Sympathetic nervous system
Say it: sim-puh-THET-ik NUR-vus SIS-tum
The sympathetic nervous system is the division of the autonomic nervous system that readies the body for action, producing the fight-or-flight response.
Also: sympathetic division, thoracolumbar division
Definition
What is the sympathetic nervous system?
The sympathetic nervous system is one of the two main divisions of the autonomic nervous system, which controls internal organs without conscious effort. Its first neurons sit in the lateral horns of spinal segments T1 to L2, so it is also called the thoracolumbar division. Their short, myelinated axons run to a chain of ganglia (clusters of nerve cell bodies) alongside the vertebral column or to ganglia in front of it, where they hand the signal to second neurons with long axons that reach the target organs. Some fibers go straight to the adrenal medulla instead.
Its connections branch widely, so many organs respond at once. Sympathetic activity speeds the heart and strengthens its beat, widens the airways and pupils, diverts blood to skeletal muscle while narrowing vessels in the gut and skin, slows digestion, increases sweating and releases glucose into the blood. The preganglionic neurons release acetylcholine, most postganglionic neurons release norepinephrine onto adrenergic receptors, and the adrenal medulla adds epinephrine (adrenaline) to the circulation.
Many drugs act on this system. Beta blockers blunt its effects on the heart in high blood pressure and heart rhythm disorders, while epinephrine itself is given in severe allergic reactions.
Word parts
Where does the term “sympathetic nervous system” come from?
- sym-
- together, withGreek syn
- -pathetic
- feeling, sufferingGreek pathos, via sympatheia (fellow feeling)
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Terms that go with it
- Autonomic nervous systemThe autonomic nervous system is the division of the nervous system that runs involuntary functions such as heart rate, digestion and gland secretion.
- Parasympathetic nervous systemThe parasympathetic nervous system is the autonomic division that conserves energy and controls rest-and-digest activities such as digestion.
- Adrenal glandAn adrenal gland is one of two small endocrine glands atop the kidneys, with a cortex that makes steroid hormones and a medulla that makes epinephrine.
- Beta blockerA beta blocker is a drug that stops epinephrine and norepinephrine from acting on beta receptors, which slows the heart and lowers blood pressure.
- Spinal cordThe spinal cord is the column of nerve tissue in the vertebral canal that relays signals between the brain and the body and runs many reflexes.
- HomeostasisHomeostasis is the body's ability to keep internal conditions, such as temperature, blood glucose and pH, steady while conditions around it change.
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Checked against
Written for biofideX in plain words, after checking these references:
- OpenStax Anatomy and Physiology 2e, 15.1 Divisions of the Autonomic Nervous System
- NCBI MeSH: Sympathetic Nervous System
- NCI Dictionary of Cancer Terms: sympathetic nervous system
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