This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
A. Drugs acting on the Muscles. - Pharmacologists have devoted much attention to this class of drugs; but as the facts ascertained are not used in medicine, we need not stop to consider them. Brunton gives the following classification, founded on that of Kobert:
Class I. Irritability of muscle unaffected; total amount of work it can do diminished. - The following produce this effect: Apomorphine, delphine, saponin, copper, zinc, and cadmium, and in large doses, antimony, arsenic, platinum, and iron.
Class III. Diminish the capacity for work, and make the excitability very irregular. - Lead, emetine, and cocaine.
Class V. Increases the excitability. - Physostigmine.
Small doses of strychnine and veratrine shorten the latent period; large doses lengthen it.
Dilute alkalies diminish the extensibility; dilute acids increase it.
B. Drugs acting on the Peripheral Endings of Motor Nerves. - Of the drugs belonging to this group the action of curare has been worked out most fully. If curare is given to an animal, it is found that the muscles will respond to a mechanical stimulus, although they will not contract when the motor nerve is stimulated. If a single muscle be removed from the circulation by ligature of its vessels before the administration of curare, afterwards it will be the only one that will respond to stimulation of its motor nerve. As this was the only muscle of the body that the drug could not reach, and it is the only one not poisoned, the poison clearly acts locally on the muscles; but as the curarized muscle will respond to mechanical stimulation, curare must paralyze the motor nerves within the muscle, probably the end plates.
Drugs paralyzing the termination of the motor nerves in muscle:
(3) Belladonna (atropine).
(8) Amyl nitrite.
(9) Diluted hydrocyanic acid.
(20) Methyl-strychnine, and many others.
Curare and conium are by far the most important. Therapeutically we never desire to paralyze motor nerve endings.
Drugs stimulating the termination of motor nerves in muscle:
(5) Strychnine (slightly).
Excepting that perhaps some of the beneficial action of strychnine in certain cases may be due to its slight action on motor nerves, we do not employ these drugs for this action.
C. Drugs acting on the Peripheral Endings of Sensory Nerves (other than those of special sense). - Our knowledge of these is derived almost entirely from observations on man, for it is very difficult to experiment upon animals, as they have such imperfect means of communicating their sensations to us.