What is a neuron

The basic building blocks of the nervous system are called nerve cells, or neurons. A neuron can be defined as a cell that processes information and transmits information by means of electrochemical impulses. Each neuron has three major functions: receiving messages, responding to them, and sending messages. These functions are handled through three different structures: the dendrites, the cell body, and the axon. The neuron receives electrical and chemical messages from other cells through its dendrites, responds to that information in some way in the cell body, and sends messages on to other neurons through the axon.

More specifically, the dendrite is the part of a neuron that usually receives electrical and chemical messages from other nerve cells. The word dendrite comes from the Greek word for trees, the cell body is connected to many dendrites, which extend from it like the branches of a tree. The cell body is the central part of the neuron that responds to incoming information and manufactures chemicals required for the cell's nutrition and normal function. A single axon usually transmits electrical impulses away from the cell body to other neurons.

Electrical messages ordinarily travel in only one direction: Information comes into a cell through a dendrite, is processed in the cell body, and passes through the axon. When we say that "information is processed" in the cell body, we mean that the cell body analyzes the information being communicated by the dendrite activity, and, based on this analysis, either sends a particular kind of electrochemical message or does not respond. Ordinarily, information is transmitted from the axon of one cell to the dendrite of another. However, in some cases, an axon transmits information directly to the cell body of another neuron. Either way, the signal is transmitted at the synapse, the space across which the axon of one neuron can stimulate the dendrite or cell body of another. Neurons communicate with one another at the synapse in a complex electrochemical code that scientists are just beginning to understand.

In one sense, the neuron can be considered a miniature battery that communicates with other neurons by firing, or sending a small electrical charge along the length of its axon. This process is called electrochemical to distinguish the movement of this charge from the movement of an electrical current, such as the one that flows through the power cord of your VCR. The electrical charge in the neuron travels down the axon by means of a chemical process that is different from, and much slower than, a household electrical current.

The firing of a neuron is said to obey the "all-or-none principle"; that is, at any given moment, a neuron either transmits its maximum electrical charge, or it does not transmit any electrical charge at all. Some scientists have compared the firing of a neuron to the firing of a rifle. If you pull the trigger, the rifle either fires or it does not; there is no middle ground. A neuron can send a message by changing the pattern and rate of this firing. In one study of the visual system of the frog, for example, a particular color-sensitive neuron fired about 20 times per second when the frog looked at a blue stimulus, 7 times per second for a blue-green stimulus, and 3 times per second for a green stimulus.