neurotransmitter, chemical that transmits information across the junction (synapse) that separates one nerve cell (neuron) from another nerve cell or a muscle. Neurotransmitters are stored in the nerve cell's bulbous end (axon). When an electrical impulse traveling along the nerve reaches the axon, the neurotransmitter is released and travels across the synapse, either prompting or inhibiting continued electrical impulses along the nerve. There are more than 300 known neurotransmitters, including chemicals such as acetylcholine, norepinephrine, adenosine triphosphate, and the endorphins, and gases, such as nitric oxide. Neurotransmitters transmit information within the brain and from the brain to all the parts of the body. Acetylcholine, for example, sends messages to the skeletal muscles, sweat glands, and heart; serotonin release underlies the process of learning and consciousness.
The actions of some drugs mimic those of naturally occurring neurotransmitters. The pain-regulating endorphins, for example, are similar in structure to heroin and codeine, which fill endorphin receptors to accomplish their effects. The wakefulness that follows caffeine consumption is the result of its blocking the effects of adenosine, a neurotransmitter that inhibits brain activity. Abnormalities in the production or functioning of certain neurotransmitters have been implicated in a number of diseases including Parkinson's disease, amyotrophic lateral sclerosis, and clinical depression.