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3 Fundamentals of the Formation of Biosignals

Propagation of the Action Potential

The electrical communication of the nerve cells with each other in the brain, with

sensory cells or to stimulate the contraction of distant muscle cells occurs by means

of the action potential. Accordingly, there are nerve pathways in the body on which

the action potentials can spread, the so-called axon. Basically, a distinction is made

between two forms of conduction, the continuous, i.e. the conduction from one point

of the axon to the directly adjacent one, and the so-called saltatory conduction, in

which the action potential spreads abruptly along the axon.

Saltatory excitation conduction is mainly found in vertebrates due to the faster

propagation of the action potential, as here usually larger distances have to be over-

come and delays in the transmission of nerve stimuli would be intolerable. An ex-

ample of this is the whale, in which a nerve stimulus may have to travel over 50 m be-

fore it reaches the brain. The basic structure of saltatory nervous system differs from

the continuous form by an additional insulation of the axon in the form of the so-

called myelin sheath (cf. Figure 3.8). The saving of the myelin sheath in continuous

nerve conduction is due to space problems, which is why it is mainly absent in smaller

creatures such as insects and on the last centimetres of a vertebrate nerve conduction.

Besides the lower propagation speed, another disadvantage of continuous excitation

conduction is that action potentials are subject to strong attenuation. In contrast to

saltatory excitation conduction, the action potential is not re-formed at the approx-

imately five millimetre long node of Ranvier between the sections of the insulating

myelin sheath. Because the action potential is constantly regenerated in saltatory ex-

citation conduction, attenuation along even long pathways is negligible. This means

it doesn’t just arrive faster, it is also transmitted with constant amplitude. At the same

time, it can be concluded that the amplitude of an action potential cannot stand for its

stimulus intensity, i.e. stronger stimuli produce constant amplitudes, but an increased

frequency of action potentials. However, there is also an upper stimulus limit for this,

which can be explained with the help of the previously discussed refractory period of

the nerve cell. When the upper stimulus limit is exceeded, the frequency of the action

potentials no longer increases, although the stimulus continues to increase.

Continuous Excitation Conduction

In continuous excitation conduction, the action potential is transmitted by depolar-

isation of directly adjacent nerve cells in the axon. This means that after stimulation of

a nerve cell, e.g. by incoming signal stimuli in the synaptic cleft between the dendrites

and the nerve cell, an action potential spreads along the axon by depolarising directly

adjacent nerve cells in the axon due to the increase in potential and these in turn de-

polarise the adjacent ones and so on. Thus, each nerve cell must depolarise in turn in

the axon before the signal arrives at the other end of the nerve. The propagation velo-

cities achieved in this process range from 1 to 5 m/s. The nerve conduction velocity is