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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