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.
Nerve-trunks are, as a rule, very much less affected by poisons than the end-plates; but they may, nevertheless, be also acted upon by strong solutions of a poison. It appears necessary to apply the poison locally to them, and they are probably little if at all affected by poisons introduced into the system generally. The action of poisons is tested by placing a small piece of guttapercha tissue under the nerve-trunk, usually the sciatic of the frog, and applying the poison directly to it, or dipping the nerve into a weak solution of common salt, or of sodium phosphate, to which the poison has been added, and comparing the poisoned nerve with one dipped into a similar saline solution without the poison.
There are two methods of comparison. The first consists in using the contraction of the corresponding muscle as an index of the functional power of the nerve; the second in ascertaining the effect of the poison on the normal electrical current in the nerve.
The motor fibres of a nerve appear to have their excitability abolished more readily than that of sensory nerves by changes in the body generally, and sometimes also by the local application of drugs to them. Thus in wounded nerves the motor function may be destroyed, while the sensory function is little altered.
and where both sensibility and motion have been destroyed by a bruise of the nerve-trunk, the sensibility may reappear, while the motor power does not. In rheumatic neuralgia there is not unfrequently motor paralysis with exaggerated sensibility. When a solution of physostigmine is applied locally to the nerve-trunk for a while, and the nerve is then irritated beyond the point of application, it is found that it will produce reflex movements of the body after it has ceased to do so in the limb supplied by the nerve, which shows that the sensory fibres can still conduct impressions, though the motor fibres cannot. Longer application of the poison will destroy the sensory fibres also. When a paste of theine is applied to the sciatic nerve, or the nerve is dipped in a solution of opium, similar results are observed.
By dipping nerves in a solution of the poison Mommsen finds that atropine diminishes the irritability of the nerves, affecting first the intramuscular endings, and afterwards the trunks. Alcohol, ether, and chloroform first increase and then diminish the irritability.