Craig C. Freudenrich
Your nervous system contains billions of cells. These cells are neurons and neuroglia (simply called glial cells). The neurons, which also are called nerve fibers, are the workhorses of the nervous system. Like wires in a building or an electronic device, they make all the connections. Neurons have the same internal organelles as other cells in your body. Unlike most cells, however, they do specialized work: Neurons carry electrochemical impulses called action potentials. Each neuron a cell body, dendrites, which receive action potentials from other cells and a long projection called an axon, which transmits the action potential to other cells. The axon is like a long, thin cable. It can be as short as a few millimeters or as long as a meter or more. For example, neurons in your legs have axons that extend from the tip of your toe up the entire length of your leg into your spinal cord.
Neurons can be classified based on their functions. Sensory neurons (afferent neurons) either sense information themselves or receive information from sensory receptors. The neurons transmit sensory information to the central nervous system (CNS), which is the brain and spinal cord. The information moves through cranial or spinal nerves. Motor neurons (efferent neurons) carry information from the CNS to muscle cells through cranial or spinal nerves. Motor neurons of the sympathetic nervous system carry information from the CNS to glands and other organs through sympathetic nerves. Interneurons are short nerve fibers that connect nearby neurons within the brain and spinal cord.
Glial cells provide support functions for many neurons. There are several types of glial cells. Some are found in the CNS, and others are found in the peripheral nervous system:
- Microglia cells -- in the CNS to protect neurons from bacteria
- Astrocytes -- help CNS neurons form the blood-brain barrier that protects the CNS from harmful chemicals
- Oligodendrocytes -- CNS glial cells that make myelin, a fatty substance that insulates the axon and increases the speed at which neurons transmit impulses
- Ependymal cells -- make the cerebrospinal fluid that surrounds and cushions the CNS
- Schwann cells -- in the peripheral nervous system to make myelin around axons in peripheral nerves and help axons regenerate when damaged
- Satellite cells -- peripheral nervous system glial cells that regulate the exchange of material between neurons and the surrounding fluid
Glial cells are different from neurons, but they help protect and even insulate them.
Neurons, or nerve cells, are the basic units of the nervous system. Though the type, shape and size of a neuron vary, each cell has three parts: a dendrite, cell body and axon. Generally speaking, dendrites receive impulses from a sensory receptor or another neuron. According to Dr. Edwin Griff, associate professor of neurobiology at Purdue University, the message then moves through the cell body and along the axon to the spinal cord and the brain. It's hard to tell how many neurons exist in the average human body, but one source estimates that the brain alone consists of at least 100 billion neurons [source: Chudler]. Glial cells, on the other hand, serve a more complementary purpose. Some of these cells surround neurons, creating a layer around the axon that can increase the speed of a message.
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