| The investigation of hearing sensation mechanism for human cochlear is a major medical problem for mankind.The basilar membrane is the basic structure for supporting other biological tissue in the cochlear.Up to now,scientists have not reached a consensus on the vibrating mode of the key macrostructure---asilar membrane.The vibration mode of the basilar membrane is a key scientific problem in the interpretation of the cochlear hearing sensation mechanism.Since the Nobel Prize winner Bekesy proposed his famous traveling wave theory by measuring the motions of the basilar membrane from fresh temporal bones,many scientists have discovered that many experimental phenomena cannot be explained by the traveling wave theory(see Section 1.3 in detail).At the same time,it is so difficult to measure coupling motions of the basilar membrane and lymph in low frequencies via a vivo experiment.In particular,due to the narrowness of the cochlear apex and complex structures,most methods for measuring vibrations of the basilar membrane require the incision of other tissues,which often alters and damages the physiological environment so that the real vibrating behaviors of the basilar membrane within the cochlear in low frequencies can not be obtained.In addition,there is lack of experimental data of the basilar membrane vibration in low-frequency.Because there are plenty of questions about the vibration mode of the basilar membrane which is the main(or supporting)structure of cochlear,the hearing sensation mechanism of cochlear is still not fully understood.Therefore,the treatments of sensorineural deafness are still restricted to hearing aids and cochlear implants,which can only improve hearing ability a little rather than an automatic auditory function recovery in a true sense.Meanwhile,the in-adaptability of cochlear implants and the denaturation of cochlear neurons will eventually lead some patients to go back to the silent world.The human cochlear when subjected to sound stimuli is a quite fine and intricate nonlinear dynamic system in which there are fluid and structures interacting with each other in a multi-physical environment.For studying this dynamic system with complex structures,in this paper,based on advantages of interdiscipline,by introducing the theories,methods,and thoughts of mechanics,structure and electrics,biological systematical cochlear models,which include theoretical analytical models and numerical models,are established and performed frequency domain and time domain calculations and analysis.From macroscopic to microcosmic,cochlear mechanical behaviors that reflecting the fluid-structure-electric coupling motions are obtained.This not only fully reproduces the vibration of the basilar membrane,but also unveils new findings.The main innovations and research work are as follows:1.For unveiling the relationship and physical law between vibration frequency of the basilar membrane and the structure size and material properties,by introducing helical coordinate system,the dispersion equation of the basilar membrane was derived,which describes the intrinsic relationship between traveling wave vibrating frequency and cochlear size and mechanical properties.Some findings are as follows:(1)The dispersion characteristics of the basilar membrane are affected by different main factors in different traveling wave regions.In the long-wave region,the vibrating frequency is related to the stiffness and cochlear height.With the increase of stiffness and cochlear height,the frequency has a nonlinear increase.Thus,the stiffness and cochlear height have a cooperative modulation for the sound frequency.In addition,the stiffness has a more influence than the cochlear height.(2)In the short-wave region,the vibrating frequency of the basilar membrane is related to both stiffness and damping.With the increase of stiffness and damping,the frequency increases nonlinear and declines nonlinear respectively.Thus,the stiffness and damping have a mutual inhibition modulation for sound frequencies.In addition,the stiffness has a more effect on dispersion than damping.(3)In the cut-off region,the physical properties of the basilar membrane has no modulation for sound frequencies.This is because the fluid-structure interaction is very weak so that when the lymph density,cochlear height and the mass of the basilar membrane are determined,the wave number is a constant and is not related to frequency.2.In order to reveal the relationship and physical law between lymph motion and cochlear size,based on the method of separation of variables and conformal transformation,equations of three cases for the velocity potential were derived to solve the steady flow problem of lymph in the cochlear.Then,the distribution of pressure field on the basilar membrane(BM)was obtained.It is found that the spiral shape and physical dimension of the cochlear have a notable influence on the radial distribution of pressure that on the basilar membrane.This makes the radial distribution no more the same as the straight cochlear is.For different cases,the distribution reflects different shape.In closed to the apex of cochlear(low-frequency sensitive area),the first case plays a leading