vestibular system revised 2002 by jtc

7/29/2019 Vestibular System Revised 2002 by Jtc

1/10

The Vestibular SystemJeffrey T. Corwin, Ph.D.

Depts. of Otolaryngology - HNS and [email protected]

March 11, 2002

Objectives1) To provide an understanding of the structures and the functions of the otolithic

organs and the semicircular canals of the labyrinths.

2) To outline the structures and functions of the vestibular pathways of the nervoussystem, including the primary afferent projections, the vestibulo-spinal pathways,the interactions with the cerebellum, and some elements of the vestibulo-ocularreflex pathways.

3) To review some aspects of balance disorders, vestibular compensation, andtests of the vestibular system.

Review of Hair Cell Structure and Response

Hair cells and supporting cellsform the five sensory epithelia thatcomprise the detectors within thevestibular portions of the inner ear.Auditory and vestibular hair cells are

similar in their basic structures andfunctions. The hair bundles of thevestibular hair cells are polarized, liketheir auditory counterparts. Deflectionof the bundle in the direction of its tallend (the end near the kinocilium)causes depolarization and increasesrelease of neurotransmitter from thebase of the cell. That, in turn, causesgreater excitation in the primaryafferent neuron and results in morefrequent production of actionpotentials. Deflection of the bundle inthe opposite direction causeshyperpolarization of the hair cell andresults in less frequent actionpotentials in the postsynaptic afferentneuron.

1


2/10

It seems likely that the same mechanisms formechano-electrical transductionfunction in both auditory and vestibular hair cells, but the sensory organs that these haircells reside in differ significantly in the range of frequencies to which they respond. Thecochlea senses frequencies from 20 Hz to 20 kHz, but vestibular detectors must senseevents associated with much slower movements, the results of accelerations of the

head and body. The movements that deflect the hair bundles in the auditory systemlargely result from the properties of the basilar membrane beneath the sensoryepithelium and the gelatinous tectorial membrane that rests above the hair bundles.

In the vestibular organs, other accessory structures influence the mechanical forcesthat impinge on vestibular hair cell bundles. The otolithic masses in the utricle andthe saccule and the semicircular ducts and cupulae in the horizontal, anterior, and

posterior canal organs have important influences, because they transform headmovements into forces that deflect the bundles of the hair cells in their associatedsensory epithelia.

2


3/10

Otolithic Organ Structure and Function

Between the cochlea and the semicircular canal organs are two sac-like organs,the utricle and the saccule, that communicate with the endolymph-containing space ofthe cochlear duct via the ductus reuniens (a narrow tube that runs between the base

of the cochlea and the saccule). When the head is erect, most of the utricle's hair cellsensory epithelium, termed a macula, is in the horizontal plane, whereas most of thesaccule's sensory epithelium (its macula) is in a nearly vertical plane. Both organs arereferred to as otolithic detectors because the hair bundles in their sensory epithelia lieembedded in gelatinous acellular otolithic "membranes" beneath otolithic massescomprised of thousands of microscopic football-shaped crystals of calcium carbonatecalled otoconia.

The movements of the otolithic masses in those organs are subject to gravity.When the otolithic masses are pulled away from their rest positions that can cause thehair bundles beneath the otolithic membranes to be deflected. The deflection causes

the generation of neural signals that the brain interprets to provide a sense of the head'sposition in relation to the earth's gravitational field (as shown in B and B' below). Sincethe otoconia have significant mass, they also act as inertial elements. When the head isaccelerated the otoconia lag behind (as shown in C), so the hair bundle deflectionscaused by their relative movements in those situations can be interpreted by thenervous system as indicators for linear accelerations of the head.

3

B'


4/10

As the small arrows in the figure illustrate the hair cells in the utricular and saccularmaculae are distributed in a range of orientations. (The arrows point in the direction offorces that cause depolarization of those hair cells.) Also, the hair cells on the two sides

of each macula's striola have opposite directions of hair bundle orientation. Adeflection in nearly any direction will be detectable as a stimulus that can causedepolarization in some portion of the hair cell population in either organ. Therefore, wecan detect a range of head orientations in relation to the pull of gravity.

