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.
Although involuntary muscular fibre consists of short cells and not of long fibres like voluntary muscle, yet the contraction wave may be propagated along a strip of involuntary muscular tissue in both directions from the point of irritation, just as in voluntary muscle or in the contractile tissue of medusae. This wave is transmitted more slowly in involuntary than in voluntary muscle; and its rate in the involuntary muscle of the heart, though slower than in ordinary striated muscle, is quicker than in unstriated muscle, so that in this respect the heart is intermediary between the two.1
1 Sigmund Meyer, Hermann's Handb. d. Physiol., Bd. 5, Theil ii., p. 476.
2 Szpilman and Luchsinger, Pflilger"s Archiv, Bd. 26, p. 459.
3 Ibid. p. 249.
The passage of contraction waves in involuntary muscular fibre is affected by the same conditions as voluntary muscle, the conduction of the contractile wave being rendered slower by fatigue and cold, while it is quickened by heat.
Cold and fatigue also render the rhythmical pulsations smaller and longer, while heat has an opposite effect. The passage of the contraction wave may also be diminished or arrested by section or pressure, just as in the contractile tissue of medusae,2 so that instead of each contraction wave passing the block produced by the sections or compression, only one out of several, or none at all, may pass. The proportion passing the block depends upon its completeness. If the tissue forming the bridge be dry as well as narrow, the block becomes more complete, and may be again diminished by moistening. Variations in the strength of the stimulus do not affect the passage of the contraction wave over the block, so that it would appear that the injury caused by the section, along with the narrowing of the conduction path, retards the re-establishment of the conductive power.
In experiments made upon the heart of a tortoise cut into a strip, it has been found by Gaskell that stimulation of the vagus removes the block, quickens the recovery of the tissue, and causes every contraction wave to pass. The effect upon the muscle therefore seems to be trophic.
A weak interrupted current applied to the muscle directly has the same action as stimulation of the vagus, i.e. it increases the conducting power of the muscle. Sometimes, however, both the vagus and a weak interrupted current have an opposite effect, and diminish instead of increasing the conducting power.
An artificial rhythm may be induced in a strip of involuntary muscular fibre cut from the heart of the tortoise by passing a weak interrupted current through it and then stimulating it at one end by induction shocks, at intervals of about five seconds. After a while, if the induction shocks are discontinued, the muscle still continues to contract rhythmically at the same rate. These contractions, at first weak, afterwards become strong, and may last for many hours. Both the conducting and the contractile power of the muscle are diminished by muscarine. When a strip of it is stimulated by induction-shocks applied to one end, the contraction wave passes quickly along; but muscarine appears to Mock its transmission, so that while the part of the muscle between the electrodes contracts at every shock, the rest of the muscle contracts only at every second one. A weak interrupted current then sent through the muscle may lower its conducting power and still further reduce the force of the contractions, and not only block the passage of most of fhe contraction waves from the point of excitation, but may even prevent the contraction of the excited part itself.
1 Hermann's Handbuch d. Physiologic, Bd. 1, p. 56.
2 Engelmann, Pfluger's Archiv, 1875, Bd. 11, p. 465; Gaskell, Journal of Physiology, vol. iii. p. 367.
Atropine has an opposite action and appears to increase the conducting power of involuntary muscle, so that when applied to a strip of the heart, the conducting power of which has been diminished by muscarine, the contractility is at once increased, and each contraction wave passes over the whole muscular strip each time that a single point is irritated. Large doses, however, appear to have a depressant action on the muscle.