role.3.For revealing the relationship and physical law between cochlear size,physical properties and lymph,firstly,the curvature and torsion of the spiral cochlear center-line were parametrically determined.Then a three-dimensional spiral cochlear fluid-structure model was established.Using WKB method,the spatial distribution of lymph pressure and vibration of the basilar membrane were obtained.The effect of spiral curvature and torsion on fluid pressure and vibration behaviors were analyzed.It is found that the spiral shape can reduce the inertia damping of fluid motion.At the apex of cochlear,the inertia damping declines the most.In addition,the radial distribution of fluid pressure appears a skew shape because of the spiral shape.At the apical end of cochlear,the skew shape distribution is the most significant.Furthermore,the spiral shape has a distinct influence on vibration of the basilar membrane in low-frequency.With the increase of cochlear curvature and torsion,the influence grows.4.Based on a real human right ear cochlear within the fresh temporal bone,through the third generation synchrotron light source CT imaging technology in Shanghai Synchrotron Radiation Facility,the DICOM data were obtained and imported into MIMICS to segment and process.Then by adopting the Geomagics software,the triangular facet model was refined,denoised,surface fitted,smoothing processed and finally formed a solid geometrical model.Using multi-physics software COMSOL,a three-dimensional cochlear fnite element model with a real spatial shape(spiral-shape),bioactive materials(anisotropic basilar membrane,viscoelastic round window)and physiological environment(compressible and viscous lymph)was established(no simulation model like this has been reported at home and abroad so far.)5.Based on the established cochlear model above,the frequency domain calculation was performed and simulated the spatial distribution of lymph pressure and vibration of the basilar membrane within the cochlear in different frequencies.This simulation not only reproduces the known traveling wave behaviors that first observed by Nobel Prize winner B6kesy,but also reveals a surprising new finding,that is the standing-wave vibration occurring in the basilar membrane at low frequencies.This finding has not been reported so far,which not only makes up for the low frequency vibration data of the basilar membrane membrane which are lack of in previous experiments,but also provide a critical answer to the questions(see Section 4.4.3 in detail)about traveling wave theory over years.6.Based on the established cochlear numerical model,the time domain calculation was further performed and simulated the time-varing displacement,velocity and stress of the basilar membrane and the spiral lamina as well as the time-varing pressure and velocity of lymph in response to a pure tone stimulus.The dynamic response process of cochlear was described.In addition,the time domain vibration characteristics of the basilar membrane was analyzed when subjected to different common sound signals.It is found that:(1)If the velocity is as the analyzing indicator,the vibration velocity of the spiral lamina can not be neglected and it can also be regarded as the important sensing structure of the cochlear.Due to the large amount of fluid mass and pressure that borne by the spiral lamina,the stress of it is very large and transfers from the basal end to the apical end with the increase of time.When the stress is spread into the apical end,the stress distribution extends from the spiral lamina to the base and the apex of the basilar membrane and then the stress concentrates at both ends.Thus,it can be predicted that the base and the apex of the basilar membrane are prone to damage and the positions corresponding to the minimum and maximum characteristic frequencies are out of function.This is why the minimum and maximum sound perception frequencies are easy to be lost.(2)When the cochlear is subjected to click sounds,the basal and apical end of the basilar membrane can successively vibrate,which may make our ears remember what we hear.This may explain why the babies can produce the sounds they heard before.(3)The hearing sensation for road environment noises will cause the basilar membrane vibrate irregularly at different locations,and this vibration behaviors will lead to unpleasant auditory perception in human ears.Therefore,it can be predicted that if the ear is exposed to the noise environment for a long time,it will not only brings fatigue damage for the basilar membrane,but also brings harm for human minds.(4)The hearing sensation for beautiful music will cause the basilar membrane vibrate regularly and synchronously at different locations,and this vibration behaviors will lead to perception of sound resonance and bring pleasant auditory perception.Even though,if the ear is exposed to the beautiful music environment for a long time,it will also bring damage for the basilar membrane and fatigue for hearing.