11. Vestibular system Flashcards
Which 3 systems maintain balance?
- Proprioception
- Vision
- Inner ear
What is the only thing the balance system can detect?
• Only acceleration
- angle acceleration (turning)
- linear acceleration (forward/backward)
What is angular acceleration governed by?
3 semi-circular canals (head motion)
What is linear acceleration governed by?
2 otolith organs
What are the normal functions of the vestibular system?
- Subserve perception of movement in space and tilt with respect to gravity
- Provide reflex balance reactions to sudden instability of gait or posture (vestibulo-spinal reflexes)
- Stabilise the eyes on earth fixed targets - preserving visual acuity during head movements (vestibulo-ocular reflexes)
- Assist control of blood pressure and heart rate during rapid up-down tilts
- Assist synchronisation of respiration with body reorientations
- Provokes motion sickness
- Provide a reference of absolute motion in space - spatial orientation
What is the disorder relating to a false perception of movement in space?
Vertigo
vestibulo-cortical
What is the disorder relating to instability of gait and posture?
Vestibular ataxia
vestibulo-spinal
What are the 2 disorders relating to the inability to stabilise the eyes?
- Vestibular nystagmus - unilateral lesions
- Oscillopsia - during head movement in bilateral vestibular lesions
(vestibulo-ocular)
What problems can arise in the acute phase of vestibular loss, and unilateral vestibular loss, respectively?
- Slight impairment of orthostatic control
* Severe nausea and vomiting
What can a disorder of the vestibular system lead to, relating to orientation and motion?
Loss of co-ordination on:
• directional reorientation
• motion intolerance
• oversensitivity to visual motion in the environment
Which vestibular projection is related to nausea?
Vestibulo-autonomic
What are the 2 main parts of the vestibular labyrinth?
• Bony labyrinth - bounded by petrous temporal bone filled with perilymph
• Membranous labyrinth
- filled with endolymph containing the receptor cells
- located within the utricle, saccule and semicircular canals
Where are the epithelial hair cells (sensory receptors) located in the vestibular labyrinth?
- Macula - utricle and saccule
* Crista ampullaris (swelling of semicircular canals)
Where are the primary afferent neurones located in relation to the vestibular labyrinth?
- Cell bodies in vestibular ganglion
- Peripheral processes end in macula and ampulla
- Central process synapse in brainstem vestibular nuclei
What causes the deflection of the hair cells in the canals and which nerve do they signal to?
- Head movement
- Acceleration causes endolymph movement => deflection of macula (where all the hair cells are)
- When you stop, this occurs in the opposite direction
- Signals to the auditory nerve (cochlear nerve - one of 2 parts of CN VIII)
What are hair cell cilia embedded in?
- Otoconia
* Gelatinous matrix containing calcium carbonate crystals
How are the sensory hair cilia arranged?
- Stereocilia - arranged in rows of varying heights
* Kinocilium - tallest one, for morphogenesis and mechotransduction
What are the different nerve endings on hair cells?
- Type I - chalice-like endings forming ribbon synapses
* Type II - simple nerve terminals
Describe the transduction mechanisms of the hair cells (hair cell receptor potential + ganglion cell discharge)
Hair cell receptor potential
• towards the kinocilium = depolarisation
• away from the kinocilium => hyperpolarisation
Ganglion cell discharge
• towards to kinocilium => increased frequency
• away from the kinocilium => decreased frequency
What is the striola?
- Dividing ridge that runs through the middle of the macula
- Divides hair cells into populations with opposing hair bundle polarities
- Kinocilia are oriented towards from it in the utricle, and away from it in the saccule
- Tilt of the head to one side has opposite effects on corresponding hair cells of the other side
Describe the orientation of the Otolith organs
Saccule
• Approximately vertical plane
• Hair cells with their overlaying layer of otoconia project normal to the plane with directional sensitivities:
- all combinations of vertical and antero-posterior directions
Utricle
• Approximately horizontal plane
• Hair cells project vertically with directional sensitivities:
- all combinations of lateral and antero-posterior directions
What force stimulates the Otolith hair cells and what give the complete signal?
