To more easily understand how cells operate, we need to review a few definitions of words.  The first is neuron.  A neuron is another name for a nerve cell.  A neuron has an axon– a thread-like part of the cell that sends signals from the cell body.  The neuron also has dendrites–a part of the cell that receives signals from other neurons. The axon of one cell does not necessarily touch the dendrites of another cell. This space between the two cells is called the synapse. The following picture illustrates this:

The brain has been compared to the world’s most sophisticated computer, which processes information by sending electric current not through circuits but through the brain’s estimated 100 billion cells.  Information is relayed from cell to cell using electricity and neurotransmitters.  Information about emotions, behavior, body temperature, appetite, or many other functions is sent, depending on what part of the brain is stimulated.

THE BLOOD-BRAIN BARRIER

The blood-brain barrier protects the brain by keeping many substances, including toxins, away from the brain.  Most drugs do not penetrate the blood-brain barrier;only certain drugs that are fat soluble can penetrate the blood-brain barrier.

NEUROTRANSMITTER

A neurotransmitter is a chemical that is stored in vesicles—hollow sac-like structures in the cells. When stimulated, the cell generates an electrical signal and causes some vesicles to release their neurotransmitters into the synapse, which attach to a receptor for that particular neurotransmitter on another cell.  This picture shows the process:

A receptor is a membrane protein on a cell that is able to bind a specific chemical substance. The receptor receives a specific signal from another cell which then produces a certain effect—like the production of endorphins for opioid/opiate receptors.  The receptor binds the specific neurotransmitter but does not bind other neurotransmitters. Generally, when neurotransmitters reach a cell’s receptors, they act like a key fitting into a lock. Channels are opened through which electrically charged particles called ions flow into or out of the cell.  Each individual cell produces one or more types of neurotransmitters. One type of neurotransmitter, dopamine, is known to affect motivation and feelings of reward in the brain.

Opening of the channels changes the electrical charges inside and outside the cell and determines if the nerve cell fires–sends an electrical signal down the axon that releases neurotransmitters into the synapse–or becomes less active. The result of this transfer of a neurotransmitter to the receptor on a cell may be increased production of a chemical like endorphins or it may be relaying something from one cell to the next.  For example, if the order is for your hand to move, then all the cells along the way carry the message to have the hand move.

Once they have activated the receptors, the neurotransmitters break away from the receptors and either:
•    Attach to another receptor;
•    Are broken down by an enzyme (estimated to be the fate of about 10% of neurotransmitters);
•    Return to the sending cell and are available to be released again (a process called reuptake).

A way to visualize the reuptake process is to think of a small wooden paddle to which is attached a rubber ball on a rubber string.  When you hit the ball it goes out a ways, seems to hang in the air and then returns to the paddle.  This is similar to what a neurotransmitter does.  It leaves one cell and goes to the receptors on another cell, stays a moment and then returns to the originating cell.  This process is continually taking place in the neurons.

HOW THE BRAIN COMMUNICATES TO THE BODY

The instructions from the brain are relayed through the cells.  As a person drives down the road, the images he sees:
•    Enter the brain through the eyes;
•    Are  converted into information that is relayed, from cell to cell, to regions that process visual input and attach meaning and memory to it;
•    Inside the cells, the information takes the form of an electrical signal;
•    To cross the synapse to the next cell, the information takes the form of a chemical signal;
•    The chemicals that carry the signals across the synapses are called neurotransmitters.

INHIBITORY AND EXCITATORY NEUROTRANSMITTERS

Neurotransmitters can be inhibitory–making it less likely that the receiving neuron will fire and send a signal down its axon and on to other neurons, or excitatory–making it more likely that a signal will be sent down the axon of the receiving neuron to other neurons in the network. Neurons are continuously receiving excitatory and inhibitory signals, and the sum of all these inputs causes the cell to send or not send a signal to the next neuron.

Excitatory neurotransmitters are vital to help us stay alert, maintain our normal memory functions, our co-ordination and normal emotional responses, our heart rate, and our blood pressure. If there is something that creates anxiety or a feeling of panic or other stress, more excitatory neurotransmitters are released and a person can feelrestlessness, higher than normal irritability, rapid heartbeats, high blood pressure, insomnia, or even have seizures.

In most situations, if we feel this anxiety or stress, inhibitory neurotransmitters are released and this calms everything down.

However, if too many inhibitory neurotransmitters are activated and stay activated, this can lead to conditions like inability to concentrate, memory loss, feelings of having no energy, sleep disorders, and many scientists now believe, Parkinson’s disease.

DRUGS AND NEUROTRANSMITTERS

Drugs can primarily affect one neurotransmitter or class of neurotransmitters, but they often affect several neurotransmitters.  For example, opioid drugs resemble the brain’s natural endorphins and stimulate many more receptors than would be stimulated normally.  This results in higher levels of endorphins being released.  Increasing endorphin production leads to increased analgesia (inability to feel pain), reduced alertness, and slowed respiration.

In many cases, a neurotransmitter stimulates or inhibits a cell and that causes the cell to produce a different neurotransmitter.  For example, nicotine stimulates dopamine-releasing cells and increases the supply of glutamate, a neurotransmitter that acts as an accelerator for cell activity throughout the brain.

After a neurotransmitter molecule binds to its receptor on the postsynaptic neuron, it comes off (is released from) the receptor and diffuses back into the synaptic space. The released neurotransmitter, as well as any neurotransmitter that did not bind to a receptor, is either degraded by enzymes in the synaptic cleft or taken back up into the presynaptic axon terminal by active transport through a transporter or reuptake pump. Once the neurotransmitter is back inside the axon terminal, it is either destroyed or repackaged into new vesicles that may be released the next time an electrical impulse reaches the axon terminal.

Different neurotransmitters are inactivated in different ways.

NEUROTRANSMITTERS IN THE BODY

The main neurotransmitters in the body and their functions are:

Acetylcholine – Acetylcholine generally is an excitatory neurotransmitter and is used to cause muscle contraction and by the brain to regulate memory.
Dopamine – Dopamine generally acts as an inhibitory neurotransmitter and is released by the brain’s reward system and produces feelings of pleasure but also has other functions.

GABA (Gamma-Aminobutyric Acid) – Gaba is the the major inhibitory neurotransmitter in the brain and is important in producing sleep, reducing anxiety, and forming memories.
Glutamate – Glutamate is necessary for learning and memory and is an excitatory neurotransmitter in the brain.

Glycine – Glycine is primarily an inhibitory neurotransmitter.

Norepinephrine – Norepinephrine is normally excitatory but is sometimes inhibitory. It is an integral part of the fight-or-flight response.  It also regulates blood pressure and calmness.

Serotonin – Serotonin affects a person’s sensory perception, appetite and mood.  Serotonin can act as an inhibitory neurotransmitter primarily in dealing with pain.