PT
● MODULE 03 / 20
Pharmacodynamics & Receptor Signalling
Connect receptor binding to response, potency, efficacy, antagonism and therapeutic margin.
Quick revision ↓ Medical Dictionary
FULL CHAPTER EXPLANATION
Deep Learning — Module 03
Read this section in sequence. It moves from drug targets to therapeutic effects, adverse reactions and safe use.
01
Targets and transduction
Medicines act on receptors, enzymes, ion channels, transporters and sometimes through physical or chemical effects. Ligand-gated channels can respond rapidly, G-protein-coupled receptors signal through second messengers, enzyme-linked receptors activate intracellular pathways, and nuclear receptors change gene expression. Antacids neutralize acid chemically rather than requiring a receptor. Receptor affinity describes binding, whereas intrinsic efficacy describes the ability of a bound ligand to activate signaling.
02
Potency and efficacy
A graded curve describes response magnitude in a tissue or individual. EC50 is the concentration producing half of the maximal response in that system; a lower EC50 indicates greater potency. Emax describes maximal effect and therefore informs efficacy. A more potent drug is not necessarily more useful or safer. A quantal curve instead describes the fraction of a population reaching a defined endpoint, which supports comparisons of effective and toxic doses.
03
Agonists and antagonists
A full agonist can produce the maximal system response; a partial agonist has a lower maximal response in that system even at high occupancy. In the presence of a full agonist, a partial agonist can reduce overall activation by competing for receptors. A reversible competitive antagonist typically shifts the agonist curve rightward while preserving the achievable maximum. Irreversible or noncompetitive antagonism can reduce the maximum, although receptor reserve can complicate the observed pattern.
04
Adaptation and variation
Repeated stimulation may produce receptor desensitization or downregulation. Tachyphylaxis is a rapid decline in response; tolerance is a broader reduction after repeated exposure. Physical dependence means withdrawal can follow abrupt cessation, and is not identical to compulsive drug use. Genetic variation, disease, age and concurrent medicines modify response through both kinetics and dynamics. Receptor selectivity is relative and often decreases at higher concentrations.
05
Therapeutic margin and clinical reasoning
The therapeutic index is commonly expressed as TD50 divided by ED50 in population studies. It is an approximation of separation between beneficial and toxic responses, not a guarantee of safety. Narrow-margin medicines require careful dosing and sometimes concentration monitoring. When a patient fails to respond, consider diagnosis, adherence, delivery, exposure and target biology before escalating. Adding a second drug may create pharmacodynamic synergy in benefit or toxicity without changing either drug concentration.
High-Yield Revision
- Potency is about concentration needed; efficacy is about maximum response.
- Partial agonists can oppose full agonists at the same receptor.
- A pharmacodynamic interaction can occur without altered drug concentrations.
Common Exam & Concept Pitfalls
- Do not infer safety solely from potency.
- Dependence and addiction are distinct concepts.
Sources & Further Reading
Curriculum scope checked 23 September 2026. These original study notes summarize established principles; current institutional guidance and product information govern clinical use. Faculty review is pending.
RAHBAR study structure: target → drug action → therapeutic use → adverse effects → monitoring. Use institutional curriculum and faculty guidance for local sequencing, practical requirements and assessment specifications.