A neurotransmitter is a chemical substance that is released by neurons in the nervous system to transmit signals to other neurons or to muscles or glands. Different neurotransmitters have different effects on the target cells they bind to, and their balance and levels in the brain can be altered by disease, injury, or drugs, leading to changes in behaviour and cognition. Some of the most well-known neurotransmitters include: acetylcholine: (muscle contractions and memory), dopamine (motivation, reward, and movement), serotonin (regulating mood, appetite, and sleep), GABA (gamma-aminobutyric acid) (inhibiting neuronal activity), and glutamate (learning and memory).