Simulation Experiments for Neural Pulses - Brain Theory from a Circuits and Systems Perspective - page 182

Appendix A

Simulation Experiments for Neural Pulses

Introduction

Electrical Parameters

Parameters may be estimated from standard values. For these experiments

parameters are assumed as in Table A.1 .
A simplified description of a neural pulse is as follows: Initially the voltage across
the membrane is

70 mV inside relative to outside. A pulse is triggered by increasing
inside voltage up to about 55 mV. Triggering in a simplified model means that

sodium currents are flowing in and that potassium currents are flowing out, at the same
time. Potassium currents are much less mainly because of the physical properties of
potassium, resulting in a buildup of positive charge inside the membrane.
Currents charge membrane capacitance, 1 F/cm 2 which translates into a certain

internal potential. When internal potential reaches the sodium cutoff voltage at
about +40 mV, sodium current cuts off, leaving potassium current to discharge
membrane capacitance. When internal voltage has dropped to the potassium cutoff
voltage of

μ

90 mV, potassium current cuts off. Once current sources are removed,
the internal voltage is assumed to drift via membrane conductance (0.3 mS/cm 2 or
3.333 k /cm 2 ) up to its rest value of 70 mV.

Generally a pulse may be approximated by straight line segments as in Fig. A.1 .
An actual pulse would have rounded tips because the cutoff voltages vary slightly
frommolecule to molecule in a membrane. The lines in an actual pulse would not be

Ω