1. Any various chemicals found in the brain and throughout the body that transmit signals among nerve cells (neurons) within the nervous system. 2. Substances that transmit or modify nerve impulses. 3. A chemical that is released from a nerve cell, which transmits an impulse from that nerve cell to another nerve cell, or to another organ (a muscle, for example). Neurotransmitters are chemical messengers that transmit neurological information from one cell to another.