ANATOMY AND PHYSIOLOGY OF

VESTIBULAR ORGAN AND NEURAL PATHWAYS

Dr. Vineet ChadhaResident

Deptt of ENT and Head & Neck SurgerySMS Medical College, Jaipur

Introduction • Vestibular system is the organ of balance and equillibrium.• It is embedded within the petrous part of temporal bone.• Function

• Detection of body motion• Detection of head in space in relation to gravity

• The five vestibular end organs, form an endolymph-filled membranous labyrinth (the endolymphatic space), which is itself contained in the perilymph-filled bony labyrinth (the perilymphatic space)

Vestibular system

Peripheral

Semicircular Canal

Horizontal(Lat.)Superior(ant.)Posterior

Vestibule orOtolith

Utricle

Saccule

Central Vestibular Nu. and its projection to

• cerebellum • Spinalcord• Extraocular Nu.

Vestibule • The central chamber of the bony

labyrinth and measures 4 mm in diameter.

• It is situated between the internal auditory meatus anteromedially and middle ear cavity laterally Lateral wall – oval window Medial wall – two recesses, a

spherical recess and an eliptical recess.

• Anterior to the vestibule sits the cochlea and is connected to the vestibule by the narrow ductus reuniens

• Posterorly lies the three semicircular canal

• The spherical recess is perforated at its anterior and inferior part, by several minute holes (macula cribrosa media) for the passage of filaments of the acoustic nerve to the saccule

• Behind this depression is an oblique ridge, the crista vestibuli, the anterior end of which is named the pyramid of the vestibule. This ridge bifurcates below to enclose a small depression, the fossa cochlearis, which is perforated by a number of holes for the passage of filaments of the acoustic nerve which supply the vestibular end of the cochlear duct.

• At the hinder part of the medial wall is the orifice of the aquæductus vestibuli, which extends to the posterior surface of the petrous portion of the temporal bone. It transmits a small vein, and contains a tubular prolongation of the membranous labyrinth, the ductus endolymphaticus

• On the upper wall or roof is a transversely oval depression, the recessus ellipticus

• The pyramid and adjoining part of the recessus ellipticus are perforated by a number of holes (macula cribrosa superior). The apertures in the pyramid transmit the nerves to the utricle; those in the recessus ellipticus the nerves to the ampullæ of the superior and lateral semicircular ducts.

• Macula cribrosa superior also c/as the “Mike’s dot” marks the passageway for superior vestibular nerve fibers to the cristae ampullares of the lateral and superior semicircular canals. It corresponds to the extreme lateral aspect of the IAC, so Mike’s dot is an important landmark in translabyrinthine surgery.

• The vestibule contains the Utricle Saccule

• Saccule: • Globular in shape and lies immediate posterior to the cochlea• Anterior part exhibits an oval thickening, the macula acustica sacculi, to which are

distributed the saccular filaments of the acoustic nerve.• From the lower part of the saccule a short tube, the Canalis reuniens of Hensen,

passes downward and opens into the ductus cochlearis near its vestibular extremi

• Utricle: • Elliptical in shape and lies posterosuperiorly. • Portion which is lodged in the recess forms a cul-de-sac, c/as the macula acustica

utriculi, which receives the utricular filaments of the acoustic nerve • It receives 5 openings of the three semicircular canal

• Utriculosaccular duct connects the utricle and the saccule• The utricle and the saccule is lined by the sensory epithelium

called the – MACULAE , which is concerned with linear accelaration and deaccelaration

Macula• It is a flat kidney shaped gelatinous organ consisting of

neuroepithilium, supporting cells, blood vessels and nerve fibres• Utricular macule – lies horizontally in the floor• Saccular macule – lies vertically on the wall

• The ciliary bundles of the sensory cells project into the overlying statoconial membrane.

• The statoconial membrane is comprised of 3 layers, as follows:– The otoconial first layer is comprised of calcareous

particles (otoconia), which are inorganic crystalline deposits composed of calcium carbonate or calcite. They vary in size from 0.5-30 mcm, with most about 5-7 mcm. The specific gravity of the otolithic membrane is much higher than that of the endolymph, about 2.71-2.94.

