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3 Fundamentals of the Formation of Biosignals

To analyse the electrodynamic potential changes on the surface of the body, an

understanding of the electrophysiological relationships of individual nerve or muscle

cells or the associations of such cells is required. These are investigated within the

framework of experimental neuro- and muscle physiology using invasive microelec-

trodes. Although the findings emerged from the interplay of theoretical and experi-

mental research, this section will introduce the physiological and relationships in an

electrical analogy. Experimental methods for measurement and common models used

are presented in section 3.2.

3.1.1 Formation and Function of Biomembranes

The cell membrane or biomembrane consists of a closed double layer of phospholipids

(cf. Figure 3.2), i.e. a chemical structure consisting of a phosphate group and one or

more hydrocarbon chains. Due to the amphiphilic¹¹ properties of the polar, hydrophilic

(water-attracting) head group and hydrophobic (fat-attracting) ends of phospholipids,

they form energetically favourable molecular aggregates (-associates), so-called asso-

ciation colloids in aqueous solutions, depending on concentration and temperature.

The phase separation is caused by the energetically favourable arrangements

between the negatively charged hydrophilic head groups and the polar water, as well

as by the contact avoidance of the water with the hydrophobic fatty acid-containing

ends by repulsive interaction. This process is called self-assembly.

Lipid bilayers, i.e. a parallel and facing arrangement of the hydrophobic ends

and a resulting membranous arrangement of the head groups on the two opposite

sides, occur at high lipid concentrations (cf. Figure 3.3). At a lower mass concentra-

tion, the critical micellar concentration (CMC) on the other hand, so-called micelles¹²

Phospholipid

Overlay

Phospholipid

Chemical Structure Phospholipid

CH2

CH3

R

C

C

C

CH2

CH2

CH3

CH3

CH3

CH2

H2C

H

H

O

O

O

O

O

P^-

N⁺

O

R

O

-P

-P

Fig. 3.2: Phospholipid in simplified representation (left), with overlay of chemical structure (middle)

and as chemical structural formula (right).

11 From Greek amphis = both and philia = love, friendship; composite with polar (water-loving) head

groups and lipophilic (fat-loving) ends.

12 From Latin, mica. mica = lump, small morsel.