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3 Fundamentals of the Formation of Biosignals

Cell

nucleus

Dentrite

Neuron

Axon

Myelin sheath

Node of Ranvier

Motor end

plate

Muscle

Fig. 3.8: Schematic representation of a nerve cell (neuron) with its dentrides and the synapses arriv-

ing there (left), an outgoing axon (nerve conduction) with myelin sheath and branches (middle) and

the muscle as a motor system with its end plates (right)

Formation of the Action Potential

In the resting state of the nerve cell, the ion concentrations inside and outside the cell

are different. This state is maintained by the sodium-potassium pump and leads to a

potential difference between −50 mV and −100 mV (inside negative, outside positive).

During the generation of the action potential, this condition changes fundamentally

due to a temporary opening-/closing of the Na⁺- and K⁺- channels. In this process,

the permeabilities for the individual ion species change in a very specific temporal se-

quence. The reason for such a change can be, for example, a brief change in the electric

field (potential) in the vicinity of a channel, which then causes an opening and thus a

further change in the potential. Such a short-term change may have been triggered by

a stimulation of the nerve cell by other nerve cells (via the release of neurotransmit-

ters in the synaptic cleft), as in peripheral somatic stimulation, or, for example, by an

optical stimulus in the visual cells. The Na⁺- and K⁺- ion fluxes are, from an electrical

point of view, a charge shift across the membrane wall (cf. Figure 3.9, left) and lead to

a change in the electrical potential. The sequence of this voltage reversal is different

for nerve and muscle cells. In the case of the nerve cell, the principle sequence (cf.

Figure 3.9) of this so-called depolarisation is as follows:

1.

depolarisation: Voltage-gated Na⁺-channels are opened due to a short-time ex-

ceeding of a threshold potential (membrane threshold), so that Na⁺ ions flow into

the cell due to the concentration gradient. Afterwards, the channels close again.

The membrane potential changes from the resting potential to a positive max-

imum value of about +20 mV (overshoot). Due to the absolute refractory period,