We discussed the nervous system of the human body and the nervous system uses a special type of chemical known as the neurotransmitter in intracellular communication. So the neurotransmitter basically travels over a very short distance basically across the synapse between the neuron and our adjacent cell. And the neurotransmitter basically only binds to specific cells and it creates a very quick and rapid response. But this response is relatively short lived. It only takes place over several seconds. Now, we're going to discuss a different type of system.