Messages, in the form of electrical impulses, constantly travel back and forth between the brain and other parts of the body. A special cell called a neuron is responsible for carrying these messages. There are about 100 billion neurons in the human brain.
A neuron has three main parts. The cell body directs all activities of the neuron. Dendrites extend out from the cell body and receive messages from other nerve cells. An axon is a long single fiber that transmits messages from the cell body to the dendrites of other neurons or to other body tissues, such as muscles. A protective covering called the myelin sheath, covers most neurons. Myelin insulates the axon and helps nerve signals travel faster and farther.
Messages travel along a single neuron as electrical impulses, but messages between neurons travel differently. The transfer of information from neuron to neuron takes place through the release of chemical substances into the space between the axon and the dendrites. These chemicals are called neurotransmitters, and the process is called neurotransmission. The space between the axon and the dendrites is called the synapse.
When neurons communicate, an electrical impulse triggers the release of neurotransmitters from the axon into the synapse. The neurotransmitters cross the synapse and bind to special molecules on the other side, called receptors. Receptors are located on the dendrites. Receptors receive and process the message.
What’s particularly interesting about neurotransmission is that each neurotransmitter can bind only to a very specific matching receptor. A neurotransmitter binds to a receptor in much the same way a key fits into a lock. After transmission has occurred, the neurotransmitter is either broken down by an enzyme (a chemical that speeds up some of the body’s processes) or is reabsorbed into the neuron that released it. The reabsorbed neurotransmitters can be reused at a later time.