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chemical that transmits information across the junction (synapsesynapse
, junction between various signal-transmitter cells, either between two neurons or between a neuron and a muscle or gland. A nerve impulse reaches the synapse through the axon, or transmitting end, of a nerve cell, or neuron.
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) 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 acetylcholineacetylcholine
, a small organic molecule liberated at nerve endings as a neurotransmitter. It is particularly important in the stimulation of muscle tissue. The transmission of an impulse to the end of the nerve causes it to release neurotransmitter molecules onto the surface of
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, norepinephrinenorepinephrine
, a neurotransmitter in the catecholamine family that mediates chemical communication in the sympathetic nervous system, a branch of the autonomic nervous system.
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, adenosine triphosphateadenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
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, and the endorphinsendorphins
, neurotransmitters found in the brain that have pain-relieving properties similar to morphine. There are three major types of endorphins: beta endorpins, found primarily in the pituitary gland; and enkephalins and dynorphin, both distributed throughout the nervous
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, and gases, such as nitric oxidenitric oxide
or nitrogen monoxide,
a colorless gas formed by the combustion of nitrogen and oxygen as given by the reaction: energy + N2 + O2 → 2NO; m.p. −163.6°C;; b.p. −151.8°C;.
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. 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; serotoninserotonin
, organic compound that was first recognized as a powerful vasoconstrictor occurring in blood serum. It was partially purified, crystallized, and named in 1948, and its structure was deduced a year later.
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 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 diseaseParkinson's disease
or Parkinsonism,
degenerative brain disorder first described by the English surgeon James Parkinson in 1817. When there is no known cause, the disease usually appears after age 40 and is referred to as Parkinson's disease; a number of genes have
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, amyotrophic lateral sclerosisamyotrophic lateral sclerosis
(ALS) or motor neuron disease,
sometimes called Lou Gehrig's disease, degenerative disease that affects motor neurons in the brain and spinal cord, preventing them from sending impulses to the muscles.
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, and clinical depressiondepression,
in psychiatry, a symptom of mood disorder characterized by intense feelings of loss, sadness, hopelessness, failure, and rejection. The two major types of mood disorder are unipolar disorder, also called major depression, and bipolar disorder, whose sufferers are
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A chemical agent that is released by a neuron at a synapse, diffuses across the synapse, and acts upon a postsynaptic neuron, a muscle, or a gland cell.