autonomic nervous system
Think of the autonomic nervous system as the body's autopilot. While you consciously decide to lift a cup, you never have to think about speeding up your heart, secreting saliva, or narrowing your pupils in bright light. The autonomic nervous system handles all of that automatically, keeping the inner workings of the body running without your attention.
It controls involuntary targets: glands, smooth muscle in blood vessels and the gut, and cardiac muscle. It is divided into two main branches, the sympathetic and parasympathetic systems, which usually oppose each other so that the body can be finely tuned between activity and recovery. A third division, the enteric system, governs the gut wall.
Pharmacologically, the autonomic nervous system is a goldmine of drug targets because so much of its signaling runs through just two transmitters, acetylcholine and norepinephrine, acting on a handful of receptor families. Drugs that imitate or block these transmitters can raise or lower blood pressure, open airways, slow the heart, dry secretions, and much more, which is why autonomic pharmacology underlies a huge fraction of everyday medicines.
When you are startled, your sympathetic system fires: heart races, pupils widen, airways open. Once the threat passes, the parasympathetic system takes over and the body settles back to rest.
The two branches in everyday seesaw balance.
A useful simplification: most autonomic effects can be traced to acetylcholine acting on muscarinic or nicotinic receptors, or norepinephrine and adrenaline acting on alpha or beta adrenoceptors. Mastering those four receptor families explains most autonomic drugs.