Vestibular system Flashcards
What is important to remember about balance
Everything is related to balance- ear (series of impulses ) and central processing.
What are the inputs to the vestibular system
Visual system Inner ear ( detecting rotation, movement and gravity) Pressure- detected by joints and limbs
What are the outputs of the vestibular system
These inputs feed into the CNS (brainstem, cortex and cerebellum)
We have reflex responses- the vestibular-ocular reflex
And also postural control ( spinal reflex)
Nausea is another potential output- if mismatch between auditory and visual perceptions
What happens if these reflexes are not quick
You would lose balance and fall over
Summarise what is meant by the vestibular system
The vestibular system is the only sensory organ specialised to transduce absolute motion in space. Angular (rotatory) motion (of the head) is sensed by the semicircular canals; acceleration
of the head and strength and direction of gravity are sensed by the otolith organs.
What does CN8 nerve conduct
Both vestibular and auditory informaiton
What is the inner ear known as
The labyrinth
Where is the inner ear located
In the petrous part of the temporal bone
This the hardest part of the temporal bone (and the body in fact)
So the inner ear is well protected
Otherwise, you would lose balance and hearing- upon trauma
The inner ear and its structures are carved within the bone- so it fits perfectly
How many parts of the vestibular organ are there
5
Or you could say there are 5 vestibular organs
What are the two otolith organs
Saccule
Utricle
Saccule is closer to and connected to the cochlea
The utricle is connected to the saccule
Describe the semi-circular canals
3 semi-circular canals (anterior, posterior and lateral)
All connected to the utricle
Everything is connected in the inner ear
Where is the perilymph found
Between the vestibular structures and the bone
Were is the endolymph found
Within the vestibular organs themselves
What do the semi-circular canals have at the end which attaches them to the utricle
An ampulla
Anterior and posterior semi-circular canals join at their non-ampulla ends
Lateral canals do not join to any other semi-circular canal
What is the function of the endolymph fluid
Help move hair cells for transduction (for sound or movement)
Why is the orientation of the vestibular organ important for its function, but not so much for the cochlea
Because the stimulus for the vestibular organ is movement- so it matters what plan and orientation you are moving in
Where abouts in the labyrinth are the vestibular organ and cochlear found
Cohclea- anterior labyrinth
Vestibular organ- posterior labyrinth
Describe the orientation of the semi-circular canals
Anterior canal: 45deg anterior
Posterior canal: 45deg posterior
Lateral canal: lie horizontally
What structures are found within the semi-circular canals and within the otolith organs
Hair cells (same as the cochlear- but not called inner or outer)
Summarise the hair cells of the vestibular organ
Type I: More in number Direct afferent, indirect efferent Round shaped Like the inner hair cells of the cochlea Type II: Direct afferents and efferents More efferents Like the outer hair cells of the cochlea
What is important to remember about the hair cells of the vestibular organ
No special distribution of hair cells- as in the cochlea
As in the cochlea, tight junctions seal the apical surfaces of the vestibular hair cells, ensuring the endolymph selectively bathes the hair cell bundle while remaining separate from the perilymph surrounding the basal portion of the hair cell
Where are the hair cells found in the vestibular organ
The two otolith organs
The ampullae
Describe the two parts of the labyrinth (the vestibular organ)
The membranous sacs within the bone are filled with fluid (endolymph) and are collectively called the membranous labyrinth.
Between the bony walls (osseous labyrinth) is another fluid, the perilymph, which is similar in composition to the CSF.
Summarise the otolith organs (i.e the static labyrinth)
§ There are 2 otolith organs:
o Utricle – senses movement in the horizontal plane.
o Saccule – senses movement in the vertical plane.
§ The otoconia is a layer of calcium carbonate on top of a gelatinous layer and as it is heavy, movements of the head displace the otoconia and thus pull the hair cells.
o Linear acceleration will also move the heavy otoconia layer.
Describe the structure of the otolith organs
The two otolith organs detect tilting and translational movements (linear, as opposed to rotational) of the head.
Both of these organs contain a sensory epithelium, the macula, which consists of hair cells and associated supporting cells (beneath the hair cells )
Overlying the hair cells and their hair bundles is a gelatinous layer; above this layer is a thin fibrous structure, the otolithic membrane, in which are embedded crystals of calcium carbonate (called otoconia)
So the otoconia contains both the gelatinous matrix and the otolithic membrane.
