Although nerve impulses are electrical, the transmission of these impulses from one neuron (nerve cell) to another, or to a muscle cell, is achieved chemically rather than electrically. The chemical that transmits the message is a neurotransmitter. Scores of different chemicals fulfill this function in different parts of the nervous system. Many neurotransmitters are similar to, or identical to, substances used by our bodies as hormones. These neurotransmitters also act as messenger molecules, but are released into the bloodstream to act on their target cells at a distance.
One of the most important nemotransmitters is acety1choline. It is released by neurons connected to skeletal muscles (causing them to contract) and by neurons that control the sweat glands and the heart beat. lt also transmits messages between neurons in the brain and spinal cord.
Interference with the action of acetylcholine on skeletal muscles is the cause of the disease myasthenia gravis; depletion of the nerve cells that release acetylcholine in the brain may be a cause of Alzheimer’s disease. Another transmitter, norepinephrine, is important in nerve pathways controlling heart beat, blood flow, and response to stress. In addition to being produced in the body by neurons, norepinephrine is made by the adrenal glands. Dopamine plays an important role in parts of the brain that control movement, and malfunction of the neurons that respond to dopamine is thought to be important in causing Parkinson’s disease. Serotonin is one of the primary neurotransmitters in parts of the brain concerned with conscious processes.
Over the last 20 years, a whole new family of transmitters, called the neuropeptides, has been discovered.
Neuropeptides are small proteins; they are larger molecules than the previously known neurotransmitters, which are all very small molecules. The best studied of these neuropeptides are the endorphins, which are used by the brain to control sensitivity to pain.