| Auditory system is used to receive and process acoustical signals by human beings. Auditory system, which is one of the most important sensory systems in human bodies, consists of the periphery and the central. Started from the ear, the periphery auditory system is the first part of sound conduction that can transfer the mechanical vibration into the electric energy for the next stage of auditory system. It is connected with the central auditory system through the auditory nerve fibers. Besides, across the brain stem, midbrain, the cerebral cortex of thalamus, the central auditory system is one of the longest central pathways in the sensory system. When arriving at the periphery auditory system along with the external auditory canal, sound is accepted and processed step by step and finally delivered to the auditory cortex which is in the top level of the auditory system. The auditory cortex receives the input from the former nuclear and makes the livings feel the presence of sound and catch the information contained in the sound by a complicated series of processing and coding. The peripheral auditory system, which is mainly composed of outer ear, middle ear and inner ear, is crucial for the transmission and coding of sound information. At the same time, it bears the role as a transformer of the energy. Mammals have a pair of symmetrical auditory periphery to expand the scope of the auditory space and realize the sound source localization and so on.There is a lot of research and study on the periphery or whole auditory system, but most is in the field of physiology. Many kinds of experimental methods are proposed to study the structures and functions of various organs and tissues in the auditory pathway. Consequently, these experiments provide rich data and lay a solid foundation for people to explore the auditory mechanism. On the other hand, we found some inevitable defects in the process of the physiological experiments. First of all, the limitations of experiment methods and technologies lead to some defects which we do not expect. For examples, the uncertainty in the animal experiments, the long experiment cycle and the poor repeatability. Secondly, the large data obtained from the animal experiments are hard to be analyzed and saved. Meanwhile, we cannot ignore the error data in that it is inescapable and unreliable, which result from the influence and interference of the external circumstance. What’s more, the physiological experiment is based on the cells or tissues. So it is impossible to target the object in the absence of influence from other structure. As a complex configuration, the function and mechanism of the periphery auditory system is uncertain and indeterminate. As a result, taking its function and properties into consideration by investigating the structure separately and completely is physiological hardly-realized.By now, people have learnt the auditory system based on the model for many years. The modeling and simulations of the periphery auditory system have been published since years ago as well. The combination of the experimental data and model turns out to have certain advantages in the study of the auditory neurons and system. Based on the physiological structure and function of the hearing system, we model the auditory system with the method of physics and mathematics. In the course of simulation, we can not only avoid the uncertainty and limitations caused by physical experiments, but also can explore the internal mechanism of model through the reasonable operations which are carried out in accordance with the research purpose of the model experiment. All of these will put forward guidance for the next study. Besides, we can do investigation of the macro mechanism of the whole system when using the model on simulation of the physiological microscopic structure. In this study, it is proved to be an effective means that in the process of model establishment and model simulation, based on the existing physical experiment and the existing model to simulate the periphery auditory system. In the process of the periphery auditory system modeling, the object of study is analyzed in separation, at the same time, the physical impact of other factors in the environment are considered.The peripheral auditory system has the ability of coding the frequency and intensity information of sound and this process mainly depends on the basilar membrane and the cochlear hair cells in together. The basilar membrane along its topology structure is equivalent to a "frequency analyzer", it can extract frequency information from the sound signal. Outer hair cells have the amplification effect to voice signal, which improves sensitivity and selectivity of the cochlea. Inner hair cells can deliver the sound signals of different intensity and length to the central nervous system. By cooperation of these structures, we transfer the acoustic vibration into electric signal and form the membrane potential to stimulate the hair cells to release the neurotransmitters into the corresponding spiral ganglion.The basilar membrane is a pseudo-resonant structure that, like strings on an instrument, varies in width and stiffness. Human’s basilar membrane is about30mm long. The "string" of the basilar membrane is not a set of parallel strings, as in a guitar, but a long structure that has different properties (width, stiffness, mass, damping, and the dimensions of the ducts that it couples to) at different points along its length. The role of the basement membrane can be summarized as lymph separation, a base for the sensory cells and frequency dispersion. Each part of the basilar membrane, together with the surrounding fluid, can be thought of as a "mass-spring" system with different resonant properties:high stiffness and low mass, hence high resonant frequencies at the near end, and low stiffness