Our bodies consist of many organ systems all controlled to a great degree by the central nervous system. The central nervous system consists of our brain and our spinal cord, which runs down our back within a bony canal that protects the delicate nerve fibers. Our brain structure is very complicated, and it is unlikely that we will ever understand it completely. We do know that various parts of the brain control different parts of our body, that the front lobes are where our thoughts seem to come from, many of our emotions arise from parts of the brain that lie deep within, and that the bottom-most part of the brain, called the “brainstem,” is where most of the automatic control centers are. These control centers determine things like body temperature, heart rate, blood pressure, sweating, and other internal systems that are beyond our control. That is why this part of the nervous system is often referred to as the “involuntary” nervous system, meaning we have little or no control over it. On the other hand, the “voluntary” nervous system refers to the brain centers that we use to control muscles like our arms and legs, language, and so on. The scientific name for the involuntary nervous system is the Autonomic Nervous System, from the Latin term meaning “self-governing.”
The autonomic nervous system is quite complex, and consists of two functional branches, one that tends to help us react to stress (physical or emotional) and one that helps our bodies relax (in a physiologic sense). The first branch, also known as the “Sympathetic” nervous system, consists of many nerve endings that control every organ system in the body. When these nerves fire, the result is often referred to as the “fight or flight” reflex, because they cause the heart to speed up, they increase the blood pressure, cause sweating, slow down stomach and intestinal activity, cause the pupils in our eyes to enlarge or dilate, etc. All of these responses to an increase in the activity of the sympathetic nervous system are geared to help us deal with a stressful situation.
The second branch of the autonomic nervous system has the opposite effect of the sympathetic nervous system, and is called the “Parasympathetic” nervous system. The parasympathetic nervous system consists mostly of nerve endings that come from a nerve that arises directly from the brain. This nerve is known as the vagus (pronounced “VAY-giss”) nerve. When the vagus nerve fires, the heart slows down, the blood pressure decreases, the stomach and intestine start pushing the food along, and so on. Thus, the sympathetic and parasympathetic nervous system tend to balance each other out – like a ying and yang – though in most circumstances one predominates over the other (depending on the body’s needs).
The term “dysautonomia” simply refers to any kind of disorder of the autonomic nervous system. It may be a rare, self-limited disturbance in autonomic function, such as that which results in a vasovagal faint (known as neurocardiogenic syncope, named after the brain (neuro-), the heart (cardio-) and the suffix –genic meaning “to give rise to.” The word syncope, of course, means fainting. In patients with vasovagal fainting, the autonomic nervous system functions normally 99% of the time, except under certain circumstances when the blood pressure and/or the heart rate drop at an inappropriate time, resulting in a sudden decrease in blood flow to the brain, resulting in the brain shutting down (fainting). This is a dramatic example of dysautonomia. A more subtle example is POTS, or the POstural Tachycardia Syndrome. A fairly common disorder of blood pressure control is known as Orthostatic Hypotension, and is discussed fully in the next sections. There are many reasons for serious, progressive autonomic problems, many of which are covered in the section on autonomic failure.