Drug Action

Medical pharmacology involves the science of chemicals (drugs) that interact with the human body. Pharmacodynamics is the effect of a drug on the body. Pharmacokinetics is the way the body affects drug with time (absorption, distribution, metabolism and excretion).

Some drugs act by virtue of their physicochemical properties and are non-specific in drug action and some drugs act as false substrates or inhibitors for certain transport systems or enzymes. Most drugs produce effects by acting on specific proteins (receptors) and normally respond to endogenous chemicals in the body (growing or originating from within). These chemicals are either synaptic transmitter substances or hormones.

Acetycholine is a transmitter substance released from motor nerve endings and it activates receptors in skeletal muscle, initiating a sequence of events that results in contraction of muscle. Acetylcholine or drugs that activate receptors and produce a response are agonists. Antagonists are drugs which combine with receptors, but do not activate them. These antagonists reduce probability of transmitter substance (or another agonist) combining with the receptor and so reduce or block its action. The activation of receptors by an agonist or hormone coupled to physiological or biochemical responses by transduction mechanisms that often (but not always) involve molecules called "second messengers".