Nervous system: neurons

You touch a hot pan and pull your hand away before you can even think "ouch." The messages that made this possible traveled through cells called neurons. Your brain alone has roughly 86 billion of them, and more run throughout your body. Neurons are the basic units of the nervous system, specialized for one thing: carrying signals quickly.

A neuron has three main parts. The cell body contains the nucleus and keeps the cell alive. Dendrites are short, branching fibers that receive signals from other cells. The axon is a long fiber that carries the signal away from the cell body toward the next cell. Some axons are very long; a single axon can stretch from your spine all the way to your toe. Many axons are wrapped in a fatty coating called the myelin sheath, which acts like insulation on a wire and makes signals travel much faster, up to about 100 meters per second.

A signal in a neuron is an electrical impulse. It travels down the axon in a wave, and it is all-or-nothing: either the neuron fires or it does not. But neurons do not touch each other. Between the end of one axon and the next cell is a tiny gap called a synapse. When the impulse reaches the end of the axon, the neuron releases chemicals called neurotransmitters, which cross the gap and attach to the next cell. This may start a new impulse there. A message is therefore electrical along a neuron and chemical across a synapse.

There are three types of neurons. Sensory neurons carry signals from sensors in your skin, eyes, ears, and other places toward the brain and spinal cord. Motor neurons carry commands from the brain and spinal cord to muscles and glands. Interneurons sit between them, inside the brain and spinal cord, and process information.

Common mistake: picturing neurons as plain electric wires. A wire carries current continuously, while a neuron sends brief impulses and uses chemical messengers across synapses. Another mistake is thinking all neurons are the same. Their structure fits their function: some are tiny, some are a meter long, and some are specialized for a single type of sensation. Next: the brain and spinal cord.