To understand how the nervous system can communicate and respond, seemingly instantly, we must first understand the most basic unit of this complex organization. These units consist of two neurons in communication, sending a message from one to the other. The presynaptic neuron is the message sender, while the postsynaptic neuron is the message reciever. The synapse is the small space between the axon of one neuron and the dendrites, cell body, or, rarely, the axon of another neuron. The presynaptic neuron lies on the side of the synapse in which the message is being sent, while the postsynaptic neuron lies on the side of the synapse that is recieving that particular message. The terms are a convention used to facilitate the understanding of what is happening in neural communication and as such are not a classification of neuron, since the majority of all neurons are simultaneously acting as both presynaptic and postsynaptic (senders and recievers of neural messages) neurons. In fact neuron generally have 1000-10,000 axonal terminals, and usually as many dendritic terminals as well.
As a nerve cell is stimulated (at the dendrites) it produces a graded potential, which changes the membrane potential. If this change in membrane potential causes the axon hillock to reach threshold, an action potential will be generated and flow down the axon. Once it reaches it's terminal, it causes the stimulation of the next nerve (or effector cell).To review the general order is as follows: stimulation, graded potential, action potential, stimulation of the next cell. The next two sections will describe graded and action potentials, followed by a third, on the synapse, the actual place where neuron to neuron communication occurs.