The brain is a complex organ comprising gray matter and white matter. Gray matter consists of the cell bodies of neurons and other support cells , and white matter consists of long tracts of axons that run between the neurons . Figure 1 shows an illustration of a neuron. Different areas of the brain have somewhat specific functions. For example, the motor cortex controls voluntary movements of the body , and the sensory cortex processes information to the senses. Different areas of the brain communicate with other areas nearby as well as more distantly.
Information travels via the axons of the neurons within the white matter areas of the brain. The brain contains billions of neurons that interact with each other electrochemically . This means that when a nerve is stimulated, a series of chemical events occur that in turn create an electrical impulse. The resulting impulse propagates down the nerve length known as the axon and causes a release of chemicals called neurotransmitters into a space between the stimulated nerve and the nerve that it wishes to communicate with, known as the synaptic cleft .
The neurotransmitters interact with receptors on the second nerve, either stimulating or inhibiting them. The interaction between the neurotransmitters and receptors can be likened to a key interacting with a lock where the neurotransmitter, or “key,” engages the receptor , or “lock,” causing it to “open.” This opening is really a series of chemical changes within the second nerve that either causes that nerve to “fire” or “not fire.” Thus brain activity is the result of an orchestrated series of nerves firing or not firing in a binary fashion. In that sense it is much like a computer where very complicated processes begin their lives as a series of 1s or 0s (on or off, fire or do not fire).
After the nerve fires, thereby releasing neurotransmitters into the synaptic cleft, the neurotransmitters must be removed from the area to turn the signal off. There are two ways that these chemicals can be removed to turn the signal off. The first is by destroying the chemical through the use of another chemical, known as an enzyme , with that specific purpose in mind. The second is by pumping the chemical back into the nerve that released it by using another special chemical known as a transporter or transport pump.
The process of pumping chemicals back into the nerve is known as reuptake. It is important to understand these basic principals of neurophysiology because all psycho-active compounds, whether neurotransmitters, hormones, medications, or addictive drugs, involve one or more of these simple mechanisms.