BME 4900 Final Report - Biomedical Engineering - University of ...

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BME 4900 FINAL REPORT 13Team 8decreases, the linearity of the voltage decay curve increases and the rate of decaydecreases.Figure 9. Percent voltage loss as a function of dendrite length. Each curve denotes a different axialresistance.The other aspect of the dendrite that must be manipulated is the time constant ofthe RC circuits. In addition to the voltage decay, this value, describing the amount oftime it takes for the capacitors to charge. If the time constant is too large, then it ispossible that the membrane potential will not reach threshold in the desired time,drastically altering the behavior of the neuron. Theoretical calculation of the equivalenttime constant for the complete dendrite circuit is possible, but is difficult to interpret asan analytical equation. Fortunately, developing a program to perform the iterativecalculations allows for rapid determination of the time constant if the component valuesare known for the system.Soma (Cell Body)The soma acts, functionally, as the final compartment of the dendritic tree,consolidating all of the input signals. Unlike the other dendrite compartments, the somamodel contains no axial resistance element. However, there is a new property that must
14 BME 4900 Final ReportTeam 8be incorporated: a current stop to prevent backflow into the dendrite from the axon. Thediode would typically be used to create such a stop, but due to the voltage loss over thediode, an alternative method is used. The precision diode operational amplifier circuithelps to eliminate the voltage drop over the diode while still allowing for the backflow tobe stopped. It is followed by a voltage follower operational amplifier circuit (see Fig. 10).Multisim analysis of the compartment’s circuit shows a current backflow ofapproximately 53 nA, which is an acceptable backflow given the amplitudes we aredealing with in the model.Figure 10. A generalized schematic of the soma compartment, including the current backflowstop. This circuit outputs to the axon compartment.AxonThe axon is final component of the neural unit and, consequently, the site ofaction potential generation and propagation. The pre-synaptic input, which crosses thepreviously discussed synapse and is conducted by the post-synaptic dendrites, finallyreaches the neuraon axon where a resulting neural action is produced. In the context ofour final design, several basic subcircuit designs can be repeated, with modification, tomimic the desired behavior of each neuron population. The design relies on theFitzHugh-Nagumo circuit model of a neuron, which is an adaptation of the empiricallydefined Hodgkin-Huxley model. This analog design provides a robust, cost-effective
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Page 32 and 33: 3 Realistic ConstraintsBME 4900 FIN
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Page 36 and 37: 6 Life-Long LearningBME 4900 FINAL
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