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.
The effect which irritation of the vascular nerves produces in the living body is also altered by the action of drugs. This effect is of two kinds - vaso-motor or vaso-contracting, and vasodilating. Fibres, having these two different actions on the vessels of a part, appear frequently to run together in the same nerve-trunk, so that sometimes we get dilatation, at other times contraction of the vessels on irritation of a nerve, and not unfre-quently we get contraction followed by dilatation. Such fibres, however, are not contained in equal proportions in different nerve-trunks. The splanchnics, for example, chiefly contain vaso-motor fibres, so that irritation of these nerves causes great contraction of the vessels in the intestine, and a rise of blood-pressure. The motor nerves of the muscles, on the contrary, appear to contain chiefly vaso-inhibitory fibres, so that stimulation of the nerve causes dilatation of the vessels in the muscle to which it is distributed. Similarly, irritation of nerves distributed to glands usually causes dilatation of the vessels in them. The chorda tympani affords a marked example of this, though the same thing is noticed also in the case of the sweat-glands in the foot on irritation of the sciatic nerve.
Most of these vaso-motor or vaso-inhibitory nerves can be stimulated reflexly by irritation of a sensory nerve, as well as directly by irritants applied to the nerves themselves.
We are not acquainted with many drugs which have the power of paralysing the ends of the vaso-motor nerves in the vessels apart from an action upon the contractile walls of the vessels, or the central nervous system. Arsenic, however, appears to be a drug of this kind, and in acute poisoning by arsenic Bohm has observed that neither irritation of the splanchnic nerves nor of the medulla raises the pressure in the way it usually does. From this effect Bohm concludes that the motor nerves contained in the splanchnics are paralysed, but some other observers have not obtained similar results. Hay has found that potash has a similar action. The method is not free from fallacy, for it is obvious that if the vessels in the intestine should happen to be already contracted either from the effect of a drug upon them or from any other cause, neither stimulation of the splanchnics nor of the medulla can have any further effect upon them or on the blood-pressure through them. For when the vessels of the intestine are contracted the blood pours into the veins from the aortic system, through the arterioles and capillaries of the voluntary muscles, and these are only to a very slight extent under the control of the vaso-motor centre in the medulla. Irritation of it will therefore have little effect on the general blood-pressure when the arterioles of the intestine are already contracted, and irritation of the splanchnics is also prevented from having much effect.
It seems probable that curare and poisons which, like it, not only paralyse the ends of the motor nerves, but also the ends of the vagus in the heart, also paralyse vaso-motor nerves, though larger doses are required for this purpose.
Vaso-dilating fibres appear also to be paralysed by curare, for irritation of the motor nerve of a muscle does not cause dilatation1 of the vessels in a muscle of an animal deeply poisoned by curare. Stimulation of the spinal cord produces contraction of the vessels of the penis instead of erection in an animal poisoned by curare,2 and stimulation of the chorda tym-pani does not cause the same amount of dilatation in a poisoned as in a non-poisoned animal, even when the dose of curare is small.3 Small doses of curare, however, and even large doses of opium, do not appear to paralyse the vaso-dilating nerves of muscles.
In some experiments which I made on the chorda tympani, I got a different result from the usual one in an animal thoroughly under the influence of opium. The vessels appeared to contract instead of dilating on irritation of the chorda tympani, so that instead of the blood gushing out of the vein, it flowed slowly, drop by drop.
1 Gaskell, Journ. of Physiol. 1878-9, vol. i. p. 273.
2 Eckhard, Beitrage, vol. vii. p. 67.
3 V. Frey, Ludwig's Arbeiten, 1876, p. 98.