– The second layer is a gelatinous area of mucopolysaccharide gel. – The third layer consists of subcopula meshwork.

• Within the macular membrane is the striola, a specialized central region that has a snowdrift-like appearance.

• In the striola, the otoconia are very small (about 1 µm) and the thickness of the otolithic membranes is either reduced, as in the utricular macula, or increased, as in the saccular macula.

• It has a higher concentration of type 1 hair cells

Structure of the otolith organs. A, Sacculus. B, Utriculus. C, Composition of otoconialmembrane of the sacculus in a section taken at the level shown in A

Semicircular canal• They are in three in no. lying postero-superior to the

vestibule and are at right angles to each other– One horizontal SCC, also c/as lateral canal– Two vertical SCC, superior or posterior canal

• They are oriented at right angles to each other and are situated so that the superior and posterior canals are at 45° angles to the sagittal plane, and the horizontal canal is 30° to the axial plane

Horizontal (LC, lateral) canal is tilted 30 degrees upward from horizontal plane at its anterior end

Vertical canals (AC and PC) are oriented at roughly 45 degrees from midsagittal plane

• Superior semicircular canal – 15 to 20 mm. in length– vertical in direction– placed transversely to the long

axis of the petrous portion of the temporal bone.

• Posterior semicircular canal – vertical – it is the longest of the three,

measuring from 18 to 22 mm– Parallel to the the long axis of

the petrous portion of the temporal bone

• Lateral or horizontal canal – is the shortest of the three(12 to

15 mm)– Arch is directed horizontally

backward and laterally

Position of the right bony labyrinth of the ear in the skull, viewed from above. The temporal bone is considered transparent and the labyrinth drawn in from a corrosion preparation.

DONALDSON’S LINE• A surgical landmark

in endolymphatic sac surgery, is derived by extending the plane of the lateral semi circular canal so that it bisects the posterior semicircular canal and contacts the posterior fossa dura the endolymphatic sac lies inferior to this line.

Each canal forms two thirds of a circle with a diameter of about 6.5 mm and a luminal cross-sectional diameter of 0.4 mm

Each canal has an ampullated limb, measuring 2 mm in diameter (It contains a saddle-shaped ridge termed the crista ampullaris, on which lies the sensory epithelium) and a nonampullated limb, which is 1 mm in diameter.

The nonampullated limbs of the posterior and superior canals fuse to form the crus commune.

All the semicircular canals open into the utricle through 5 openings

The horizontal canal is paired with the contralateral horizontal canal; however, the superior canal is paired with the contralateral posterior canal and vice versa

Crista Ampullaris

• Saddle shaped gelatinous mass located at the ampullated end of each SCC

• Consists of a crest of sensory epithelium supported on a mound of connective tissue, lying at right angles to the longitudinal axis of the canal

• Its sensory epithelium has special cells c/as the HAIR CELLS (the sensory cells of vestibular system)

• A bulbous, wedge-shaped, gelatinous mass called the cupula surmounts the crista. The cupula extends from the surface of the cristae to the roof and lateral walls of the membranous labyrinth, forming a fluid-tight partition.

• Distinct subcupular space in the region of the cupula overlying the apex of the center of the crista This subcupular space is believed to provide space for freedom of movement and more sensitive responses to endolymph flow for the stereocilia on the hair cells in the central zone.

• The specific gravity of the cupula is approximately 1.0, which is about the same as that of the endolymph. This matching of the specific gravity of the cupula and the endolymph is necessary to prevent the cupula from floating upward in certain head positions and causing an enduring nystagmus.

• Disruption of this match in specific gravity is likely the cause of postalcoholic nystagmus.

Cellular Morphology Of The Vestibular Sensory Epithelium

• Sensory epithelium is made up of:– Supporting cells– Hair cells– Afferent nerve fibers and their synaptic terminals – Efferent nerve fibers and their synaptic boutons.

