The plasma membranes of cells contain specialized proteins called receptors. Receptors allow cells to receive chemical signals that direct cellular function and activity. These chemical signals, also called signaling molecules, include hormones, neurotransmitters, and local mediators. Signaling molecules have the ability to bind to receptors and trigger a series of biochemical events in a cell's interior.

There are hundreds of different signaling molecules, each of which will only bind to a certain receptor. Neurotransmitters are the chemicals used by the nervous system to transmit nerve impulses to and from the brain. Hormones are released into the bloodstream by specialized tissues called glands. They can influence cellular events throughout the body. On a more local level, local mediators are chemicals released by many cells types to influence other cells that are nearby.

The relationship between signaling molecules and their receptors is very specific. Each receptor on a cell's surface...