The membrane electrical potential is a central element in the functioning of all living cells, and temporal variations of this potential are crucial for signaling within the nervous systems of all animals. Electrical properties of nerve cells are covered in neurobiology courses, but because electrical concepts are complex and textbook illustrations are static, students often find neurophysiology intimidating and inaccessible. In order to make neurophysiology less formidable and more accessible, we developed a comprehensive approach that begins with the elements of electrical circuits. This approach is found in both this book and in NDX II, a modeling system that is available for free online and is largely platform independent. This book, together with its associated computer simulation lessons, teaches students about neurophysiological concepts, and then the online software further expands their knowledge through modeling experiments. These hands-on simulations serve to deepen students" understanding of basic neurophysiological techniques, including intracellular recording and voltage clamp. NDX II simulates the dynamic properties of neurons at several organizational levels: the membrane patch, neuronal compartments (dendrite, soma, neurite, and axon), individual neurons, and synaptic interactions within neuronal circuits (up to 100 neurons). NDX II models present experimental results dynamically, in the sense that the results are displayed as they are generated, providing a sense of experimental verisimilitude. Based on a user-friendly, highly accessible graphics interface, the models encourage active exploration of physiological properties through the manipulation of model parameters. NeuroDynamix II embodies a tight interdependence between didactic text and online software. Section I of the book provides explicit, illustrated introductions to electrical concepts, properties of ion channels, resting and action potentials, synaptic interactions, and neuronal circuits. Each didactic subsection concludes with detailed modeling "lessons" that preconfigure NDX II models to illustrate and explore neurophysiological principles. Section II provides brief descriptions of seven integrated models, with complete glossaries of variable and parameter names and units. Sections III and IV furnish detailed descriptions of the equations for the models and summarize numerical methods. The book concludes with a brief computer guide and a bibliography.