The autonomic nervous system runs the body's automatic work, such as heart rate, digestion and pupil size. Its two branches usually act in opposite directions to keep the balance.
Sympathetic
Sympathetic nerves leave the spinal cord from the thoracic and upper lumbar segments (T1 to L2). They prepare the body for action and mostly use norepinephrine at the target organ.
Parasympathetic
Parasympathetic nerves leave from the brainstem, mainly through the vagus nerve, and from the sacral cord (S2 to S4). They support rest and digestion and use acetylcholine at the target organ.
Two-neuron chain
Both branches use two neurons in a row joined at a ganglion, and the first neuron always releases acetylcholine. Sympathetic ganglia lie near the spinal cord; parasympathetic ganglia lie near or inside the organ.
Dual control
Most organs get both supplies, and the balance between them sets the activity. At rest the heart is under strong vagal tone, which is why the resting rate is lower than the sinus node's own rate of about 100.
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Heart | Faster and stronger | Slower |
| Pupils | Dilate | Constrict |
| Airways | Dilate | Constrict, more mucus |
| Gut | Slows down | Speeds up digestion |
| Bladder | Holds urine | Empties |
At the bedside
Orthostatic vital signs test how well the sympathetic system tightens vessels and raises heart rate on standing. A blunted response suggests dehydration, medicines or autonomic failure.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).