Sympathetic vs. Parasympathetic
Both part of the autonomic nervous system, the sympathetic and parasympathetic nervous systems work involuntarily. Sympathetic is responsible for the response commonly referred to as "fight or flight," while parasympathetic is referred to as "rest and digest." Both originate in the spinal cord and branch out from there.
The sympathetic nervous system is the part of the autonomic nervous system that prepares the body to react to stresses such as threat or injury. It causes muscles to contract and heart rate to increase.
The parasympathetic nervous system is the part of the autonomic nervous system that controls functions of the body at rest. It helps maintain homeostasis in the body. It causes muscles to relax and heart rate to decrease.
While there is little that people can do to consciously control their parasympathetic nervous system, there are factors (such as exercise and experience) that can help people exert a level of control over the sympathetic responses. The two generally do not function well at the same time. That is, when faced with imminent danger, the body will divert blood flow from parasympathetic nerve functions (such as digestion) to sympathetic nerve functions (such as muscle contraction and heavy breathing to run away).
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