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Modelling of the static and dynamic mechanical properties of human otoliths [Elektronische Ressource] / vorgelegt von Rudolf Jäger

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101 pages
Modelling of the static and dynamicmechanical properties of humanotolithsDISSERTATIONzur Erlangung des Grades eines Doktorsder Naturwissenschaftender Fakult¨at fu¨r Mathematik und Physikder Eberhard-Karls-Universit¨at zu Tu¨bingenvorgelegt vonRudolf Jager¨aus Ellwangen2003Tag der mu¨ndlichen Pru¨fung: 23.07.2003Dekan: Prof. Dr. Herbert Muther¨1. Berichterstatter: Prof. Dr. Hanns Ruder/PD. Dr. Thomas Haslwanter2. Berichterstatter: Prof. Dr. -Ing. Andrew H. ClarkeFluctuat nec mergiturMotto of the city of ParisAbstractThe aim of this study is a numerical investigation of the static and dynamic prop-erties of the otoliths. The otoliths are a part of the vestibular system, located inthe inner ears. They sense accelerations of the head. In the static case, informationretrieved from them indicates the orientation of the head with respect to gravity.Under dynamic conditions, they provide information about the current directionand magnitude of head acceleration. Two important parts of the otoliths can bedistinguished with respect to their function in the sensory apparatus, the otolithmembrane and macula. The otolith membrane, a flat, layered filament structure isdeformed by head accelerations. Though materials within the inner ear have simi-lar densities, this membrane is subject to external accelerations because it containshigher density crystals, known as otoconia. The otolith macula contains hair cells,the sensory units of the otoliths.
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Modelling of the static and dynamic
mechanical properties of human
otoliths
DISSERTATION
zur Erlangung des Grades eines Doktors
der Naturwissenschaften
der Fakult¨at fu¨r Mathematik und Physik
der Eberhard-Karls-Universit¨at zu Tu¨bingen
vorgelegt von
Rudolf Jager¨
aus Ellwangen
2003Tag der mu¨ndlichen Pru¨fung: 23.07.2003
Dekan: Prof. Dr. Herbert Muther¨
1. Berichterstatter: Prof. Dr. Hanns Ruder/PD. Dr. Thomas Haslwanter
2. Berichterstatter: Prof. Dr. -Ing. Andrew H. ClarkeFluctuat nec mergitur
Motto of the city of ParisAbstract
The aim of this study is a numerical investigation of the static and dynamic prop-
erties of the otoliths. The otoliths are a part of the vestibular system, located in
the inner ears. They sense accelerations of the head. In the static case, information
retrieved from them indicates the orientation of the head with respect to gravity.
Under dynamic conditions, they provide information about the current direction
and magnitude of head acceleration. Two important parts of the otoliths can be
distinguished with respect to their function in the sensory apparatus, the otolith
membrane and macula. The otolith membrane, a flat, layered filament structure is
deformed by head accelerations. Though materials within the inner ear have simi-
lar densities, this membrane is subject to external accelerations because it contains
higher density crystals, known as otoconia. The otolith macula contains hair cells,
the sensory units of the otoliths. These cells feature small hair bundles which pro-
trude into the otolith membrane, superimposed on the macula. If the membrane is
deformed, the hair bundles are tilted and associated hair cells change their polariza-
tion. The resulting signal is subsequently reported to the brain.
Uptonow littleworkhas beendevotedtothis sensoryapparatus thoughitisknown
thatitplaysaimportantrolewithinthevestibularsystem. Thecurrentinvestigation
tries to improve the understanding of this structure by simulating the responses of
the otolith membrane, using the method of finite elements (FEM). This method is
especially well suited for this problem since the curved boundary and shape of the
otolith membrane present no problem to it. As a result of this approach, membrane
displacements are obtained which depend on the direction and magnitude of head
acceleration.
Based on recent findings with regard to the morphology of the membrane and mea-
surements of the 3 dimensional (3D) shape of the otolith macula, the numerical
model tries to mimic information processing by these sensors as close as possible.
Deformations of the membrane depend on the local orientation of the macula and
the direction of head acceleration. The model suggests that interactions within the
membrane are of minor importance. This means that different parts of the mem-
brane with different orientation interact weakly. The curved shape of the macula
only affects interactions in small membrane areas, which have high local curvature.
To verify FEM results, the dynamics of the membrane were studied using transfer
functions. We derived the functions for the case of an infinitely extended, planar
membrane. Comparison with corresponding FEM results showed only minor dis-
crepancies. From this it may be concluded that the lateral border of the otolith2
membrane has no effect on the movement of the whole structure and only leads to
appreciable effects in the vicinity of the border. Our results also predict that oto-
conia may exhibit resonant behavior within their supporting filament matrix. This
resonance is expected to occur for frequencies between 100 and 1000 Hz. If such
a resonance exists, structural damage can be imposed on the otolith membrane by
loud sounds from the frequency range mentioned above.
The polarization is determined by the tilt of hair bundles associated with the cells.
Based on the local displacement of the otolith membrane, the polarization of hair
cells was calculated. This modelling indicates that head orientation with respect to
gravity as well as time-dependent changes of head acceleration are represented in a
distributed fashion on the macula level.Contents
Index i
Figure Index iii
Table Index v
1 Introduction 1
1.1 The Vestibular System . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 The Otolith Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Hair cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 Methods 13
2.1 Continuum Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.2 Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.3 Stress-Strain Relationship . . . . . . . . . . . . . . . . . . . . 16
2.1.4 Equation of the moving continuum . . . . . . . . . . . . . . . 17
2.2 The Finite Element Method (FEM) . . . . . . . . . . . . . . . . . . . 20
2.2.1 The Discretization Process . . . . . . . . . . . . . . . . . . . . 21
2.2.2 Shape functions . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.3 Derivation of the Finite Element Method - Static Case . . . . 29
2.2.4 Derivation of the Finite Element Method - Dynamic Case. . . 35
2.2.5 Time-Integration of Dynamic Problems . . . . . . . . . . . . . 37
2.3 Transfer Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.4 Fractional Calculus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
i3 Mechanical Results 47
3.1 Mechanical Properties of the Otolith Membrane . . . . . . . . . . . . 47
3.2 Static Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.1 Displacement Curves . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.2 Lateral Boundary of the Otolith Membrane . . . . . . . . . . 49
3.2.3 Parameter Studies . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2.4 Curvature Effects . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2.5 Discussion of Static Mechanical Effects . . . . . . . . . . . . . 54
3.3 Dynamic Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3.1 Curvature Effects . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3.2 Discussion of Dynamical Curvature Effects . . . . . . . . . . . 58
3.3.3 Effects at Higher Frequencies . . . . . . . . . . . . . . . . . . 59
3.3.4 Discussion of the Effects at Higher Frequencies . . . . . . . . . 62
4 Hair Cell Results 65
4.1 Hair cell Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.2 Hair cell responses under static conditions . . . . . . . . . . . . . . . 67
4.2.1 Excitation Maps . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.2.2 Response Curves . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.2.3 Discussion of Static Hair cell Responses . . . . . . . . . . . . . 69
4.3 Hair cell responses under dynamic conditions . . . . . . . . . . . . . . 71
4.3.1 Spatio-Temporal Excitation Maps . . . . . . . . . . . . . . . . 71
4.3.2 Coding of Tilt Direction . . . . . . . . . . . . . . . . . . . . . 72
4.3.3 Discussion of Dynamic Hair cell Responses . . . . . . . . . . . 74
5 Conclusions 77
References 79
A Curriculum Vitae 85
B Acknowledgements 87
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