This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
Cut the posterior nerve-roots to prevent afferent impulses from getting to the cord, strychnize the frog, and no convulsions result. Stimulate the central cut end and convulsions follow, whether the roots have been cut peripheral or central to the ganglia.
These experiments show - (1) That the drug does not act upon the peripheral nerves or the posterior root ganglia. (2) That it does not of itself produce motor effects. (3) That it causes increased motor response to afferent impulses, i. e., to external stimuli.
The convulsions are, therefore, reflex in nature, the strychnine acting on structures in the cord itself and resulting in greatly increased reflex excitability.
What is a reflex? If the eye is exposed to a light, the pupil contracts; if some irritating dust gets into the nose, it causes sneezing. These are motor reflexes. If about dinner time the appetizing odor of food is recognized, the stomach begins to secrete gastric juice; if a substance of bitter taste gets into the mouth, the saliva flows. These are secretory reflexes. In each case there is some peripheral stimulus, these actions not occurring otherwise, and the response is involuntary. A reflex, then, is an involuntary secretory or motor response to an afferent impulse.
Reflex actions are usually purposeful and definite, the same kind of response regularly following stimulation at a given place. A piece of dust on the conjunctiva ordinarily results in instant closure of the eye; a teaspoonful of mustard placed in the stomach regularly results in vomiting; the dipping of a frog's hind leg in acetic acid regularly results in a drawing of the leg away from the offending substance and an attempt to wipe it away with the other leg. The afferent impulses, therefore, do not travel at random to any motor cells, but would seem to travel to those motor cells which can produce the proper purposeful motor response. That is, for each afferent impulse there seems to be in the cord one particular path or group of paths along which it travels to reach the motor or secretory cells, this one path ordinarily being open to it, while all other paths are closed to it. By training, certain new paths are opened up, or, in other words, actions which are at first voluntary become reflex, as in piano-playing, skating, and most of our activities. At first the will is necessary to insure the desired response to the stimulus, as that the finger shall strike a certain key of the piano when the eye sees a certain printed note. But by constant repetition a path is established so that the player comes to strike the proper key involuntarily as soon as the eye perceives the note.
Reflexes are of three kinds, viz.:
(1) The simple reflexes, which involve only one muscle, as in winking the eye. (2) The coordinated reflexes, in which, during the contraction of one set of muscles, there is inhibition of the opposing muscles; these are the ordinary purposeful reflexes of our bodies. (3) The convulsive reflexes, which are incoordinated because all the muscles are stimulated, and there is no inhibition. Since all the muscles contract, the stronger predominate. Convulsive reflexes are exaggerated, purposeless, and harmful, and are due to some derangement of coordination.
How does strychnine produce convulsive reflexes? Baglioni (1900) performed an experiment which has become classic. He exposed the spinal cord of a decapitated frog at the brachial plexus, and removed the pia with its vessels to cut off circulatory connection with the parts of the cord above and below. He then painted the denuded area with a solution of strychnine, and thus poisoned the part of the cord through which afferent impulses from the fore-limb would have to pass, but did not poison the rest of the cord.
1. On stimulating the hind-limb, he got the usual normal reflex response, the poisoned area being beyond the influence of such a stimulus. When he pinched the foot, the leg was drawn up; if he placed a drop of acetic acid upon the leg, the other leg would be drawn up to wipe it off. This proved that the sensory nerves, the synapses, and the motor cells in the lower part of the cord were unpoisoned and acting normally.
2. But when he pinched or pricked one of the fore-limbs or dipped it in acid, there resulted a convulsion of the whole body, both hind-limbs and fore-limbs being involved. In other words, when the afferent impulse passed through an unstrychnized portion of the cord, the response was the usual one; but when the impulse passed through a strychnized area, there was an abnormal response, not only in the muscles usually affected by such an impulse in an unpoisoned animal, but also in a large number of the other muscles of the body. These muscles went into a convulsive state, whether their motor cells were in the poisoned area or not. Therefore the action of the strychnine is neither on the motor cells themselves nor on the synapses about the motor cells; and is in all probability on either the intermediary neurons in the cord or the first synapses of the afferent system.
If the dose given is just a little less than enough to produce convulsive twitching, the response of the usual muscles is greater than normal, but in the usual purposeful way; and this is believed to be due to the greater transmission of the afferent impulses. There is no satisfactory evidence that the motor cells themselves are stimulated.
Hence the action of strychnine upon the spinal cord may be thought of as not only to facilitate the passage of afferent impulses to their usual motor cells, but to open up the paths to the other motor cells, so that the impulses may reach and affect cells ordinarily beyond their influence. In other words, strychnine increases reflex activity by facilitating the passage of afferent impulses in the cord (across and up and down the cord). It may directly stimulate the motor cells themselves, but this is not proved.