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 1

Where the vestibular system is 2

It contains 5 balance organs each side 3 semicircular canals: angular acceleration (turning) 2 macular organs: linear acceleration (gravity/tilt) 3

Detecting linear acceleration: which way is up? 4

Here’s how molluscs do it Which way is up? Here’s how molluscs do it Statocyst organ from Hermissenda 5

Here’s how molluscs do it Which way is up? Here’s how molluscs do it heavy stone hair cell to detect how the stone moves 6

We do it much the same way...Human otolith organ Otoconia from human ear Otolith = “ear stone” Otoconia = “ear dust” 7

There are two otolith (macula) organs Utricular macula organ (utricle): horizontal Saccular macula organ (saccule): vertical 8

Otolith organs detect linear acceleration 9

Hair cells detect movement of the otolith membrane 10

Structure of a vestibular hair cell Kinocilium “Hair bundle”: stereocilia Synapse 11

Vestibular hair cell transduction Move the hair bundle at different angles... Membrane potential changes depend on angle 12

Vestibular hair cell transduction Depolarisation Hyperpolarisation Glutamate 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 link 15

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 16

Endolymph and perilymph Endolymph Perilymph 17

Endolymph and perilymph High K+ +80 mV Perilymph (extracellular fluid) Low K+ 0 mV Intracellular High K+ -80 mV 18

What happens when the channel opens? Endolymph High K+ +80 mV K+ Ca2+ Intracellular High K+ -80 mV Large driving force for K+ K+ enters hair cell and depolarises it 19

Vestibular hair cell and nerve activity 20

Vestibular hair cell responses in the otolith organs 21

All orientations are represented in the otolith organs Arrows point towards the kinocilium So moving the otoconia in that direction stimulates the hair cells Utricle Saccule 22

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 23

Angular acceleration 24

Angular acceleration is detected by the semicircular canals 25

How the semicircular canals work Cupula Turn Inertia of fluid 26

How the semicircular canals work All hair cells in the cupula are oriented in the same direction So a given fluid movement stimulates (or inhibits) all of them Cupula Turn Inertia of fluid 27

Effect of a turning motion 28

Firing of a vestibular nerve fibre Firing rate increases when we start turning (acceleration) Firing rate decreases when we stop (deceleration) 29

Firing of a vestibular nerve fibre This is all explained by fluid inertia (Imagine turning a bowl of soup!) 30

Orientation of the semicircular canals (about 45°) 31

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 32

Summary: 5 balance organs each side 3 semicircular canals: angular acceleration (turning) 2 macular organs: linear acceleration (gravity/tilt) 33

Caloric test 34

How warm water affects semicircular canals convection current warm perceived turning motion external auditory meatus 35

How cold water affects semicircular canals convection current cold perceived turning motion external auditory meatus 36

Keeping the eyes on target: vestibulo-ocular reflexes 37

Vestibulo-ocular reflexes 38