Now, let's begin by recalling some physics. Now, if we study the movement of the action potential along the axon from a physics perspective, we see that the action potential is nothing more than a moving electric current and the axon is nothing more than a biological wire. So recall that what determines the velocity with which the current moves inside the wire is the resistance of that wire to the flow of that current. Now, resistance itself depends on three important factors. So recall that the resistance of an electric current moving inside a wire depends on the cross sectional area of that wire, its thickness. It depends on the length of that wire, and it also depends on an internal property known as the resistivity, which basically depends on the type of material that that wire is made of.