A leading neuroscientist argues that the peripheral nervous system, long understood to play a key role in regulating basic bodily functions, also signals the onset of illness. The central nervous system, consisting of the brain and the spinal cord, has long been considered the command center of the body. Yet outside the central nervous system, an elaborate network of nerve cells and fibers extends throughout our bodies, transmitting messages between the brain and other organs. The peripheral nervous system, as it’s known, regulates such vital functions as heart rate, digestion, and...