Lecture 32: Vestibular physiology Flashcards
Why do we often see symptoms that affect both hearing and vestibular parts?
Vestibular organs and cochlear share the same systems (fluids and nerve fibres going through same tracts),so we often see symptoms that affect both hearing and vestibular parts.
Describe the role of vestibular system and what maintenance of balance involves
Humans possess a highly sophisticated integrated system for maintaining balance (steady position).
- This sense of balance is not prominent in our consciousness but is essential to co-ordination of motor responses, eye movement (e.g. maintaining fixation on an object while moving e.g. move your head up and the eyes will look down) and posture.
- This achieves dynamic (functions when you move) and static (where you are relative to gravity) equilibrium.
Maintenance of balance involves the integration of
1) visual,
2) vestibular,
3) proprioceptive inputs (receptors in joints) and superficial sensory information.
These then integrate with outputs to maintain posture.
Maintenance of balance involves the integration of….
1) visual,
2) vestibular,
3) proprioceptive inputs (receptors in joints) and superficial sensory information.
These then integrate with outputs to maintain posture.
Proper balance and posture require continuous information about position and motion of all body parts including head and eyes. However, feedback information about head and eyes must be independent of each other as the eyes can be fixed on a target when head is moving
Describe the role of balance/vestibular syetem in cognition
Role in cognition include
1) self-motion perception,
2) bodily self-consciousness,
3) spatial navigation,
4) spatial learning,
5) spatial memory
6) objective recognition memory
What are some consequences of damages to the vestibular system?
ACUTE and CHRONIC
The vestibular system is exquisitely sensitive and finely balanced.
- Acute minor derangements in vestibular function (i.e. mild infection in the inner ear) can have catastrophic effects on balance, causing vertigo (sense of losing balance, movement) disorientation and nausea, which results in some people fall to the ground.
- Chronic and gradual loss of vestibular function may have limited symptoms of disequilibrium with compensation at central nuclei (brain adaptation).
The vestibular system senses…..
The vestibular system senses dynamic and static position of the head and body.
It provides information on direction of movement of the head and body by detecting linear** and **angular acceleration (NOT detecting velocity). (if you’re in constant acceleration, you won’t sense speed)
This information allows conscious awareness of position of head and body in space, enables reflex control of eye movements, posture and body motion.
Children with middle ear disease can impair vestibular processes leading to problems with the development of motor skills.
What is the word meaning “sense of losing balance, movement)”
Vertigo
What are the symptoms of vertigo?
Vertigo is perception of motion either of person or environment (room is spinning) when there is none. It has directionality to it.
- Differs from light-headedness (e.g. due to change in BP) or dizziness
- Often accompanied by visceral autonomic symptoms e.g. pallor, sweating, nausea and vomiting.
What is Motion Sickness?
Conflict between senses. Information from vestibular, visual, proprioceptive systems compared with an internal model of expected congruity.
Mismatch of inputs (stimuli) promotes the symptoms associated with vertigo and motion sickness.
(e. g. if the eyes say you’re not moving but vestibular system tells you that you are- mismatch of inputs)
* For example, there is no gravity in space, so no vestibular function. Astronauts sees things moving but brain tells them that they are not moving, which also affect spatial memory (atrophy of vestibular inputs and hippocampus due to lack of stimulation)*
Describe the Organisation of the vestibular system
The vestibular system comprises sensory organs in the peripheral system located in inner ear which projects to vestibular nuclei in brainstem via vestibular portion of vestibulocochlear nerve (CN VIII).
The vestibular nucleus occupies a large portion of the medulla beneath the floor of fourth ventricle and is divided into four distinct nuclei (lateral, medial, superior, inferior vestibular nucleus). Fibres project from vestibular nuclei to cerebral cortex, cerebellum, oculomotor nuclei, motor spinal cord, autonomic nervous system.
Describe the structure of the Sensory Venstibular Apparatus
The inner ear comprises bony and membranous labyrinths.
- The bony labyrinth is a series of cavities in the temporal bone housing both the auditory and vestibular sensory organs.
- It comprises:
- cochlea (auditory portion), and
- vestibule and
- three semicircular canals (vestibular system).
- It contains perilymph a fluid that is high in Na+ and low in K+ (similar to extracellular fluid).
- Within bony labyrinth lies membranous labyrinth that is filled with a fluid known as endolymph that is high in K+ and low in Na+ (unlike other extracellular fluids).
- The membranous labyrinth contains the specialised auditory (organ of Corti) and _balance sensory organ_s, one in each of semicircular canals and two within the vestibule.
Because of the _________ sound is directed into the cochlear because____
Round window
Because it is the only place where there is pressure relief
Therefore we don’t get motion sickness with sound. But there are conditions where there’s a thinning of the bone and window which allows sound to go into the vestibular organs and so people spin with loud sound.
