Pharmacology Principles and the Autonomic Nervous System

Step 1 pharmacology rewards concepts over memorized numbers. If you understand how a drug moves through the body, how it engages its receptor, and how the two branches of the autonomic nervous system are wired, most questions become predictable pattern recognition rather than brute recall.

Core Ideas

• Pharmacokinetics is what the body does to the drug — absorption, distribution, metabolism, and excretion (ADME) — while pharmacodynamics is what the drug does to the body at its receptor.
• The autonomic nervous system runs on two transmitters: acetylcholine (cholinergic) and norepinephrine/epinephrine (adrenergic), acting at receptors whose location determines the physiologic effect.
• Sympathetic = "fight or flight," parasympathetic = "rest and digest," and most organs receive opposing input from both, so predicting a drug's effect means knowing which receptor and which tissue.

Pharmacokinetics: ADME

Absorption depends on route and on how much drug survives before reaching systemic circulation. Orally administered drugs pass through the gut wall and liver first, where a fraction is metabolized — the first-pass effect — which lowers bioavailability. IV drugs bypass this entirely, giving 100% bioavailability.

Distribution is captured by the volume of distribution (Vd), a conceptual volume relating the amount of drug in the body to its plasma concentration. A large Vd implies the drug leaves plasma for tissues (lipophilic, tissue-bound drugs); a small Vd implies it stays in the blood (large, charged, plasma-protein-bound drugs).

Metabolism occurs mainly in the liver in two conceptual phases. Phase I reactions (oxidation, reduction, hydrolysis; largely cytochrome P450) add or expose a functional group, often yielding a still-active metabolite. Phase II reactions (glucuronidation, acetylation, sulfation) conjugate the drug to make it water-soluble for excretion. Geriatric patients tend to lose phase I capacity first.

Excretion is mostly renal. Two other kinetic parameters matter: clearance, the volume of plasma cleared of drug per unit time, and half-life, the time for concentration to fall by half. A drug reaches steady state in about 4–5 half-lives, and the same rule governs how long it takes to fully eliminate.

Pharmacodynamics: Drugs at the Receptor

An agonist binds and activates a receptor; an antagonist binds and blocks it without activating. Competitive antagonists compete for the same site and can be overcome by more agonist — they shift the dose-response curve right and reduce potency without lowering maximal effect. Noncompetitive antagonists bind irreversibly or at another site and reduce efficacy (the maximal achievable effect), which cannot be overcome by adding agonist.

Distinguish two properties of the dose-response curve. Potency is the dose required to produce an effect (position on the x-axis); efficacy is the maximal effect the drug can produce (height of the curve). A very potent drug is not necessarily more effective. The therapeutic index (TI) compares the dose that is toxic to the dose that is effective — a high TI means a wide safety margin, a low TI (e.g., warfarin, digoxin) demands monitoring.

Autonomic Wiring and Receptors

Both divisions use a two-neuron chain with a ganglion. All preganglionic neurons release acetylcholine onto nicotinic receptors. Postganglionically, the parasympathetic system releases acetylcholine onto muscarinic receptors, while the sympathetic system releases norepinephrine onto adrenergic receptors — with two exceptions: sweat glands (sympathetic but cholinergic/muscarinic) and the adrenal medulla (releases epinephrine into blood).

Key receptor effects: M3 causes smooth-muscle contraction, glandular secretion, and pupillary constriction; alpha-1 causes vasoconstriction and mydriasis; beta-1 increases heart rate and contractility; beta-2 relaxes bronchial and vascular smooth muscle.

High-Yield Exam Patterns

• Given a route change (oral to IV), predict bioavailability rising because first-pass metabolism is bypassed.
• A curve shifted right with the same maximum = competitive antagonist; a lowered maximum = noncompetitive antagonist.
• "Cannot be overcome by more agonist" is the phrase that signals noncompetitive/irreversible blockade or reduced efficacy.
• Match the symptom cluster to a receptor: bradycardia + bronchoconstriction + salivation = muscarinic (parasympathetic) excess.
• Beta-2 activation relaxes bronchi (bronchodilation); muscarinic activation constricts them — a classic paired distractor.
• Expect a drug to reach steady state in 4–5 half-lives regardless of dose.

Common Traps to Avoid

• Confusing potency with efficacy — the leftmost curve is most potent, the highest curve is most efficacious.
• Assuming all sympathetic fibers are adrenergic — sweat glands use acetylcholine.
• Thinking a competitive antagonist lowers maximal effect; it lowers potency but the max is preserved with enough agonist.
• Forgetting that phase II (conjugation), not phase I, produces the water-soluble form for excretion.
• Equating a large volume of distribution with a large plasma concentration — a large Vd means the drug has left the plasma.