Differences between Parasympathetic nervous system and Sympathetic nervous system

Differences between Parasympathetic and Sympathetic nervous systems

The parasympathetic and sympathetic nervous systems are the two main divisions of the autonomic nervous system, which controls involuntary bodily functions. The parasympathetic system is responsible for the body's "rest-and-digest" or "feed-and-breed" activities, while the sympathetic system governs the "fight-or-flight" response. Though they often have opposing effects, their actions are complementary, working together to maintain the body's internal balance, or homeostasis.

The parasympathetic nervous system conserves energy and manages bodily functions during periods of rest. It slows the heart rate, constricts the pupils, stimulates digestion, and increases saliva production. Nerves of the parasympathetic system originate in the brainstem and the sacral region of the spinal cord.

Conversely, the sympathetic nervous system prepares the body for stressful or dangerous situations. It increases heart rate and blood pressure, dilates the pupils, slows digestion, and redirects blood flow to muscles. The nerves for this system emerge from the thoracic and lumbar regions of the spinal cord.

Comparison Table

Category	Parasympathetic nervous system	Sympathetic nervous system
Primary function	"Rest-and-digest"	"Fight-or-flight"
Nerve origin	Craniosacral (brainstem and sacral spine)	Thoracolumbar (thoracic and lumbar spine)
Neuron length	Long preganglionic, short postganglionic	Short preganglionic, long postganglionic
Effect on heart rate	Decreases heart rate	Increases heart rate
Effect on digestion	Stimulates digestion	Inhibits digestion
Effect on pupils	Constricts pupils	Dilates pupils
Postganglionic neurotransmitter	Acetylcholine	Norepinephrine (mostly)

Venn diagram comparing Differences between Parasympathetic nervous system and Sympathetic nervous system

The key neurotransmitter for preganglionic neurons in both systems is acetylcholine. However, for postganglionic neurons, the parasympathetic system primarily continues to use acetylcholine, while the sympathetic system mainly uses norepinephrine. An exception within the sympathetic system is its innervation of sweat glands, where acetylcholine is the postganglionic neurotransmitter.

Structurally, the two systems differ in the length of their nerve fibers. The parasympathetic system has long preganglionic neurons that extend from the central nervous system to ganglia near or within the target organs, and very short postganglionic neurons. The sympathetic system features short preganglionic neurons that synapse in ganglia located in a chain alongside the spinal cord, and long postganglionic neurons that then travel to the target organs. This arrangement allows the sympathetic system to produce a widespread response quickly, while the parasympathetic system's actions are typically more targeted and slower to activate.