Physiology of Balance and Physiology of Balance and Vestibular FunctionsVestibular Functions

Prof. Vajira Weerasinghe Prof. Vajira Weerasinghe

Dept of PhysiologyDept of Physiology

Major functionsMajor functions

1. The primary organ of equilibrium1. The primary organ of equilibrium

2. Major role in the subjective sensation 2. Major role in the subjective sensation of motion and spatial orientationof motion and spatial orientation

3. Adjustments of muscle activity and 3. Adjustments of muscle activity and body position to allow for upright posture body position to allow for upright posture

4. Helps stabilise the eyes in space during 4. Helps stabilise the eyes in space during head movementshead movements

Vestibular apparatusVestibular apparatus

Semicircular canalsSemicircular canals UtricleUtricle sacculesaccule

Vestibular apparatusVestibular apparatus Sense organs of the vestibular system are Sense organs of the vestibular system are

mechanoreceptorsmechanoreceptors The semicircular canals are so arranged that The semicircular canals are so arranged that

they lie in planes orthogonal to one anotherthey lie in planes orthogonal to one another

Semicircular canalsSemicircular canals

Semicircular canals sense angular Semicircular canals sense angular acceleration acceleration

The three semicircular canals have The three semicircular canals have swellings, called ampullae and within each swellings, called ampullae and within each ampulla is the sense organ, called the ampulla is the sense organ, called the cristacrista

In the cristae the hairs of the hair cells are In the cristae the hairs of the hair cells are embedded in a gelatinous mass, called the embedded in a gelatinous mass, called the cupula, which extends across the ampullacupula, which extends across the ampulla

During movementDuring movement

Rotational angular acceleration of the Rotational angular acceleration of the whole canal causes fluid to be left behind whole canal causes fluid to be left behind on account of its inertiaon account of its inertia

Fluid inertia during angular acceleration Fluid inertia during angular acceleration results in displacement of the cupula and results in displacement of the cupula and bending of the sensory hairsbending of the sensory hairs

This is the adequate stimulus for exciting This is the adequate stimulus for exciting the hair cell the hair cell

During movement During movement

movement of the cupula and its embedded hairs during rotation first in one direction and then in the opposite direction

Transduction in semicircular Transduction in semicircular canalscanals the resultant bending of the cilia causes the resultant bending of the cilia causes

cell cell

– excitation when the bending is toward the excitation when the bending is toward the kino-ciliumkino-cilium

– with a resultant increase in the firing with a resultant increase in the firing frequency of vestibular nerve fibres frequency of vestibular nerve fibres

– inhibition when away from the kino-cilium inhibition when away from the kino-cilium – with a resultant decrease in the firing with a resultant decrease in the firing

frequency of vestibular nerve fibresfrequency of vestibular nerve fibres

Transduction in semicircular Transduction in semicircular canalscanals the sensory cells exhibit a "resting discharge" the sensory cells exhibit a "resting discharge" modified (increased or decreased) depending modified (increased or decreased) depending

on the direction in which the cupula is deflected on the direction in which the cupula is deflected depending on the direction of rotation depending on the direction of rotation

the cilia-kinocilium complexes of the hair cells the cilia-kinocilium complexes of the hair cells in the semicircular canals are oriented in one in the semicircular canals are oriented in one directiondirection

all the hair cells in each canal one of the three all the hair cells in each canal one of the three canals are maximally excited by the same canals are maximally excited by the same direction of angular rotationdirection of angular rotation

Semicircular canalsSemicircular canals the neural output from ampulla represents the the neural output from ampulla represents the

velocity at which the canal is being rotated over velocity at which the canal is being rotated over the range of normal head movements the range of normal head movements

the canal mechanism therefore preforms a the canal mechanism therefore preforms a mathematical integration of the input signal mathematical integration of the input signal (the integral of acceleration = velocity) (the integral of acceleration = velocity)

due to the very small size of the canal due to the very small size of the canal (diameter of about 0.3 mm), which results in a (diameter of about 0.3 mm), which results in a large increase in the viscous properties of the large increase in the viscous properties of the fluidfluid

Semicircular canalsSemicircular canals semicircular canal system acts as a semicircular canal system acts as a

precise angular (rotational) precise angular (rotational) speedometerspeedometer

its neural output is directly its neural output is directly proportional to the angular (rotational) proportional to the angular (rotational) velocity of head movementsvelocity of head movements

By combining the input from each of By combining the input from each of the three canals, the brain can create the three canals, the brain can create a representation of the vector which a representation of the vector which describes the instantaneous speed of describes the instantaneous speed of head rotation relative to 3D spacehead rotation relative to 3D space

: a 3D speedometer!: a 3D speedometer!