Why do we not get motion sickness with loud sound if the vestibular and cochlear organs share similar systems
Because of the Round window, sound travels into the cochlear because it is the only place where there is pressure relief
Therefore we don’t get motion sickness with sound.
But there are conditions where there’s a thinning of the bone and window which allows sound to go into the vestibular organs and so people spin with loud sound.
Describe the structure of the vestibular organs
1) Three semicircular canals (superior/anterior, posterior, lateral/horizontal)
- The semicircular canals lie in three planes, each at right angles to each other, and hence detect motion in each plane.
- The later (horizontal) canal lies at an angle of 30° to the horizontal, slopping downwards and backwards.
- The superior (anterior) and posterior canals lie vertically, and at right angles to each other.
2) Utricle
3) Saccule
- (The Utricle and the Saccule are in the vestibule of the inner ear between cochlea and vestibular system)
- Contains the otolithic organs, the macula sacculus and utriculus that detect static head position and linear acceleration.
4) Ampullae
- (of semicircular canals. Contains the _crsitae ampullari_s which detects angular acceleration)
5) Superior and Inferior Vestibular Nerves
- Cell bodies in scarpa’s ganglion exits as part of vestibulocochlear nerve in interanl auditory meatus
What contains the crsitae ampullaris and what does it do?
Ampullae contains the crsitae ampullaris which detects angular acceleration
Within a swelling (ampulla) at one end of each semi-circular canal lies crista ampularis, a specialised epithelial ridge that contains vestibular sensory cells which are innervated by vestibular nerve, (it is also made up of supporting cells).
Crista ampullaris is covered by gelatinous structure called the cupula that stretches to roof of the ampulla. The displacement of this cupula (same density as endolymph) by flow of endolymph stimulates vestibular sensory cells. Crista ampullaris detects angular acceleration.
What Contains the otolithic organs, the macula sacculus and what does it do?
Utricle and the Saccule contains the otolithic organs, the macula sacculus and utriculus that detect static head position and linear acceleration.
Within the vestibule lie the otolithic organs (gravity receptors) that detect linear acceleration.
The otolithic organs are macula utriculus and macula sacculus.
- The machine which contain sensory hair cells (sterocilia) arranged in “button-like” (macula) sheets, in a matrix of supporting cells. Sterocilia project into otolithic membrane, which are made up of a gelatinous mass (does not have the same density as the surrounding endolymph).
- On top of otolithic membrane, there are crystals of calcium carbonate (otoconia/otoliths) (cf. copula of semi-circular canals).
What does the ampulla do?
Ampullae contains the crsitae ampullaris which detects angular acceleration
Within a swelling (ampulla) at one end of each semi-circular canal lies crista ampularis, a specialised epithelial ridge that contains vestibular sensory cells which are innervated by vestibular nerve, (it is also made up of supporting cells).
Crista ampullaris is covered by gelatinous structure called the cupula that stretches to roof of the ampulla. The displacement of this cupula (same density as endolymph) by flow of endolymph stimulates vestibular sensory cells. Crista ampullaris detects angular acceleration.
Describe the pairinng of the semicircular canals
Important: Vestibular sensory orgns operate bilaterally as pairs
The 2 lateral semi-circular canals are paired
The anterior of one and posterior of the other work together.
The semicircular canals lie in three planes, each at right angles to each other, and hence detect motion in each plane.
- The later (horizontal) canal lies at an angle of 30° to the horizontal, slopping downwards and backwards.
- The superior (anterior) and posterior canals lie vertically, and at right angles to each other.
Describe the features of the sensory cells inside the vestibular system
Instead of outer and inner hair cells, there are type 1 and 2 cells.
- Type 1 cells has really big nerve innervation that provides very tight coupling between hair cell and nerve ending.
- Type 2 cells receive a lot of descending or efferent inputs.
Main difference is the large kinocilium (large single specialised ‘stereocilium’ on cell). It sits on 1 side of the cell- create a polarity.
- Stereocilia are not true cilia, they are graded in height with tallest nearest the kinocilium.
- Bundle of sterocillium sitting in fluid, have the same tip links which connect stereocilia together and they are the site of transduction (mechanical input of fluid moves against sterocilium bending towards/away from kinocilium so stimulate/inhibit cell).
Polarisation of Sensory Cells
All of the cells in the crista ampularis are polarised, so kinocilium is on the same side of cell throughout that organ.
Movement of stereocilium in one direction leads to activation, movement in the other direction will result in inhibition.
What is the main difference between the sensory cells in the cochlear and the vestibular system?
Main difference is the large kinocilium (large single specialised ‘stereocilium’ on cell).
