Brainstem generators Flashcards
What does conjugate mean?
Same direction eye movements
What type of conjugate eye movements do you get?
Vestibular ocular reflex
Optokinetic reflex
Smooth pursuit
Saccades
What does dysjunction mean?
Opposite direction eye movements
What type of opposite eye movements do you get?
Vergences
What vergences are there?
Convergence and divergence
What is the purpose of speed for eye movements?
To do slow gaze maintaining eye movements
To do fast fixation changing movements
How fast is slow maintaining gaze?
50 deg per second
How fast is fast changing fixation?
1000 deg per second
What does slow eye movements do?
Compensate for self or object motion
Reduce image blur and improve acuity
What brainstem generators nuclei are there for slow conjugate eye movements?
Medial vestibular nucleus ( MVN)
What is MVN used for?
Slow conjugate movement for VOK, OKR and pursuit
What brainstem generators nuclei are there for slow dysjunction eye movement?
Mesencephalic reticular formation (MRF)
What is MRF used for?
Slow dysjunction eye movements for convergance and divergence
What type of brainstem generators nuclei are there for fast conjugate saccades eye movements?
Paramedian pontine reticular formation PPRF- horizontal
Rostral interstitial nucleus of medial longitudinal fasciculus (riMLF) -Vertical
What is PPRF nuclei used for ?
Horizontal fast saccades
What is riMLF nuclei used for?
Vertical fast saccades
How are brainstem generator nuclei and inter nuclear connected?
Connections via the medial longitudinal fasciculus (MLF)
How to you achieve Horizontal directional eye movements?
Simultaneously contract lateral rectus muscle of one eye and medial rectus of the opposite eye and relax antagonist in each eye.
?What law is applied ?
Sherringtons law
How do you achieve vertical directional eye movements?
Simultaneously contract the superior rectus, inferior oblique of both eyes for elevation or Superior oblique and inferior rectus of both eye for depression
relax their antagonist in each eye
What is the inter nuclear pathway?
Axons highly myelinated and forms connection between brainstem generators nuclei and the ocular motor nuclei
Via medial longitudinal fasciculus
What are the axons of MLF?
Heavily myelinated and fast conductors
Where does the MLF extend to?
From the medulla,
through the pons
to the upper midbrain
How many neurone types does the abducens nucleus have?
2 neurone types
What are the two neurone types for abducens nucleus?
60% motor neurones
40% inter neurones
What do the motor neurones in abducens nucleus do?
Innervates the ipsilateral Lateral Rectus
What does the inter neurones in abducens nucleus do?
Innervated the neurones in the opposite ocularmotor nucleus supplying the synergist MR, via crossed projections in the MLF
Damage to cross pathway of MLF causes what disorder?
INO (Inter nuclear ophthalmoplegia
How does INO occur ?
weakness off adduction of MR ipsilateral to the lesion on attempt gaze shifts
vascular infarts, brainstem tumours, demyelinated disease, drug, toxicity, viral infection, trauma
What do you see in INO - right MLF lesion?
PX cannot move both eyes to the right
- full abduction of left eye
- weakness of adduction of right eye
- left eye may show instability -nystagmus when moved out
What is nystagmus outwards and paresis inwards known as?
NO PIE - nystagmus outwards
paresis inwards
How do you make horizontal saccades?
Via Paramedian pontine reticular formation PPRF
What do PPRF contain?
Excitatory burst cells (EBCs) which fire at high frequency 1000Hz just before saccades towards same side
Where do EBCs of PPRF project?
Project to the ipsilateral abducens nucleus , activating motor neurones to LR and inter neurones to opposite ocular motor nucleus (MR pool)
What can unilateral PPRF lesion cause?
Result in loss of saccades towards lesion side
What nucleus makes vertical saccades ?
riMLF- rostral institial nucleus of medial longitudinal fasiculus
what does the riMLF contain?
Excitatory burst cells fires high frequency rate just before vertical saccades made
Where does the EBCs for the riMLF project to?
When making downwards gaze projects in the MLF to the ipsilateral trochlear and ocular motor nuclei
When making upward gaze projects to contralateral oculormotor nucleus which crosses the midline in posterior commissure
What happens if you get large bilateral riMLF lesions?
All vertical saccades effected
What happens if you get small bilateral riMLF lesions?
Selectively impair up and down gaze
How is upward saccades effected?
Lesions of the lateral riMLF or posterior commissure -pineal tumour , perinauds syndrome
How are downwards saccades effected?
Lesions of the medial riMLF
What disorder do you get if you have no VOR?
Oscillopsia- image spins round as head movements made
What two sets of organs are in the vestibular and inner ear?
- Otoliths -static
- Semi circular canals- dynamic motion
What are the 3 semi circular canals?
Horizontal - excited by lateral motion towards that side
Anterior - activated by forward (down) motion
Posterior - activated by backward (Up) movements
What is resting potential of vestibular ?
100-300 spikes per second
How do you activate the vestibular hair cells?
Increased activity when cilia are bent towards kinocilium and decreased activity when bent away
What does rightward head motion do?
Activates hair cells in the Horizontal Simi circular canal in right inner ear
-Excites vestibular ganglion cells that innervate them via 8th nerve
Excitatory input to 2 ipsilateral MVN then crossed input 3 left abducens nucleus
Activation of left LR and right MR to 4 oculomotor nucleus
compensatory leftwards eye movement
How do you get down VOR?
UP head movement -activates posterior SCC in both inner ears
-pathway - vestibular ganglion cells ->MVN and crossed inputs to trochlear and oculomotor IR nuclei
How do you get UP VOR
Down head motion - activates anterior SCC both inner ears
-pathway- vestibular ganglion cells->MVN and crossed inputs to the oculomotor (SR & IO) nuclei
How can oscillopsia occur ?
Damage to :
Vestibular apparatus
Vestibular nuclei
MLF damage
