This book is intended for undergraduates studying the biological and medical sciences. The field of excitable cell physiology is one which is found quite baffling by a significant minority of these students. My aim here is to provide a brief introductory account, based on a conceptual approach, rather than on a mathematical or historical description. Once the student has grasped certain basic ideas concerning excitable cell function, the individual examples which follow fit into a well-defined pattern. No attempt has been made to give credit in appropriate measure to the many scientists who have contributed to this field. The further reading cited has been chosen with the reader alone in mind. I would like to thank Tim Cripps for help and advice, also Charlie Tomson and Michael Hart for their careful reading of the manuscript. Finally I am indebted ·to Jenny Kenyon for her excellent typing of the work. Part One THE CONCEPT OF EXCITABILITY INTRODUCTION 1 1.1 The Excitable Tissues living organisms are able to respond to changes in their environment.