Definition: A neurotransmitter is a chemical messenger that carries signals between neurons as well as other cells in the body. These chemicals are released from the end of one neuron and cross the synapse to receptor sites in the next neuron. While scientists are not quite sure exactly how many neurotransmitters exist, more than 100 have been identified. Certain diseases, including Alzheimer's and Parkinson's, are associated with problems with specific chemical messengers. Learn more about neurotransmitters.
Image courtesy Piotr Bizior