The two phases of the action potential are depolarization and repolarization.
Depolarization is characterized by:
* The initial phase of the action potential;
* An excitatory response; and
* An influx of sodium and calcium ions into the cell.
Repolarization is characterized by:
* The restoration phase;
* An inhibitory response; and
* Potassium leaving the cell or chloride entering the cell.

Syndrome of inappropriate antidiuretic hormone (SIADH) leads to hyponatremia due to excessive fluid retention. Increased levels of sodium are caused by dehydration, diabetes insipidus, and gastroenteritis.

The brainstem includes the reticular formation system, which controls involuntary functions in the body, such as breathing.
The basal ganglia, also called the corpus striatum, supports learning and automatic actions like walking and driving a car. The hypothalamus regulates appetite, water balance, circadian rhythm, body temperature, and libido.

* Pharmacology is the study of what drugs do and how they do it.
* Pharmacokinetics is the study of what the body does to drugs, including absorption, distribution, metabolism, and excretion.
* Pharmacodynamics is the study of what drugs do to the body, including target sites for drug actions such as receptors, ion channels, enzymes, and carrier proteins.

Tryptophan increases the risk of serotonin syndrome when taken with SSRI medication, MAOI medication, or the herbal supplement St. John's wort.
Vitamin E interacts with warfarin, stains, and antiplatelet drugs. Melatonin interacts with aspirin, NSAIDs, beta-blockers, and steroids. Omega-3 fatty acids interact with warfarin.