(5)The hearing sensation for two men voices in different frequencies at the same time will cause the basilar membrane vibrate in different amplitude at different locations.This vibration behavior can make people distinguish different frequencies of sound so that the sounding objects can be identified.By contrast,when the cochlear is exposed to the same frequency sounds that produced by two men,the basilar membrane will vibrate with large amplitude at the same location.This will makes it difficult for people to distinguish different sounds thus easily causing confusion of hearing.7.By introducing active mechanism into the established cochlear numerical model,a fluid-structure-electric coupling analysis model was built.The main factors that influencing active mechanism were analyzed.The nonlinear mechanical behaviors within the active cochlear were also analyzed.It is found that:(1)The cochlear active mechanism are affected by many factors.The longitudinal electrical cable,maximum saturation capacitance of hair bundles,tilt projecting distance all have a great impact on cochlear hearing sensitivity and characteristic frequency.The force-electric coupling coefficient and stiffness of outer hair cells both have remarkable influence on cochlear hearing sensitivity,but have a little influence on the characteristic frequency.The stiffness of hair cells has a obvious effect on the characteristic frequency,but has little effect on cochlear hearing sensitivity.(2)The nonlinear hearing sensation behaviors exist in the cochlear.It is reflected that the cochlear can autonomously amplify the low-level sound,which make people can perceive weak sounds.The cochlear can autonomously abate the high-level sound,which can protect our ears from damages when subjected to high intensity sound to some extent.The cochlear can also compress the high-level sound which restricts the sensing structures from responding too fast so that the cochlear structure can perceive sounds within the physiological range of response speed.(3)Two-tone suppression phenomenon exists in human ear when subjected to two simultaneous sounds.The interfering sound with different frequencies will inhibit the perceiving probe sound.More level the interfering sound is,more obvious the inhibition effect is.The low-frequency interfering sound has a comparatively small inhibition effect,but can make the nonlinear compression effect disappear with the increase of the sound level.(4)As a complex nonlinear system,the phenomenon of harmonic distortion can not be avoided in the cochlear.The production of harmonic distortion make the basilar membrane vibration mode very complex,which wil:l make people perceive distorted sounds.When higher order harmonics bring about clustering sounds,it will be difficult for people to distinguish from the distorted sounds with different frequencies and original principal tone.(5)When subjected to two pure tones,the ear can perceive some distortion products that are related with the principal tone.The frequencies of the distortion products mainly appear in a form of linear combination of different frequencies(nfi+mf2,n and m are integers),more notable in the form of 2f1-f2.The larger the difference of two principal tones frequencies is,the greater the frequency tuning range of the distorted tones will be.The human ear can perceive the distortion products as well as the principal tone at the same time,which will make the perceiving sounds more wonderful.8.Based on the established active cochlear numerical model,the sensorineural deafness caused by damaged basilar membrane,round window hardening,damaged outer hair cells and perilymph fistula were simulated.Then the effect of different abnormal pathological states on the cochlear behaviors were analyzed.It is found that:(1)The fracture and thickening of the basilar membrane not only make the characteristic position different from that in the normal state,but also make the basilar membrane resonance at the same location in response to different frequencies.In addition,the velocity of the basilar membrane declines the most in the cochlear apical end,which will cause delayed perception of the sound in the cochlear when people communicate with each other.(2)The fixation of the cochlear implant into the round window will make hearing lose about 13-25dB within the whole perception frequencies.The low-frequency hearing loss is more serious than the high-frequency.When the problems of tissue fibrosis and hyperosteogeny are serious,the hearing loss in the characteristic frequency will reach 50dB,which will significantly influence the perception of hearing.(3)The functional loss,structural damage and totally loss of outer hair cells will cause serious impact on the cochlear hearing sensation.The totally loss of outer hair cells influence the most.The damage of outer hair cells at different locations will cause the most serious impact on the cochlear hearing at the corresponding frequency that determined by the location.(4)When the fistula is located in the oval window,the vibration amplitude of the basilar membrane declines dramatically.When the fistula is located in the round window,the vibration amplitude of the basilar membrane is not affected.When the fistula is located at different positions in the cochlear duct,the effect is remarkable only in corresponding place. |
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