What is the key role of the otoconia
To facilitate the movement of the hair cells- as the endolymph fluid is not dense enough to move the hair cells effectively.
The otoconia makes the otolithic membrane heavier than the structures and fluids surrounding it; thus, when the head tilts, gravity causes the membrane to shift relative to the macula.
The resulting shearing motion between the otolithic membrane and the macula displaces the hair bundles, which are embedded in the lower, gelatinous surface of the membrane. This displacement generates a receptor potential in the hair cells.
A shearing motion between the macula ad the otolithic membrane also occurs when the head undergoes translational movements; the greater relative mass of the otolithic membrane causes it to lag behind the macula temporarily, leading to transient displacement of the hair bundle.
What is a key feature of the crystals
They are loose
So when you move in a given direction or tilt- the crystals will move and will displace the membrane, which will displace the hair cells
So they transmit the pressure
Summarise the striola
Striola: opposing hair bundle polarities
movement in any direction stimulate a distinct subset of cells
Describe the striola
A specialised area of each otolith organs (the central part of each macula) which divides the hair cells into two populations having opposing polarities.
The striola demarcates the overlying layer of otoconia and forms an axis of symmetry such that hair cells on opposite sides of the striola have opposing morphological polarisations.
Thus, a head tilt along the axis of the striola will excite the hair cells on one side while inhibiting the cells on the other side.
Describe the different orientations of the saccular macula and the utricle macula
The saccular macula is orientated vertically and the utricular macula horizontally, with continuous variation in the morphological polarisation of the hair cells located in each macula.
Inspection of the exciatatory orientations in the macula indicates that the utricle responds to translational movements of the head in the horizontal plane and to sideways head tilts, whereas the saccule responds to vertical translations movements of the head and to upward or downward head tilts.
Why is the striola particularly important
The system needs to be quick and resistant to damage- so it needs a lot of information quickly.
So the striola ensures that we receive the same information twice, albeit a different response, one stimulatory and one inhibitory, this is important as it improves the velocity of the processing and the response.
But, it also aids in recovery as another part is helping to transmit the same signal- so you would still get a response.
Where are the hair cells found in the utricle and saccule
Utricle- at the bottom
Saccule- to one side ( the side towards the cochlea)
Summarise the semi-circular canals (ie the kinetic labyrinth)
Ampulla: Hair cells in crista
Gelatinous projection: Cupula
Kinocilia: same direction on each side of the head
Describe the structure of the semi-circular canals
At its base- a bulbous expansion- the ampulla- which houses the sensory epithelium, or crista, that contains the hair cells.
The hair bundles extend out of the crista into a gelatinous mass, the cupula, that bridges the width of the ampulla, forming a viscous barrier through which endolymph can circulate. As as a result, movements of the endolymphatic fluid distort the relatively compliant cupula.
When the head turns in the plane of one of the semi-circular canals, the inertia of the endolymph produces a force across the cupula, distending it away from the direction causing the head movement and causing a displacement of the hair bundles within the crista.
Describe the effect of translational movements of the head on the semi-circular canals
They produce equal forces on each side of the cupula, so that hair bundles within the ampulla are not displaced.
Describe the kinocilia of the semi-circular canals
Unlike the saccular and utricular maculae, all of the hair cells in the crista are organised with their kinocilia pointing the same direction. Thus, when the cupula moves in the appropriate direction, the entire population of hair cells is depolarised and the activity in all the innervating axons increases When the cupula moves in the opposite direction, the population is hyperpolarised and neuronal activity decreases. Deflections orthogonal (perpendicular) to the excitatory- inhibitory direction produce little or no response.
Describe how the semi-circular canals work together
Each semi-circular canal works in concert with the partner located on the other side of the head that has its hair cells arranged oppositely. There are three such pairs: the two (right and left) horizontal canals, and the anterior canal on each side working with the posterior canal on the other side.
Head rotation deforms the cupula in opposite directions for the two partners, resulting in opposite changes in their firing rates. Thus, the orientation of the horizontal canal makes them selectively sensitive to rotation in the horizontal plane. More specifically, the hair cells in the canal toward which the head is turning are depolarised, while those on the other side are hyperpolarised.
What is important to remember about the cupula
It is not as dense as the gelatinous matrix of the otolith organs
What is the arrangement of the semi-circular canals similar to
A corner of a room
Describe how a stroke can present with inner ear symptoms
Communication between brain and inner ear in terms of their blood supply.
