Autonomic Nervous System 101The autonomic nervous system is the control center of your vital organs. It helps prepare the body for the environment it 's in by directing resources to the most pressing needs. It does this through it's three branches: The sympathetic, parasympathetic, and enteric branches. It used to be broken down in to the sympathetic and parasympathetic branches until it was learned that the enteric branch, which controls the digestive system, can function without assistance from the parasympathetic branch. Since we know very little about the specifics of the enteric branch outside of how it's controlled by the sympathetic and parasympathetic branches, we'll simply consider those 2 branches.
The sympathetic branch of the autonomic nervous system is also referred to as fight or flight. When sympathetic nervous system activity increases it diverts blood flow to muscles and away from the organs of digestion, increases heart rate and blood pressure to increases oxygen to muscles, and mobilizes energy for use to either fight or flee a stressor. The parasympathetic nervous system directs blood flow away from muscles and towards the organs of digestion while decreasing heart rate and blood pressure. The parasympathetic nervous system is referred to as rest and digest. The picture below shows the primary functions of each branch.
Looking at this figure, it would be easy to think of the 2 branches as being antagonistic to one another, but that isn't the whole story. While the effects of increased activity of each system is antagonistic of the other, they are actually complementary to one another in the grand scheme of things. While fight or flight allows you to battle a stressor in the environment, rest and digest allows you to recover from that stressor and be prepared for the next. In this way, proper function of one branch is dependent on proper function of the other.
The sympathetic and parasympathetic branches also have other effects throughout the body, but they are much more nuanced. During the initial stages of increased sympathetic nervous system activity, the immune system is enhanced, but if sympathetic nervous system activity is increased chronically, it has a depressive effect on the immune system. This is thought to be one of the processes underlying the increased inflammation found in the metabolic syndrome and heart disease. In the reproductive system, increased parasympathetic activity promotes erection while increased parasympathetic activity induces orgasm.
At any time, most people are at varying degrees of both sympathetic and parasympathetic activity. The autonomic nervous system doesn't function in an on/off manner; when sympathetic nervous system increases it doesn't completely shut off parasympathetic activity. Instead, they are both typically activated to some degree with increased sympathetic activity typically causing a decrease in parasympathetic activity and vice versa. Under extreme circumstances in either direction, one can basically be forced in to the other due to dysregulation in the system. In other words, if someone is under constant, unabating stress that is never recovered from due to poor parasympathetic "tone", they will be forced in to a long term parasympathetic state. In converse, someone who completely removes stress and physical activity from their life is likely to see a lowered threshold for activating the sympathetic nervous system. In other words, things that pose a minor threat are handled as major ones.
ConclusionThe autonomic nervous system is comprised of three branches with opposing, but complementary, functions that allow us to adapt to the environment we are in. Increased sympathetic activity prepares the body to respond to a stressor while increased parasympathetic activity helps the body recover from it. Under normal circumstances, the sympathetic and parasympathetic nervous systems work together to help you respond to the world you live in. Under extreme conditions, the autonomic nervous system can become dysregulated making the individual's response to the environment exaggerated and maladaptive.
In my next blog I will go over how lifestyle factors can contribute to dysfunction in the autonomic nervous system.