Tetanus (physiology)
The term tetanus (from ancient Greek τέτανος (tetanos) = tension, spasm; also tetanic contraction or electrotonus ) describes in physiology the form of muscle spasm caused by electrical current. A distinction is to incomplete from complete and smooth tetanus.
Emergence
Note: The frequencies mentioned differ greatly in the literature. This is often due to the type of muscles used experimentally or their original organism (e.g. human, frog).
At stimulation frequencies in the low range (~ 5 Hz ), the skeletal muscle reacts with single twitches, as the calcium released as part of the electromechanical coupling , which enables the contraction through the influx into the cytosol of the muscle cell, is completely removed from the cytosol by a Ca 2+ - ATPase the muscle cell can be removed.
If the frequency of neural excitations increased to about 10 Hz, either by the central nervous system or in the experiment by direct irritation leading to the respective muscle efferent neural pathway, the result is the superposition ( superposition ) of individual twitches. The tension maxima increase, but can still be distinguished from one another ( incomplete tetanus). This effect is based on a prolonged increased cytosolic calcium concentration, since the capacity and speed of the Ca 2+ -ATPase at this stimulus frequency are not sufficient to reduce the calcium concentration to the resting value of approximately 10 −7 mol / l . From a frequency of approx. 30 Hz a smooth or complete tetanus occurs, in which the individual twitches can no longer be separated from one another. The increased contractility due to the longer elevated cytosolic calcium level is accompanied by an increase in the force of contraction by 2 to 8 times.
Fusion frequency
The stimulus frequency at which a transition from incomplete to complete tetanus takes place is known as the fusion frequency or fusion frequency . The interval between the triggering nerve stimuli must be less than 1 / 3–1 / 4 of the time that a single twitch of the muscle lasts. However, the refractory period of the muscle fibers has a limiting effect , i.e. H. the time span in which no further action potential can be triggered after a previous excitation on the cell membrane . In the case of skeletal muscles, this refractory period is a few milliseconds. Incoming stimuli during this phase are extinguished.
The heart muscles cannot be tetanised due to their long refractory phase of 0.2 to 0.3 seconds. This enables the vital regular heart action, which consists of phases of relaxation (filling) and contraction (ejection of blood).
In the skeletal muscles, voluntary contractions are usually superimposed single twitches. If a higher force is to be generated, either additional (individually twitching) motor units are recruited (spatial summation) or the motor units are activated with increasing frequency (temporal summation). With this more rapid stimulation, the individual twitches can be superimposed with an increase in strength up to complete tetanus, but persistent tetanic contractions are not the rule, especially with low forces generated. The contractions of the skeletal muscle appear "smooth"; H. no single twitches can be resolved and the contraction is homogeneous. A smooth contraction is not automatically tetanic, but the complete tetanic contraction is always smooth the other way around (for the definition of the tetanic contraction, see above).
Individual evidence
- ↑ Pflüger EFW: Studies on the physiology of Electrotonus: with 5 copper plates , Verlag Hirschwald, 1859 online here