Tetanic contraction

The term tetanus referred to in the literature typically the disease tetanus. However, particularly in the older literature he referred to that described herein, caused by electric current shape of the muscle cramp (also tetanic contraction or electrotonus ). We distinguish the incomplete from complete or smooth tetanus.

Formation

Note: The frequencies mentioned vary greatly in the literature. Often this is due to the nature of the experimentally used muscles or their source organism ( eg, human, frog) conditionally.

Irritable frequencies in the low range (~ 5 Hz) of the skeletal muscle with a single jerk respond because the liberated in the electromechanical coupling calcium, which is made ​​possible by the influx into the cytosol of muscle cell contraction, with a Ca2 -ATPase completely from the cytosol of the muscle cell can be removed.

If the frequency of neural impulses increased to about 10 Hz, either by the central nervous system or in the experiment by direct stimulation of the muscle leading to the respective efferent neural pathway, it is over- (superposition ) of individual twitches. Here, take the stress maxima, but are still distinguishable from each other ( incomplete tetanus ). Basis of this effect is longer-lasting increased cytosolic calcium concentration, since the capacity and speed of the Ca2 ATPase in the stimulation frequency are not sufficient to reduce the calcium concentration to the steady state value of about 10-7 mol / l. At a frequency of about 30 Hz, there is a smooth and complete tetanus, in which the individual contractions are no longer distinguishable from each other. The increased contractility by the more elevated cytosolic calcium levels is an increase of contraction force on the 2 - to 8 -fold accompanied.

Fusion frequency

The stimulation frequency, wherein a transition from the whole tetanus incomplete takes place is referred to as fusion frequency. The interval of the trigger neural stimulation has for less than 1/3-1/4 of the time continues for a Einzelzuckung the muscle amount. However, a limiting effect, the refractory period of the muscle fibers, i.e. the time period in which after the previous energization to the cell membrane, no further action potential is triggered. In skeletal muscle, this refractory period is a few milliseconds. During this phase, incoming stimuli are extinguished.

The heart muscle is not tetanisierbar due to their long refractory period of 0.2 to 0.3 seconds. Thus the vital regular heart action is enabled, which consists of phases of relaxation ( filling) and contraction ( ejection of the blood).

In skeletal muscle voluntary contractions are usually superimposed single twitch. If a higher power are generated either more (single twitch ) motor units recruited (spatial summation) or the motor units with increasing frequency active ( temporal summation ). In this faster irritation may occur superposition of the individual twitches with a power increase to a complete tetanus, sustained tetanic contractions but are not particularly at low forces generated the rule. The contractions of skeletal muscle appears "smooth", ie it can not single twitches are dissolved and the contraction proceeds homogeneously. However, a smooth contraction is not automatically tetanus, but the full tetanic contraction is the other way around always smooth (for definition of tetanic contraction see above).

Itemization

• Motor activity