Information is constantly exchanged between the brain and other parts of the body by both electrical and chemical impulses. A cell called a neuron is responsible for carrying this information. The human brain is made up of 100 billion neurons.
A neuron has three main parts. The cell body directs all of the neuron’s activities. Dendrites, short branches that extend out from the cell body, receive messages from other neurons and pass them on to the cell body. An axon is a long, single fiber that transmits messages from the cell body to the dendrites of other neurons or to other tissues in the body, such as muscles. A protective covering called the myelin sheath covers the axons of many neurons. Myelin insulates the axon and helps messages from nerve signals travel faster, farther, and more efficiently.
The exchange of information from the axon of one neuron to the dendrites of another is called neurotransmission. Neurotransmission takes place through the release of chemicals into the space between the axon of the first neuron and the dendrites of the second neuron. These chemicals are called neurotransmitters. The space between the axon and the dendrites is called the synapse.
When neurons communicate, an electrical impulse traveling down the axon causes neurotransmitters to be released from the end of the axon into the synapse. The neurotransmitters cross the synapse and bind to special molecules on the other side, called receptors. Receptors are found on the dendrites and cell bodies of all neurons. These receptors convert the information into chemical and/or electrical signals for processing in the neuron.
Our body produces many different types of neurotransmitters. Each neurotransmitter has a specific role to play in the functioning of the brain. A neurotransmitter binds to a receptor in much the same way that a key fits into a lock. A specific neurotransmitter only binds to certain receptors. Once the neurotransmitter has bound to a receptor, a series of events follow. First, the message carried by the neurotransmitter is received and passed on to the receiving nerve cell. Second, the neurotransmitter is inactivated and either broken down by an enzyme or reabsorbed from where it was released. The reabsorption is completed by other molecules called transporter molecules. These molecules are located in the cell membranes of the axon that releases the neurotransmitters. They pick up specific neurotransmitters from the synapse and carry them back across the cell membrane into the axon. The neurotransmitters are then recycled for use at a later time. Note that this process is true for most neurotransmitters, but not for all of them.