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.
Its conducting power for motor impulses is assumed to be impaired when it is noticed that any drug causes partial paralysis of the hinder extremities of an animal before the anterior extremities.
It is usually tested by irritating the spinal cord at its upper end, either mechanically with the point of a needle, or by a galvanic or faradaic current passed through electrodes inserted into it close together, and observing whether irritation of the cord itself in this way causes contraction in the muscles of the legs.
When no contraction is produced by irritation of the cord itself, while direct irritation of the motor nerves can still produce vigorous contraction, it is evident that the cause of the paralysis must be that the spinal cord has lost its power to conduct motor impulses.
These experiments may be made in a frog, the cerebrum of which has been previously destroyed; and they may be confirmed in warm-blooded animals where sensibility has been destroyed by a section of the cord, just below the medulla, and respiration is kept up artificially. The spinal cord is then exposed, and the anterior columns are irritated in the ways already mentioned.
The power of the cord to conduct sensory impressions is ascertained by exposing it under anaesthetics and allowing their influence to pass so far off that the animal is capable of giving evidence of sensation. The posterior roots are then irritated before and after the injection of the poison into the circulation.
When it is found that after the poison is injected the irritation of the posterior roots which previously caused evidence of sensation no longer produces any effect, while irritation of the anterior columns still produces motion, the conclusion appears to be just, that the poison has paralysed the conducting power of the sensory columns of the cord.
This action appears to be possessed by caffeine, for Bennett found that while irritation of the posterior roots of the cord caused violent struggles and loud cries in a rabbit before the injection of caffeine into the circulation, similar irritation, after the injection, caused only a slight quiver. That this effect was not due to motor paralysis was shown by the fact that irritation of the anterior columns caused violent muscular contractions after the injection as well as before it.1 painful impressions in the spinal cord.
Fig. 57. - Diagram to show the effect of chloroform, chloral, and other anaesthetics on conduction of
Ordinary impressions of touch, temperature, and muscular action are transmitted through the posterior roots of the spinal cord to the ganglia of the posterior horn of the grey substance, and thence upwards by the fibres of the lateral columns. Painful sensations, however, appear to be transmitted upwards through the grey substance of the cord. The afferent nerves, which transmit impressions from one part of the cord to another, so as to produce co-ordinated reflex movement, are contained in the posterior columns of the cord.
It is evident that any injury or poison which chiefly affects the grey matter so as to diminish its conducting power may abolish pain while reflex action still persists. This condition may be produced by division of the grey matter of the cord, and it occurs also at a certain stage of the action of anaesthetics such as chloroform and ether.
The action of drugs on the power of the spinal cord to conduct reflex stimuli both transversely and longitudinally has been carefully investigated by Wundt. He first ascertains the time which elapses between the application of a stimulus to a motor nerve and the contraction of a muscle, the nerve used being the sciatic, and the muscle the gastrocnemius of a frog. This time, which includes that requisite for the stimulus to travel down the motor nerve and to set the muscle in action, he terms the direct latency. He next stimulates a sensory root of the spinal nerve at the same level and on the same side as the motor nerve, taking care that the stimulus does not act on the motor nerve directly, but only reflexly through the cord. The time between the application of the stimulus and the commencement of contraction he terms the total latency. By deducting the direct latency from the total latency, he ascertains the time required for the stimulus to pass through the grey matter of the cord from the posterior to the anterior horn of the same side. This he calls the reflex time.
1 Hughes-Bennett, Edin. Med. Journ., Oct. 1873.
The time required for transverse conduction is ascertained by applying the stimulus to a posterior root on the other side and comparing the latency with that of stimulation to a posterior root on the same side.
The time required for longitudinal conduction is ascertained by applying a stimulus to the brachial nerve, so that it has to travel down the greater part of the length of the spinal cord before it can excite the sciatic nerve. By comparing the latent period of excitation in the brachial nerve with that of the sciatic on the same side 1 the length of time required for longitudinal
Fig. 58. - Diagram to show the method of investigating reflex and transverse conduction in the spinal cord. The motor nerve is first irritated at 1. As the cylinder revolves at a known rate, and a mark is made upon it by an electro-magnet at the instant the nerve is irritated, the distance between this mark and the commencement of the muscle curve indicates the time required for the irritation to travel down the motor nerve to the muscle and set it in action. The irritation is next applied to the posterior root on the same side (2). The distance between the commencement of contraction in this case and in that where the motor nerve was irritated gives the time required for simple reflex transmission of the stimulus from the posterior to the anterior horn of the cord. The stimulus is then applied to the posterior root on the opposite side at 3, and the distance between the commencement of the consequent contraction and that of the curve obtained by irritating at 2 gives the time required for transmission across the cord.