Anterior inferior cerebellar artery has two branches- one to the cerebellum and another, the labyrinthine artery- which has many branches to supply the structures of the inner ear.
Thus a stroke can present with dizziness if the clot is in this artery- need to spot this quickly and thrombolyse the patient.
What are the two parts of the vestibular nerve
Superior and inferior
Clinically important as only 80% of tests available test the superior part of the nerve- so if you present with symptoms relating to pathology of the inferior vestibular nerve- then this won’t be picked up by many tests.
Describe the vestibular nerve and the targets of their afferents
Primary afferents end in vestibular nuclei (in the brainstem) and in cerebellum
Vestibular nuclei superior lateral medial inferior
Organisation Static labyrinth (otoliths) lateral & inferior Kinetic labyrinth (SCC) superior & medial
Where are all the reflexes for the vestibular system generated
In the brainstem.
The brainstem will receive information from the inner ear, visual system, cerebellum and proprioception senses too
Connected to motor nuclei to generate response
What are the projections of the vestibular nuclei
spinal cord
nuclei of the extraocular muscles
Cerebellum
Centers for cardiovascular + respiratory control
Describe the pathway from the hair cells to the vestibula nuclei
As with the cochlear nerve, the vesitublar nerves arise from a population of bipolar neurones, the cell bodies of which in this instance reside in the vestibula nerve ganglion (Scarpa’s ganglion). The distal processes of these cells innervate the hair cells of the kinetic and static labyrinth, while the central processes project via the vestibular portion of the vestibulocochlear nerve to the vestibular nuclei (and also directly to the cerebellum).
Hair cells — vestibular nuclei (via vestibular nerve)
Describe the convergence of both canal and otolith afferents
Although the canal and otolith afferents are largely segregated in the periphery, a large amount of central-otolith convergence is found in the vestibular nuclei, a feature that ultimately enables the unambiguous encoding of head orientation and motion through the environment.
Indeed, although head tilts and translational movements of the head can similarily excite otolith organs, the semi-circular canals are excited only by rotations that accompany head tilts and not by purely translational movements.
Therefore, integration of information from the otolith organs and canals in the vestibular nuclei and cerebellum can be used to distinguish head tilts from translational head movements.
Which reflexes do the central projections of the vestibular system participate in
Those responsible for maintaining equilibrium and gaze during movement
Those responsible for maintaining posture
Summarise the vestibulo-spinal reflex
Lateral and medial vestibulospinal tracts to limb and trunk and upper back and neck respectively.
Summarise the vestibulo-cerebellar reflex
Inferior cerebellar peduncle
To the vestibulo-cerebellum (Flocconodular lobe)
Summarise the vestibulo-ocular reflex
Medial longitudinal fasciculus to: Oculomotor nucleus (superior, medial and inferior rectus) Abducens nucleus (lateral rectus) Trochlear nucleus (Superior oblique)
Summarise the functions of the vestibulo cerebellar pathways
Movement coordination
Posture regulation
VOR modulation
Info from medial vestibular nuclei (rostral medulla) and lateral vestibular nucleus (mid pons)
Describe the vestibulo cerebellar pathways
The cerebellum is a major target of ascending vestibular pathways and also provides descending input to the vestibular nuclei, resulting in a recurrent circuit architecture that plays an important role in modulating vestibular activity.
The vestibular-cerebellar circuits play a critical role in integrating and modulating vestibular signals to enable adaptive changes in the VOR, distinguish head tilts from rotational movements, and distinguish passive movements form the head and body from those that are self-generated.
Major vestibular targets in the cerebellum include the flocuculus, paraflocculus, nodulus, uvula, rostral fastigial nucleus and vermis all play distinct roles in vestibular plasticity and multimodal integration.
What is important to remember about damage to the cerebellum
Coordinates motor activity- but does not initiate it
So in pathology of the cerebellum- you will still be able to walk- but you will walk funny- ataxia
Eye reflexes will work differently.
Summarise vestibular pathways to the thalamus and cortex
Superior and lateral vestibular nuclei— ventroposterior nucleus of the thalamus (via the medial lemniscus)– vestibular cortex via internal capsule
The neurone projecting to the thalamus projects both ipsilaterally and contalaterally
Summarise the role of the thalamus and the cortex in the vestibular system
Vestibular nuclei: project to thalamus
Thalamic nuclei: project to the head region of the primary somatosensory cortex
Also to superior parietal cortex: ‘vestibular cortex’ concerned with spatial orientation.
