Humans and animals have very similar nervous systems. Even though humans have a much more complex system, the basic functions of our nervous system is quite similar to that of animals, even leeches. Within the nervous system there are neurons. Different neurons have different functions within the nervous system. Sensory neurons tell the brain about the internal and external environment. Motor neurons contract muscles and mediate behavior. Communication neurons transmit signals from one area of the brain to another. It is important for there to be different neurons because the brain must perform many different tasks. The lab showed how neurons respond to different stimuli. For example, you would not want all of your neurons to respond to the slightest touch, such as the feather. Nor would you want all of your neurons to be able to feel the harshest touch, like the probe. Many different types of specialized neurons are needed in order to be able to pick up specific stimuli. The lab showed that leeches, and humans, have neurons that can pick up all types of different stimuli.
On the left is a picture of a X cell, which responds to no stimuli. The right a picture of a P cell, which responds only to probe. Sources: http://media.hhmi.org/biointeractive/vlabs/neurophysiology/index2.html