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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:
However, neurons differ from other cells in the body because:
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.
|"Neuron" | "Axon" | "Dendrite" ||
|"Nissl" | "Mitochondria" | "Endoplasmic reticulum"|
There are several differences between axons and dendrites:
What is inside of a neuron? A neuron has many of the same organelles such as mitochondria, cytoplasm and a nucleus, as other cells in the body.
Did you know?
Neurons are the oldest and longest cells in the body! You have many of the same neurons for your whole life. Although other cells die and are replaced, many neurons are never replaced when they die. In fact, you have fewer neurons when you are old compared to when you are young. On the other hand, data published in November 1998 show that in one area of the brain (the hippocampus), new neurons CAN grow in adult humans.
Neurons can be quite large - in some neurons, such as corticospinal neurons (from motor cortex to spinal cord) or primary afferent neurons (neurons that extend from the skin into the spinal cord and up to the brain stem), can be several feet long!Happy 117th Birthday to the Golgi apparatus! In 1898, the famous neuroanatomist Camillo Golgi reported his discovery of a ribbon-like apparatus inside neurons of the cerebellum. This structure now bears his name as the "Golgi apparatus."
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