Vestibular Systems Flashcards
Which structures in the body are responsible for angular (rotational) motion of the head?
Semi-circular canals
Which structures in the body sense the acceleration of the head and the strength of gravity?
Otolith organs
What are the two otolith organs?
Saccule
Utricle
What are the main functions of the vestibular system?
Subserve perception of motion in space and tilt (with respect to gravity).
Provide reflex balance reactions to sudden instability of gait/posture (vestibulo-spinal reflexes).
Stabilise the eyes on fixed targets during head movement, preserving acuity (vestibulo-ocular reflexes).
Assist in control of heart rate and blood pressure during rapid up-down tilts.
Assist synchronisation of respiration with body reorientations.
Provokes motion sickness
Define vertigo.
False perception of movement in space
Define vestibular ataxia
Instability of gait or posture
What happens to the ability of the brain to stabilise the eyes in unilateral vestibular lesions?
Vestibular nystagmus.
The eyes start moving in the direction of the lesion
What happens to the ability of the brain to stabilise the eyes in bilateral vestibular lesions?
Oscillopsia.
Everything appears to be shaking – the ability to stabilise the eyes is lost
What are some other consequences of vestibular loss?
Slight impairment of orthostatic control.
Severe nausea and vomiting.
Loss of coordination on directional reorientation, motion intolerance, oversensitivity to visual motion in the environment
What type of cell is involved in the detection of movement in the vestibular system?
Hair cells
Describe the cilia of these hair cells.
There is one kinocilium and the rest are stereocilia
What does the hair cell fibre synapse with and where does it project?
It synapses with a primary neurone dendrite (cell body in Scarpa’s ganglion).
They project to the vestibular nuclei in the brainstem
What stimulates hair cells?
In Otoliths: deflection by forces of inertial resistance to acceleration.
In Semi-circular Canals: endolymphatic fluid rotation
Describe how the hair cell receptor potential can be changed.
Depolarisation = movement towards the kinocilium.
Hyperpolarisation = movement away from the kinocilium
Describe how ganglion cell discharge from otolith organs can be changed.
Towards the kinocilium = increased firing frequency.
Away from the kinocilium = decreased firing frequency
Describe the orientation and sensitivities of the saccule.
Saccule is oriented vertically with the hair cells projecting normal to the plane.
They are sensitive in all combinations of vertical and antero-posterior directions
Describe the orientation and sensitivities of the utricle.
Utricle is oriented almost horizontally with the hair cells projecting vertically.
Directional sensitivities in all combinations of lateral and antero-posterior directions
How do the otolith organs give a signal of linear acceleration in all 3-dimensional directions?
Vector sum of utricular and saccular stimulation patterns gives signal of linear acceleration in all 3-dimensional directions
Describe the structure and function of the semi-circular canals.
Hair cells project from the ampulla in the wall of the canal and are uni-directionally oriented so that acceleration to a particular side stimulates the canals on that side
(e.g. rotation of the head to the right stimulates the right canal, rotation in the other direction inhibits the right canal activity).
When head rotation decelerates to a stop, the canal on the other side (left side) is stimulated
Describe the firing of the canals when the head is still.
Each canal has a tonic firing rate so that they equal out when the head is still
What are the effects of loss of canal function on one side? (Physiology)
There is unopposed signal coming from the intact side meaning that there is partial impairment of sensitivity to rotation in the ‘on’ direction of the defunct canal
Why would a unilateral canal lesion cause vertigo?
The unopposed tonus of the intact canal gives a signal as if the head is rotating to the intact side.
Patient may feel like they’re spinning even though they’re not.
Why would acute unilateral vestibular disorder cause vestibular nystagmus?
Unopposed tonus of the intact canal causes the eyes to be driven to the lesioned side – this is a vestibulo-ocular reflex (because it thinks that your head is rotating towards the intact side)
Where do superior and medial vestibular neurones project?
They project to the motor nuclei supplying extraocular muscles.
What does the vestibular system consist of?