• 1) Supporting cells• Extend from the basement membrane to the apical surface• The upper part of the supporting cells contains large

numbers of round or ovoid granules. The function of these secretory granules is uncertain, but it is thought that they are responsible for the formation of the cupula and otolithic membrane

• 2) Hair cells (the sensory cells of the vestibular system)– Characterised by a bundle of

stereocilia attached to their apical surface and grouped in a stair-case arrangement

– In addition, each hair cell has a single long kinocilium. This kinocilium is longer than the stereocilia and is eccentrically located

Schematic drawing of the two types of sensory cells in the mammalian labyrinth showing fine structural organization of type I and type II sensory cells and their innervation.

Type I Type II

Flask shaped Cylindrical

Calyx +nt Calyx -nt

Single aff. Ending Multiple aff & eff endings

• The location of kinocilium relative to the stereocilia imparts a certain polarization to the

hair cell.• Displacement of hair bundle

toward the kinocilium results in an increase in the firing rate of the afferent fiber(s) contacting the hair cell whereas displacement of the hair bundle away from the kinocilium results in a decrease in firing rate.

– Disp. Of bundle towards kino. Opening of K+ channels along the cilia Depolarization of hair cells Ca+ influx at the base of hair cells Inceased neurotransmitter inflow into synapses Stimulation of nerves

• Since the stimulation of sensory cells by deflection of hair bundle away or towards kinocilium is what initiates signal transduction , the spatial oreintation of cilia is such that every position in space and every movement of head stimulates or inhibits certain receptors

• Horizontal canal: on the side facing utricle

• Vertical canals: side away from the utricle

• Otolith organs: hair cells are in two bands separated by striola – In utricle towards striola– In saccule away from striola The red arrow indicates the polaruty of the cilia

i.e each of the arrow heads points to the direction of kinocilium in that field

• 3) Vestibular nerve afferents• All vestibular afferents have a resting discharge rate.

This enables the afferents to respond to stimuli that cause excitation as well as inhibition

• There are three group of afferent nerve endings:- – Boutons: Afferents exclusively on type II hair cells in

• Regular discharging • Low rotational sensitivity

– Pure Calyx: Exclusively on calyx ending on type I cells in central zone • Irregular discharging• Low rotational sensitivity

– Dimorphic

Blood supply to the vestibular apparatus

The main blood supply to the vestibular apparatus is from the INTERNAL AUDITORY ARTERY which in 45% of cases arises from the Anterior cerebellar artery. It can also arise from the superior cerebellar(35%) or basilar artery(20%).

Vestibular Nerve

Crista of sup. and horizontal SCCUtricular macula

Crista of post. SCCSaccular macula

Superior vestibular Nv. Inferior vestibular Nv.

Vestibular nerve At the level of int acostic meatus

Vestibulocochlear nerve

Enters brain stem at CP angle

Ends at VESTIBULAR NUCLEUS at the floor of IV th ventricle

The course of Vestibular nerve through the internal acoustic meatus

• The vestibular (Scarpa's) ganglion sits at the bottom of the internal auditory meatus and acts as a relay station for nerve fibres of the vestibular nerve.

• It has two parts, the superior vestibular ganglion and the inferior vestibular ganglion

• Each vestibular nv. has 25,000 aff. fibres which are bipolar neurons having there cell bodies located in the scarpa’s ganglion

• Anastomosing branches– Voit’s anastomosis– Oort’s anastomosis– Facial-vestibular anastomosis

Central Vestibular System• It includes the vestibular nucleus and its various connection• The vestibular nucleus lies on the floor of the IV th ventricle

• The vestibular nu. has four divisions– Superior V.N of Becheterew– Lateral V.N of Deiters – Medial V.N of Schwalbe– Inferior or Descending V.N

VentrallyNu. & spinal tract of V th nv.

Laterally Restiform bodies

VESTIBULAR NUCLEUS

Medially Pontine reticular

formation

Dorsally Brachium conjunctivuum

• Afferent supply to divisions of vestibular nv.

• Efferents from divisions of vestibular nu.

S LM D

Cristae (SCC)cerebellum

Utricle CerebellumSpinal cord

Cristae Cerebellumutricle

Utricle Saccule

S LM D

Thalamus Ocular nu.

