Vestibular System Flashcards
Define the vestibular system
A sensory system essential in the control of posture and balance. Found in the inner ear, it is a series of fluid-filled membraneous tubes, (labyrinths), which are embedded in the temporal bone.
What does the vestibular apparatus consist of?
3 semi-circular canals, the utricle to which the semi-circular canals all connect and the saccule
Describe the structure of the semicircular canals
They sit at right angles to each other (3 dimensions) and have swellings at their bases, called ampulla, containing sensory hair cells.
What do the utricle and saccule contain?
Sensory hair cells that detect movement.
What are the otolith organs?
Collective terms for utricle and saccule
What is the function of the otolith organs?
Detect linear acceleration and encode information about the position of the head in space - any movement in a straight line
What movement is detected by the utricle?
Back/front tilt
What movement is detected by the saccule?
Vertical (up and down) movement
What movement do the semicircular canals detect?
Rotational acceleration
What is the macula of the otolith organs?
Contain sensory hair cells which detect movement
What is the sensory component of the semicircular canals?
Ampulla
What is the sensory component of the otolith organs?
Macula
What is found inside the ampullae of the SSCs?
Sensory receptors called Cristae, which consist of a flexible gelatinous structure called the cupula that stretches across the entire width of the ampulla and responds to movement of the endolymph fluid with the canals as it moves and then moves the hair cells within cupola
How does cristae send information back to the brain?
Movement of endolymph pushes on the gelatinous cupula and activate the receptors cells
Hair cells synapse directly with the sensory neurones of he vestibular nerve (CN VIII)
What is the inertia of fluid?
If the skull is rotated left or right from rest, the endolymph at first does not move because of its inertia. However the ampulla moves instantly because it is embedded in the skull.
How is an action potential triggered from the cupula through movement of the head?
As you turn your head your bony skull does, but due to inertia of endolymph, and so it initially doesn’t move.
As skull and cupula move, but fluid doesn’t, this cures drag against the cupula.
The drag distorts the cupula and the cilia imbedded inside it, in the opposite direction to movement, which fires action potentials along the vestibulocochlear nerve to brain, so this is how the brain knows that you’re moving
What happens to the endolymph after the initial phase of inertia?
If rotate at constant velocity, the endolymph catches up and rotates at the same speed, removing the shearing forces, but this takes several seconds
What happens when you suddenly stop the movement of your head?
Causes endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness (mixed messages sent to brain)
Momentum of endolymph continues causing shearing of cupula in the opposite direction to that of the start
What are the two types of hair cells?
- Kinocilium - single, very large
* Stereocilia - progressively smaller stereocilia
What is the effect on the number of APs fired if the stereocilia are distorted AWAY from the kinocilium?
Hyperpolarisation and decreased discharge of APs in the vestibular nerve
What is the effect on the number of APs fired if the stereocilia are distorted TOWARDS the kinocilium?
Depolarisation and increased discharge of APs in the vestibular nerve
What is the benefit of the change in APs fired due to direction of the stereocilia to the kinocilium?
During movement, some of the ampulla in the SSCs increase AP and some decrease.
This allows the brain to determine movement in time and space.
The orientation of the cupulae are slightly different so the brain can bind a 3D image of body position using pattern of firing and inhibition received.
Where in the brain does the integration of sensory info from the ampullae take place?
Cerebellum
How is the maculae orientated in each otolith organs?
- Utricle - horizontal plane
* Saccule - vertical plane
Describe the mucula
Contains cilia (one kinocilium and series of sterocilium) which protrude into a gelatinous massed called the otolith membrane.
Embedded in the otolith membrane are CaCO3 crystals called otoliths.
What is the otolith membrane?
Gelatinous material in muculae
What are otoliths?
CaCO3 crystals in maculae
Why are ololiths affected more by gravity compared to endolymph?
They have a greater density so they exaggerate the movement of the otolith membrane to distort the cilia
Describe how tilting the head stimulates signals sent to the brain?
Detected by the macula in the utricle (horizontal plane)
Tilting the heads moves the otoliths and creates drag on the otolith membrane in which they are embedded -> distorts the jelly and moves the cilia
In which direction of the stereocilia in the macula increased and decreases discharge of APs?
- Backward tilt -> moves otolith (and stereocilia) in direction of kinocilium causing depolarisation and increased discharge of AP
- Forward tilt -> movement away from kinocilium and hyper polarisation and decreases AP discharge
What do macula in the saccule respond to?
Vertical forces i.e. movement in a lift, and provides information on orientation of head when lying down
Why do the vestibular centres of medulla and cerebellar centres coordinate?
Vestibular centre determine position in time and space and the cerebellar centre coordinate the postural muscle required to maintain balance
What three components allow coordination between the vestibular and cerebellar centres?
