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A Realistic Nerve Model for Electrical Stimulation

Author(s):

Thomas Loser
Conference/Journal:

Semester/Bachelor Thesis, WS 05/06
Abstract:

Electrical stimulation in general is the excitation of muscles due to artificially applied currents. A model for axons with various diameters has previously been created but the simulation of this model could only be performed for one specific axon. The second issue which has not yet been explored is the influence of a nerve fiber onto the current distribution inside the arm model. Up to now one made the assumption that nerves could be neglected when observing the current distribution in the model.
The purpose of this thesis was to firstly develop a multi-axon-model in NEURON which could simultaneously simulate an arbitrary number of nerve fibers of different diameters. The resulting voltage distribution data was then processed in MATLAB. In MATLAB one could analyze the time dependent run of the potential curve inside the nerve and also make a statement of the number of generated action potentials. Whether APs were generated or not depended on the pulse length of the stimulation current as well as on the amplitude of the current. It has to be added that the applied voltage distribution along the nerve (extracellular potential) was simulated and computed from a model which neglected the influence of nerve fibers on the distribution.
To compare the obtained results with the reality an experiment at Paralab in Balgrists was carried out. The subject (a human being) had to sit onto the Multi- Moment-Chair (MMC) where its left leg was fixed to a force-sensor. Then several current pulses of different pulse length were applied to the leg. As a result the muscles where excited and they produced a force onto the sensor. The sensor recorded the time run of the force curve which was basically zero except when a pulse was applied to the leg. After this the results where analyzed and plotted. Finally, the experimental plots were compared to the plots from the simulation. One could say that the model was not too different from the experiment.
The second task of this semester thesis was to analyze the influence of the nerve onto the extracellular potential and also the transmembrane potential (potential difference between the extracellular region and the axoplasm) using an FE-model. To do this three geometric models of the human forearm were built in ANSYS. The first was a cuboid which consisted entirely of tissue. No nerves were integrated. The second one was the same cuboid but this time containing a nerve fiber. The third was basically the same as the second model but the nerve was fully surrounded by Myelin, in other words it possessed no Nodes of Ranvier. The three models were then stimulated and the potential distribution was saved into a file. This data was again analyzed in MATLAB. As a matter of fact, one could not make a clear statement whether the potential differences between the three models where significant or not.

Supervisors: A. Kuhn, Dr. T. Keller, Prof. M. Morari

Year:

2006
Type of Publication:

(13)Semester/Bachelor Thesis
Supervisor:

M. Morari

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% Autogenerated BibTeX entry
@PhdThesis { Xxx:2006:IFA_2816
}
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