We can model some of the electrical properties of an unmyelinated axon as an ele

Question

We can model some of the electrical properties of an unmyelinated axon as an electric cable covered with defective insulation so that the current leaks out of the axon to the surrounding fluid. We assume the axon consists of a cylindrical membrane filled with conducting fluid. A current of ions can travel along the axon in this fluid and can also leak out through the membrane. The inner radius of the cylinder is 5.0 um; the membrane thickness is 8.0 um. a) If the resistivity of the axon fluid is 2.0 Ohm*m, calculate the resistance of a 1.0-cm length of axon to current flow along its length. b) If the resistivity of the porous membrane is 2.5X10^7 Ohm*m, calculate the resistance of the wall of a 1.0-cm length of axon to current flow across the membrane. c) Find the length of axon for which the two reistances are equal. This length is a rough measure of the distance a signal can travel without amplification.

Explanation / Answer

a) R = rho L/A = 2*0.01/(pi*5.0E-6^2)=2.55E8

b) now thickness is L and surface area is A

R = 2.5E7 * 8.0E-6/(2*pi*0.01*5.0E-6)=6.37E8

c)

here if equal

2*L/(pi*5.0E-6^2)=2.5E7 * 8.0E-6/(2*pi*L*5.0E-6)
L=0.0158 m

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