- Projections from the Vest. nuclei on one side project ipsilaterally, bilaterally and contra-laterally to descending motor pathways (and also the extraocular muscle nuclei)
- Vest. nuclei receive input from proprioceptors signalling limb and body position, also from neck and eye muscles.
- Vest. nuclei project via the thalamus to the cerebral cortex -> perception of movement and body position = kinaesthetsia
What two tracts does the vestibular system reflexes involve?
Vestibulocoritcal and vestibulospinal tracts
What are three vestibular systems reflexes?
- Tonic labyrinthine reflexes
- Dynamic righting reflexes
- Vestibulo-ocular reflexes
What is the function of the Tonic Laryrinthine Reflexes?
Keeps the axis of the head in a constant relationship with the rest of the body
Uses info from maculae and proprioceptors
What is the function of the Dynamic righting reflexes?
Adjusts position to maintain balance when moving.
Initiate rapid postural adjustments that are made to stop you falling when you trip.
Long reflexes, involving extension of all limbs.
What is the function of the Vestibulo-ocular reflexes?
Afferents from the semi-circular canals project and connect (within the vestibular nuclei) to afferent fibres travelling to the extraocular nuclei to influence eye movement.
The visual system also sends descending projections which control posture. Demonstrated by the increased difficulty to balance when you close your eyes
What occurs with destruction of the vestibular apparatus in relation to balance (i.e. vestibule-ocular reflexes)?
Can still maintain balance if movement is relatively slow and eye are open.
Balance is lost immediately on closing eye.
Name two vestibulo-occular reflexes
Static reflex and Dynamic Vestibular Nystagmus
What is the static reflex of vestibulo-occular reflexes?
When you tilt your head, the eyes intort/extort to compensate, so that over a certain range, the image stays the right way up.
What is the Dynamic Vestibular Nystagmus reflex of the vestibulo-occular reflexe?
A series of saccadic eye movements that rotate the eye against the direction of passive rotation of the head and body
This allows the original direction of gaze whilst head in rotating and until eyeball come to end of it’s range of movement and it rapidly flicks to the zero position (straight ahead)
If rotation continues, the slow phase starts agains, then flicks back.
What is a saccadic eye movement?
Movement during passive movement of body - slow movement to keep gaze of original direction whilst head is rotating and then fast movement of eyes to the new direction of gaze
Define nystagmus
Fast flicking between the direction of gaze
A Right rotation -> R nystagmus
What is nystagmus used to test?
Vestibular function
Name two tests to test vestibular function
Post-rotatory nystagmus and caloric stimulation
Describe process of post-rotatory nystagmus test
Subjects are rotated in a Barany chair. If rotate to the left then during acceleration get a left nystagmus. At the end of rotation, for about 20 seconds, during deceleration get a right nystagmus.
Due to endolymph in SSCs catching up and now pushing the cupula in the opposite direction.
What the the caloric stimulation test test?
Function of horizontal semicircular canal
How do you carry out a caloric stimulation test?
Outer ear is washed with either cold or warm fluid, the temperature difference from core 37C, gets through the thin bone and sets up convection currents which affect the endolymph.
What do the results of the caloric stimulation test indicate?
Warm fluid causes nystagmus towards affected side, cold causes nystagmus away from affected side (COWS – Cold Opposite, Warm Same)
Warm into R ear -> R Nystagmus, Cold into R ear -> L nystagmus
What is it important to remember if irrigating the outer ear (wrt temp affecting nystagmus)?
Use fluid of appropriate temperature. The stimulation of the vestibular system in the absence of movement can cause nausea and vomiting.
What can powerful maintained stimulation of the vestibular system cause?
Kinetosis = motion sickness, caused by mixed signals to the brain
What are the symptoms kinetosis?
Autonomic NS symptoms: • Nausea and vomitnig • Decrease BP • Dizziness • Sweating and pallor
What is the cause of kinetosis (motion sickness)?
If visual and vestibular system inputs to the cerebellum are in conflict
I.e. when in a car, if you look at the headrest your eyes send signals to brain to say you’re not moving, but your ears signal that you’re accelerating
The cerebellum generate ‘sickness’ signal to the hypothalamus to bring about ANS changes
Name three conditions related to abnormal vistublar function
- Labrynthitis
* Menieres disease
What is labrynthitis?
Acute interference with normal vest. function due to infection = all ANS symptoms and vertigo
What is vertigo?
Perception (hallucination) of movement in the absence of movement. There may also be nystagmus. Gross impairment of posture and balance.
What are the features of Meniere’s disease?
Vertigo, nausea, nystagmus and tinnitus = subjective noise. Associated with over production of endolymph causing increased pressure.
What drug is the inner ear sensitive to?
Streptomycin
