Types of Neurons (Nerve Cells)
The human body is made up of trillions of cells. Cells of the nervous system, called nerve cells or neurons. are specialized to carry "messages" through an electrochemical process. The human brain has approximately 100 billion neurons. To learn how neurons carry messages, read about the action potential .
Neurons come in many different shapes and sizes. Some of the smallest neurons have cell bodies that are only 4 microns wide. Some of the biggest neurons have cell bodies that are 100 microns wide. (Remember that 1 micron is equal to one thousandth of a millimeter!).
Neurons are similar to other cells in the body because:
- Neurons are surrounded by a cell membrane.
- Neurons have a nucleus that contains genes.
- Neurons contain cytoplasm, mitochondria and other organelles.
- Neurons carry out basic cellular processes such as protein synthesis and energy production.
However, neurons differ from other cells in the body because:
- Neurons have specialize cell parts called dendrites and axons. Dendrites bring electrical
signals to the cell body and axons take information away from the cell body.
- Neurons communicate with each other through an electrochemical process.
- Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).
One way to classify neurons is by the number of extensions that extend from the neuron's cell body (soma).
Bipolar neurons have two processes extending from the cell body (examples: retinal cells, olfactory epithelium cells).
Pseudounipolar cells (example: dorsal root ganglion cells). Actually, these cells have 2 axons rather than an axon and dendrite. One axon extends centrally toward the spinal cord, the other axon extends toward the skin or muscle.
Multipolar neurons have many processes that extend from the cell body. However, each neuron has only one axon (examples: spinal motor neurons, pyramidal neurons, Purkinje cells).
Neurons can also be classified by the direction that they send information.
Check out the Gallery of Neurons to see some pictures of real neurons or "Sidewalk Cells" to see photographs of neurons on the street.