3.1 Physiology and Electrical Activity of Muscle and Nerve Cells

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RP

Axon

AP

U/mV

U/mV

- - + + + + + + +

MS

0

0

0

0

Time

U/mV

U/mV

+ + - - + + + + +

- - + + - - - - -

+ + - - - - - - -

Time

L1

RP

Fig. 3.10: Propagation of the action potential in continuous excitation conduction: the action poten-

tial propagates as a wave along the axon with a nerve conduction velocity of 1 to 5 m/s; the partial

figures on the left and right refer to different times.

indeed a diagnostic parameter that is used in the daily practice of neurologists for the

diagnosis of numerous nerve diseases such as multiple sclerosis or amyloidosis.

In contrast to the invasive measurement with the help of microelectrodes as

shown in Figure 3.10, the doctor usually measures withsurfaceelectrodes(cf.chapter4)

and determines the speed of propagation from the running time and the distance

between the measuring points. If this is reduced, this indicates a pathological situ-

ation of the measured nerve. Figure 3.10 also shows the depolarisation and charge

shift of the ions associated with the conduction. Analogous to the electrical resist-

ance of a conductor, a conductance and an attenuation or a decay interval of the

action potential can also be determined in the case of the axon. The specific resist-

ance ga of an axon moves around 30 Ωm, the associated decay distance l, at which

the input potential has decayed to half, is a few millimetres.

Saltatory Excitation Conduction

The saltatory excitation conduction, which only occurs in vertebrates, has been op-

timised by evolution with regard to the paths and response times to be overcome with

it by a periodic isolation from myelin sheath and a node of Ranvier. With this excita-

tion conduction, the action potential is not transmitted to the directly adjacent nerve

cells in the axon by depolarisation, but due to the field propagation in the axon, it

jumps across the sections of the myelin sheath and is regenerated in the next node of

Ranvier. This type of propagation is advantageous not only with regard to the speed of

propagation, which differs by a factor of about ten, but also because of the preserva-

tion of the amplitude of the action potential. However, nerve conductors require more

space due to their isolation, which is why they only occur in larger organisms. This

is also the reason why continuous nerve conduction is unavoidable in the last milli-

metres from the nerve end, even in vertebrates. However, the associated attenuation is