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.
1 For convenience sake both the sciatic and the brachial nerves are taken in this experiment on the opposite side from the muscle, so that the time of longitransmission of stimuli in the cord is ascertained. The mode of ascertaining the time of ordinary reflex and transverse transmission in the cord is shown diagrammatically in Fig. 58.
The differences in the latent period and in the form of the muscle curve obtained by irritation of the motor nerve, and by simple transverse, and longitudinal reflex stimulation, are shown diagrammatically in Fig. 59. Wundt found that when a motor nerve was irritated at a point distant from the muscle the resulting contraction had not only a longer latent period, but was less in height and longer in duration than when the nerve was irritated close to the muscle. From a comparison of the curves it will be seen that a small portion of grey matter has a similar effect upon the stimulus which passes through it that a great length of nerve-fibre would have. In all reflex actions, therefore, in the normal animal, the contraction of the muscle has a longer latent period, less height, and longer duration than that produced by direct irritation of the motor nerve. The increase in the latent period, diminution in height, and longer duration are greater in the case of transverse than of simple reflex, and greater still in the case of combined transverse and longitudinal reflex.
Fig. 59. - Diagram to show the difference between the length of the latent period and form of the curve in contraction induced, B, by direct irritation of the motor nerve; c, by simple reflex from irritation of the cord on the same side; and D, by cross reflex from irritation of the cord on the opposite side to that from which the motor nerve proceeds, as shown in Fig. 58. e shows combined transverse and longitudinal reflex; A indicates the moment at which the stimulus was applied in each case.
In the normal frog a stronger stimulus is necessary to produce reflex contraction than would be sufficient if it were applied directly to the motor nerve, and strong and weak stimuli will produce strong and weak muscular contractions. The spinal cord has a power of summation similar to that already referred to in the case of contractile tissue of medusae, so that a stimulus which would be powerless to produce a reflex contraction if applied once to a posterior root or to a sensory nerve will be effectual if repeated several times in close succession.
Strychnine has an effect on the conducting power of the spinal cord which we should hardly expect, and so have other convulsant poisons. It increases the excitability so much that slighter stimuli than before will produce reflex action, and it destroys to a considerable extent the power of summation, so that instead of each stimulus producing a contraction in proportudinal conduction is ascertained by deducting the transverse from the combined transverse and longitudinal conduction.
tion to its strength, all have the same effect - a weak one, which is just strong enough to produce an effect at all causing as great a contraction as the most powerful. The time required for the transmission of stimuli through the cord is enormously increased, so that the latent period of ordinary reflex, and still more of transverse and longitudinal reflexes, is greatly increased, sometimes, indeed, to as much as ten times the normal. The retardation of transverse conduction is not ahsolutely greater than of longitudinal conduction; but, as the distance through which the stimulus has to pass in the former case is much less than in the latter, it follows that strychnine increases the resistance more transversely than longitudinally. Morphine in small doses has no very marked action upon the cord, but larger doses have an action almost exactly like that of strychnine, causing increased reflex irritability, tetanic contractions, and prolonged latency. Veratrine has a similar action. Nicotine and coniine in small doses have a similar action to strychnine, but this is quickly masked by the rapid appearance of paralysis. When large doses are used, paralysis occurs almost immediately, and is usually accompanied by fibrillary twitchings. Atropine has at first an action similar to strychnine in causing increased excitability, prolonged latency, and tetanic contraction. It differs from strychnine in causing more rapid diminution in the irritability of the grey substance of the spinal cord and in diminishing the conducting power of peripheral nerves. In consequence of this, irritation of the sciatic nerve in a frog poisoned by atropine causes two contractions, one direct and one reflex, separated from each other by a distinct interval, whereas, in a frog poisoned by strychnine, these two contractions begin almost at the same moment and appear superimposed upon each other.1
The effect of drugs upon the reflex action of the spinal cord is usually estimated by the time which elapses between the application of a stimulus and the occurrence of reflex action, before and after the administration of a drug. Longer time indicates diminished, and shorter time increased, excitability of the cord.
Since the spinal cord in mammals quickly loses its excitability when deprived of oxygenated blood (as shown by Stenson's experiment, p. 164), frogs are used for experiment. The method usually employed is called Turck's method. The cerebral lobes in a frog are destroyed, and after sufficient time has elapsed to allow it to recover from the shock, it is suspended either by the head or fore-legs, so that the hind-legs hang down. A very dilute solution of sulphuric acid, the acid taste of which can be little more than perceived by the tongue, is put in a small beaker and raised until one foot of the frog is completely immersed in it.
1 According to W. Stirling, the latent period of reflex action in the spinal cord is increased by the chloride and bromide of potassium and ammonium, by lithium salts, and by chloral and butyl-chloral; it is decreased by the chloride, bromide, and iodide of sodium. - Stirling and Landois' Physiology, 2nd ed., vol. ii. p. 909.
The time is then counted by means of a metronome, between the immersion of the foot in the acid solution and the time when the leg is drawn up out of it. As soon as the foot is drawn up, the acid is carefully washed off with some fresh water in order to prevent any injury to the skin, and after a minute or two, the experiment may be repeated. "When the time seems constant the drug is injected into the lymph-sac, and the experiment is repeated again. The greater or less time which is required for the withdrawal of the foot from the acid after the injection of the poison, as compared with the time required before, shows the extent to which the reflex action of the spinal cord has been diminished or increased by the poison.