Ocular nu.Cervical cordCerebellumc/l V.N

ThalamusReticulospinal tractVestibulospinal tract

Cerebellum c/l V.N (med, lat)Reticular formation

Principles of Applied Vestibular Physiology

• Principle 1• The vestibular system primarily drives reflexes

to maintain stable vision and posture

• Clinical Importance: The brainstem interprets imbalances in vestibular input resulting from pathological processes in the same way that it interprets imbalances resulting from physiological stimuli. Therefore, the cardinal signs of vestibular disorders are reflexive eye movements and postural changes

VOR Gaze stabilization

Head rotationLinear accelaration

VESTIBULAR NUCLEUS

VCR cervical spinal

motor neurons

Maintains neck

positionStimulates

OrInhibits

VSR lower spinal

motor neurons

Maintains body

position

VESTIBULO OCULAR REFLEX• Helps in gaze fixation and keeps the object on fovea with

change in head positionSudden counterclockwise head rotation

Clockwise rotation of endolymph

Disp. of cupula towards utricle in Lt HC (AMPULLOPETAL)

Excitation of hair cells

Increased depolarization and consequent stimulation of V.N

Stimulates Excitatory interneurons

Stimulates Inhibitory interneurons

Stimulation of Ipsilateral III – MR

Contralateral VI - LR

Inhibition of Ipsilateral VI – LR

Contralateral III - MR

VESTIBULO OCULAR REFLEX• The polarity of stereociliary bundles in the right horizontal

canal is a mirror image of the arrangement on the left, so

In Rt HC clockwise movt. of endolymph is away from the utricle

(AMPULLOFUGAL)

Inhibition of hair cells

Increased hyperpolarization and consequent inhibition of V.N

inhibits Excitatory interneurons

inhibits inhibitoryInterneurons

Inhibition of Ipsilateral III – MR

Contralateral VI - LR

Stimulation of Ipsilateral VI – LR

Contralateral III - MR

VESTIBULO SPINAL REFLEX• Pathway :–

Lateral and medial vestibulo spinal tractReticulospinal tract

Sudden change in posture to lt side

Excites the vestibular apparatus on rt side

Inhibits the vest. app. on lt side

Increased aff activity and stimulation of V.N

Deccreased aff activity and inhibition of V.N

Decreased tone of ExtensionIncreased tone of flexion

Increased tone of ExtensionDecreased tone of flexion

• Principle 2• By Modulating the Non-Zero Baseline Firing of

Vestibular Afferent Nerve Fibers, Semicircular Canals Encode Rotation of the Head, and Otolith Organs Encode Linear Acceleration and Tilt

– Semicircular canals primarily sense rotational acceleration of the head.

– Utricle and Saccule primarily sense linear acceleration in horizontal and vertical directions, respectively.

A.The cupula spans the lumen of the ampulla from the crista to the membranous labyrinth. B. Head acceleration exceeds endolymph acceleration.The relative flow of endolymph in the canal is therefore opposite to the direction of head acceleration. This flow produces a pressure across the elastic cupula, which deflects in response.

A, At rest there is baseline release of excitatory glutamate from the hair cell synapses B, Hair cells are depolarized C, This occurs because the stretched tip links open cationic channels The influx of potassium ions raises the hair cell's membrane potential. D, Activation of voltage-sensitive calcium channels in the basolateral membrane of the cell. Synaptic release of glutamate increases which in turn increases afferent firing rate.

• Principle 3• Stimulation of a Semicircular Canal Produces Eye

Movements in the Plane of that Canal• The semicircular canals are perpendicular to each

other and the canals in two labyrinth are arranged in complemantary coplanar planes – Two horizontal canals are roughly in one plane, which is

nearly horizontal when the head is in an upright position– The left anterior canal is roughly coplanar with the right

posterior canal in the left-anterior-right-posterior (LARP) plane

– The right anterior canal is roughly coplanar with the left posterior canal in the right-anterior-left-posterior (RALP) plane

• This is c/as Ewald’s First law

• Clinical Importance:• BENIGN PAROXYSMAL POSITIONING

VERTIGO (BPPV). – In the most widely accepted current model of BPPV,

otolith crystals displaced from the utricular otoconial mass come to rest in the posterior semicircular canal

– When the patient lies down and turns the head toward the affected side, aligning the posterior canal with the pull of gravity (THE DIX-HALLPIKE MANEUVER), the otolith crystals fall toward what is now the "bottom" of the canal.

