sympathetic nervous system
Summary
The sympathetic nervous system is the division of the autonomic nervous system that mobilizes the body for threat or exertion ("fight or flight"). Heart rate, breathing, and alertness rise while digestion slows. It generally opposes the parasympathetic division, which supports rest and digestion.
Detail
Meaning: The sympathetic nervous system is one division of the autonomic nervous system, which controls involuntary activity of smooth muscle, cardiac muscle, and glands. When a stressor is perceived, it raises heart rate and blood pressure, dilates the pupils and airways, sends blood toward skeletal muscle, and slows digestion. It also stimulates the adrenal medulla to release epinephrine and norepinephrine, which amplify and prolong arousal. The parasympathetic division generally produces the opposite "rest and digest" state.
On the MCAT: A passage describes a person facing a threat, an exam, or a measured response such as heart rate, pupil size, or skin conductance, and asks which division is active or which way a variable shifts. Sympathetic arousal also appears in emotion theories: James-Lange (arousal comes first, then the emotion), Cannon-Bard (arousal and emotion occur together), and Schachter-Singer (arousal plus a cognitive label). It also appears in the alarm stage of the general adaptation syndrome.
Common confusion: The word is not about sympathy or feeling for others. The sympathetic system is also not the somatic system, which controls voluntary skeletal muscle. The two autonomic divisions are not simple on and off switches, because both are typically active at some level and the balance shifts. Cortisol release through the HPA axis is a slower, separate hormonal stress pathway.
Sources
- Psychology (Myers)
- Biological Psychology (Kalat)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.