Vasomotor Nerves

Vasomotor nerves (blood vessel-moving nerves). Nerve effects on blood vessels manifest as changes in vessel caliber. To the extent that this influence in terms of decreasing arterial caliber can be considered firmly established, the question of their active dilation under the influence of the nervous system cannot be resolved so categorically. The reason for this circumstance must be sought in the difficult-to-explain mechanism of such active dilation of the caliber. Nevertheless, facts obtained by experiments compel most physiologists to recognize in certain areas of the animal organism the existence of special vasodilator nerves, alongside the generally recognized and firmly established system of vasoconstrictor nerves. As for the innervation of veins and capillaries, the study of it encounters considerable difficulties in that active changes in their caliber are difficult to separate from their natural passive reaction to changes in blood flow in adjacent arterioles. However, recent works by Donegan apparently prove the existence of vasoconstrictors for veins of the hind limbs of dogs and cats. The mechanism of changes in capillary caliber, despite numerous studies, cannot be considered finally clarified to this day. The reaction of vessels to histamine (dilation of arteries and veins and constriction of capillaries) apparently suggests that the mechanism of variations in vessel caliber of these systems is not constructed identically. Changes in vessel caliber under the influence of nerve irritations were first noted by Pourtour du Petit in 1772, long before the discovery by Henle in 1840 of the muscular layer of vessels. Claude Bernard and Brown-Séquard in 1852, independently of each other, discovered vasoconstrictor fibers in the cervical sympathetic nerve and thereby laid the foundation for the study of vascular innervation. Thanks to innervation-related changes in caliber, organs receive a quantity of blood in accordance with their state and need for nutrient material. Vasoconstrictors belong to the autonomic sympathetic nervous system. Irritation of the peripheral end of vasoconstrictors causes constriction of the caliber, while their sectioning increases the normal caliber of vessels. The latter circumstance gives reason to assume the existence of a special center, through which vasoconstrictors maintain vascular tone at a certain level. Experiments with successive sections have established that the vascular center is located in the medulla oblongata, in the upper part of both sides of the rhomboid fossa. This point should be considered rather as a convergence point of fibers coming from many scattered centers in the brain. The course of vasomotor nerves, in general, follows one plan. In the composition of the lateral columns, their fibers pass through the spinal cord, and upon exiting, in the composition of the anterior roots, they proceed through the white rami to the sympathetic trunk. Fibers emerging from the ganglia of the sympathetic trunk partly go directly to the vessels (a. subclavia and a. thyroidea ima), and partly through the gray rami approach the somatic nerve of the corresponding territory and go with it in a common trunk. Some of the vasoconstrictors pass through the sympathetic trunk without interruption to the prevertebral ganglia (gangl. coeliacum, mesenter. sup. et inf.), from where, as postganglionic parts, they reach their organs. Vasoconstrictors for the head come from the upper part of the thoracic section of the spinal cord and through the sympathetic trunk enter the composition of the superior cervical ganglion and from the latter, as the peripheral postganglionic part, proceed to the vessels: 1) of the retina—together with the n. trigeminus; 2) of the tongue—in the trunk of the n. hypoglossi; 3) to the vessels of the skin and organs of the head, including the salivary glands, together with the vascular plexuses of the arteries (a. carotis ext., maxillar. ext. and their branches). The question of vasoconstrictors of the brain, apparently, is resolved in the affirmative sense by the works of Weber and Gildemeister. For organs of the abdominal cavity, vasoconstrictors go in the trunks of the nn. splanchnici major et minor. For the upper extremities, they emerge through the anterior roots from the thoracic section (D. IV-X) through the sympathetic trunk to the gangl. stellatum and from there to the vessels. For the lower extremities—from the lower thoracic and lumbar nerves (D. XI-L. III). For pelvic organs—from the lumbar nerves (L. III-V) through ganglion mesenteric. infer, to the nervi hypogastrici. Vasodilators. The dilation of vessels associated with irritation of certain nerves gives reason to recognize the existence of special vasodilator nerves. However, the mechanism of vessel dilation in this case is still not clear and apparently it is not a matter of active muscular dilation of the caliber, but rather a weakening of the tone of vasoconstrictors. A considerable number of such dilator nerves belong to the parasympathetic nervous system, however, experiments have established the presence of dilators also in the posterior roots of the spinal cord, fibers of which cannot be attributed to the autonomic system either by anatomical or pharmacological analysis. Upon sectioning of the posterior roots, upon irritation of the peripheral end of the latter, as well as upon irritation of any section along the course of a sensory nerve, dilation of vessels always occurs in the territory of the peripheral branching of this nerve. Langley and Bayliss gave this phenomenon the name of antidromic action, without, however, explaining its essence. The reaction is expressed exclusively by dilation of vessels of the corresponding area. Antidromic action, associated with the functions of centripetal nerves, is particularly clearly manifested in the experiments of Bayliss and Head. They operated on the radial nerve, which includes sensory spinal nerves and fibers of the sympathetic nerve and gangl. stellati. After removal of the stellate ganglion and subsequent degeneration of sympathetic nerves, irritation of the radial nerve caused dilation of vessels on the periphery, which could be attributed exclusively to the irritation of the centripetal sensory nerve, in other words, to be ascribed to antidromic action. Leaving aside the mechanism of action on vessels, it must be recognized that each organ of our body, in terms of regulation of blood supply, depends on two types of nerve effects and that, in addition to vasoconstrictor nerves, it is also subject to nerves, the excitation of which can cause the vessels of the organ to increase their caliber by one means or another. As mentioned above, part of the nerves of the latter type passes in the posterior roots of the spinal cord and possesses the property of antidromic action, and part belongs to the parasympathetic nervous system. In their distribution, there is no definite plan as is the case in the system of vasoconstrictors—a circumstance due to which the vasodilator center, as a unifying principle of their function, has not been recognized by many physiologists. To individual organs, vasodilators go either by separate branches or together with other nerves. Organs of the head receive them with branches of the nn. trigemini, faciales and glosso-pharyngei. The vagus nerve carries them for the thyroid gland. For the upper extremities, vasodilators emerge through the posterior roots of the cervical and thoracic sections (C. VI-VIII and D. I); for the lower extremities—through the same roots of the lumbar and sacral sections (L. V-VII and S. I) of the spinal cord. Vasodilators for the penis, irritation of which causes erection, emerge from the anterior roots of the sacral section (S. I-IV) and pass into the nn. erigentes. In the latter alone, apparently, true vasodilators are contained, irritation of which directly causes dilation of the vessels of the cavernous and spongy bodies of the penis. Dilation of vessels can also be caused not by direct nerve effects on the muscular layer of vessels, but by the influence on it of metabolic products entering the blood, such as carbonic acid, lactic acid, etc. (the humoral pathway). On nervus depressor as a regulator of general blood pressure—see Depressor nervus.—Vasomotor syndrome, see Friedman's vasomotor syndrome.