The Neuron

Very Special(ized) Cells

Your nervous system is composed of specialized cells called neurons. The structure of a neuron can give you a clue to its function. Each has a compact cell body and long, slender processes - a little like antennae. The processes that pick up messages are called dendrites. Those that conduct messages to the next cell are called axons.

Let's see how a message travels down an axon. The neuron has the special ability to build up a charge - much like a battery - across its membrane. More sodium ions (Na⁺) and potassium ions (K⁺) build up outside the membrane, so the inside of the cell is more negative. If you could put a microscopic meter on each side of the membrane, you could measure the potential energy (about -70 millivolts), like a tiny battery.

When the neuron is stimulated, sodium ions can enter the cell. The potential energy (voltage) across the membrane drops. Even though the cell membrane quickly begins to restore the ions to their proper position, this change, in turn, affects the membrane next to it. Like fire along a fuse, the electrical change moves down the axon. By the time the membrane restores the charge across the membrane at one point, the signal is moving ahead.

Axons are the longest parts of any cell. A single axon can be only a few micrometers or as long as a meter. But eventually the electrical signal reaches the end. The axon of one neuron doesn't touch the dendrites of the next neuron. Instead there is a tiny gap called a synapse. A chemical neurotransmitter is released from the end of the axon, which jumps the gap and starts the process all over again, stimulating the next nerve cell.

How fast does this occur? Don't be confused by the word "electric." Unlike electrons in a conductor (which move at nearly the speed of light) the messages that are sent by movement of charged ions move at about 100 m/second.