Otolith organs (utricle and Otolith organs (utricle and saccule)saccule) The utricle and the saccule are two sac-The utricle and the saccule are two sac-

like structures like structures

each of which contains a specialized each of which contains a specialized region (the macula) region (the macula)

which is made up of a ciliated sensory which is made up of a ciliated sensory epithelium (the vestibular hair cells epithelium (the vestibular hair cells

the hair cells in the vestibular system differ from the hair cells in the vestibular system differ from those in the auditory systemthose in the auditory system

Each vestibular cell has a number of thin Each vestibular cell has a number of thin stereocilia and one thicker longer kinocilium stereocilia and one thicker longer kinocilium positioned at one end of the cell’s hair-bearing positioned at one end of the cell’s hair-bearing surfacesurface

Hair cells exhibit a constant "resting discharge Hair cells exhibit a constant "resting discharge activity" even in the absence of a stimulusactivity" even in the absence of a stimulus

Thus, stimulation is sensed by the CNS as a Thus, stimulation is sensed by the CNS as a change in this resting, "spontaneous" discharge change in this resting, "spontaneous" discharge raterate

The cilia which emerge The cilia which emerge from the hair cells are from the hair cells are embedded in a gelatinous embedded in a gelatinous matrix (the otolith matrix (the otolith membrane) containing membrane) containing solid CaCO3 crystals (the solid CaCO3 crystals (the otoconia) which overlies otoconia) which overlies the cells the cells

During linear acceleration, During linear acceleration, the crystals (being denser the crystals (being denser than the surrounding fluid) than the surrounding fluid) will tend to be left behind will tend to be left behind due to their inertiadue to their inertia

resultant bending of the resultant bending of the cilia causes cell cilia causes cell

excitation when the excitation when the bending is toward the bending is toward the kinociliumkinocilium– with a resultant increase in with a resultant increase in

the firing frequency of the the firing frequency of the afferent sensory fibres afferent sensory fibres

inhibition when away inhibition when away from the kinocilium from the kinocilium – with a resultant decrease in with a resultant decrease in

the firing frequency of the the firing frequency of the afferent sensory fibresafferent sensory fibres

Macula of the utricle lies in the horizontal Macula of the utricle lies in the horizontal planeplane– Respond to horizontal linear force Respond to horizontal linear force

Macula of the saccule lies in the vertical Macula of the saccule lies in the vertical planeplane– Respond to vertical linear force Respond to vertical linear force

Within each Within each organ, the cilia-organ, the cilia-kinocilium kinocilium complexes of hair complexes of hair cells are spatially cells are spatially arranged such arranged such that all possible that all possible directions of linear directions of linear movement are movement are representedrepresented

Since they are sensitive to acceleration, Since they are sensitive to acceleration, the otolith organs detectthe otolith organs detect

– the direction and magnitude of gravitythe direction and magnitude of gravity

– transient linear accelerations due to transient linear accelerations due to movement movement

Central connectionsCentral connections The maculae and cristae are innervated by The maculae and cristae are innervated by

bipolar neurons of the vestibular ganglion bipolar neurons of the vestibular ganglion

The central processes of these cells form The central processes of these cells form the vestibular nerve which enters the brain the vestibular nerve which enters the brain stem at the cerebellopontine angle medial stem at the cerebellopontine angle medial to the cochlear nerve to the cochlear nerve

The vestibular nerve bifurcates into short The vestibular nerve bifurcates into short ascending and long descending branches ascending and long descending branches which are distributed to the vestibular which are distributed to the vestibular nucleinuclei

Some vestibular nerve fibers continue without Some vestibular nerve fibers continue without interruption to the ipsilateral cerebellar cortex interruption to the ipsilateral cerebellar cortex and one of the deep cerebellar nucleiand one of the deep cerebellar nuclei

Most primary vestibular fibers Most primary vestibular fibers terminate differentially in the four main terminate differentially in the four main vestibular nuclei in the floor of the fourth vestibular nuclei in the floor of the fourth ventricleventricle

The vestibular nuclei give rise to secondary The vestibular nuclei give rise to secondary vestibular fibers which project to specific vestibular fibers which project to specific portions of the cerebellum, certain motor cranial portions of the cerebellum, certain motor cranial nerve nuclei and to all levels of the spinal cordnerve nuclei and to all levels of the spinal cord

