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The sense of balance
Sensory modalities that
contribute to maintaining balance
The vestibular system (labyrinth)
The inner ear is divided into vestibular division, which contains the
organs of equilibrium (the utricle, saccule, and semicircular canals) and
cochlear division, which contains the organs of hearing (the cochlea).
The vestibular system (labyrinth)
The otolith organ is detecting linear
acceleration. It consists of two sacs and the
receptors are grouped in a macula in each sac.
Three semicircular canals are
detecting rotation (angular
acceleration). Each semicircular
canal has an enlargement (ampulla)
within which the sensory cells are
grouped.
Hair cells
Hair cells are receptors responsible for the sense of equilibrium and for audition. The
hair cells of the receptor organs in the human internal ear are of similar form and
function. They transduce mechanical stimuli into receptor potential.
From the cell's apical surface extends the hair bundle,
the mechanically sensitive organelle. Afferent (from
receptors) and efferent (to receptors) synapses occur
upon the basolateral surface of the plasma membrane.
This scanning electron micrograph of a hair cell's apical
surface. The hair bundle comprises some 60 stereocilia,
arranged in rows of varying length and the single
kinocilium at the bundle's tall edge. Deflection of the
hair bundle to the right, the positive stimulus direction,
depolarizes the hair cell; movement in the opposite
direction elicits a hyperpolarization.
Tip links
Stereocilia are connected with each other by elastic structures within the hair bundle
called tip links.
The scanning electron micrograph of a hair bundle's top surface. The links that connect each
stereociliary tip to the side of the longest adjacent stereocilium are visible.
A model for the mechanism of mechanoelectrical transduction by hair cells
The ion channels that participate in
mechanoelectrical transduction in hair cells are
gated by elastic structures in the hair bundle.
Opening and closing of these channels is
controlled by the tension in the gating spring, that
senses hair-bundle displacement.
The membrane potential of the receptor cell
depends on the direction in which the hair bundle
is bent. Deflection toward the kinocilium causes
the cell to depolarize and thus increases the rate of
firing in the afferent fiber. Bending away from the
kinocilium causes the cell to hyperpolarize, thus
decreasing the afferent firing rate.
Adaptation in the hair cells
The electrical response to a positive stimulus displays an initial depolarization, followed by a decline to a
plateau and an undershoot at the cessation of the stimulus. Negative stimulation elicits a complementary
response. Bundle movement in response to positive stimulation increases tip link tension and opens
transduction channels. As stimulation continues, the tip link's upper attachment moves down the stereocilium,
allowing each channel to close during adaptation. During negative stimulation tension is restored to the
initially slack tip link by active ascent of the link's upper insertion.
Mechanism of transduction
1.
Movement
2.
Streching of the gating string
3.
Increase membrane conductance to K+
4.
Influx of K+ ions into the cell, the
depolarization spreads into the cell
5.
Release of neurotransmitter at the hair cell
synapse onto vestibular nerve sensory terminal.
The Otolith Organs
In the otolith organs, the sensory epithelium is
overlain with a sheet of gelatinous extracellular
matrix, the otholitic membranne. Embedded in this
structure are otoconia, rocklike cristals of calcium
carbonate.
When the head undergoes linear acceleration the
otoconial mass lags behind movement of the head,
because of its inertia. The motion of the otoconia is
communicated to the otolithic membrane, which thus
shifts with respect to the underlying epithelium. This
motion in turn deflects the hair thus exciting an
electrical response in the hair cells.
Otholitic membranne
Spatial organization of the otolith organs
The hair cells in each organ are localized to a roughly
elliptical patch, called the macula. The hair bundles
are oriented orthogonally to a curving midline (the
striola). This arrangement allows for detection of all
possible stimulus orientations within the plane of the
macula. Any particular horizontal acceleration
maximally depolarizes one group of hair cells and
maximally inhibits a complementary set. Because the
organs are bilateral, the brain receives additional
information from the contralateral labyrinth.
The utricles are oriented horizontally, the
saccules are oriented vertically. The hair
cells in the utricles respond to
accelerations in the horizontal plane;
saccules are especially sensitive to
vertical accelerations, of which gravity is
the most important.
The semicircular canals
A thickened zone of epithelium, the
ampula, contains the hair cells. The
hair bundles of the hair cells extend
into a gelatinous cupula, which
stretches from the crista to the roof
of the ampulla. The cupula is
displaced by the flow of
endolymph when the head moves.
