Every person’s body contains billions of nerve cells (neurons). There are about 100 billion in the brain and 13.5 million in the spinal cord. The body’s neurons take up and send out electric and chemical signals (electrochemical energy) to other neurons.

Neurons receive signals in a short antennae-like part called the dendrite, and send signals to other neurons with a long cable-like part called the axon. An axon can be up to a meter long.

In some neurons, axons are covered with a thin layer of fat called myelin, which acts as an insulator. It helps transmit nerve signals, or impulses, down a long axon. The main part of a neuron is called the cell body. It contains all of the important parts of the cell that allow it to function properly.

Neurons come in a variety of shapes and sizes depending on where they’re located in the body and what they’re programmed to do. Sensory neurons have dendrites on both ends and are connected by a long axon that has a cell body in the middle. Motor neurons have a cell body on one end and dendrites on the other end, with a long axon in the middle.

There are four types of neurons:

• Sensory: Sensory neurons deliver electrical signals from the outer parts of the body — the glands, muscles, and skin — into the CNS.
• Motor: Motor neurons carry signals from the CNS to the outside parts of the body.
• Receptors: Receptor neurons sense the environment (light, sound, touch, and chemicals) around you and convert it into electrochemical energy that is sent by sensory neurons.
• Interneurons: Interneurons sends messages from one neuron to another.