3.1 Physiology and Electrical Activity of Muscle and Nerve Cells

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proportions of the membrane voltage. The Hodkin and Huxley model includes only

the ion species Na⁺ and K⁺ and their conduction across the ohmic resistances RNa⁺

and RK⁺ and a leakage current that can flow across the ohmic resistance RLDS. The re-

spective fractions of membrane voltages UA^± result from the ion distributions between

intra- and extracellular space (cf. Figure 3.9); these are counter-directional in the case

of Na⁺ and K⁺, consequently the voltage sources are also counter-directional. In sub-

section 3.1.3, the emergence and propagation of action potentials of the nerve cell is

described in more detail. There, an action potentials is triggered by a stimulus (coming

from outside) under certain conditions, which propagate along the axons. During this

process, the permeabilities or the conductances gA^± = R⁻¹

A^{± change for individual ion}

species with time. In the electrical equivalent circuit, this behaviour is represented by

variable resistances. The instantaneous total current Im through the cell membrane is

obtained analogous to electrical engineering:

Im = INa⁺ + IK⁺ + ICl−/Ca2+ + ILDS + CLDS

dULDS

dt

.

(3.2)

In the course of the book, further equivalent circuit diagrams, for example for nerve

conduction in the axon (cf. subsection 3.1.3), and basic models of heart excitation (cf.

section 3.2) are modelled in detail using Scilab/COS. In chapter 4, their significance in

the measurement of potentials at the body surface becomes apparent.

3.1.3 Emergence and Propagation of Action Potentials

In the section 3.1 the electrophysiological basics of the resting potential were clarified

under the assumption of a thermodynamic equilibrium and the temporal processes in

the emergence of nerve impulses, the so-called actionpotentials,weredeliberately neg-

lected. This section will now deal with the mechanisms of the emergence and propaga-

tion of nerve impulses in the body.

Physiology of the Nervous System

The basic building blocks of the central nervous system (CNS) and the peripheral

nervous system (PNS) and the sensory and motor systems are the neuron or nerve cells.

A neuron has several synapsessynapses (signal inputs) and an axonal (signal output)

and is therefore often compared to a multi-input single-output-system (MISO system)

of signal processing (cf. Figure 3.8). Signal processing in neuron networks takes place

on an electrochemical basis. Thus, through the release of chemical messengers in the

synaptic cleft of a neuron and internal processing (decision) by the nerve cell, an elec-

trical nerve signal is generated and transmitted through the axon to other nerve cells.