Tetanus - Tetanus

This is a form of muscle contraction. When a muscle or the nerve that controls the muscle is stimulated once, the muscle contracts once in response and then relaxes. This type of contraction is called a twitch. However, when the muscle or the nerve to the muscle is stimulated continuously with an appropriate interval between each, the individual twitches merge and add up, resulting in a large, sustained contraction. This type of contraction is called tetanus. The maximum tension of tetanus is several times that of a twitch, and the contraction state continues as long as the stimulation continues.

When the stimulation frequency is low, the individual twitches do not completely fuse and the contraction curve fluctuates, which is called incomplete tetanus, and when the twitches are completely fused and there is no fluctuation, it is called complete tetanus. The minimum stimulation frequency required to obtain complete tetanus is called the critical fusion frequency, and in the case of frog skeletal muscles, stimulation at 20 Hz or more is required at 15°C.

In the body, skeletal and smooth muscles usually contract by tetanic contraction, whereas cardiac muscle does not undergo tetanic contraction and always performs twitch contractions.

[Hidenobu Mashima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

筋収縮(きんしゅうしゅく)の一形態をいう。筋あるいは筋を支配する神経に1回刺激を加えると、筋はそれに応じてただ1回収縮し、ついで弛緩(しかん)する。このような収縮は単収縮とよばれるが、それに対して、適当な間隔をおいて、筋、または筋への神経を連続刺激すると、各単収縮は融合・加重し、持続の長い大きな収縮が得られる。このような収縮を強縮とよぶ。強縮の最大張力は単収縮の数倍に達し、刺激が続く限り、収縮状態も持続する。

　刺激頻度が少なく、各個の単収縮が完全に融合せず、収縮曲線が動揺するものを不完全強縮、単収縮が完全に融合し、動揺のないものを完全強縮とよぶ。完全強縮を得るために必要な最少の刺激頻度を、臨界融合頻度といい、カエル骨格筋の場合は、15℃では20ヘルツ以上で刺激する必要がある。

　生体内での骨格筋や平滑筋の収縮は、通常強縮であるのに対し、心筋は強縮をおこさず、つねに単収縮を行う。

［真島英信］

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