Chemicals involved in the transmission of signals from nerve cells to other nerve cells or other types of cells. Nerve cells receive information from a stimulus — a touch, for example, or a source of heat — and send the information to the brain. The brain then sends signals back to the body telling it how to react. For example, if you touch a hot stove, nerve cells in your fingers send pain signals to your brain, and your brain sends signals to your muscles telling them to move your hand away from the stove.

For a signal to be passed along, nerve cells, or neurons, must communicate with each other. The signal travels down a part of the neuron called the axon, and then it jumps across a junction called the synapse to another neuron’s dendrites. It’s a neurotransmitter that carries the signal across the synapse.

There are many different types of neurotransmitters. Some of them help to stimulate actions in the body, such as muscle contractions and the release of hormones. Some suppress actions in the body, such as those that constrict blood vessels or bring on sleep.

Low levels of the neurotransmitters dopamine, norepinephrine, and serotonin are known to play a role in depression. One of the ways depression is treated is with medicines that raise levels of these neurotransmitters in the brain. Drugs called tricyclic antidepressants, for example, raise levels of serotonin and norephinephrine, while selective serotonin reuptake inhibitors (SSRIs) raise levels of serotonin.

Low levels of serotonin and norepinephrine have also been implicated in fibromyalgia syndrome, a condition characterized by tenderness in certain points throughout the body along with muscle pain, fatigue, and insomnia. Antidepressant medicines are sometimes used to treat fibromyalgia. Research has shown mixed results, but some people find they help to relieve pain and, in the case of the tricyclics, improve sleep.

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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