because they are electrical impulses. Think about flipping on a light switch when you enter
a room. When you flip the switch, it closes an electrical circuit. With the circuit closed,
electricity can flow to the light through wires inside the walls. The electricity may have to
travel many meters to reach the light, but the light still comes on as soon as you flip the
switch. Nerve impulses travel equally fast through the network of nerves inside the body.
Figure 20.2: The axons of many neurons, like the one shown here, are covered with a fatty
layer called myelin sheath that insulates the axon like the plastic covering on an electrical
wire, and allowing nerve impulses to travel faster along the axon. ( 19 )
What Does a Neuron Look Like?
A neuron has a special shape that lets it pass signals from one cell to another. As shown
inFigure 2, a neuron has three main parts: cell body, dendrites, and axons. Thecell
bodycontains the nucleus and other organelles. Dendrites and axons project from the
cell body. Dendritesreceive nerve impulses from other cells, and axonspass the nerve
impulses on to other cells. A single neuron may have thousands of dendrites and axons, so
it can communicate with thousands of other cells.
Types of Neurons
Neurons are usually classified based on the role they play in the body. Two types of neurons
are sensory neurons and motor neurons.
- Sensory neuronscarry nerve impulses from sense organs and internal organs to the
central nervous system (see below).