Cortical projections may account for feeling of dizziness (vertigo) during certain kinds of vestibular stimulation
What is important to remember about the vestibular cortex
There is no single primary vestibular cortex
Rather, there is a vestibular cortical system, involving a distributed set of cortical areas, involving a distributed set of cortical areas in the parietal and posterior insular regions. Each of these areas contains neurones that are modulated by the vestibular signals, interconnected across areas, and give rise to subcortical connections with the vestibular nuclear cortex of the brainstem. Of particular importance is the parietoinsular vestibular cortex, which integrates multimodal proprioceptive signals and generates a ‘head-in-space’ frame for the body orientation and motor control.
Other areas contributing to the vestibular cortical system include the ventral premotor cortex (central sulcus) and the cingulate motor area, as well as the visual posterior Sylvian area
Summarise the physiology of the vestibular system
Sensory inputs (visual, vestibular and proprioceptive) These feed into the central processes ( the prmary processor (vestibular nuclear complex) and the adaptive processor (cerebellum)- both the central processes raly information with each other. The primary processor has output to the motor neurones which control both eye and postural movements.
Summarise the functions of the vestibular system
To detect and inform about head movements.
To keep images fixed in the retina during head movements.
Postural control.
Describe the depolarisation of the hair cells
Depolarised- by movement of kinocilia K+ channels open Ca2+ influx Binding of vesicles with membrane Release of NT to nerve
Describe the firing rate of the hair cells in the otolith organs
Hair cells in the vestibular organ are always discharging- they have a resting firing rate- as gravity is always a stimulus (an acceleration of force)- important otherwise you wouldn’t be able to maintain balance even when standing still.
Prior to the movement, the axons will have a high firing rate (resting rate) which either increases or decreases depending on the direction of the movement (depolarisation or hyperpolarisation)
The response remains at a high level, as long as the tilting force (for otolith organs) remains constant, such neurones faithfully encode the static force being applied to the head.
When the head returns to the original level, the firing level of the neurones returns to baseline value
Conversely, when the tilt is in the opposite direction, the neurones respond by decreasing their firing rate below the resting level.
Describe the firing of the cells in the semi-circular canals
Exhibit a high level of spontaneous activity (resting rate)
Thus, they can transmit information by increasing or decreasing their firing rate, hence more effectively encoding rotational head movements.
When rotated continuously in one direction, there are three phases:
Initial period of acceleration (maximum firing rate)
Period of constant velocity (resting rate)
Sudden deceleration (minimum firing rate- cupula deflected in opposite direction- leading to hyperpolarisation)
Which structures in the body sense the acceleration of the head and the strength of gravity?
Otolith organs
– stimulated by LINEAR acceleration and GRAVITY force.
o This gives a signal of head acceleration and tilt.
What is important to remember about the otolith organs on each side of the head
The saccular and utricular maculae on one side of the head are mirror images of those on the other side.
Thus, a tilt of the head to one side has opposite effects on corresponding hair cells of the two utricular maculae.
This concept is important in understanding how the central connections of the vestibular periphery mediate the interaction of inputs from the two side of the head
Which structures in the body are responsible for angular (rotational) motion of the head?
Semi-circular canals
Summarise how the semi-circular canals work
Angular acceleration
Endolinph intertia
Cupulla moves and displaces hair cells
Superior and Inferior SCC: (+) Away from Utricule
Horizontal SCC: (+) Towards the Utricule
Output signal on VIIIth nerve is velocity
Endolymphatic fluid circulation stimulates the hair cells- the fluid moves opposite to the direction of angular acceleration ( like when you shake water in a bottle)
Describe the role of the brainstem in the vestibular system
One side has faster discharge (the direction the stimulus is moving in)
The other side has an inhibited discharge
The brainstem senses this difference and thus tells you the direction in which your head has moved,
Why is it important that the semi-circular canals respond to acceleration as opposed to velocity
If it responded to velocity- it would respond constantly to constant velocity and so it would feel like you are constantly moving.
Summarise the two vestibulo spinal pathways
Lateral vestibulo spinal tract
Ipsilateral
Motor neurons to limb muscles
Medial vestibulospinal tract
Bilateral
Motor neurons to neck and back muscles
What is important to remember about balance
Balance defects become more pronounced in low light, or while walking on an uneven surface, indicating that balance normally is the product of vestibular, visual and proprioceptive inputs.
