Neurotransmitters and Neurons
To understand what happens in the brain when a person becomes clinically depressed as well as how antidepressant medications work, it is first important to learn a bit about the function of neurons and neurotransmitters. Within the brain, there are special chemicals called neurotransmitters that carry out many very important functions. Essentially, they help transfer messages throughout structures of the brain's nerve cells. These nerve cells, called neurons, are organized to control specialized activities. We each have somewhere between 10-100 billion neurons within our brains. Whenever we do anything, react, feel emotions, think, our neurons transmit messages in the form of electrical impulses from one cell to another. These electrical impulses travel across the neurons at an amazing rate of speed- less than 1/5,000 of a second. Because they move so quickly, our brains can react instantaneously to stimuli such as pain.
A neuron is made up of a cell body, an axon, and numerous branching dendrites. Chemical messages pass through the brain by traveling through these neuronal structures. First, it begins as an electrical impulse that is picked up by one of the dendrites of the neuron. Next, the impulse moves through the cell body then travels down the axon. When it reaches the axon the electrical impulse is changed to a chemical impulse. These chemical impulses, or neurotransmitters, released by the axon have the duty of carrying messages from one neuron to another. When the message is picked up by the dendrite of a neighboring neuron, it is changed back in to an electrical impulse and process begins again. Neurons do not actually touch one another. Instead, the chemical messenger passes from one neuron to another through a small narrow gap, called a synapse, which separates the neurons.
Neurotransmitters travel from neuron to neuron in an orderly fashion. They are specifically shaped so that after they pass from a neuron into the synapse, they can be received onto certain sites, called receptors, on a neighboring neuron. Neurotransmitters can fit a number of different receptors, but receptor sites can only receive specific types of neurotransmitters. Upon landing at the receptor site of neuron, the chemical message of the neurotransmitter may either be changed into an electrical impulse and continue on its way through the next neuron, or it may stop where it is. In either case the neurotransmitter releases from the receptor site and floats back into the synapse. It is then removed from the synapse in one of two ways. The neurotransmitter may be broken down by a chemical called monoamine oxidase, or it may be taken back in by the neuron that originally released it. The latter case is called reuptake.
Of the 30 or so neurotransmitters that have been identified, researchers have discovered associations between clinical depression and the function of three primary ones: serotonin, norepinephrine, and dopamine. These three neurotransmitters function within structures of the brain that regulate emotions, reactions to stress, and the physical drives of sleep, appetite, and sexuality. Structures that have received a great deal of attention from depression researchers include the limbic system and hypothalamus.
Theories about how neurotransmitters may be related to a person's mood have been based upon the effects that antidepressant medications can have on relieving depression in some people. It is believed that these medications are effective because they regulate the amount of specific neurotransmitters in the brain. However, the role that neurotransmitters play in the development or treatment of clinical depression is not completely clear. For instance, it has been shown that many people who are depressed have low levels of the neurotransmitter norepinephrine. The use of some antidepressants can increase the level of norepinephrine in the brain, and subsequently relieve depressive symptoms. One the other hand, it has also been shown that some other people who are depressed have high levels of norepinephrine. This same scenario may be true for other neurotransmitters. Another reason that the effects of neurotransmitters are not clear-cut has to do with the fact that antidepressant medications do not work for everyone. If there were a direct causal link between the level of a neurotransmitter in the brain and depression, then we would expect a much higher rate of success with medication. Further, although antidepressant medications can change the level of a neurotransmitter in the brain immediately, it normally takes a few weeks for a person with depression to feel better. What is seems to boil down to is that there appears to be a strong relationship between neurotransmitter levels in the brain and clinical depression, and that antidepressant medications work for a great many people, but we are not absolutely certain of the actual relationship between neurotransmitters and depression.
The reason we do not know more about the effects of neurotransmitters has to do with that fact that they are so difficult to study. Neurotransmitters are present in very small quantities, they are only available in certain locations within the brain, and they disappear very quickly once they are used. Because they are removed so fast, they cannot be measure directly. Researchers can only measure what is left over after their use in the brain. The substances that remain are called metabolites and they can be found in blood, urine, and cerebrospinal fluid. By measuring these metabolites, researchers can gain an understanding of the effects of changes in neurotransmitters in the brain.
It is unknown whether changes in levels of neurotransmitters cause the development of depression or depression causes changes in neurotransmitters. It may happen both ways. Researchers believe that our behavior can affect our brain chemistry, and that brain chemistry can affect behavior. For instance, if a person experiences numerous stressors or traumas this may cause his or her brain chemistry to be affected, leading to clinical depression. On the other hand, that same person may learn how to change depressed thoughts and behavior and cope with stressful events. Doing this may also change brain chemistry and relieve depression.