The ability to record the real-time movement of a protein is quite enthralling; in effect, this is what we achieve when we record the activity of an individual ion channel. Clearly we do not “see” the amino acids that make up the ion channel undergo the conformational changes that must occur, but we observe the consequences—the current flow through the ion channel pore. The currents that flow are very small, a few picoamperes (10⁻¹² A), and, in the case of the nicotinic acetylcholine receptor found at the muscle end plate, for example, they represent the net movement of about 25,000 monovalent ions per millisecond through the channel pore. These currents are recorded using an electrophysiological technique termed patch-clamp recording. The word patch is self-evident—we record ion channel activity from a small area of membrane. The word clamp refers to the fact that we maintain this membrane at a constant potential (for a description of the electronics that achieve the voltage clamp, see Chapter 2).