physiology and physics of the vestibular system

physiology and physics

is the labyrinth the perfect sensor

for motion gravity detection ?

physiology of the vestibular system

sea sickness: why ? vestibular loss: only short term impact ?

vestibular system

- old sensory system, embedded in many places

in the brain and takes part in many functions

- works in synchrony with all other sensory systems

to detect motion and gravity

- works in silence:

but … you will know when it does not work anymore

the vestibular labyrinth

- works likes a glass of water that a little child

tries to bring to you without spoiling a drop …

- responds to any tilt or movement

- only quiet at very very low accelerations

the vestibular labyrinth

ciliated receptor cells

cilia

statolith

sensory fibres

statocyst: sensitive for gravity and sound

two labyrinths on either side of the headfilled with endolymphe and otoconia

video adapted from AVOR-app: Hamish MacDoughall

vestibular organ(vestibular labyrinth)

hearing organ(cochlea)

hearing organ: high frequency motions (sound)vestibular organ: low frequency motions (movement)

nerve fibre

kinocilium

stereocilia

tip links

A

B

C

nerve:

action potentials

0 100 400 spikes/sec

80 mV 60 mV120 mV

Ewalds’s 2nd Law: hair cells: asymmetric mechano-receptors

ion channels

myosine

filaments

sacculus

utriculus

HC

AC

PC

2 vestibular labyrinths sense head movement and tilt

any motion and tilt: utriculus + sacculus

angular accelerations: 3 canals HC+PC+AC

3 canals

labyrinth• translations + tilt + rotations: statolith systems

• rotations: canal system

statoliths

otoconia = calcium carbonate crystals

supporting cells haircellsnerve

utriculus + sacculusaccelerometers

• function based on inertia of statoconia mass

• multi-directional sensitivity

• highest sensitivity for low frequencies

Fg

0

velocity

Fg

- no discrimination between translation and tilt possible

- detects also (fast) rotations (centrifugal force)

constant velocity

acceleration deceleration

utriculus

sacculus

lateralmedial

forwards-backwards,

up and downs translations

forwards-backwards,

sidewards translations

gain = membrane shift / head acceleration

frequency (Hz)1 / Tlow =

I mass

B friction (visc)

K elasticity

K

B

B

I1 / Thigh=

1.0 10.0

optimal sensitivity for the gravity vector

0.1

//

0

impact viscosity B and elasticity K on statolith function

• mechanical changes

viscosity B

elasticity K

specific mass otoconia: gain

<30 30-49 50-69 ≥70 <30 30-49 50-69 ≥70

Agrawal et al, 2013

sacculus utriculus

c-VEMP-ampl. (µV) o-VEMP-ampl. (µV)

left utricular nerve stimulation: tilt, shift and torsion

sacculus

utriculus

HC

AC

PC

2 vestibular labyrinths sense head movement and tilt

any motion and tilt: utriculus + sacculus

angular accelerations: 3 canals HC+PC+AC

3 canals

endolymphe

rotation

Ewald’s 2nd Law:asymmetry

cupula

acceleration / inertia of masselasticity

viscosity (friction)

latency SP = 0.8 ms

max. deflectioncupula = 2 ms

latency VOR = 8 ms

maximum deflection 1°

elasticityviscosity (friction)

mass

backcupula = 20 s

0 10 20 sec

cupula deflection

- acceleration: cupula reaches maximum deflection within 2 msec

- constant rotation: cupula returns back in 20 secs

cer

vn

omn

nph

velocity storage memory

- increase of LF sensitivity

- calculation of velocity

- integration canal-otolith input

duration 20 s 60 s

durationdeflection cupula = 2 ms

durationcupula back = 20 s

durationvelocity storage = 60 s

(time constant 11-26 sec)

durationcentral adaptation > 300 s

velocity storage: mainly for horizontal canals

0 100 200 300 sec

25

50

75

100

0

velocity step37%

T

slow phase velocity

- a normal canal only senses a change in rotation velocity- a normal canal does not sense constant rotation, linear acceleration or gravity

canals are not sensitive for gravity(specific mass endolymphe ≈ specific mass cupula)

