The nervous system is made up of two parts. Each part contains billions of neurons. The first part is the central nervous system. It contains the brain and spinal cord, which is a fibrous, ropelike structure that runs through the spinal column down the middle of the back.

The other part is the peripheral nervous system. It consists of thousands of nerves that connect the spinal cord to muscles and sensory receptors. The peripheral nervous system is responsible for reflexes, which help the body avoid serious injury. It's also responsible for the fight or flight response that helps protect you when you feel stress or danger.

Let's examine an individual neuron up close.

Here is a peripheral nerve. Each one of the nerve bundles, or fascicles, contains hundreds of individual nerve.

Here's an individual neuron, with its dendrites, axon, and cell body. The dendrites are tree-like structures. Their job is to receive signals from other neurons and from special sensory cells that tell us about our surroundings.

The cell body is the headquarters of the neuron. It contains the cell's DNA. The axon transmits signals away from the cell body to other neurons. Many neurons are insulated like pieces of electrical wire. The insulation protects them and allows their signals to move faster along the axon. Without it, signals from the brain might never reach muscle groups in the limbs.

Motor neurons are responsible for voluntary control of the muscles all over the body. The operation of the nervous system depends on how well neurons communicate. For an electrical signal to travel between two neurons, it must first be converted to a chemical signal. Then it crosses a space about a millionth of an inch wide. The space is called a synapse. The chemical signal is called a neurotransmitter.

Neurotransmitters allow the billions of neurons in the nervous system to communicate with one another. That's what makes the nervous system the body's master communicator.