The PPRF contains Excitatory Burst Cells (EBCs) which fire at very high frequency (up to 1000Hz) just before a saccade towards the same side that you want to look
Do EBCs also project high frequency action potentials (known as bursts) to the contralateral synergist?
Yes so we can get that rapid eye movement of both eyes
What do unilateral PPRF lesions lead to (lesions on one side)?
And as a result what does the patient do to compensate?
Unilateral PPRF (Paramedian Pontine Reticular Formation) lesions result in loss of saccades towards the lesioned side (paralysis of ipsi-versive gaze’).
Patients make whole-head saccades instead
When you make a saccade you must ofcourse innervate the synergist (via the EBCs-excitatory Burst cells) but how do we relax the antagonist at the same rate as we innervated the synergist?
We use inhibitory burst cells
What does the name ‘burst cell’ refer to?
Cells which fire at an incredibly high frequency (1000Hz)
How are vertical saccades produced?
Upwards saccades are prodcued by the lateral riMLF via MLF projections to the contralateral Oculomotor (IO, SR) Nucleus, which cross the midline in the Posterior Commissure (PC)
Downwards saccades are produced by the medial riMLF probably mediates Down-gaze via projections in the MLF to the ipsilateral Trochlear & Oculomotor Nuclei
What saccades are affected by large bilateral riMLF lesions?
Large bilateral riMLF lesions abolish all vertical saccades.
But smaller lesions selectively impair up- or down-gaze
Where does the Posterior Commissure sit?
Just under the pineal gland (so tumours of the pineal gland can cause damage).
What would a lesion affecting the Posterior Commissure or lateral riMLF affect?
Up-gaze palsy
+ convergence-retraction on attempted up-gaze (i.e. you struggle to contract on upwards gaze)
+ near-light dissociation: pupil constriction absent on attempted
Convergence, but present in response to light. (near reflex absent but light reflex present)
[Lesions of the lateral riMLF affect upwards saccades , Lesions of the medial riMLF affect downward saccades]
If someone has a lesion affecting downward saccades can they not look down at all?
No - technically they can still make slow downward movements in response to head movements (i.e. VOR is still present)
What is Oscillopsia?
When the VOR (Vestibular Ocular Reflex) is absent and so your eyes cannot account for head movements thus when you move your head the world spins.
Summarise the structure of the vestibular organs in each ear
The vestibular organ in each ear is made up of the otoliths and semi-circular canals.
The otoliths are made up of the utricle and saccule and respond to head position in respect to gravity (static movement).
The semi circular canals repsond to dynamic motion of the head
Describe what movment activates which semi-circular canal
Horizontal canal is excited by lateral motion towards that side;
Anterior canal is activated by forward (down) motion;
Posterior canal is activated by backward (up) motion.
How can we detemine the direction of head movement based on impulses from vestibular hair cells?
Bi-directional signalling: increased activity when cilia are bent towards the kinocilium & decreased activity when bent away.
[At resting potential there is constant transmitter release]
Describe the pathway of the horizontal Vestibular reflex (VOR) for a right side head movement
- A Rightward Head Motion, activates Hair Cells in the Horizontal SCC in Right inner ear (and inhibits horizontal SCC in left ear)
- This leads to excitation of the vestibular ganglion cells and the 8th cranial nerve which supplies
- Excitatory input to the ipsilateral (same side) MVN (as route of 8th cranial nerve is uncrossed). Excitatory Burst cells in the MVN both types of neurones in the abducens nerve – the ones that travel to the LR and the synergist (via the MLF).
- This causes Activation of left LR & right MR via the oculomotor pathway.
- Thus we get Compensatory Leftward Eye Movements
[Inhibitory input from opposite horizontal scc results in relaxation of antagonist muscles to improve precision]
Describe the pathway of the Vestibular ocular reflex for a backward head movement
- Backward (up) head motion activates the Posterior scc’s in both inner ears
- 3 neuron pathway: Vestibular Ganglion Cells are innervated which innervate MVN & crossed inputs to the Trochlear (to innervate SO) & Oculomotor Nuclei (to innervate IR)
- This pathway is bilateral, so both eyes move down to compensate
- (Anterior scc are inhibited by backward head motion, resulting in relaxation of antagonistic Elevator muscles)
Describe the pathway of the Vestibular ocular reflex for a forward head movement (i.e. your face moving towards the ground)
- Forward (down) head motion activates the Anterior scc’s in both inner ears
- 3 neuron pathway: Vestibular Ganglion Cells innervated which innervate MVN & crossed inputs to the Oculomotor Nuclei (which innervate SR, IO)
- This pathway is bilateral, so both eyes move up to compensate
- (Posterior scc’s are inhibited by forward head motion, resulting in relaxation of antagonistic Depressor muscles)
How does oscillopsia occur?
[Full name of the condition is Head movement Dependent Oscillopsia (HMDO)]
from damage to:
- Vestibular Apparatus
- Vestibular Nuclei (+ problems with balance/posture)
MLF damage (+ other eye movement disorders)
If Inter-Nuclear Ophthalmoplegia (INO) is a result of lesions in the MLF does that mean oscillopsia is always experienced with Inter-Nuclear Ophthalmoplegia (INO)?
No due to possible sparing from a projection from the Lateral Vestibular Nucleus
