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
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.