The Sympathetic and Parasympathetic Nervous Systems

In this lesson, you’ll learn about two subdivisions of the autonomic nervous system – the sympathetic and parasympathetic nervous systems. Watch as a hiker, Phil, runs away from a terrifying bear and see how these systems react to the impending danger!
Nervous Systems
So, you may recall that the somatic nervous system is the part of the nervous system that voluntarily responds to external stimuli and that the autonomic nervous system is the part of the nervous system that involuntarily regulates internal body functions. Additionally, the autonomic nervous system can be further subdivided into two divisions: the sympathetic nervous system and the parasympathetic nervous system.
Both of these systems control the same group of body functions, but they have opposite effects on the functions that they regulate. The sympathetic nervous system prepares the body for intense physical activity and is often referred to as the fight-or-flight response. The parasympathetic nervous system has almost the exact opposite effect and relaxes the body and inhibits or slows many high energy functions. The effects of the parasympathetic nervous system can be summarized by the phrase ‘rest and digest’. Let’s look at an example of how these two systems function in response to changes in the environment.
Sympathetic Nervous System
Hiker avatar
This is Phil. Phil is hiking in the wilderness all by himself and enjoying the tranquility of the great outdoors when he turns a corner and finds himself only feet away from a full-grown bear. In a fraction of a second, the brain recognizes the animal in front of him as a bear and classifies it as a very big threat. Immediately, the sympathetic nervous system is activated and without any conscious control by Phil at all, several body functions are affected all at once:
1. Heart rate is increased. Increased heart rate results in an increase in oxygen and nutrients that reach the brain and muscles, preparing them to deal with whatever Phil will have to face.
2. The liver is stimulated to release glucose into the bloodstream providing more energy that will be ready to power the muscles in case it is needed.
3. The bronchioles in the lung are dilated to allow more air into the lungs, which will increase oxygenation of the blood and keep up with the increased flow of blood through the lungs due to the increased heart rate.
4. The pupils of the eyes are dilated. Because the sympathetic nervous system is often activated when people are surprised, pupil dilation is a visual cue that we use to read surprise on people’s faces.
5. The adrenal glands are stimulated to secrete epinephrine and norepinephrine. The adrenal glands are a pair of hormone-producing glands located on top of the kidneys that respond to stress. Together, the epinephrine and norepinephrine secreted by the adrenal glands have the same basic effects as the nerves of the sympathetic nervous system by increasing heart rate, increasing bronchiole dilation, and increasing glucose release from the liver. In addition, norepinephrine is also known to increase alertness. It may seem redundant that these hormones have the same actions as the sympathetic neurons, but hormones have longer lasting effects than nerve impulses, so while the initial fight or flight response is mediated by neurons, these hormones serve to reinforce and help to sustain the response.
6. Digestive activity is inhibited. In the moment of truth, a person may need every last ounce of energy they can muster. If Phil makes it through this ordeal, then he’ll have plenty of time to complete digestion of whatever he ate for breakfast.
7. The bladder is relaxed. After all, this isn’t the time for a person to relieve themselves; there are more pressing matters at hand. Even so, I’ll bet that some of you out there are thinking, ‘Hold on, I thought that people wet themselves when they get scared!’ It turns out that the sympathetic nervous system is only activated in situations where the brain decides that there is a reasonable chance of survival. In very extreme cases of danger where the chances of survival seem remote, crippling fear often takes over and people can lose bladder and bowel control. They may also go limp and play possum as last-ditch efforts to survive.
This is not a complete list of all of the functions regulated by the sympathetic nervous system, but it does include most of the major functions that would come into play in Phil’s situation with the bear. As it turns out, Phil opted for flight over fight, and ran away. He ran for almost a whole half-mile before he dared to stop and turn around, but he could have easily run further if the need was there because his sympathetic nervous system had prepared him almost instantly to run for his life!
The Parasympathetic Nervous System
When Phil turns around and sees that the bear is nowhere to be found, his parasympathetic nervous system begins the process of relaxing and calming the body so that it can recover from the heightened physical and metabolic activity.
Some of the effects of the parasympathetic nervous system are:
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