Balance:

•The vestibular system: detector of acceleration- Transduction in vestibular (and auditory) hair cells- Linear acceleration (gravity: which way is up?):the macula (otolith) organs- Angular acceleration (head turning):the semicircular canals

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Where the vestibular system is

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It contains 5 balance organs each side

3 semicircular canals: angular acceleration (turning)

2 macular organs: linear acceleration (gravity/tilt)

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Detecting linear acceleration: which way is up?

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Which way is up?Here’s how molluscs do it

Statocyst organ from Hermissenda5

Which way is up?Here’s how molluscs do it

heavy stone

hair cell to detect how the stone moves

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We do it much the same way...Human otolith organ

Otoconia from human ear

Otolith = “ear stone”Otoconia = “ear dust”

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There are two otolith (macula) organs

Utricular macula organ (utricle): horizontalSaccular macula organ (saccule): vertical

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Otolith organs detect linear acceleration

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Hair cells detect movement of the otolith membrane

Hair cells

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Structure of a vestibular hair cell

Kinocilium

“Hair bundle”:stereocilia

Synapse

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Vestibular hair cell transduction

Move the hair bundle at different angles... Membrane potential

changes depend on angle12

Vestibular hair cell transduction

Depolarisation HyperpolarisationGlutamate release No glutamate release 13

Vestibular hair cell transduction

What happens when the hair bundle bends?14

“Tip links” between adjacent stereocilia

Bending the hair bundle stretches the tip link15

Vestibular hair cell transduction

•Tip link is directly coupled to an ion channel•Stretching the tip link opens the channel•That lets ions flow: but what happens to membrane potential?•For that we need to understand the fluids bathing the hair cell: endolymph and perilymph

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Endolymph and perilymph

Endolymph Perilymph17

Endolymph and perilymph

Perilymph (extracellular fluid)Low K+

0 mV

IntracellularHigh K+

-80 mV

EndolymphHigh K+

+80 mV

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What happens when the channel opens?

IntracellularHigh K+

-80 mV

EndolymphHigh K+

+80 mV

K+

Ca2+

•Large driving force for K+

•K+ enters hair cell and depolarises it

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Vestibular hair cell and nerve activity

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Vestibular hair cell responses in the otolith organs

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All orientations are represented in the otolith organs

Saccule

Utricle

•Arrows point towards the kinocilium•So moving the otoconia in that direction stimulates the hair cells

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Otolith organ summary

•Detectors of linear acceleration•Major source of linear acceleration is gravity•So they detect head tilt and tell us which way is up•Also stimulated by e.g. starting to walk, car starting to move, plane taking off•All orientations are represented in the otolith organs: so they can detect acceleration/gravity/tilt in any direction

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Angular acceleration

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Angular acceleration is detected by the semicircular canals

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How the semicircular canals work

Turn

Cupula

Inertia of fluid

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How the semicircular canals work

Turn

Cupula

Inertia of fluid

•All hair cells in the cupula are oriented in the same direction•So a given fluid movement stimulates (or inhibits) all of them

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Effect of a turning motion

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Firing of a vestibular nerve fibre

•Firing rate increases when we start turning (acceleration)

•Firing rate decreases when we stop (deceleration)

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•This is all explained by fluid inertia

•(Imagine turning a bowl of soup!)

Firing of a vestibular nerve fibre

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Orientation of the semicircular canals

(about 45°)

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Semicircular canal summary

•Detectors of angular acceleration•Major stimulus is head turning•Respond only at onset and cessation of a turning movement•Semicircular canals work in pairs and each member of a pair is stimulated by turns in the opposite direction•Cupula has same density as endolymph: so semicircular canals are unaffected by gravity

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Summary: 5 balance organs each side

3 semicircular canals: angular acceleration (turning)

2 macular organs: linear acceleration (gravity/tilt)

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Caloric test

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How warm water affects semicircular canals

warm convection current

external auditory meatus perceived turning motion

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How cold water affects semicircular canals

cold convection current

external auditory meatus perceived turning motion

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Keeping the eyes on target: vestibulo-ocular reflexes

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Vestibulo-ocular reflexes

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