THE AUTONOMIC NERVOUS SYSTEM / TWO BRANCHES
THE AUTONOMIC NERVOUS SYSTEM
Much of what the brain does takes place beyond our ability to sense it-or appreciate it. In the midbrain’s pons and medulla lie the centers that regulate the vital, everyday functions of life. Think about it: How fortunate you are that you don’t have to concentrate in order to breathe, or make your heart pump blood.
The first rule of the living brain is to go on living. Thus, these crucial areas of the midbrain, called the autonomiC (“involuntary”) nervous system, are not easily overruled by the higher functions of the cortex. While it’s possible to hold your breath while underwater or throwing a tantrum, the midbrain will eventually overrule the efforts of the cortex and force the lungs to inhale. However, some drugs, such as tranquilizers and stimulants, can affect the autonomic nervous system, altering things like the heart rate and blood pressure for good or ill.
Like day and night, the autonomic nervous system has two equally important halves. They are reciprocal and complementary. The day- light side of wakefulness and work is called the sympathetic branch. It works when the body’s sense of self-preservation, developed over eons of evolution, calls for energy. In extreme cases, the sympathetic branch triggers the so-called fight or flight response. When a threat looms, the body prepares to meet it or quickly escape from it. Blood pressure and heartbeat skyrocket, breathing speeds up, and in a multitude of other ways the midbrain signals to the body to prepare itself for action.
The parasympathetic branch is the calmer, quieter side of the nervous system. It’s responsible for the so-called relaxation response. The midbrain signals to the body to lower breathing rate, heartbeat, and blood pressure. As a result, the brain promotes and recognizes a feeling of well-being.
Modern pharmacology can bring about a similar result, but much of the self-help books of the past few decades have focused on meditation and other forms of stress management to stimulate the parasympathetic branch while soothing the sympathetic.