18.2
The Parasympathetic Nervous System
Overview
The parasympathetic nervous system is one of the two major divisions of the autonomic nervous system. This parasympathetic system is responsi…
The parasympathetic nervous system restores the body during calm times when there are no threats, and is often called the rest and digest system.
It is one of the major divisions of the autonomic nervous system, which controls involuntary functions by regulating the activity of smooth muscle in visceral organs, cardiac muscle in the heart, and certain glands, such as the salivary glands.
When the parasympathetic nervous system is activated, signals from preganglionic neurons in the brain stem, and sacral part of the spinal cord are sent to ganglia, clusters of neuronal cell bodies in the peripheral nervous system, near the organs or glands being controlled.
Here, the message is transmitted to postganglionic neurons, which typically use the neurotransmitter acetylcholine to regulate activity of target tissues. For example, they lower heart rate, relaxing the body, and stimulate muscle contraction in the intestines and bladder, promoting digestion and excretion.
As a result, the parasympathetic system helps rebuild energy stores and maintain homeostasis at times when the body is able to rest.
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Q1: What is the parasympathetic nervous system and what does it do?
The parasympathetic nervous system is a major division of the autonomic nervous system that restores the body during calm times when there are no threats. Often called the rest and digest system, it regulates involuntary functions by controlling smooth muscle in visceral organs, cardiac muscle in the heart, and certain glands. It helps rebuild energy stores and maintain homeostasis when the body can safely rest.
Q2: How does the parasympathetic nervous system transmit signals to target organs?
Preganglionic neurons in the brainstem and sacral spinal cord send signals to ganglia, clusters of neuronal cell bodies in the peripheral nervous system located near target organs. Here, messages transfer to postganglionic neurons, which typically use acetylcholine to regulate target tissue activity. This pathway allows precise control of heart rate, digestion, and other involuntary functions.
Q3: What specific effects does parasympathetic activation have on the body?
Parasympathetic activation lowers heart rate and constricts pupils, promoting relaxation. It stimulates digestion by increasing salivation and peristaltic contractions in the stomach and intestines, and promotes bladder contraction for urination. It also increases pancreatic insulin secretion to rebuild energy stores and enhances blood flow to reproductive organs.
Q4: How does the parasympathetic nervous system differ from the sympathetic nervous system?
While the parasympathetic nervous system promotes rest, digestion, and energy restoration during calm times, the sympathetic nervous system activates during threats or stress. The parasympathetic system lowers heart rate and stimulates digestion, whereas the sympathetic nervous system increases heart rate and inhibits digestion. Together, these opposing systems maintain homeostasis by responding to different body states.
Q5: What role does acetylcholine play in parasympathetic signaling?
Acetylcholine is the primary neurotransmitter used by postganglionic parasympathetic neurons to regulate target tissue activity. Released at the synapse and neurotransmitters junction, acetylcholine binds to receptors on smooth muscle, cardiac muscle, and glands to produce parasympathetic effects like lowering heart rate and stimulating digestion.
Q6: Why is proper parasympathetic signaling important for standing up?
The parasympathetic nervous system controls vasodilation of cerebral arteries, allowing blood vessels to relax and maintain adequate blood flow to the brain during postural changes. Improper parasympathetic signaling can cause orthostatic intolerance, leading to lightheadedness or fainting when standing. This demonstrates how parasympathetic control is essential even for routine movements.
Q7: How does the hypothalamus control parasympathetic nervous system activation?
The hypothalamus activates the parasympathetic nervous system by sending signals that trigger preganglionic neurons in the brainstem and sacral spinal cord. These neurons then relay messages through ganglia to postganglionic neurons, which innervate target organs and glands. This hierarchical control allows the brain to coordinate parasympathetic responses that promote rest and digestion during appropriate times.