physiology and physics of the vestibular system
TRANSCRIPT
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