This chapter will cover the phenomenon of excitability or the ability of certain cells to respond to electrical stimuli by undergoing rapid changes in membrane potential so that the polarity reverses; in other words, a cell which is normally positive on the outside becomes momentarily negative (for a particular region of the cell). This reversal of polarity is a basis of communication within different parts of a nerve or a muscle cell. The basis for communication from nerve to nerve or from nerve to muscle cell, involves information being transferred across a specialized gap or synapse between adjacent cells. This is concerned with the conversion of electrical signals into a chemical signal and then back to an electrical signal (although in some specialized synapses, there is no chemical step).