Vestibular system - organ of position and balance sense - placed in the semicurcular canals in petrous bone lying in three mutually perpendicular planes. The canals start in utricle, which is connected with sacculus. Both parts are placed in vestibulum communicating with ductus cochlearis.

One outlet of each canal is transformed in ampulla, divided by the ampullary crist into two parts. Macula utriculi is in the lower part of utricle, the macula sacculi in sacculus. The crists and ampullae are covered by sensory epithelium composed of hair-cells. There are also gelatinous cupulae on ampullary crists and the statoconia membranes in maculae. Their function is to stimulate stereocilia of sensory cells. The statoconia are crystals of CaCO3 - it increases the mass of gelatinous membranes.

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The semicircular canals allow analyse the rotational motion of the head. Receptors of ampullary crists react on angular acceleration. The cupulas of crists work as valves, which are deflected by streaming endolymph and stimulate the hairs of sensory cells by bending – depolarisation or hyperpolarisation takes place.

The receptors of utricle and sacculus react on linear acceleration and gravitation. When changing the head position, the membrane with statoconia shifts against hairs of sensory cells - excitation arises. Important for keeping erect position - static reflexes.

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generates compensatory responses to head motionpostural responsesocular-motor responsesvisceral responses

To achieve this the vestibular system measuresHead rotationHead acceleration

Einstein’s equivalency theorem states that an accelerometer cannot distinguish between translational accelerations and tilts

►Spatial arrangement of the 6 SSC cause 3 coplanar pairingsR & L lateral, L anterior and R posterior; l

posterior & R anterior; R & L horizontal

►Allows for a Push-Pull arrangement of the two sides (e.g., as head turns right, right SSC will increase firing rate & the left SSC will decrease firing rate)

►Advantagessensory redundancycommon mode rejection/noiseassist in compensation for sensor overload

►Depolarization of the ipsilateral hair cells occurs during angular head movements

►Hyperpolarization of contralateral hair cells occurs at the same time

►Hair cells are only able to hyperpolarize to what they were at rest = cut off of inhibitory influences from the movement going in the opposite direction even if the ipsilateral hair cells continue to spike higher firing rates

Purves 2001

►Utricle and saccule►Otolith sensory structures

MaculaeOtolithic membraneOtoconia

►Movement of gel membrane & otoconia cause a shearing action to occur over the hair cells → sensitivity of otoliths

►Respond to:Linear head motion on accelerationStatic tiltTwo organs respond to respective

accelerations or tilts in their respective planes► Saccule has vertical orientation of maculae► Utricle has horizontal orientation of maculae

Bear 1996

►2 types: kinocilium & stereocilia►Sensory structures for the peripheral

end organs (maculae and ampula)►Hyperpolarized or depolarized

depending upon the direction of deflection of the stereocilia (movement of stereocilia towards the kinocilium causes depolarization of the hair cell)

►Affect the firing rate of the primary vestibular afferents to the brainstem

Bear 1996

►Striola serves as a structural landmark►Contains otoconia arranged in narrow

trenches, dividing each otolith►Orientation of the hair cells change over

the course of the macula►Allows otoliths to have multidirectional

sensitivity

►Tonic firing rate►Vestibular Ocular Reflex►Push-pull mechanism► Inhibitory cutoff►Velocity storage system

Vestibular apparatus and cochlea form the inner ear

Vestibular apparatus – provides sense of equilibrium consists of

otolith organs (utricle and saccule) and semicircular canals

Sensory structures located within membranous labyrinth filled with endolymph and located within bony labyrinth

Utricle and saccule provide info about linear acceleration

Semicircular canals, oriented in 3 planes, give sense of angular acceleration

Hair cells – receptors for equilibrium Each contains 20-50 stereocilia (hair-like extensions)

1 of these is a kinocilium---a true cilium

Stereocilia bend toward kinocilium – hair cell depolarizes releases NT that stimulates CN VIII

When bent away from kinocilium – hair cell hyperpolarizes In this way, frequency of APs in hair cells carries

information about movement

When stereocilia are bend away from kinocilium, hair cell is hyperpolarized, i.e. inhibited. It occurs because acceleratory force acts to flow of fluid in semicircular canals during circular motion of the head or whole the body.

Hair cells are located along crista ampularis and protect their cilia in cupula. Hair cells are secondary sensor cells, which synapse with neurons. Axons of these nerve cells compose vestibular nerve.