Give inputs and outputs. DRAW
Includes three main inputs:
- visual
- proprioceptive
- vestibular information (detects gravity and rotation of head)
- outputs are mainly reflexes to maintain a stable posture and stable gaze
- CNS integrates this information and generates the responses
What is the location of the vestibular organ?
vestibular organ is in the posterior area of the inner ear
What is the structure of the vestibular organ?
- utricule and saccule are located in the vestibule and are joined by a conduit
- saccule is also joined to the cochlea
- three semicircular canals on each ear: anterior, posterior and lateral
- semicircular canals have an ampulla on one side, and they are connected to the utricle
What is the significance of the location of the labyrinth?
- location of the vestibular organ draws planes for anterior and posterior canals
- planes determine which structure will be stimulated with a specific head movement
- head movements in different planes stimulate different parts of the vestibular organ
- important for generating the correct response
What is the role of the hair cells in the vestibular system?
- vestibular hair cells have a kinocilium (the biggest cilium) and stereocilia
- cilia allows depolarisation of the cells with movement of the endolymph generated by head movement
- present in all vestibular organs.
Describe the otolith organs
- They are the utricle and saccule
- Cells are located on maculae containing gelatinous matrix and hair cells
- Cells are placed horizontally in the utricle and vertically in the saccule
- these hair have otoliths on top:
- carbonate crystals
- weight of crystals helps deflection of hairs
Describe the structure and function of the semicircular canals
- NO otoliths here (pathology can arise here)
- hair cells in the canals are located in the ampulla
- rest of the canal only has a liquid high in potassium called endolymph
- ampulla has the crista, where the hair cells are located
- cells are surrounded by the cupula which helps the hair cell movement
Describe the planes of the semi circular canals
- orientation of the canals in the head defines three planes
- anterior and posterior canals form a 90° angle
- lateral canals are horizontal to the other canals
What are the projections of vestibular nuclei?
Give name of specific tracts you can rememebr. DXraw if possible
- Primary afferents end in vestibular nuclei and in the cerebellum
- vestibular nuclei have projections to:
- spinal cord
- nuclei of the extraocular muscles
- cerebellum (feedback on coordination)
- centres for cardiovascular + respiratory control
- autonomic centre
Describe the vestibular pathways.
Draw
- Hairs cells receive inputs, then there’s afferents to vestibular nuclei.
- superior + lateral
> ventroposterior nucleus > vestibular cortex. - lateral + medial + inferior
> vestibulospinal - inferior
> vestibulocerebellar reflexes
> vestibulo-ocular reflex (VOR)
What is the vestibular cortex?
- not one specific area
- since many inputs and integrators are involved, many cortical areas participate
- main processing centre thought to be in the parietal lobe, in the Parieto-Insular Vestibular Cortex (PIVC)
Summarise vestibular physiology.
Draw if possible
How do hair cell potentials change?
- Hair cells have a resting potential which has a basal discharge to the nerve
> when upright system should still be active to keep yourself straight. - Hairs moving towards the kinocilium generates depolarization and an increase in nerve discharge.
- Hairs moving away from the kinocilium generates hyperpolarization and a reduction in nerve discharge
How do the otolith organs work?
Respond to linear acceleration and tilt: due to otolith movement
Utricule: horizontal movement
Saccule: vertical movement
How do the semicircular canals work? Describe in detail
- Respond to angular acceleration
- cupulla moves and displaces hair cells
- causes either depolarisation or hyperpolarisation
- output signal on VIIIth (vestibulcochlear) nerve is velocity
- info integrated in the brainstem
- SCC work in pairs according to the planes (both laterals)
- anterior from one side with posterior of the opposite side
What are the vestibular reflexes?
- Vestibulo-ocular reflexes: fastest reflex in the body
- Vestibulo spinal reflexes
What is the vestibulo-ocular reflex?
- keeps images fixed in the retina
- connection between vestibular nuclei and oculomotor nuclei
- allows compensatory movement
- eye movement in opposite direction to head movement, but same velocity and amplitude
What is the vestibulo spinal reflex
connection between vestibular nuclei and spinal cord
- motor neurons to limb muscles (lateral tract)
- motor neurons to neck and back muscles (medial tract)
This leads to postural control, avoidance of falls and compensatory body movement according to the head position ◦