– As the otoliths fall, they push endolymph ahead of them, causing cupular deflection and exciting hair cells on the posterior canal crista.

– Nystagmus develops during the time that endolymph moves.

– Ewald's first law predicts the direction of that nystagmus, It will be in the plane of the affected posterior canal, independent of pupil position or head position.

Excitation of the left posterior canal (PC) by moving canaliths in benign paroxysmal postioning vertigo (PC-BPPV) causes slow phase eye movements downward in the plane of the affected PC

• Principle 4 • A Semicircular Canal Is Normally Excited by Rotation in

the Plane of the Canal Bringing the Head Toward the Ipsilateral Side

• Keeping track of ampullopetal and ampullofugal flows is unnecessary, instead one needs to only recall that a semicircular canal is excited by rotation in the plane of the canal bringing the head toward the ipsilateral side.– The right horizontal canal is excited by turning the head toward the right in the

horizontal plane.– The right anterior canal is excited by pitching the head nose down while rolling

the head toward the right in a plane 45 degrees off of the midsagittal plane. – The right posterior canal is excited by pitching the head nose up while rolling it

toward the right in a plane 45 degrees off of the midsagittal plane

• Clinical importance: This principle eliminates the need to memorize the orientations of stereocilia in particular ampullae and whether ampullopetal or ampullofugal flow excites a given canal

• Principle 5 • Any Stimulus that Excites a Semicircular Canal's

Afferents will Be Interpreted as Excitatory Rotation in the Plane of that Canal

• A pathological asymmetry in input from canals causes the eyes to turn in an attempt to compensate for the "perceived" head rotation.

• However, given the mechanical constraints imposed by the extraocular muscles, the eyes cannot continue to rotate in the same direction that the canals command for very long. Instead, rapid, resetting movements occur, taking the eyes back toward their neutral positions in the orbits. The result is nystagmus, a rhythmic, slowly forward-quickly backward movement of the eyes.

• This nystagmus has two phases– Slow vestibular driven phase– Fast resetting movement

• By convention it is the fast component which is the direction of nystagmus

• CALORIC TESTSubject is placed supine with head tilted up by 30°

Irrigation of EAC bywarm and cold water

Temp. transfer to the lateral part of horizontal SCC and change in density of endolymph

COLD WATERWARM WATER

Density ↓ ed Density ↑ ed

Lighter fluid moves up towards ampulla

Stimulation of hair cells and VOR

Heavier fluid moves away from ampulla

Inhibition of hair cells and VOR

Eye moves to opp. side (slow ph.)

Eye moves to same side (slow ph.)

NYSTAGMUS TO SAME SIDE

NYSTAGMUS TO OPP. SIDE

COWSCold oppositeWarm same

• FISTULA TEST• In cases of fistula in HCC air pressure changes

in external canal is transmitted to HCC producing NYSTAGMUS

• Positive test– Erosion of lateral SCC– Fenestration operation

• Negative test– Normally – Dead labyrinth

+ve pressure Stimulates hair cells

Nystagmus to same side

• SUPERIOR CANAL DEHISCENCE SYNDROME• Another example of a disorder causing isolated

stimulation of a single semicircular canal• When the sup. SCC is eroded a third window is created

through which loud sounds stimulate the SCC– Applying a loud sound to the left ear through a headphone causes her

to develop vertigo and nystagmus. When she is directed to look 45 degrees to her left, one observes that the slow phases of her nystagmus move her pupils up

– In this case, the eyes are moving in the LARP plane and in the direction anticipated for excitation of the left anterior canal or inhibition of the right posterior canal. Since only the left ear is receiving the sound stimulus, the problem must lie in the left anterior superior canal.

