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6 Applications and Methods in Biosignal Processing

determines whether an action potential is triggered in the cell. If this is the case, the

action potential spreads from the dendrites along the axon and triggers the release of

neurotransmitters at the end of the axon (synapse) which are deposited there in ves-

icles. These neurotransmitters in turn react with the receptors of the next neuron and

control the cell membrane there (cf. chapter 3).

Stimulus propagation through a nerve cell occurs as the action potential advances

along the axon. If depolarization is triggered at one point, Na⁺ ions flow from the

environment outside the cell into the interior. This results in a local decrease in the

concentration of Na⁺ ions in the external cellular ambience, which is compensated by

diffusive ion currents from the environment. This, however, decreases the Na⁺ concen-

tration there, which briefly increases the transmembrane voltage above the threshold

voltage¹. By this the action potential is also triggered in the vicinity. However, this is

only possible in the area behind the the depolarization site. The area in front of the de-

polarization site is still in the refractory state at this point, because the action potential

has passed through there before. In the refractory phase the action potential cannot be

triggered again. Therefore, the action potential, and thus the electrical stimulus, con-

tinues in only one direction of the axon fiber. The speed of this process and thus the

conduction velocity is approx. 3 m/s. For neurons that excite skeletal muscles, how-

ever, this transmission speed would be too low, because due to the great length of the

connection between nerve center and muscle fiber of up to one meter, the reaction e.g.

to a dangerous situation would be too slow. In fact, in peripheral neurons that estab-

lish the connection to motor muscle units, a significantly higher conduction velocity

of up to 120 m/s is achieved. The peripheral neurons have a different structure com-

pared to brain neurons. The axon fiber is surrounded by Schwann cells, which prevent

the triggering of the action potential. The sheath is called myelin sheath. At intervals of

0.2 to 1.5 mm, the myelin sheath is interrupted by constrictions, the Ranvier constric-

tions. Only there can the action potential be triggered. Since, during depolarization

at a constriction, the cell-external compensatory currents extend to the next constric-

tion, the action potential jumps from one constriction to the next. This mechanism

significantly increases conduction velocity vs. brain neurons (cf. subsection 3.1.3).

The compensating currents along the axon fiber generate electric fields that

propagate in the neurons of the cerebral cortex to the surface of the skull surface.

The result is a potential difference between arbitrary locations on the scalp. The

overall potential distribution on the scalp results from the superposition of the po-

tentials of all active neurons and glial cells, with nearby cell regions making the

strongest contribution. A spatial potential distribution is created on the scalp that

varies in time with the activity of the cell regions involved. The voltage amplitude

measurable on the scalp is up to 100 μV. Recording by means of electrodes is called

1 The direction of the transmembrane voltage is from the cell interior to the -exterior. Thus, a lack of

positive ions in the cell exterior increases the transmembrane voltage.