This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
When the stimulation is exceedingly strong, the relaxation after contraction may become very slow, and the descent of the curve may be divided into two parts. At first it descends for a short time pretty quickly, and then falls very slowly indeed. This long contraction of the muscle is known as contracture. It is very strongly marked in muscles poisoned by veratrine or barium. It occurs, though to a less extent, in muscles poisoned by salts of calcium and strontium, by ammonia, and by the chloride, iodide, nitrite, nitrate, and cyanide of ammonium.2
The cause of contracture is not known; it is considered not to be a tetanic contraction, because unlike an ordinary tetanised muscle it does not give rise to secondary tetanus in another frog's muscle, when the nerve of the latter is placed upon it. It is, however, an active contraction, not a mere alteration in the elasticity of the muscle preventing its relaxation; for, as Fick and Boehm have shown, a much greater amount of heat is developed during the long-continued contracture than in an ordinary contraction. Sometimes, and indeed not unfrequently, the contracture, instead of consisting of a single prolonged contraction, appears in the form of a prolonged contraction added on to an ordinary contraction before relaxation has had time to occur. This gives rise to a peculiar hump in the curve, as is well seen in the middle curve in Fig. 49. This appears to show that the contracture is really a double phenomenon, like the two contractions observed after a single stimulation in the muscle of the crayfish by Richet (Fig. 45). How far the contracture may depend upon irritation of the muscle by its own current has yet to be determined.
1 Kronecker, Ludwig's Arbeiten, 1871, p. 183. 2 Brunton and Cash, Proc. Roy. Soc, 1883.
Fig. 45. - Secondary contraction in the muscle of a crayfish. The thick part of the lower line shows the time during which the muscle was irritated by a tetanising current. It will be noticed that the secondary contraction occurs after the irritation has ceased, and after the tetanus caused by it has relaxed. It is not a simple continuous rise, but exhibits several waves indicative of a kind of rhythm. (After Richet.)