Semicircular Canal Structure and Function

Each semicircular canal has aswelling, called the ampulla, which containsits own hair cell sensory epithelium, termed acrista. The crista forms part of the inner

lining which covers a crest of tissue thatprotrudes into the expanded, endolymph-filled lumen of the ampulla. The bundles ofthe crista's hair cells project into thatendolymph fluid and adhere to an acellular,gelatinous cupula. The cupula appears to besimilar in composition to the acellular,gelatinous tectorial membrane in the cochleaand the acellular, gelatinous otolithicmembranes in the utricle and the saccule.Like those accessory structures it helps to

transform a physical stimulus into a deflectionthat can be detected by sensory hair cells.The cupula is positioned like a gate spanningthe lumen of the ampulla and hinged at thelevel of the hair cell bundles.

4


5/10

The three semicircular canals (Horizontal, Anterior, and Posterior) are inperpendicular planes. Like the cochlear duct, the utricle, and the saccule, they are filledwith endolymph. The ampullae are innervated by neurons that course into the crista.The semicircular canals detect angular accelerations (i.e., rotational movements) of thehead.

Rostral

Semicircular canal functions can be illustrated by considering the example of thehorizontal semicircular canals on the two sides of the head. The endolymph in eachsemicircular canal has inertia, and it is not directly coupled to the movement of the

head, because it is a fluid. Therefore, when the head is rotated to the left within theplane of the horizontal canal (i.e. the head rotates counterclockwise in the view above),the inertial fluid in the canal tends to lag the behind the movement of the head and thewalls of the semicircular canals. In effect, when the head rotates the fluids inertia tendsto make it stay put, so the walls of the canal (which are, of course, integral parts of thehead) initially move with the head around the endolymphatic fluid.

As noted above, the cupula is positioned like a gate that is hinged by attachmentto the hair bundles in the crista beneath it. The gate-like cupula projects across theendolymph fluid in the ampulla.

If we consider the left horizontal canal, in the illustration above, we see that whenthe head is rotated to the left the lagging inertial mass of fluid inside the canal tends topush the cupula to the right, toward the rostral end of that semicircular canal. The forceon the cupula thereby deflects the hair bundles in the crista in the direction of therelative fluid motion in the canal. The fluid in the right ear's horizontal canal also lagsbehind the leftward rotation of the canal walls, causing the right canal's cupula to bedeflected away from the rostral end of its ampulla because of the mirror symmetry of theleft and right canals.

Posterior


6/10

How are the relative motions between the fluid and the canal walls sensed?

In both the left and the right horizontal canal ampullae, all the hair cells areoriented so that they will be depolarized by deflection of the cupula directed toward the

rostral end of the ampulla. They are hyperpolarized by deflection of the cupula awayfrom the rostral end. Because the right and the left horizontal canal cristae are mirroropposites of each other they always have opposing (push-pull) responses tohorizontal rotations of the head. Rapid rotation of the head toward the left causesdepolarization of hair cells in the left horizontal canal's ampulla, and increased firing ofaction potentials in the neurons that innervate the left horizontal canal. The sameleftward rotation of the head simultaneously causes a hyperpolarization of the hair cellsin the right horizontal canal's ampulla and a decrease in the rate of firing of actionpotentials in the neurons that innervate the horizontal canal of the right ear

As illustrated in the figure below, the vertical semicircular ducts from the two

labyrinths send two signals to the brain resulting from the angular acceleration of thehead in the plane of those ducts. The right anterior canal and the left posterior canalfunction together (as one push-pull pair), as do the left anterior and the right posteriorcanals.

Central Vestibular Pathways

Primary vestibular projections: The cell bodies of the bipolar afferent neurons thatinnervate the hair cells in the maculae and cristae in the vestibular labyrinth reside inthe vestibular ganglion (also called Scarpa's ganglion) near the internal auditorymeatus. The centrally projecting axons from the vestibular ganglion come together with


7/10

axons projecting from the auditory neurons to form the eighth nerve. The eighth nerveruns through the internal auditory meatus together with the facial nerve. The primaryafferent vestibular neurons project to four nuclei that comprise the vestibular nuclearcomplex in the floor of the medulla beneath the fourth ventricle.