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cellbio.utmb.edu/.../Ear/ organization_of_the_inner_ear.htm.

Have a macula that contains hair cells Hair cells embedded in

gelatinous otolithic membrane contains calcium

carbonate crystals (otoliths) that resist change in movement

Utricle sensitive to horizontal acceleration Hairs pushed

backward during forward acceleration

Saccule sensitive to vertical acceleration Hairs pushed upward

when person descends

Provide information about rotational acceleration

Project in 3 different planes

Each contains a semicircular duct

Crista ampullaris – where sensory hair cells are located

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Ampula is enlargement at one end of semicircular canal. It has a small crest on top of which is a gelatinous mass known as cupula. Hair cells have two kinds of cilia – kinocilium and stereocilia.

Kinocilium is large cilium located at one end of hair cell. Stereocilia are small. When stereocilia are bent towards kinocilium, hair cell is depolarized, i.e. stimulated.

Hair cell processes embedded in cupula of crista ampullaris

When endolymph moves cupula moves Sensory processes

bend in opposite direction of angular acceleration

From Kandel and Schwartz

►Vestibular nerve►Vestibular nuclei►Cerebellum►Oculomotor complex

CN 3, 4, and 6Along with vestibulospinal reflexes coordinate

head and eye movements

►ThalamusConnection with vestibular cortex and reticular

formation → arousal and conscious awareness of body; discrimination between self movement vs. that of the environment

►Vestibular Cortex Junction of parietal and insular lobeTarget for afferents along with the cerebellum

► Both process vestibular information with somatosensory and visual input

►Vestibular nerve and vestibular nuclei have a normal resting firing rate (70-100 cycles/sec)

►Baseline firing rate present without head movement

►Tonic firing is equal in both sides; if not, a sense of motion is felt e.g., vertigo, tilt, impulsion, spinning

►Excitation and inhibition of the vestibular system can then occur from stimulation of the hair cells

►Spontaneous recovery with light

►Causes eyes to move in the opposite direction to head movement

►Speed of the eye movement equals that of the head movement

►Allows objects to remain in focus during head movements

►VOR►Optokinetic reflex►Smooth pursuit reflex, saccades, vergence►Neck reflexes

combine to stabilize object on the same area of the retina=visual stability

►Keeps eye still in space while head is moving

►Ratio of eye movement to head movement (equals 1)

►Perseveration of neural firing in the vestibular nerve by the brainstem after stimulation of SSC to increase time constant (10sec.)SSC respond by producing an exponentially

decaying change in neural firing to sustained head movement

►Otolith & somatosensory input also drive mechanism

►Direction of gaze will shift with the head movement

►Cause degradation of the visual image► In severe cases, visual world will move

with each head movement

►Visual illusion of oscillating movement of stationary objects

►Can arise with lesions of peripheral or central vestibular systems

► Indicative of diminished VOR gainmotion of images on foveadiminished visual acuity

►Monitors vestibular performance►Readjusts central vestibular processing of

static & dynamic postural activity►Modulates VOR►Provides inhibitory drive of VOR (allows for

VORc)

►Provide motor output from the vestibular system to:Extraocular muscles (part of VOR)Spinal cord & skeletal muscles (generate

antigravity postural activity to cervical, trunk & lower extremity muscles)

►Response to changing head position with respect to gravity (righting, equilibrium responses)

►Generates compensatory body movement to maintain head and postural stability, thereby preventing falls

Vestibular nystagmus – involuntary oscillations of eyesoccur when spinning person stops Eyes continue to move in direction opposite to

spin, then jerk rapidly back to midline

Vertigo – loss of equilibrium Natural response of vestibular apparatusPathologically, may be caused by anything that

alters firing rate of CN VIII Often caused by viral infection

•The otolith organs transduce acceleration

•Thus, translational accelerations and tilts of the head cause similar activity on otolith afferents.

•Otolith information is therefore ambiguous (Einstein’s equivalency principle) and must be resolved in order to provide proper stabilizing and orientation responses.

Acceleration

Translation

Tilt

Both Angles are equal.

Otolith Ambiguity

Consists of three semi-circular canals

Monitors the position of the head in space

Controls balance Shares fluid with the

cochlea Cochlea & Vestibular

system comprise the inner ear