Johannes J. Struijk
Bioelectric Modelling, Lecture 1, 2008
Exercises Lecture 1
1. In what respect is the strength duration curve for a spherical cell of 20 micron diameter different
from a cell of 40 micron diameter (for intracellular electrical stimulation). What about the
difference in rheobase and chronaxy in those two cases?
2) Consider a myelinated nerve fiber with the following properties
Fiber diameter D = 10 m
Axon diameter d = 0.6D
Internodal distance L = 100D
Length of node of Ranvier l = 2 m
Membrane capacitance cm = 0.1 F/m2
Membrane resistivity rm = 0.5 m2
Intracellular resistivity  = 0.7 m
Calculate the length constant and the time constant for this fiber.
Give a physical interpretation of length constant and time constant.
3) An unmyelinated fiber (in an ideal homogeneous, large extracellular medium) is stimulated with
a point electrode (cathode) at 1 mm away from the fiber. consider the following three situations
for the reference electrode (anodal point electrode – see the figures).
a) The reference electrode is far away;
b) The reference electrode is at 1 mm on the other side of the fiber;
c) The reference electrode is at 1 mm from the fiber and 1mm from the cathode on the same
side as the cathode.
a)
b)
c)
Which situation has the highest stimulus threshold and which one has the lowest threshold,
based on an analysis of the activating function?