As a result, the hair bundles
extending into the cupula are also
displaced. Because all the hair
bundles in each semicircular canal
share a common orientation,
angular acceleration in one
direction depolarizes hair cells
while acceleration in the opposite
direction hyperpolarizes the
receptor cells and diminishes
spontaneous neural activity.
Orientation of the semicircular canals
In each labyrinth the three canals are almost precisely perpendicular to one another, so that the canals
represent accelerations about three mutually orthogonal axes. The planes in which the semicircular canals lie
do not, however, correspond with the head's major anatomical planes.
Responses of receptors in semicircular canals
Menière Disease
•
•
•
•
•
Menière disease affects the vestibular
labyrinth.
It is characterized by relapsing vertigo
(dizziness), with attacks lasting from tens of
minutes to tens of hours. The vestibular
symptoms are often accompanied by noise
in the ears (tinnitus) and distorted hearing.
It is believed to be linked to an excess of
endolymph in the inner ear.
Removal of the affected labyrinth surgically
may relieve severe vertigo.
Vincent Van Gogh may have suffered from
Menière disease.
Vincent van Gogh, Selfportrait, 1889
K. Arenberg, L. F. Countryman, L. H. Bernstein and G. E. Shambaugh Jr, Van Gogh had Meniere's disease and not epilepsy, JAMA, Vol.
264 No. 4, July 25, 1990
Arnold, Wilfred N. (1992). "Vincent van Gogh: Chemicals, Crises, and Creativity". Birkhäuser Boston.
Vestibular pathways
The axons that carry the hair cells responses to the central nervous system terminate in several large cell
groups (vestibular nuclei) in the brainstem. From there the signals go to neck and trunk motoneurons, to limb
motoneurons, to cerebellum, thalamus and extraocular motoneurons. This network of vestibular connections
is responsible for the various reflexes that the body uses to compensate for head movement and the
perception of motion in space.
Circuits involved in control of eye movements
The vestibulo-ocular reflex
Rotation of the head in a counterclockwise
direction causes endolymph to move clockwise
with respect to the canals. This reflects the
stereocilia in the left canal in the excitatory
direction, thereby exciting the afferent fibers on
this side. In the right canal the hair cells are
hyperpolarized and afferent firing there decreases.
The vestibulo-ocular reflex keeps the visual images fixed on the
retina, by moving the eyes when the head moves. Vestibular
nerve signals head velocity to the vestibular nuclei and
motoneurons that control ocular muscles. E.g., counterclockwise
head rotation excites the left horizontal canal, which then excites
neurons that evoke rightward eye movement.
Caloric stimulation
Zero gravity
1.
Pleasant and intriguing
experience
2.
Space adaptation syndrome
(SAS) – vomiting, headache,
nausea
3.
Fast adaptation (< 72 hours)
4.
Upon return to Earth after longterm weightlessness, slow
adaptation, e.g., in maintaining
standing balance with eyes
closed.
5.
After more than 1 month flights
astronauts experience vision
problems due to redistribution
of fluids and increased pressure
on the eyeballs, causing optic
nerve crush.
Hearing
Sounds
Hearing ranges of different animals
Outer ear
The ear has three functional parts. The main part of the external ear, the auricle, captures sound
efficiently and sends it into the ear canal. Our capacity to localize sounds in space, especially along the
vertical axis, depends critically upon the sound - gathering properties of the external ear.
Middle ear
Middle ear transmits mechanical energy to the ear's receptive organ. The three tiny ossicles, or
bones: the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup pass the motions of
the tympanic membrane to the oval window – the bony covering of the cochlea.
