parasympathetic nervous system

Meaning: The parasympathetic nervous system is a branch of the autonomic nervous system, which is part of the peripheral nervous system and controls involuntary functions. It is dominant in calm conditions, often called "rest and digest": it slows heart rate, constricts the pupils, and promotes digestion and salivation. It generally opposes the sympathetic branch ("fight or flight"). Much of its output travels in the vagus nerve, and its neurons release acetylcholine onto target organs.

On the MCAT: Stress and emotion passages ask which branch is active in a scenario, such as a person startled by a threat versus one relaxing after a meal. Expect to predict the direction of change in heart rate, pupil size or digestive activity. Parasympathetic activity helps return the body to baseline after a stressor, so a falling heart rate after danger passes points to a shift toward this branch.

Common confusion: Students treat the two branches as on/off switches. Both usually carry some activity at once, and the body's state reflects the balance between them. Another slip is mixing up autonomic and somatic: the parasympathetic branch is autonomic (involuntary control of organs and glands), while the somatic system controls voluntary skeletal muscle. To keep the branches straight, link parasympathetic with calm and sympathetic with arousal.