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Nothing is more crucial to the survival and independence of organisms—be they elephants or protozoa—than the maintenance of a constant internal environment. Claude Bernard, the great French physiologist, said everything on this matter when, in the 1850s, he wrote, “La fixité du milieu intérieur est la condition de la vie libre.” Maintaining such constancy is called homeostasis. The basics of homeostasis are relatively simple but miraculously efficient at the cellular level, where ion pumps in cell membranes allow the chemical interior of cells to remain constant, whatever the vicissitudes of the external environment. More complex monitoring systems are demanded when it comes to ensuring homeostasis in multicellular organisms—animals, and human beings, in particular.

Homeostatic regulation is accomplished by the development of special nerve cells and nerve nets (plexuses) scattered throughout our bodies, as well as by direct chemical means (hormones, etc.). These scattered nerve cells and plexuses become organized into a system or confederation that is largely autonomous in its functioning; hence its name, the autonomic nervous system (ANS). The ANS was only recognized and explored in the early part of the twentieth century, whereas many of the functions of the central nervous system (CNS), especially the brain, had already been mapped in detail in the nineteenth century. This is something of a paradox, for the autonomic nervous system evolved long before the central nervous system.

They were (and to a considerable extent still are) independent evolutions, extremely different in organization, as well as formation. Central nervous systems, along with muscles and sense organs, evolved to allow animals to get around in the world—forage, hunt, seek mates, avoid or fight enemies, etc. The central nervous system, with its sense organs (including those in the joints, the muscles, the movable parts of the body), tells one who one is and what one is doing. The autonomic nervous system, sleeplessly monitoring every organ and tissue in the body, tells one how one is. Curiously, the brain itself has no sense organs, which is why one can have gross disorders here, yet feel no malaise. Thus Ralph Waldo Emerson, who developed Alzheimer’s disease in his sixties, would say, “I have lost my mental faculties but am perfectly well.”

By the early twentieth century, two general divisions of the autonomic nervous system were recognized: a “sympathetic” part, which, by increasing the heart’s output, sharpening the senses, and tensing the muscles, readies an animal for action (in extreme situations, for instance, life-saving fight or flight); and the corresponding opposite—a “parasympathetic” part—which increases activity in the “housekeeping” parts of the body (gut, kidneys, liver, etc.), slowing the heart and promoting relaxation and sleep. These two portions of the ANS work, normally, in a happy reciprocity; thus the delicious postprandial somnolence that follows a heavy meal is not the time to run…