The four nuclei of the vestibular nuclear complex are thelateral vestibular nucleus(alsocalled Deiter's nucleus), the medial vestibular nucleus, the superior vestibularnucleus,and the inferior vestibular nucleus.

1) The lateral vestibular nucleus(or Deiter's nucleus) receives inputsfrom the utricle and the semicircularcanals. It plays an important role in themaintenance of posture. Its second-order neurons send axons through thelateral vestibulospinal tract down to

the ipsilateral ventral horn of the spinalcord. The tonic activation of thoseneurons has facilitatory effects on thealpha and gamma motor neurons thatinnervate the gravity-opposing musclesof the limb. Normally the cerebellarvermis and higher centers contributeinhibitory projections to the lateral vestibular nucleus that tends to counterbalance itsexcitatory influence on those muscles. When the brainstem is transected above thelevel of the lateral vestibular nucleus, its excitatory influence over the innervation ofthe antigravity muscles contributes to decerebrate rigidity in the limbs.

2) The medial vestibular nucleus and 3) the superior vestibular nucleusreceive inputs primarily from the semicircular canals. The neurons in the medialvestibular nucleus make monosynaptic connections with motor neurons in thecervical spinal cord that innervate the muscles of the neck. Those pathways areimportant in mediating reflex movements of the neck that tend to stabilize theposition of the head in space.

Other neurons in the medial vestibular nucleus and the neurons of the superiorvestibular nucleus participate in controlling the vestibulo-ocular reflexes.Relatively large diameter axons from those cells ascend through the brainstem in afiber bundle termed the medial longitudinal fasciculus, (often simply referred to asthe "MLF"). The medial longitudinal fasciculus is located just beneath the floor of theventricle, bilaterally near the midline. Fibers from vestibular nuclei that inhibit alphamotor neurons run in the portion of the MLF that descends through the spinal cord.The medial longitudinal fasciculus can be an important landmark in brain sections. Itis somewhat vulnerable to damage because of its location, and it is crucial fornormal visual motor function.


8/10

4) The inferior vestibular nucleus receives inputs from the three semicircularcanals, the saccule, and the utricle. It also receives fibers from the vermis of thecerebellum. This nucleus appears to be a site where vestibular inputs are integratedwith inputs from other sensory systems and inputs from the cerebellum. Outputsfrom the inferior vestibular nucleus project into the vestibulospinal and

vestibuloreticular pathways.

Vestibular Function

The Vestibulo-Ocular Reflex Pathways:

The horizontal vestibulo-ocular reflex (VOR) pathway coordinates the action offour muscles (the left and right lateral recti and the left and right medial recti) to stabilizevisual gaze when the head is rotated in the plane of the horizontal semicircular canals.The basic pathway for the horizontal VOR is a three-neuron arc. The first elements are

the primary afferent vestibular neurons that project to the medial and superior vestibularnuclei. The second elements are neurons that reside in those nuclei and project to theabducens nuclei and the oculomotor nuclei. The third elements are the motor neuronsthat reside in those nuclei and innervate the oculomotor muscle fibers. The VOR resultsin movements of the eyes that are timed to coordinate with the movement of the head,so that the fovea can maintain its gaze on whatever target it is imaging.

Nystagmus is a rythmic alternation of slow and fast-phase eye movements. In avestibular nystagmus the eyes move slowly in one direction (so gaze is stabilized) andthen quickly move back in the opposite direction (so that the system in reset and anew region of gaze can be stabilized on the retina during the next slow-phase

movement of the eyes). By convention nystagmus is named for the direction of itsfast phase.

Diagnosis of vestibular and eye-movement disorders can be aided by a number ofobjective measures. For instance, the alternating slow-phase and fast-phasemovements of the eyes can be monitored electrophysiologically as theelectronystagmogram (ENG) or via infrared video imaging.