Impedance matching
water– inner ear
air – environment
Amplitude reflection and
transmission coefficients
r12 
B Z1  Z 2

A Z1  Z 2
C
2Z1
t12  
A Z1  Z 2
Z  v
Mechanical impedance

Mass density
v
Wave propagation speed
Energy reflection and
transmission coefficients
R  r2
T  t2
Impedance matching
rair =1.21 kg/m 3
rwater =103 kg/m 3
vair = 340 m/s
vwater =1480 m/s
Energy reflection and
transmission coefficients:
T = 1- R = 0.0011
fraction of power transmitted
R = 0.9989
~99.9% is reflected
If only 1 part in 1000 makes it through, the loss is:
10log101/1000 = -30 dB
Transformer Action #1
Incus
Malleus
Stapes
~ 3.2 mm2
Tympanic membrane
~ 55 mm2
Area ratio about 17:1
Pressure = Force/area
Stapes pressure is 17 times TM [PS = PTM (AreaTM/AreaS)]
Sound pressure level is 20log10P/Pref
Sound pressure increase is approximately
20log1017/1 = 25dB
Transformer Action #2
Incus
7 mm
Malleus
9 mm
F1 x L1
=
F2 x L 2
Malleus/Incus length ratio is 9:7 so force and hence the pressure is increased by 1.3:1
Sound pressure increase is approximately
20log109/7 = 2 dB
Eardrum displacement at threshold of hearing
Y(x, t) = A cos(kx - w t)
Acoustic travelling wave
æ ¶Y ö
1
2 2
2
DE = 2 rDxS ç
÷ = rDxSA w sin (kx - w t)
è ¶t ø
2
Total wave energy
2
I=
dE
r vdtSA w
=
dtS
dtS
2
2
1
sin 2 (kx - w t) = rvA 2w 2
2
I = I 0 = 10 -12 W/m 2
A=
1
2pn
2I
rv
1
2 *10 -12
A=
= 1.26 *10 -11 m
2764 1.29 * 340
D =1.06*10 -10 m
(Epot = Ekin)
Wave intensity
Wave intensity at the
threshold of hearing 0 dB
Wave amplitude
Wave amplitude at 440 Hz
Hydrogen atom diameter
The eardrum displacement at threshold of hearing is ~ 1/10 the diameter of a hydrogen
atom!
Sound Intensity and Sound Pressure Levels
Human ear responds to sound intensities in the range of 10-12– 100 W/m2. Sound intensity level is
defined as:
I
  (10dB) log
I0
where:
 – sound intensity level
I – sound intensity
I0 – standard reference sound intensity 10–12 W/m2
Sound pressure level (SPL) or sound level is a logarithmic measure of the effective sound pressure
of a sound relative to a reference value:
L = (20dB)log
p
p0
Sound Intensity and Sound Pressure Levels
Intensity
dB
Pressure
Examples
108 W/m2
200
2*105 Pa
Volcano erruption
102 W/m2
140
2*102 Pa
Jet aircraft, 50 m
1 W/m2
120
2*101 Pa
Rock concert
10-2 W/m2
100
2 Pa
Disco, 1m from the speaker
10-4 W/m2
80
2*10-1 Pa
Highway, 5 m
10-6 W/m2
60
2*10-2 Pa
Speech, 1 m
10-8 W/m2
40
2*10-3 Pa
Background in quiet library
10-10 W/m2
20
2*10-4 Pa
Background in TV studio
10-12 W/m2
0
2*10-5 Pa
Hearing threshold
Threshold of pain 130 dB
Discomfort threshold, hearing damage possible 120 dB
limit Apple iPod volume in Europe 85 dB
How cochlea works ?
How cochlea works ?
How cochlea works ?
How cochlea works ?
How cochlea works ?
How cochlea works ?
High frequency
Low frequency
Cochlea in the inner ear is shaped like a snail shell. Net
effect is to boost pressure created by sound so that the
inner ear fluid can be displaced. Its spiral shape
effectively boosts the strength of the vibrations caused
by sound, especially for low frequencies.
Cochlea and basilar membrane
Cochlea contains basilar membrane, which separates two liquid-filled tubes that run along the
cochlea, the scala media and the scala tympani. Basilar membrane is narrow at the base and widens
at the tip. Different frequencies are coded by the position along the membrane – high frequencies
displace the membrane at the base, low frequencies displace the membrane at the apex. The organ of
Corti sits on top of the basilar membrane. It transduces pressure waves to action potentials.
Helmholtz’s resonance theory of hearing
 ( x, t )  ( A sin kx  B cos kx) cos(t   )
 (0, t )  0  B  0
 ( L, t )  0  kL  
v
  kv 
L
Different frequencies of sound are encoded by their precise position along the basilar
membrane. Short strings at the base would resonate in response to high notes, and the long
strings (at the apex) to low notes.
The travelling wave theory - Von Bekesy (1928). Nobel 1961
The sound pressure applied to the oval window is
transmitted as a travelling wave along the basilar
membrane. The peak displacements for high frequencies
are toward the base, and for low frequencies are toward
the apex.
Georg von Békésy (1899 – 1972)
Envelopes induced by sound at 3 different
frequencies
Problem: envelopes of the travelling waves are wide while we are hearing pure tones
There must be additional mechanism for tunning of the auditory system to the
sound frequency.
Proof: movements of the basilar membrane
Effect of cochlear amplifier. c) The peak due to cochlear amplifier. d)
Amplitude of the passive movement of basilar membrane in the absence of
the cochlear amplifier.