Twitch/Tetanus

Twitch is a muscle contraction caused by a single action potential, whereas tetanus is a sustained muscle contraction caused by a series of repetitive action potentials. The amplitude of tetanus contraction is larger than that of twitch, due to mechanical summation. 

Figure 2. Muscle stimulation, a) a single nerve impulse (stimulus) causes a single contraction (a twitch). There is a small delay following the stimulus before force rises called the latent period, b) A train of stimuli at a low frequency causes an unfused tetanus. Force increases after each progressive stimulus towards a maximum, as calcium levels in the myofibrillar space increase. But there is enough time between each stimulus for calcium to be partially taken back up into the sarcoplasmic reticulum allowing partial relaxation before the next stimulus occurs, c) A train of stimuli at a higher frequency causes a fused tetanus, and force is maximum. There is not enough time for force to relax between stimuli. In the contractions shown here, the ends of the muscle are held fixed the contractions are isometric.

Tetanus occurs when Cl. tetani, ubiquitous in the soil and faeces, contaminates wounds, especially deep puncture-type lesions. These might be minor traumas such as a splinter, or major ones such as battle injury. At these sites, tissue necrosis and possibly microbial growth reduce the oxygen tension to allow this anaerobe to multiply. Its growth is accompanied by the production of a highly potent toxin which passes up peripheral nerves and diSuses locally within the central nervous system. It acts like strychnine by affecting normal function at the synapses. Since the motor nerves of the brain stem are the shortest, the cranial nerves are the first affected, with twitches of the eyes and spasms of the jaw (lockjaw). 

As mentioned previously, a single action potential lasting only 2 msec causes a muscle twitch that lasts approximately 100 msec. If the muscle fiber has adequate time to completely relax before it is stimulated by another action potential, the subsequent muscle twitch will be of the same magnitude as the first. However, if the muscle fiber is restimulated before it has completely relaxed, then the tension generated during the second muscle twitch is added to that of the first (see Figure 11.3). In fact, the frequency of nerve impulses to a muscle fiber may be so rapid that there is no time for relaxation in between stimuli. In this case, the muscle fiber attains a state of smooth, sustained maximal contraction referred to as tetanus. 

Figure 11.3 Muscle twitch summation and tetanus. A single action potential (represented by A) generates a muscle twitch. Because duration of the action potential is so short, subsequent action potentials may restimulate the muscle fiber before it has completely relaxed, leading to muscle twitch summation and greater tension development. When the frequency of stimulation becomes so rapid that no relaxation occurs between stimuli, tetanus occurs. Tetanus is a smooth, sustained, maximal contraction.

Paralysis can take place anytime there is a failure or interference in the transfer of biochemical impulses from nerve to muscle. On the other hand, hyperactivity of neuromuscular transmission can lead to minor twitches and cramps or to severe spasms as in tetanus (lockjaw) or amyotrophic lateral sclerosis (Lou Gehrig disease). There is still much to learn about both hyperactive and paralytic cases, but new research on DNA and immunology is proving helpful. 

The improvement in regrowth is reflected in the improved electrophysiological parameters of the EDL muscle in the melanocortin-treated, injured neonates. Many of the contractile parameters of both the twitch and tetanus are in the normal range force amplitude, contraction rate, halfrelaxation time, post-tetanic twitch amplitude, and rate of rise (Zuccarelli and Strand, 1992). Of considerable significance is that a-MSH also increases the number of motor units, presumably restoring fine motor control in the traumatized, developing neuromuscular system. 

Eddinger TJ, Murphy RA (1991) Developmental changes in actin and myosin heavy chain isoform expression in smooth muscle. Arch Biochem Biophys 284 232237 Eddinger TJ, Wolf JA (1993) Expression of four myosin heavy chain isoforms with development iii mouse uterus. Cell Motil Cytoskeleton 25 358368 Edman KA (1980) Depression of mechanical performance by active shortening during twitch and tetanus of vertebrate muscle fibres. Acta Physiologica Scandinavica 109 1526... 

A typical skeletal muscle reacts to an artificially induced electrical impulse with a brief contraction, resulting in a mechanical deflection imder suitable experimental conditions (see Fig. 9.6). An increase in the excitation results in a superposition of contractions when the successive impulse takes place before the muscle has dilated completely incomplete tetanus. Single twitches are no longer detectable at an excitation frequency of about 50 impulses per second complete tetanus. 

Fig. 9.6. Description of a muscle contraction from single twitch to tetanus...

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