When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released.

They include chemicals secreted within the brain, in a part of the hypothalamus called the arcuate nucleus.

With sclerotherapy, doctors inject a liquid chemical called a sclerosing agent directly into a varicose vein to close it off.

They terrify patients with a "halimeter," a new gadget that measures a nasty smelling chemical called methyl mercaptan.

When a beetle finds a suitably weakened tree it sends out a chemical called a pheromone to attract others.

In order for the eye to function propertly, a special chemical called a chromophore is bound to a protein known as an opsin.

The first cell releases a chemical called a neurotransmitter into the synaptic gap.

For a long time, investigators thought a chemical in St. John's wort called hypericin was responsible for its effects against depression.

Just as unexpected is its number one cause: wet nail polish, or, more precisely, a chemical in it called formaldehyde.

It works by temporarily blocking the transmission to the brain of a naturally occurring chemical called acetylcholine.