Neurons and Neurotransmitters: The "Brains"
of the Nervous System

The nervous system serves three functions

The nervous system is the body's control and communication network. In humans, this system:

The nervous system itself has two main parts: The central nervous system includes the brain and spinal cord, acts as a "control center." The peripheral nervous system includes all other nerve elements. These elements connect the brain and spinal cord to muscles and glands.

Neurons "power" these functions

Your body is made up of billions of cells. Cells are the basic unit of all living things. Even single-celled organisms such as bacteria can perform the basic functions needed to sustain life. These basic functions include gathering energy from food, reproducing, and producing waste material.

Nearly all cells include three parts:

  • An outer wall called a membrane.
  • A nucleus that contains essential chemicals.
  • A body of clear fluid called the cytoplasm.
Picture of Cell
An example of a cell.
Plants, animals and human beings are multicellular (many-celled) creatures. Our bodies include billions of cells that specialize in certain functions. For example, some cells become part of muscle tissue and help us to move. Other cells make up organs, glands, blood, veins, arteries, and bones.

Picture of a neuron
A nerve cell: A neuron.
To serve its three functions, the nervous system includes vast circuits of delicate cells that are elaborately interconnected. In fact, the brain, spinal cord, and nerves throughout the body are all made up of one kind of cell. These are nerve cells, also called neurons. Your brain includes billions of neurons. So does your spinal cord and all the nerves that fan out from the spinal cord to your glands, organs, and muscles.

Neurons are specialized. Their specific function is to allow our brains to learn, reason, and remember. Through the activity of neurons, the body responds and adjusts to changes in the environment. These changes, called stimuli, set off impulses in our sense organs: the eye, ear, organs of taste and smell, and sensory receptors located in the skin, joints, muscles, and other parts of the body.

Every time you feel something-including the effects of a drug-millions of neurons are "firing" messages to and from one another. Those messages consist of chemicals and electrical impulses.

Neurotransmitters cross the gaps between neurons

Each neuron may have thousands of branches that connect it to other neurons. The branches are called dendrites or axons . Dendrites carry messages toward the cell body; axons carry messages away from the cell body to another neuron. Axons extend for as long as four feet in humans. In some animals, axons are even longer.

At first, we thought that axons and dendrites simply ran through the body continuously, like wires. Then we discovered a space between each axon and dendrite. We call this space a synaptic gap, or synapse. The synapse is the space between the axon of one neuron and the dendrites of the next neuron in a nerve pathway. That gap is extremely small-about one-millionth of an inch.

Picture of neurons
The connection between neurons.

Researchers originally thought that electrical impulses jumped these gaps, like electricity jumps across the gap in a spark plug. Now we know this is not true. Chemicals-not electrical impulses-travel across the gaps. These chemicals are neurotransmitters. Today we know of about 50 neurotransmitters. Undoubtedly there are more waiting to be discovered.

Our bodies synthesize (make) neurotransmitters. Some of the chemical building blocks for neurotransmitters, such as amino acids, come from the foods we eat.

Neurons include places to store neurotransmitters. These storage areas, called vesicles, are located close to the ending of each axon. Neurons synthesize some neurotransmitters right in the vesicle. Others neurotransmitters are synthesized in the body of the cell and shipped down to the vesicle.

spacer Picture of the synapse
The synapse.

Most addictive drugs change the effect of neurotransmitters on neurons. To understand how these drugs work, we need to know about neurotransmitters and how they act as chemical "messengers."

Neurotransmitters meet three criteria

Neurotransmitters are molecules-groups of atoms, joined by a chemical bond, that act as a unit. In order to be called a neurotransmitter, a molecule must meet three criteria:

Picture of Molecule
A molecule.

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Addiction Science Research and Education Center
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