• This is an example of superior semicircular canal dehiscence syndrome causing a Tullio phenomenon

• Principle 6• For High Accelerations, Head Rotation in the

Excitatory Direction of a Canal Elicits a Greater Response than Does the Same Rotation in the Inhibitory Direction

• Movement of endolymph in the "on" direction for a canal produced greater nystagmus than an equal movement of endolymph in the "off" direction.

• This is called as Ewald's Second Law, indicates an excitation-inhibition asymmetry

• This can occur at multiple levels• Hair cells• Vestibular Nv. Aff.

• HEAD THRUST TEST• In it the examiner simply asks the subject to stare at the

examiner's nose while the examiner turns the subject's head quickly along the excitatory direction for one canal.

• If the function of that canal is diminished, the VOR will fail to keep the eye on target, and the examiner will see the patient make a refixation saccade after the head movement is completed, thus inducing nystagmus to same side.

• In contrast, when the head thrust is in the excitatory direction of an intact canal (and nerve), the patient's gaze remains stable on the examiner's nose throughout the movement.

A through C show a head thrust to the left, exciting the left horizontal canal (HC). The eyes stay on the examiner's nose throughout the maneuver, indicating normal left HC function. D through F show a head thrust to the right, exciting the right HC. The eyes do not stay on target, but move with the head during the head thrust (D through E). A refixation saccade brings the eyes back on target after completion of the head movement (F). This is a "positive" head thrust sign for the right HC, indicating hypofunction of that canal

• Principle 7 • The Response to Simultaneous Canal Stimuli Is

Approximately the Sum of the Responses to Each Stimulus Alone– Most of the rotations of the head stimulate two or all

three SCC pairs. The motion of endolynph in each canal will detemine the degree to which the hair cell in that canal are stimulated

– Max. motion of endolymph will occur in that canal which is relatively more in the plane of head movt.

– Thus the eye movt. +nt due to any head movt. is the sum of vectors from every stimulated canal

A, Excitation of the LH canal causes rightward slow phases due mainly to strong activation of right LR and left MR.B, Excitation of the LA canal causes upward/clockwise (from patient's perspective) slow phases, due to combined action of the right IO and SR and the left SO and SR.C, Excitation of the LP canal causes downward/clockwise (from patient's perspective) slow phases, due to combined action of the right IO and IR and the left SO and IR. D Equal stimulation of LH and RH canals elicits antagonistic contraction of MR and LR bilaterally, yielding no nystagmus. E Combined equal excitation of LA and LP canals excites muscle activity that is the sum of each canal's individual effect; upward and downward pulls cancel, resulting in a purely clockwise nystagmus. F Combined equal excitation of all three left canals causes a right clockwise slow phase, the expected result of summing activity for each individual canal

• Clinical Implications:

• This nystagmus as seen in fig. F can be seen when the labyrinth is irritated– Early in an attack of ménière's disease – After stapedectomy procedures – Early in the course of viral labyrinthitis

• “Fetter and Dichgans” measured 3D eye movements in 16 patients with spontaneous nystagmus 3 to 10 days after the onset of vestibular neuritis. – Their spontaneous nystagmus axes clustered between the direction

expected from hypofunction of the horizontal canal and the direction expected from hypofunction of the anterior canal on the affected side.

– Hypofunction of the posterior canal did not seem to contribute to the nystagmus, and head thrusts in the plane of the ipsilateral posterior canal showed preserved function.

– The authors proposed that vestibular neuritis is therefore usually a disorder of the organs innervated by the superior vestibular nerve (i.e., the horizontal and anterior canals and the utricle).

• Principle 8 • Nystagmus Due to Dysfunction of Semicircular Canals

Has a Fixed Axis and Direction with Respect to the Gaze

• Clinical Implications: – This principle helps to distinguish nystagmus resulting from

a peripheral vestibular disorder from nystagmus resulting from a central disorder.• In peripheral disorder:- The direction or axis remains the same• In central disorder:- The axis or direction of nystagmus may change

depending on the direction of gaze.

