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3 Fundamentals of the Formation of Biosignals

Cell

Molecular

Structures

Atomic

Structures

Pipette

Salzlösung

Silber-

chloriddraht

Zelle

-

QRS

P

T

R

-

+

+

+

Organism

Fig. 3.1: Biophysical approach and model descriptions of the electrophysiologic mechanisms at the

different scales.

right in his statements, the foundations of electrophysiology that are still valid today

were made much later.

A number of approaches have been established over time to describe the under-

lying processes, Figure 3.1 considers them according to their scaling. In this, the view-

point differs in the richness of detail of the models, the associated questions to be

answered as well as their validity.

The mode of action of the electrical activity of nerve and muscle cells inside the

body and outside a living organism is described by the laws of electrophysiology

and electrostatics/-dynamics by the Maxwell-equations.⁶ The latter is applied, for

example, in the measurements of electrical potentials on the surface of the body or

by needle or microelectrodes on the scale of the living organism down to individual

cell clusters. Diffusive processes and movements of individual ions at the cell scale,

on the other hand, are addressed by the Nernst-Planck-equation⁷ of phenomenolo-

gical thermodynamics. Here, the processes in the cell, for example the electrodiffus-

ive movement of ionic currents, are considered down to individual ions and their

effects on the scale above. The arrangement of supermolecular structures, as they

occur, for example, in the formation of a lipid membrane or the arrangement of mem-

brane proteins in a cell wall, requires the methods of statistical thermodynamics,

the laws of which are described by the Poisson-Boltzmann-equation⁸. Interactions

of atomic and molecular processes are ultimately quantum mechanically explained

by the Schrödinger-equation⁹ or simplified molecular dynamics described by New-

6 Fundamental equations of electricity and magnetism, named after the Scottish physicist James C.

Maxwell (1831–1879).

7 Fundamental equation of the movement of ions taking into account the electric field, named after

the Nobel Prize winners Walter H. Nernst (1864–1941) and Max Planck (1858–1947).

8 Fundamental equation of electrostatic interactions between molecules in liquids with ions dis-

solved in them, named after Simeon D. Poisson and Ludwig Boltzmann.

9 Fundamental equation of quantum mechanics, named after the Austrian physicist and Nobel laur-

eate Erwin R.J. Schrödinger (1887–1961).