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The Human Brain: How Brain Cells Communicate With Each Other

The Chemical Messaging System of the Human Brain

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Updated February 01, 2009

The Human Brain: How Brain Cells Communicate With Each Other
Photo © Sheryl Ankrom

Weighing in at only about three pounds, the brain is the most complicated part of the human body. As the organ responsible for intelligence, thoughts, sensations, memories, body movement, feelings and behavior, it has been studied and hypothesized for centuries. But, it is the last decade of research that has provided the most significant contributions to our understanding of how the brain functions. Even with these advancements, what we know so far is probably only a fraction of what we will, undoubtedly, discover in the future.

The human brain is believed to function in a complex chemical environment through various types of neurons and neurotransmitters. Neurons are brain cells, numbering in the billions, which are capable of instant communication with each other through chemical messengers called neurotransmitters. As we live our lives, brain cells are constantly receiving information about our environment. The brain then attempts to make an internal representation of our external world through complex chemical changes.

Neurons (Brain Cells)

To get a better idea of how the brain functions through chemical communication, let’s start by looking at figure 1.1, which shows a basic schematic of a single neuron.

The center of the neuron is called the cell body or soma. It contains the nucleus, which houses the cell’s deoxyribonucleic acid (DNA) or genetic material. The cell’s DNA defines what type of cell it is and how it will function.

At one end of the cell body are the dendrites, which are receivers of information sent by other brain cells (neurons). The term dendrite, which comes from a Latin term for tree, is used because the dendrites of a neuron resemble tree branches.

At the other end of the cell body is the axon. The axon is a long tubular fiber that extends away from the cell body. The axon acts as a conductor of electrical signals.

At the base of the axon are the axon terminals. These terminals contain vesicles where chemical messengers, also known as neurotransmitters, are stored.

Neurotransmitters (Chemical Messengers)

It is believed that the brain contains several hundred different types of chemical messengers (neurotransmitters). Generally, these messengers are categorized as either excitatory or inhibitory. An excitatory messenger stimulates the electrical activity of the brain cell, whereas an inhibitory messenger calms this activity. The activity of a neuron (brain cell) -- or whether or not it continues to release, or pass on, chemical messages -- is largely determined by the balance of these excitatory and inhibitory mechanisms.

Scientists have identified specific neurotransmitters that are believed to be related to anxiety disorders. The chemical messengers that are typically targeted with medications commonly used to treat panic disorder include:

Serotonin. This neurotransmitter plays a role in modulating a variety of body functions and feelings, including our mood. Low serotonin levels have been linked to depression and anxiety. The antidepressants called selective serotonin reuptake inhibitors (SSRIs) are considered to be the first-line agents in the treatment of panic disorder. SSRIs increase the level of serotonin in the brain, resulting in decreased anxiety and inhibition of panic attacks.

Norepinephrine is a neurotransmitter that is believed to be associated with the fight or flight stress response. It contributes to feelings of alertness, fear, anxiety and panic. Selective serotonin-norepinephrine reuptake inhibitors (SNRIs) and tricyclic antidepressants affect the serotonin and norepinephrine levels of the brain, resulting in an anti-panic effect.

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter that acts through a negative feedback system to block the transmission of a signal from one cell to another. It is important for balancing the excitation in the brain. Benzodiazepines (anti-anxiety drugs) work on the GABA receptors of the brain inducing a state of relaxation.

Neurons and Neurotransmitters Working Together

When a brain cell receives sensory information, it fires an electrical impulse that travels down the axon to the axon terminal where chemical messengers (neurotransmitters) are stored. This triggers the release of these chemical messengers into the synaptic cleft, which is a small space between the sending neuron and the receiving neuron.

As the messenger makes its journey across the synaptic cleft, several things may happen:

  1. The messenger may be degraded and knocked out of the picture by an enzyme before it reaches its target receptor.
  2. The messenger may be transported back into the axon terminal through a reuptake mechanism and be deactivated or recycled for future use.
  3. The messenger may bind to a receptor (dendrite) on a neighboring cell and complete the delivery of its message. The message may then be forwarded to the dendrites of other neighboring cells. But, if the receiving cell determines that no more of the neurotransmitters are needed, it will not forward the message. The messenger will then continue to try to find another receiver of its message until it is deactivated or returned to the axon terminal by the reuptake mechanism.

For optimal brain function, neurotransmitters must be carefully balanced and orchestrated. They are often interconnected and rely on each other for proper function. For example, the neurotransmitter GABA, which induces relaxation, can only function properly with adequate amounts of serotonin. Many psychological disturbances, including panic disorder, may be the result of poor quality or low quantities of certain neurotransmitters or neuron receptor sites, the release of too much of a neurotransmitter or the malfunctioning of the reuptake mechanisms of the neuron.

Sources:

Antidepressant Use in Children, Adolescents, and Adults. Revisions to Product Labeling. 02 May 2007 U.S. Food and Drug Administration.

Kaplan MD, Harold I. and Sadock MD, Benjamin J. Synopsis of Psychiatry, Eighth Edition 1998 Baltimore: Williams & Wilkins.

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