Describe the microanatomy of the cristae ampullaris
Within a swelling (ampulla) at one end of each semi-circular canal lies crista ampularis, a specialised epithelial ridge that contains vestibular sensory cells which are innervated by vestibular nerve, (it is also made up of supporting cells).
Crista ampullaris is covered by g_elatinous structure/mass_ called the cupula that stretches to roof of the ampulla. The displacement of this cupula (same density as endolymph) by flow of endolymph stimulates vestibular sensory cells.
Crista ampullaris detects angular acceleration.
The cupula has the same density as the fluid. So if the fluid moves, the cupula moves at the same acceleration.
All of the cells in the crista ampularis are polarised, so kinocilium is on the same side of cell throughout that organ.
Movement of stereocilium in one direction leads to activation, movement in the other direction will result in inhibition.
All of the cells in the crista ampularis are ____________
Therefore…
All of the cells in the crista ampularis are polarised, so kinocilium is on the same side of cell throughout that organ.
Movement of stereocilium in one direction leads to activation, movement in the other direction will result in inhibition.
(Very important point!) Hair cells of the vestibular organs are___________ and _________
(Very important point!) Hair cells of the vestibular organs are polarised and arranged so that they are aligned in the same direction.
Describe the arrangement of the ortolitic organs
Within the vestibule lie the otolithic organs (gravity receptors) that detect linear acceleration.
The otolithic organs are macula utriculus and macula sacculus.
- The machine which contain s_ensory hair cells_ (sterocilia) arranged in “button-like” (macula) sheets, in a matrix of supporting cells. Sterocilia project into otolithic membrane, which are made up of a g_elatinous mass (does not have the same density as the surrounding endolymph)._
- On top of otolithic membrane, there are crystals of calcium carbonate (otoconia/otoliths) (cf. copula of semi-circular canals).
Otocoina can potentially can come under the influence of gravity as they have mass. As you move your head, gravitational field will pull otoconia, which will tend to open or shut transduction channels inside hair cells because of the sensory cells.
- Application of any pressure onto the membrane can distort the stereocilia and lead to inhibition/stimulation of the cells.
- These relies upon mass (gravity) whereas semi-circular canal which relies on fluid movement.
The two maculae are oriented at right angles: the utricle is in horizontal plane and the saccule is in vertical plane.
Describe the difference between Stimulation of Hair cells and Nerve fibres in the Cochlear system vs vestibular sensory cells
As in the auditory system, vestibular hair cells are polarised.
Like hair cells of cochlea, c_onstant low-level current flowing through hair cells causing resting discharge in vestibular nerve._
- Displacement of stereocilia t_owards kinocilium_ causes hair cell depolarisation and increase in discharge of vestibular nerve.
- Displacement of stereocilia away from kinocilium results in hyperpolarisation and decrease in discharge of vestibular nerve.
The main difference between stereocillia and hair cells:
- The vestibular sensory cells have a massive standing current
- Some of these channels (mechanically gated channels) are open all the time when at rest
- So the brain receives steady info from the vestibular sensory cells and the brain equalises these when at rest.
- If you rotate your head, some cells will become depolarised and increase the rate of activity of the vestibular nerve.
- The channels that were open to produce a steady state will close (inhibition)
Describe the physiological significant of polarisation and pairings
Because of hair cell polarisation, rotation of head to one side leads to stimulation of activity on same side and decreased activity on the other side.
Comparison of bilateral vestibular nerve discharge pattern and rate provides information on direction and acceleration of head movement.
e.g. if you move your head to the left, the fluid will appear to move in the opposite direction. So the fluid will push the stereocillia along axis of polarity and cause an excitation.-Increase in firing. (differential firing rates between when you move yhour head to the left or to the right)
Describe the stimulation of the otolithic organs
When tilting head, gravitational field pull/push on otoconia, which bends sterocilia, and its direction determines level of position
What are the projections from the vestibular nuclei?
Projections from Vestibular Nuclei
· 1) Vestibulo-oculomotor pathways (VOR) via medial longitudinal fasciculus
(vestibular-occular reflex)
· 2) Vestibulo-spinal pathways lateral throughout spinal cord, medial LF mostly to cervical and thoracic regions innervating musculature of neck
· 3) Hippocampus
4) Cerebellum
5) Autonomic nervous system
6) Cerebral cortex
The central nervous system uses the information frm the vestibular system to…..
· Maintain equilibrium and gaze with movement (vestibulo-ocular reflex (VOR))
· Maintain posture (vestibulo-cervical (VCR) and vestibulo-spinal reflex (VSR))
· Maintain muscle tone
Why do you get vertigo from acute peripheral damage?
Eyes tell you you are not moving, ears telling you you are moving, proprioreceptors are telling you you are standing up.
These conflict of balance causes vertigo. Your brain feel that you are moving, then you start to move in the opposite position to counteract the perception of movement, which causes fall.