The VOR pathways will be covered in greater detail during the lecture on eyemovements. For our purposes it is worth noting that the VOR is remarkably plastic.The gain of the reflex changes in response to changes in the visual system, such aschanges in the magnifying power of eyeglasses. Experiments conducted by G. Melville-Jones demonstrated that the direction of the reflex movements of the eyes would evenreverse in volunteers who wore prisms that reversed their visual image of the world.(Fortunately it changed back to normal soon after the prisms were removed!)


9/10

Balance Disorders

Dizziness can have many causes other than vestibular system dysfunctions.Fluctuations in blood pressure and visual system abnormalities can lead to feelings of

lightheadedness. For that reasons it is important to take an accurate patient history.Even with an accurate history it is difficult to diagnose dizziness. However, if a patientreports feeling as though the environment is spinning or feeling as though they aremoving through space those are often indications of vestibular system involvement.

It is important to be aware that the hair cells of the ear are particularly vulnerableto aminoglycoside antibiotic toxicity. Vestibular hair cells are especially susceptible toaminoglycoside poisoning, and once lost, they do not appear to be replaced. Patientson i.v. aminoglycosides are at risk of bilateral chemical lesioning of their vestibular andauditory organs. The increasing incidence of resistant forms of tuberculosis appears tobe putting more patients at risk for antibiotic induced ototoxicity.

Labyrinthectomy

The magnitude of symptoms caused by labyrinthectomy depends on whetherthe loss is bilateral or unilateral and the rapidity of the loss. Patients who have had aslow-onset loss of vestibular function bilaterally, e.g. as the result of streptomycintoxicity, may not complain of symptoms related to vestibular function. However, theirloss of vestibulo-ocular reflexes and vestibulo-spinal reflexes will result in oscillopsiawith head movements, and instability when walking in the dark. (Those patients arecompletely resistant to seasickness.) In contrast, the loss of one labyrinth can result insevere acute symptoms, including extreme dizziness and nausea, vomiting, deviation

toward the side of the lesion when walking (if forced), and a brisk spontaneousnystagmus that interferes with vision. The nystagmus that results has a slow-phaseeye movement directed toward the side of the lesion and a fast-phase eye movementdirected toward the intact side. Skull fractures that pass through the internal auditorymeatus can sever the eighth nerve and result in such a rapid unilateral removal of thefunction of one labyrinth.

Recovery from unilateral labyrinthine loss occurs through vestibularcompensation that begins almost immediately. Within one week, a young patient maybe able to walk without difficulty and may be able to suppress the spontaneousnystagmus. After one month, most patients have few residual symptoms, althougholder patients may not recover as well. Symptoms may return when patients have acold. The basis for this appears to be a learned modification in reflexes, so thatunbalanced inputs from the vestibular system are ignored and visual and proprioceptiveinputs are relied upon exclusively.


10/10

Nucleus Inputs Outputs Function/Interactions

1. The Lateral VestibularNucleus

(Deiter's Nucleus)

Utricles andSemicircular Canals

Project ipsilaterally viathe lateral

vestibulospinal tract

Facilitate activity of ventralhorn alpha & gamma motor

neurons that innervate gravity-opposing muscles of the limbfor maintenance of posture

2. The Medial VestibularNucleus and

and

3. The Superior VestibularNucleus

Primarily from theSemicircular canals

MVN neurons makemonosynapticconnections withcervical motor neuronsthat innervate neckmuscles.

Other neurons in theMVN & the SVN send

large diameter axonsinto the MLF(ascending anddescending).

MVN neurons to cervical cordfunction in stabilizing the headin space.

Ascending fibers in the MLFgive rise to the VOR bysynapsing in the III, IV, and VInerve nuclei. Descendingfibers in the MLF project to the

ventral horn at many levels(ipsilaterally at spinal levels) tosynapse on alpha motorneurons and inhibit them.

4. The Inferior VestibularNucleus

Semicircular canals,Utricles, Saccules,and CerebellarVermis

Project into thevestibulospinal andvestibuloreticularpathways.

Functions in integration ofvestibular, multisensory, andcerebellar inputs.

vestibular system revised 2002 by jtc

Documents