How do chemicals work in the brain?
We begin with a short introduction to how the brain works in general and how chemicals interact with neurons to alter communication between nerve cells. This will help you to understand how the brain responds to the ingestion of alcohol. The brain is a complex organ that is comprised of gray matter and white matter. Gray matter consists of the cell bodies of neurons and other support cells. White matter consists of long tracts of axons, like telephone lines, that run between the neurons. Figure 1 shows the brain and its general divisions, and Figure 2 shows a single neuron. Different areas of the brain have somewhat different 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 starts in the gray matter and travels via the axons of the neurons, making up the white matter in the brain.
Gray matter the part of the brain that contains the nerve cell bodies, including the cell nucleus and its metabolic machinery.
White matter tracts in the brain that consist of sheaths (called myelin) covering long nerve fibers.
Neuron a nerve cell made up of a cell body with extensions called dendrites and the axon.
Figure 1 Brain and general divisions. From brainconnection.com. Used with permission. Copyright © 1999 Scientific Learning Corporation. All rights reserved.
Figure 2 Single neuron. From brainconnection.com. Used with permission. Copyright © 1999 Scientific Learning Corporation. All rights reserved.
Neurons and Neurotransmitters
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 occurs that in turn creates 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 (see Figure 3). 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 ultimately either causes that nerve to "fire" or not to "fire." Brain activity is the result of an orchestrated series of nerves firing or not firing in binary fashion. It is much like a computer where very complicated processes begin as a series of l's or O's (on or off, fire or do not fire).
Motor cortex an area on the outer part of the brain that is responsible for voluntary motor control.
Sensory cortex an area on the outer part of the brain that is responsible for organizing sensory input into a coherent perception at the level of consciousness.
Axon that part of the neuron or nerve cell that is a long tube conducting signals away from the cell body.
Electrochemical the means by which a nerve conducts signals through the body and axon. This causes a release of chemicals.
Neurotransmitters chemical released by nerves that communicate with other nerves causing electrochemical changes in those nerves to continue to propagate a signal.
Synaptic cleft the gap between nerves where neurotransmitters are released that allow nerves to communicate with one another.
Receptors specific areas of protein on a neuron that are configured to respond only to specific neurotransmitters.
Transporter also known as a transport pump. Transporters are made up of proteins that act as "vacuum cleaners," taking up leftover neurotransmitters from the synaptic cleft and transporting them back into the nerve cell that originally released them.
After the nerve fires, releasing neurotransmitters into the synaptic cleft, the neurotransmitters must be removed from the area in order to turn the signal off. There are two ways that these chemicals can be removed in order 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 up 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 (see Figure 3). It is important to understand these basic principals of neurophysiology because all psychoactive compounds, whether neurotransmitters, hormones, medications, addictive drugs, or alcohol, involve one or more of these mechanisms. The differences between their effects stem from the particular receptor and neurotransmitter with which it interacts. Alcohol works in the brain in a manner similar to other chemicals, as a "key" that fits into a specific "lock" that opens a door for further communication. Alcohol, unlike many other drugs of abuse, however, is not a magic bullet targeting a specific area of the brain and a specific neurotransmitter or receptor system. Alcohol works on both the motor and sensory components of the brain and has multiple neurotransmitter effects, although it does have a receptor that appears to be specifically designed for it. These effects are not unique to alcohol. Opiates have opiate receptors, and marijuana has marijuana receptors, which means that the body produces chemicals with similar activity as their ingested cousin, alcohol.
Figure 3 Synaptic Cleft, wikipedia.com.