Vagotropic Substances

VAGOTROPIC SUBSTANCES, medicinal substances and poisons possessing selective action on the endings of the vagus nerve, wherein the action is not limited to the vagus nerve but extends to the endings of other parasympathetic nerves, consequently these poisons are otherwise called parasympathetic. V. (resp. parasympathetic) poisons are divided into those that paralyze parasympathetic endings and those that stimulate them. The most important representatives of the first group are: atropin, hyoscyamin, scopolamin, and homatropin. They paralyze the cardiac endings of the vagus nerves, causing increased heart rate in humans and in those animals which, like humans, have a constant vagus nerve tone. The aforementioned poisons also paralyze the endings of the motor and secretory branches of the vagus nerve innervating the digestive tract, and in this respect they act by relaxing intestinal spasm, decreasing peristalsis and secretion to the extent that these phenomena depend on excitation of the vagus nerve apparatus. By paralyzing the endings of the vagus nerves in the bronchial muscles, the same poisons dilate the bronchial lumen, especially in cases where the bronchi are in spasm caused by excitation of the vagus nerves. Atropin and similar substances paralyze the endings of other parasympathetic nerves: 1) oculomotorius, as a result of which the pupil dilates, 2) chorda tympani and parasympathetic fibers of the glossopharyngeus, whereby salivation ceases, 3) pelvici, as a result of which the influence of this nerve on the smooth muscle of pelvic organs is eliminated. Producing the described action, the poisons of this group do not paralyze the contractile substance of smooth muscles and the secretory function of the glandular cells themselves, which continue to respond to direct irritation by electrical current. To the group of V. s. that stimulate parasympathetic endings belong cholin and its derivatives (acetylcholin, muscarines, natural and artificial), pilocarpin, arecolin, and physostigmin. The poisons of this group stimulate the endings of those nerves which are capable of paralyzing the action of atropin, which explains their characteristic action: pupil constriction, salivation, increased peristalsis and complete contraction of the uterine and bladder muscles, slowing of heart rate and bronchospasm. Certain peculiarities, compared with other members of the group, is presented by physostigmin, differing, among other things, in that in small doses it stimulates the endings of parasympathetic nerves less than it increases their excitability to central impulses (e.g., a moderately physostigminized pupil in darkness is dilated, but already responds to weak light with maximum constriction). Clarifying the localization of action of parasympathetic poisons, it should be considered that they act on the intermediate substance connecting the nerve endings with the contractile (resp. secretory) substance, and not on the nerve endings of the parasympathetic nerves themselves, because after cutting these nerves and their complete degeneration, the muscles and glands continue to respond to stimulating V. poisons (with the exception of physostigmin), and this action, as usual, can be stopped by atropin. Acting on the peripheral apparatus of parasympathetic nerves, V. poisons, as a general rule, do not affect the endings of the sympathicus and the elements innervated by it. An exception is the action of V. poisons on the sweat glands (which, as is known, are innervated by the sympathetic, not the parasympathetic system). Under certain special conditions, V. poisons produce a perverted 'sympathetic' effect on other organs as well. Thus, on isolated organs, with increased excitability of sympathetic endings (e.g., after the effect of large doses of adrenalin), these endings begin to respond to V. poisons, and the latter give an effect opposite to the usual. In addition to the substances considered, V. action is also to varying degree characteristic of ions of certain (alkali) metals, among which potassium exhibits this action most sharply, showing it especially clearly on the heart.