An extracellular substance (as the hormone epinephrine or the neurotransmitter serotonin) that binds to a cell-surface receptor and initiates intracellular activity.

An intracellular substance (as cyclic AMP) that mediates cell activity by relaying a signal from an extracellular molecule (as of a hormone or neurotransmitter) bound to the cell's surface.

A lipophilic hormone that passes through the plasma membrane of a cell, binds to an intracellular receptor, and changes gene expression.

Amines and proteins that do not passively diffuse through the cell membrane.

A small gland that makes steroid hormones, adrenaline, and noradrenaline. These hormones help control heart rate, blood pressure, and other important body functions.

The area of the brain that secretes substances that influence pituitary and other gland function and is involved in the control of body temperature, hunger, thirst, and other processes that regulate body equilibrium

A tongue-shaped glandular structure involved in the secretion of fluid containing protease, carbohydrase, and lipase, into the duodenum to aid in food digestion, as well as the secretion of insulin and glucagon into the bloodstream in order to regulate sugar levels in blood.

Four small glands of the endocrine system which regulate the calcium in our bodies.

A small endocrine gland shaped like a pinecone, located in the epithalamus, and involved in melatonin production.

A small pea-sized gland that plays a major role in regulating vital body functions and general wellbeing. It is referred to as the body's 'master gland' because it controls the activity of most other hormone-secreting glands.

A small organ that lies in the upper chest under the breastbone. It makes white blood cells, called lymphocytes, which protect the body against infections.

The process by which a cell increases its response to a substance or signal from outside the cell to carry out a specific function.

A decrease in the number of receptors on the surface of target cells, making the cells less sensitive to a hormone or another agent.

Any of a class of cell membrane proteins that are coupled to cell surface receptors and upon stimulation of the receptor by an extracellular signaling molecule (such as a hormone or neurotransmitter).