The nervous system is a vast, complex communication network. In many ways, our nervous system is like the man-made communication systems we're all familiar with. How does cable television reach your home? Electrical signals are sent across a network of wires. Your television interprets those electrical signals, enabling you to see your favorite programs.

Our nervous system works in a similar manner. Electrical signals (nerve impulses, or action potentials) are sent across a network of wires (neurons). These impulses are interpreted by the brain and by our arms, legs, and vital organs. The animation in this activity provides an example. The needle prick starts a neural signal that is transmitted via neurons to the brain. The brain interprets this signal as pain and sends a neural signal as a response to the hand; the muscles in the hand interpret the neural signal as an instruction to move the hand away from the needle.

The rest of this activity presents some of the details of how signals are transmitted within and between neurons. There are two important things to notice. First, there is no such thing as a "strong" or a "weak" action potential. We refer to this aspect of neural activity as the all-or-none law. A stimulus is either strong enough to elicit an action potential ( all ) or it is too weak to produce any neural response ( none ). Second, neurons do not communicate with one another directly. There are gaps, called synapses, between neurons. The electrical signal within the neuron is transmitted across these gaps by chemicals that are released into the synapse. These chemicals are called neurotransmitters.