Modeling of Biomedical Systems: Bioelectric 

For this assignment you will use your work from the EBME 359 Bioelectric Labs (Labs 6 and 7) that modeled stimulation from an extracellular electrode and examined the distance from the electrode to the nerve axon (height). Extracellular stimulation and volume conduction will also be discussed in the lectures.

In medical devices such as pacemakers, the relative position between the electrode and the target (nerve fiber in this case) depends on many factors such as surgical implantation. The stimulation parameters required to activate the target depend on the distance between the electrode and nerve fiber.

As discussed in class, nerve fiber sizes (diameters) can range from ~1-20 µm. The size of the nerve fiber (the neural target) affects the amount of current or charge that must be injected (into the fiber or electrode) to generate an action potential. Therefore the relationship between the nerve fiber size and the threshold must be known to accurately determine the stimulation parameters. In Bioelectric HW 1 (and 359 labs 4 and 5) you examined the effect of fiber diameter on threshold for current injected into the nerve fiber (similar to EBME 359 Labs 6 and 7). Use your work from EBME 359 Lab 7.

1. Determine the effect of fiber diameter on threshold for a given pulse width (cathodic stimulation). You may choose the same pulse width as for the 359 labs. Plot the fiber diameter (x-axis) vs. the current threshold (y-axis). You must use an appropriate range of fiber diameters. Describe the relationship and choose a curve fitting equations that represents the relationship. What is the difference between small and large nerve fibers? What is the fundamental reason that explains the difference in stimulation thresholds for nerves of different diameter? Why is this relationship different from what you observed in the previous 359 lab using intracellular current injection?

2. Plot on one graph the second difference of membrane potential along the fiber for each electrode individually and combined. Identify the node where the action potential originates. Plot the second difference of membrane potential for a 5 um and 20 um fiber (fibers half and twice as big). Determine the stimulation thresholds for the 3 fibers. Explain why the stimulation thresholds are different. Plot the curves for different electrode to nerve distances. What do you notice about the effect of distance from the nerve on the shape of the second difference of the potential?

3. Determine the threshold current amplitude for a range of pulse widths and plot the current threshold (Y axis) vs. the pulse width (X-axis). Explain why this relationship exists. Identify which point (stimulus pulse) on your curve uses the minimum charge.