18.2: The Parasympathetic Nervous System
The parasympathetic nervous system is one of the two major divisions of the autonomic nervous system. This parasympathetic system is responsible for regulating many unconscious functions, such as heart rate and digestion. It is composed of neurons located in both the brain and the peripheral nervous system that send their axons to target muscles, organs, and glands.
The “Rest and Digest” System
Activation of the parasympathetic system tends to have a relaxing effect on the body, promoting functions that replenish resources and restore homeostasis. It is therefore sometimes referred to as the “rest and digest” system. The parasympathetic system predominates during calm times when it is safe to devote resources to basic “housekeeping” functions without a threat of attack or harm.
Control of the Parasympathetic System
The parasympathetic nervous system can be activated by various parts of the brain, including the hypothalamus. Preganglionic neurons in the brainstem and sacral part of the spinal cord first send their axons out to ganglia—clusters of neuronal cell bodies—in the peripheral nervous system. These ganglia contain the connections between pre- and postganglionic neurons and are located near the organs or glands that they control. From here, postganglionic neurons send their axons onto target tissues—generally smooth muscle, cardiac muscle, or glands. Typically, the neurotransmitter acetylcholine is used to regulate the activity of these targets.
Activation of the parasympathetic system causes a variety of effects on the body. It lowers heart rate and causes the pupils to constrict—restoring the body to a more relaxed state. It also stimulates digestion and excretion—for instance, by promoting salivation, peristaltic contractions in the stomach and intestines, and contraction of the bladder to expel urine. It helps rebuild energy stores by causing the pancreas to secrete more insulin. Finally, it even promotes reproduction by increasing blood flow to the genitals.
The control of heart rate and blood flow is essential even for mundane tasks such as standing up. People who suffer from orthostatic intolerance (OI) can experience chronic lightheadedness and fainting from the simple act of getting into an upright posture, called orthostasis. The autonomic nervous system controls the necessary changes to the vasculature and heart rate when we engage in orthostasis. In particular, the parasympathetic system is responsible for the signals that allow vasodilation—the relaxing of the muscles around lining the blood vessels—of the cerebral arteries. Improper signaling by the parasympathetic nervous system can cause loss of consciousness due to inadequate blood flow to the brain.