Describe the lateral vestibulospinal tract
descends ipsilaterally in ventral funiculus of spinal cord
axons terminate in lateral part of ventral horn and influence motor neurons to limb (especially extensor antigravity) muscles
The axons terminate monosynaptically on extensor muscles, and they disynaptically inhibit flexor motor neurones; the net result is a powerful excitatory stimulus for the extensor (antigravity) muscles.
When hair cells in the otolith organs are stimulated, signals reach the medial part of the ventral horn. By activating the ipsilateral pool of motor neurones innervating extensor muscles in the trunk and limbs, this pathway maintains balance and the maintenance of upright posture
Describe the medial vestibulospinal tract
descend bilaterally in medial longitudinal fasciculus (MLF) to cervical and upper thoracic spinal cord
axons terminate in medial part of ventral horn and influence motor neurons to neck and back muscles (axial muscles)
ipsilateral projection more dense
This pathway regulates head position by reflex activity of neck muscles in response to stimulation of the semicircular canals caused by rotations of the head. For example. during a downward pitch of the body (i.e tripping), the superior canals are activated and the head muscles reflexively pull the head up.
The dorsal flexion of the head initates other reflexes, such as forelimb extension and hindlimb flexion, to stabilise the body and protect against a fall
What are the key properties of the vestibular ocular reflex
Function: To keep images fixed.
Connection between vestibular nuclei and oculomotor nuclei.
5 - 7 msec latency
Eye movement in opposite direction to head movement (but hopefully with the same velocity and displacement as the head)
Quickest reflex in the body!
Describe the pathway of the vestibular ocular reflex in response to rotation
Vestibular nerve fibres originating in the left horizontal semi-circular canal project to the medial and superior vestibular nuclei. Excitatory fibres from the medial vestibular nucleus cross to the contralateral abducens nucleus, which has two outputs.
One of these is a motor pathway which causes the lateral rectus of the right eye to contract; the other is an excitatory projection that crosses the midline and ascends via the medial longitudinal fasciculus to the left oculomotor nuclueus (midbrain), where it activates neurones that cause the medial rectus of the left eye to contract.
Finally, inhibitory neurones project from the medial vestibular nucleus cross to the left abducens nucleus (pons), directly causing the motor drive on the lateral rectus of the left eye to decrease and also indirectly (via oculomotor) the right medial rectus to relax.
The consequence of these several connections is that the excitatory input from the horizontal canal one one side produces eye movements to the opposite sides.
Describe the VOR in the vertical directions
Superior vestibular neurones from the vertical canals project ipsilaterally to the IIIrd and IVth nuclei to generate vertical vestibular-ocular reflexes.
Give an example of a unilateral lesion
Horizontal nystagmus
Problem in inner ear
Outline some diagnostic methods
Anamnesis- pay attention to their stories Cranial nerves Balance and Gait assessment Cerebellum- bring finger to nose Gaze assessment: eye movements- nystagmus Vestibular tests: Caloric test vHIT VEMP Rotational test Imaging: CT Scan, MRI Subjective assessment (questionnaires)
Describe some symptoms that the patient may experience
Vertigo: Illusion of movement usually rotational or ‘true vertigo’ Dizziness, giddiness: more vague Unsteadiness: off balance= BVF Self –motion perception
Describe the epidemiology of balance disorders
1/4 people experienced dizziness at some time
80% severe enough to see a doctor (Kroenke 1992)
1/2 experience dizziness in people 75+ years (Downton and Andrews 1990)
1/4 referrals to ENT and neurology clinics
Describe the different causes of central and peripheral vestibular disorders
Peripheral vestibular disorders: labyrinth and VIII nerve
e.g., vestibular neuritis, BPPV, Meniere’s disease BVF, UVF.
Central vestibular disorders: CNS (brainstem/cerebellum)
e.g., stroke, MS, tumours
Describe the time scale of the different vestibular disorders
- Acute: Vestibular Neuritis (‘labyrinthitis’)
Stroke - Intermittent: Benign Paroxysmal Positional Vertigo (BPPV)
- Recurrent: Meniere’s Disease - rare
Migraine - common - Progressive: Acoustic Neuroma (8th nerve)
Degeneration
Describe the issue with the word dizzy
Vague and non specific…not always vestibular:
* Heart disorders * Presyncopal episodes * Orthostatic hypotension * Anaemia * Hypoglycaemia * Psychological * Gait disorders