- Inertial resistance of the otoconial mass to linear head acceleration
- Vector sum of utricular and saccular stimulation patterns give a signal of linear acceleration in all 3D directions
How does the vestibular labyrinth convey information of a static upright body (head) position and a head tilt?
- Vestibular ganglion neurones that innervate the saccule have tonic discharge
- Due to constant hair displacement imposed by gravity
- Input continues to produce action potentials over the duration of this stimulus
What is the ampullary cupula and how are the hair cells related to it?
- Structure providing the sense of spatial orientation
- Located within the ampullae of each of the three semicircular canals
- Gelatinous component of the crista ampullaris that extends from the crista to the roof of the ampullae
- Cilia of the hair cells are embedded in this, and are stimulated by the differential movement of the endolymph and crista
Which canal does rotation to the right stimulate?
Right canal
Which canal does stopping a rightward rotation (deceleration to the right) stimulate?
Left canal
What are the (4) different vestibular nuclei and how are they organised?
Somatotopic organisation • Kinetic labyrinth (SCCs) - superior - medial • Static labyrinth (otoliths) - lateral - inferior
What 4 main targets do the vestibular nuclei project to?
- Spinal cord
- Nuclei of the extraocular muscles
- Cerebellum
- Centres for cardiovascular + respiratory control
What does the vestibular pathway from the static labyrinth (otoliths) influence?
- Vestibulospinal tract
- Antigravity muscles that contribute to upright posture and equilibrium
- Blood pressure
What does the vestibular pathway from the kinetic labyrinth (SCC) influence?
- Medial longitudinal fasciculus
- Eye movements that ensure that the retinal image is kept stationary when the head moves
- Postural adjustments
- Cardiovascular + respiratory function
Describe the pathway of the lateral vestibulo-spinal tract
• Descends ipsilaterally in the ventral funiculus of the spinal cord
• Axons terminate in the lateral part of the ventral horn
- influence motor neurones to the limb muscles (especially extensor antigravity muscles)
Describe the pathway of the medial vestibulo-spinal tract
• Descends bilaterally in the medial longitudinal fasciculus (MLF) to the cervical and upper thoracic spinal cord
• Axons terminate in the medial part of the ventral horn
- influence motor neurones to the neck and back muscles
What is the vestibular-ocular reflex?
- Reflex that functions to stabilise images during head movement
- Done by producing eye movements in the direction opposite to head movement
- This preserves the image on the centre of the visual field
- With continuing head rotation frequent, saccades reposition the eyes more centrally to form an overall pattern of normal or ‘physiological vestibular nystagmus’
- Nystagmus = involuntary eye movement
How is the vestibular-ocular reflex carried out?
• Axons of the medial vestibular nucleus cross the midline and project to the contralateral abducens (VI) nucleus
- abducts the eye (in the opposite direction to head rotation
• Axons from the VI cross and ascend in the MLF and excite the contralateral oculomotor (III) nucleus
- adducts the other eye (in the opposite direction to head rotation)
Which neurones project to the motor nuclei supplying extraocular muscles?
Superior and medial vestibular neurones
How can you test the vestibulo-ocular reflex?
• Horizontal v.o. reflex can be elicited by warming or cooling the endolymph in the SCC
• Warm caloric test applied to the right ear (warm water pumped into the ear)
• Should produce a slow drift of the eyes away from the stimulated side
• Followed by a fast saccade towards the stimulated side
- direction of nystagmus named in accordance to this fast saccadic phase
Describe the vestibulo-cerebellar pathways?
