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Mimics and Blockers: The Four Functional Drug Classes

Two messengers times two actions gives four great drug families: sympathomimetics, sympatholytics, parasympathomimetics, and anticholinergics. Here is how to predict each one's effects — and the partner effect of its opposite.

Two by two: the whole map on one page

Everything in autonomic pharmacology fits a tiny grid. Pick a messenger (cholinergic or adrenergic) and pick an action — turn it on (agonist, a mimic) or turn it off (antagonist, a blocker). That gives exactly four functional classes. A mimic of one branch looks like a blocker of the other, because the two branches oppose each other on most organs.

                 TURN ON (agonist)          TURN OFF (antagonist)
CHOLINERGIC      parasympathomimetic        anticholinergic
(acetylcholine)  'wet & slow'               'dry & fast'

ADRENERGIC       sympathomimetic            sympatholytic
(norepinephrine) 'fight-or-flight up'       'fight-or-flight down'
The four functional classes. Diagonally opposite boxes produce similar bedside pictures.

The cholinergic pair

A parasympathomimetic mimics acetylcholine, so it reproduces the wet-and-slow picture from guide 2: small pupils, watery secretions, a slow heart, an active gut and bladder. These drugs are used to empty a sluggish bladder, treat glaucoma, or boost a weak gut. The mirror image is the anticholinergic (a muscarinic blocker), which produces a dry-and-fast patient: dry mouth, blurred near vision, a faster heart, urinary retention. Memorably: 'dry as a bone, blind as a bat, red as a beet, mad as a hatter.'

The adrenergic pair

A sympathomimetic turns up fight-or-flight, but its precise effect depends on which adrenoceptor it favours — exactly the table from guide 3. A beta-2 agonist opens airways for asthma; an alpha-1 agonist tightens vessels to raise blood pressure or clear a stuffy nose. The opposite is the sympatholytic. Its most famous member, the beta-blocker, shields the heart from adrenaline: it slows the rate, lowers blood pressure, and protects after a heart attack — while its non-selective forms can also narrow airways, which is why it is used cautiously in asthma.

  1. Name the messenger: cholinergic (ACh) or adrenergic (NE)?
  2. Name the action: agonist (mimic) or antagonist (block)?
  3. Name the subtype if it matters: alpha vs beta, M vs N — that decides the target organ.
  4. Read the predicted effects off your two tables, then flip them for the side effects.