Role of vestibular systemRole of vestibular system

Role in postureRole in posture

Vestibular postural reflexesVestibular postural reflexes

– Tonic labyrinthine reflexTonic labyrinthine reflex

– Labyrinthine righting reflexLabyrinthine righting reflex

Postural reflexesPostural reflexes

tonic labyrinthine reflextonic labyrinthine reflex Stimulus: gravitational pullStimulus: gravitational pull Response: contraction of limb extensorsResponse: contraction of limb extensors receptors: vestibular organsreceptors: vestibular organs (work through vestibulospinal tract)(work through vestibulospinal tract)

labyrinthine righting reflexlabyrinthine righting reflex Stimulus: gravitational pullStimulus: gravitational pull Response: attempt to maintain head level Response: attempt to maintain head level receptors: vestibular organsreceptors: vestibular organs

Video

Vestibulo-ocular reflexVestibulo-ocular reflex The is an important mechanism by which unblurred vision The is an important mechanism by which unblurred vision

is made possible during the head movements that are is made possible during the head movements that are generated during every day lifegenerated during every day life

If the head is turned to the left, the balance of the afferent If the head is turned to the left, the balance of the afferent neural information from the two horizontal canals on each neural information from the two horizontal canals on each side of the head would cause the eyes turn to the right side of the head would cause the eyes turn to the right – opposite direction to head movementopposite direction to head movement

The leftward head movement causes The leftward head movement causes – an increase in the activity of left horizontal canal hair cells and an increase in the activity of left horizontal canal hair cells and

afferent fibers afferent fibers – a decrease in the activity in the horizontal canal hair cells and a decrease in the activity in the horizontal canal hair cells and

afferent fibersafferent fibers

This difference in activity between the left and right This difference in activity between the left and right afferent fibers is responsible for generating an oppositely afferent fibers is responsible for generating an oppositely directed eye movement at the same velocity as the head directed eye movement at the same velocity as the head movementmovement

This eye movement "reflex" is very importantThis eye movement "reflex" is very important

It allows us to keep our retina fixed on the same point in It allows us to keep our retina fixed on the same point in visual space both during and following naturally generated visual space both during and following naturally generated head movementshead movements

Video

Eye movementsEye movements Saccadic movementSaccadic movement

Smooth pursuit Smooth pursuit movement movement

What is nystagmus?What is nystagmus? Periodic rhythmic Periodic rhythmic

oscillatory oscillatory movements of the movements of the eyeballeyeball

Various types of nystagmusVarious types of nystagmus

Jerk nystagmus (with fast and slow Jerk nystagmus (with fast and slow phases)phases)

Pendular nystagmus (like sine waves)Pendular nystagmus (like sine waves) Horizontal, vertical or rotatoryHorizontal, vertical or rotatory Downbeat or upbeatDownbeat or upbeat PathologicalPathological

– Vestibular disorder, cerebellar disorderVestibular disorder, cerebellar disorder Physiological Physiological

– Optokinetic Optokinetic

Optokinetic nystagmusOptokinetic nystagmus

Physiological basis of Physiological basis of nystagmusnystagmus Visual fixationVisual fixation

Vestibulo-ocular reflexVestibulo-ocular reflex

Neural integratorNeural integrator– cerebellum, ascending vestibular pathways, cerebellum, ascending vestibular pathways,

and oculomotor nuclei are important and oculomotor nuclei are important components of the neural integrator components of the neural integrator

Oculocephalic reflex (Doll’s Oculocephalic reflex (Doll’s head phenomenon, Doll’s head phenomenon, Doll’s manoevre)manoevre)

Doll’s head head phenomenonphenomenon

•In comatose or severely lethargic patients, the vestibulo-ocular reflex can be used to test whether brainstem eye movement pathways are intact

•The oculocephalic reflex, a form of the vestibulo-ocular reflex, is tested by holding the eyes open and rotating the head from side to side or up and down

•this test consists of the rapid rotation of the patient's head in a horizontal or vertical direction

• the eyes move conjugately in the opposite direction of the head turn

• vestibular nuclei and medial longitudinal fasciculi should be intact

Caloric testingCaloric testing Instilling cold or warm water into Instilling cold or warm water into

the external auditory canal can the external auditory canal can produce nystagmusproduce nystagmus

This produce the same This produce the same movement of endolymph in the movement of endolymph in the semicircular canals produced by semicircular canals produced by rotations of the headrotations of the head

Cold water induces nystagmus Cold water induces nystagmus away from the ear being away from the ear being irrigatedirrigated

Water water induces nystagmus Water water induces nystagmus towards the ear being irrigatedtowards the ear being irrigated

Video

VertigoVertigo

Feeling of rotationFeeling of rotation

May be positional or non-positionalMay be positional or non-positional

CausesCauses– Inner ear disordersInner ear disorders– Vestibular disordersVestibular disorders– Brain stem lesions Brain stem lesions