Fg

Fg

Fg

translation

exceptions: alcohol, canalolithisis, cupulolithiasis etc

alcohol changes

density ratio

cupula / endolymphe

a change of orientation to gravity is

mistaken for a rotation positional nystagmus

calcium carbonate crystals

detached from the utriculus / sacculus

maximal

minimal

Ewald’s 1st Law: optimal sensitivitywe need 3 canals for 3 dimensions

3D asymmetry

1st and 2nd law of Ewald

left and right HC left HC right HC

left PCright PCwhy two and not one labyrinth?

requirements

- sensitivity for very small and very high accelerations (large dynamic range)- sensitivity in all directions

possible solutions

1 sensor for all accelerations and all directions: never optimal

1 sensor for very small accelerations in all directions2nd sensor for very high accelerations in all directions

1 sensor for small and very high accelerations in 1 directiondifferent sensors for different directions

statocyst utriculus 2 (utriculus + sacculus) 6 canals

AD +

AS - OUT

left and right HC

amplifier

two asymmetric labyrinths? brainstem = differential amplifier!

- opposite input signal: output = 2x- large dynamic range (DR)- reduces common disturbances

symmetric

asymmetric

DR

frequency dependence

semicircular canals ?

which parameters play a role ?

1. inertia of mass (J)

• relative displacement

2. viscosity (B)

• friction and damping

3. elasticity cupula (K)

• cupula returns back at

constant velocity

theoretical model canal: 2nd order system

B

head

endolymphe

K

B friction / viscosity (max. friction: endolymphe moves with canal)

K elasticity cupula (no elasticity: cupula does not bend back)

I endolymphe mass, size (no inertia: no movement)

cupula

I

cupula deflection depends on viscosity, elasticity and mass

theoretical model canal: 2nd order system

leads to the following differential equation

q = Θ + Θ + ΘB K

I I

q angle head rotation

Θ angle cupula deflection

I endolymphe mass, size

B friction (viscosity)

K elasticity cupula frequency frequency

gain phase

cupula deflection depends on viscosity, elasticity and mass

0.1 Hz 10 Hzsensitivity

frequency (Hz)

frequency dependence canals: gain

B

I

K

B

I mass

B friction (visc)

K elasticity

canal senses acceleration, cupula deflection indicates head velocity

VS

BI

KB

frequency

-90°

+90°

1 / Tlow = 1 / Thigh=

0.1 Hz 10 Hz

canal senses acceleration, cupula deflection indicates head velocity

frequency dependence canals: phase

I mass B friction (visc) K elasticity

VS

impact viscosity B and elasticity K on canal function

• mechanical changes

viscosity B

elasticity K

specific mass (e.g. alcohol intake, canaloliths)

HC

PC AC

DVA reduction (logMar)

<30 30-39 40-49 50-59 60-69 ≥70 years

<30 30-39 40-49 50-59 60-69 ≥70 <30 30-39 40-49 50-59 60-69 ≥70

0.0

0.00.0

0.5

0.5

0.5

Agrawal et al, 2013presbyo-vestibulopathy ?

sacculus

utriculus

HC

AC

PC

2 mirror-symmetric vestibular labyrinths sense head movement and tilt

any motion and tilt: utriculus + sacculus

angular accelerations: 3 canals HC+PC+AC

3 canals

is the labyrinth a pefect sensor ?

no the brain needs help from other senses !

0.2 Hz 2 Hz 20 Hz

sensitivity

frequency (Hz)

vision and/or

propriocepsiscanals

statolith

- statolith system detects any acceleration and tilt- canals detect only angular accelerations- no visual or proprioceptive input: brain neglects labyrinthine input

some facts of life that still need to be explained

- divers under water can’t orient themselves without vision !

- loss of orientation when covered by an avalanche

submersion in water or under snow:

principle of inertia of mass = gravity detection in labyrinth remains→ normal detection of accelerations and gravity should be possible

the brain needs multi-sensory input or pre-knowledge(LF?) utriculars/saccular input alone seems to be neglected

are the labyrinths and their sensory friends pefect sensors ?

no sometimes the brain still fails: motion sickness