• It is important to note that the magnitude of the nystagmus is not fixed depending on gaze.

• Principle 9:• Brainstem Circuity Boosts Low-Frequency VOR

Performance Through "Velocity Storage" and "Neural Integration“

• Clinical Implications – Post-rotatory nystagmus. – Head-shake nystagmus. – Alexander's Law

HEAD-SHAKE NYSTAGMUS • If the head is rotated side to side in the horizontal plane

in normal subjects, the velocity storage mechanism is charged equally on both sides.

• There is no post-rotatory nystagmus as the stored velocities decay at the same rate on either side.

• However, nystagmus does occur after head shaking in subjects with unilateral vestibular hypofunction.

• When the head stops rotating, the nystagmus is for continued rotation toward the intact side.

ALEXANDER’S LAW. After unilateral vestibular loss, a central process (called the “leaky integrator”) contributes to eye motion and nystagmus by allowing the eye to drift to center, regardless of its position.

When the eyes look to the direction of thefast phase (right, B), the leaky integrator causes the eye to drift to the left. This drift adds to the vestibular slow phase, and the net slow phase velocity (SPV) increases.

When the eyes look to the direction of the slow phase (left, C), the leaky integrator causes the eye to drift to the right. This drift subtracts from the vestibular slow phase, and the net SPV decreases.

• Principle 10 • The Utricle Senses Both Head Tilt and Translation, but

Loss of Unilateral Utricular Function Is Interpreted by the Brain as a Head Tilt to the Opposite Side

• Clinical Implications: An isolated loss of utricular nerve activity elicits a stereotyped set of

static responses called the OCULAR TILT REACTION– (1) A head tilt toward the lesioned side – (2) A disconjugate deviation of the eyes such that the pupil on the intact side is

elevated and the pupil on the lesioned side is depressed (a so-called skew deviation)

– (3) A static conjugate counter roll of the eyes—rolling the superior pole of each eye away from the intact utricle

• Each of these signs can be understood as the brain's compensatory response to a perceived head tilt toward the intact utricle.

• The ocular tilt reaction can also occur from interruption of central otolithic pathways as, for example, in multiple sclerosis.

• The full ocular tilt reaction is not often observed in peripheral vestibular lesions because the brainstem compensates for some aspects very rapidly

The otolith tilt reaction forloss of left utricular function

• Principle 11: • Sudden Changes in Saccular Activity Evoke Changes in

Postural Tone – The saccule is almost planar and lies in a parasagittal orientation.

– Hair cells of the saccule, are polarized so that they are excited by otoconial mass displacements away from the striola, and sense linear accelerations.

– Thus, sudden excitation of hair cells across the saccular macula would likely be interpreted by the brain as a sudden loss of postural tone (i.e., falling). The appropriate compensatory reflex would be one that activates the trunk and limb extensor muscles and relaxes the flexors to restore postural tone.

• Saccular excitation probably underlies the test of VESTIBULAR-EVOKED MYOGENIC POTENTIALS (VEMPS)– VEMPs are transient decreases in flexor muscle electromyographic

(EMG) activity evoked by loud acoustic clicks or tones applied to the ear. – Sufficiently loud sounds applied to the ear excite saccular afferents. The

predicted reflexive response would include relaxation of flexor muscles. – Sternocleidomastoid is the preffered site– Because the saccule is the only end organ that mediates VEMP

responses, absence of VEMP responses may indicate saccular dysfunction

• Postural tone change that may be related to saccular activity is the drop attack, also know as the "OTOLITHIC CRISIS OF TUMARKIN," – It is a dramatic loss of postural tone that can occur in ménière's disease

independent of other vestibular symptoms at the time of the fall.– It is not clear what causes the sudden loss of postural tone, but sudden

deformations of the saccular macula associated with the hydropic changes of the labyrinth have been invoked.

• Principle 12• The Normal Vestibular System Can Rapidly

Adjust the Vestibular Reflexes According to the Context, but Adaptation to Unilateral Loss of Vestibular Function May Be Slow and Susceptible to Decompensation

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