- Afferents from the vestibular ganglion and nuclei
- They go to the cerebellar cortex of the flocculonodular lobe
- Cerebellar efferents from fastigial nucleus (deep) to all vestibular nuclei
- Role in postural regulation (via vestibulo-spinal tracts) and eye movements (via MLF)
Describe the vestibular pathways to the thalamus and cortex
- All vestibular nuclei project to ventral posterior and ventral lateral nuclei of the thalamus
- Thalamic nuclei project to 2 cortical areas (2V and 3a) - part of the ‘head region’ of the primary somatosensory cortex
- Projection also to the superior parietal cortex: ‘vestibular cortex’ - spatial orientation
- Cortical projections may account for feeling of dizziness (vertigo) during certain kinds of vestibular stimulation
What are peripheral vestibular disorders caused by and give some examples?
Problems with the labyrinth and vestibulocochlear nerve
• vestibular neuritis (acute)
• BPPV (intermittent)
• Meniere’s disease (recurrent) bilateral, unilateral
What are central vestibular disorders caused by and give some examples?
Problems with the CNS (brainstem/cerebellum) • Stroke • MS (acute) • Tumours • Cerebellar degeneration (chronic)
What is oscillopsia?
- Marked loss of vestibular function
- Impaired eye stabilisation during rapid head movement - vestibulo-ocular reflex is the only mechanism that can drive fast compensatory eye movements
- Visual world will seem to oscillate or lag behind during active or passive head movements
How can you test for oscillopsia?
- Head shaking test
- Focus on target
- Normally eyes remain fixed
- In oscillopsia, eyes will be taken off target, and multiple catch up saccades will be made to regain the target
- May affect one side
What are the different types of vestibular ataxia?
Bilateral vestibular disorder
• Mild gait ataxia
• Worse at speed, when negotiating rough ground or when vision is reduced
Unilateral vestibular disorder
• Tendency for the body and head to lean or fall to the lesioned side
How does a loss of vestibular tone affect BP?
- Provokes hypotensive episodes
* Patient feels faint as well as dizzy
What is the likely cause of vestibular vertigo that lasts seconds or minutes?
Seconds
• Benign positional vertigo
• BPPV (due to debris in canals)
Minutes
• Vertebrobasilar insuffiency
What is the likely cause of vestibular vertigo that lasts hours or days?
Hours
• Meniere’s syndrome
• Hearing disturbance
Days
• Vestibular neuritis
• Infarction of labyrinth
What is Benign, Paroxysmal Positional vertigo (BPPV) caused by?
- Normally there are little, dense crystals in the hair cells
- If you bang your head, crystals fall off hair cells and fall into the fluid in the canals
- Otoconial debris stimulate the ampulla causing false signals of head rotation
- Results in short-lived dizziness with head movements
What is Meniere’s syndrome caused by?
- (Reissner’s/basilar) membrane between endolymph and perilymph
- Membrane splits and the fluids mix => loss of balance function on affected side
- Vicious attack of vertigo and hearing loss
- Triad: vertigo, tinnitus, deafness
- Very rare
What is vestibular neuritis?
- Sudden, unilateral vestibular loss
- Hearing is spared
- No CNS symptoms or findings
- After URTI; mini-epidemics (viral related)
What is a vestibular migraine?
- Most common cause of recurrent spontaneous vertigo attacks
- Migraine symptoms during attack
- Hearing usually spared
- Response to treatment
How can you test for BPPV?
- Dix-Hallpike manoeuvre
- Positive test - patient reports of reproduction of vertigo and nystagmus is observed
- Modified test is done with patient moved sideways, done due to anxiety or discomfort with conventional (supine) manoeuvre
What is the manoeuvre is done to move debris out of the sensitive part of the SCCs?
- Semont repositioning manoeuvre
* Patient is rapidly moved from lying on one side to another
Where do the vestibular projections go to, involved in perception of motion in space, and how can a unilateral canal lesion affect this?
• Via thalamus to temporo-parietal ‘spatial’ cortex
- The tonus of the intact canal gives a signal as if the head is rotating to the intact side
- Patient may experience symptoms of intense spinning or feelings as if on a boat (vertigo)
- Eyes are drive to the lesioned side because of this
- Brainstem detects this and causes saccades to the intact side - seen in the acute phase of loss and minimised by visual suppression mechanisms
