Finally, there is the essential role that the hypothalamus plays in the initiation and regulation of autonomic activity, both sympathetic and parasympathetic. Sympathetic responses are most readily obtained by stimulation of the posterior and lateral regions of the hypothalamus, and parasympathetic responses from the anterior regions. The descending sympathetic fibers are largely or totally uncrossed. According to Carmel, fibers from the caudal hypothalamus at first run in the prerubral field, dorsal and slightly rostral to the red nucleus, and then ventral to the ventrolateral thalamic nuclei; then they descend in the lateral tegmentum of the midbrain, pons, and medulla to synapse in the intermediolateral cell column of the spinal cord. In the medulla, the descending sympathetic pathway is located in the posterolateral retroolivary area, where it is frequently involved in lateral medullary infarctions. In the cervical cord, the fibers run in the posterior angle of the anterior horn (Nathan and Smith). According to the latter authors, some of the fibers supplying sudomotor neurons run outside this area but also remain ipsilateral. Jansen and colleagues, by the use of viral vectors in rodents, were able to label certain neurons of the hypothalamus and the ventral medulla that stimulated sympathetic activity in both the stellate ganglion and the adrenal gland. They hypothesized that this dual control underlies the fight-or-flight response, as described in Chap. 24. By contrast, the pathways of descending parasympathetic fibers are not well defined.
Afferent projections from the spinal cord to the hypothalamus have been demonstrated in animals and provide a potential route by which sensation from somatic and possibly visceral structures may influence autonomic responses.
The function of the autonomic nervous system in its regulation of the visceral organs is to a high degree independent of voluntary control and awareness. Furthermore, when the autonomic nerves are interrupted, these organs continue to function (the organism survives), but they are no longer as effective in maintaining homeostasis and adapting to the demands of changing internal conditions and external stresses.
Viscera have a double-nerve supply, sympathetic and parasympathetic and in general these two parts of the autonomic nervous system exert opposite effects. For example, the effects of the sympathetic nervous system on the heart are excitatory and those of the parasympathetic inhibitory. However, some structures—sweat glands, cutaneous blood vessels, and hair follicles—receive only sympathetic postganglionic fibers, and the adrenal gland, as indicated earlier, has only a preganglionic sympathetic innervation. Also, some parasympathetic neurons have been identified in sympathetic ganglia.
All autonomic functions are mediated through the release of chemical transmitters. The modern concept of neurohumoral transmission had its beginnings in the early decades of the twentieth century. In 1921, Loewi discovered that stimulation of the vagus nerve released a chemical substance (Vagusstoff) that slowed the heart. Later this substance was shown by Dale to be acetylcholine (ACh). Also, in 1920, Cannon reported that stimulation of the sympathetic trunk released an epinephrine-like substance, which increased the heart rate and blood pressure. He named this substance “sympathin,” subsequently shown to be norepinephrine, or NE. Dale found that ACh had pharmacologic effects similar to those obtained by stimulation of parasympathetic nerves; he designated these effects as “parasympathomimetic.” These observations placed neurochemical transmission on solid ground and laid the basis for the distinction between cholinergic and adrenergic transmission in the autonomic nervous system.