and high mass, hence low resonant frequencies, at the far end. In the mammalian cochlea, some10-30afferent fibers each form a single bouton-like ending on one inner hail cell, and most such contacts are served by a single synaptic ribbon. This kind of structure has many vesicles on it. The data suggest that transmission operates through the coordinated release of multiple vesicles from each active zone. A mechanism that is different from that in the central system could ensure reliable transmission during the ongoing sound signaling. Perception of sound is initiated in the inner ear by the conversion of vibrational energy into a neural code, via a two-step process. First, the mechanical displacement of stereocilia generates a change in the cochlear hair cell’s membrane potential. Second, these voltage changes cause release of neurotransmitter onto postsynaptic processes (termed’ afferent boutons’), which extend from inner-ear ganglion cells. These ganglion cells then give rise to action potentials which are sent to the brain via the auditory nerve. In this course appear several times of energy transformation:from mechanical energy into electrical energy, and then from electrical energy into chemical energy, again from chemical energy into action potential. It is worth noting that the receptor potential of mammalian inner hair cells recorded in the cell has a very good followed feature only in low frequency, thus we can speculate that in the process of energy conversion signal frequency information especially high frequency information may be lost.In the study, we use computer simulation to research the cochlea basilar membrane and inner hair cells. The experiment process is mainly divided into two steps. Firstly, according to the physiological structure and characteristics of the basilar membrane and hair cells to set up a mathematical modeling; secondly, using the well-built model to explore the response characteristics of the basilar membrane and hair cells. During which, the MATLAB is used to code and the Origin (a kind of analysis software) is used to data procession. The sine wave is regarded as input to the models, and when necessary, joined the noise in the signal to simulate the voice. The investigation to the sound signal mainly concentrates in intensity, frequency and duration. In addition, this study also contains the modeling and simulation of the cochlear nucleus, which is the improvement of existing cochlear nucleus neuron model. The process of the simulation is similar to before.In the paper, we not only establish new models of cochlear basilar membrane and inner hair cell, improve the cochlear nucleus model, but use these models to simulate the real structure as well. What we get is consistent with the experimental data. Firstly, the differentiated all-pole gamma-tone filter is a new improved filter that is based on the gamma-tone filter. It remains all properties of the former filter in modeling the basilar membrane, but brings the asymmetry. Besides, its expression in frequency domain is much simpler for digital implementation. We model a filter group with the differentiated all-pole gamma-tone filter to select frequency from the sound vibration more fine-tuning and subtly. Secondly, the inner hair cell model we propose in the paper is on account of the fact that inner hair cell can release multi-vesicular simultaneously triggered by calcium ion influx. The deflection of stereocilia activates the ion channels on the top of inner hair cell, which generates a change in the cochlear hair cell’s membrane potential to depolarize the cell. The depolarization makes the inner hair cell open the L-type calcium ion channels at the bottom of it, as a result, the extracellular calcium ions flow into the cell through this channel and gather near the ribbon structure to make the vesicles release into the gap. Neurotransmitters in the vesicles combine with glutamate receptors at postsynaptic membrane, which generate action potentials. When the frequency of input is200Hz, every output voltage waveform of depolarization is corresponding to the vibration, which means the output may well follow the input and the frequency information of input signal is preserved. When the input frequency is20KHz, the high frequency information of input signal has not been preserved.The results from our modeling and simulation are consistent with the real data in the animal experiments. Our researches not only verify the response characteristics of each structure, but also carry on some reasonable inference. They also provide certain guiding for later physiological experiments. The Basilar membrane is widest and least stiff at the apex of the cochlea, and narrowest but most stiff at the base. The parameters of the basilar membrane at a given point along its length determine its characteristic frequency at which it is most sensitive to sound vibrations. The frequency code founds an essential premise for auditory system to process sound. Besides the frequency dispersion, the basilar membrane possesses biological characteristics in rich like active amplification, nonlinearity and asymmetry. All these make the basilar membrane more sensitive and precise. With regard to inner hair cell, the course of vesicle movement, exocytosis needs time, which leads to the low-pass filter property of inner hair cells. In the simulation of the cochlear nucleus, by modifying the parameters of the model we find the interchanges among "primary-like","chopper", and "onset" response patterns. Furthermore, we simulate the "pauser" response pattern by adding an extra input in our model. The results indicate that the synaptic integrations and the input modes can give rise to different characteristics of CN neurons, which eventually determine the response patterns of CN neurons. |
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