The human internal environment is regulated in large measure by the integrated activity of the autonomic nervous system and endocrine glands. Their visceral and homeostatic functions, essential to life and survival, are involuntary. Why the forces of evolution favored this separation from volition is an interesting question. Claude Bernard expressed this idea in sardonic terms when he wrote, "nature thought it prudent to remove these important phenomena from the caprice of an ignorant will."
Although only few neurologic diseases exert their effects primarily or exclusively on the autonomic–neuroendocrine axis, there are numerous medical diseases that implicate this system in some way: hypertension, asthma, and certain dramatic disorders of cardiac conduction, to name some of the important ones. However, many general neurologic diseases involve the autonomic nervous system to a varying extent, giving rise to symptoms such as syncope, sphincteric dysfunction, pupillary abnormalities, erectile dysfunction, diaphoresis, cardiac dysrhythmias, and disorders of thermoregulation. Finally, in addition to their central role in visceral innervation, autonomic parts of the neuraxis and parts of the endocrine system are engaged in all emotional experience and its display, as discussed in Chap. 25.
Breathing is unusual among nervous system functions. Although continuous throughout life, it is not altogether automatic, being partly under volitional control. Current views of the central and peripheral control of breathing, and the ways in which it is altered by certain diseases are of considerable interest to neurologists, if for no other reason than respiratory failure is common in neurologic conditions such as coma, cervical spinal cord injury, and a large number of acute and chronic neuromuscular diseases. Many of these same comments pertain to the function of swallowing, which is largely automatic and continues at regular intervals even in sleep but is also initiated voluntarily. Furthermore, swallowing fails in ways similar to breathing as a consequence of neurologic diseases.
The autonomic, endocrine, and respiratory systems, although closely related, give rise to disparate clinical syndromes. This chapter deals more strictly with the autonomic nervous system and the neural mechanisms of respiration and swallowing, and the next chapter, with the hypothalamus and neuroendocrine disorders. The following discussion of anatomy and physiology serves as an introduction to both chapters.
The most remarkable feature of the autonomic nervous system is that a major part of it is located outside the brain and spinal cord, in proximity to the visceral structures that it innervates. This position alone seems to symbolize its relative independence from the cerebrospinal system. In distinction to the somatic neuromuscular system, where a single motor neuron bridges the gap between the central nervous system (CNS) and the effector organ, in the autonomic nervous system there are always two efferent neurons serving this function, one (preganglionic) arising from its nucleus in the brainstem or spinal cord and the other (postganglionic) arising from specialized nerve cells in peripheral ganglia. Figure 26-1 illustrates this fundamental ...