Sir Andrew Fielding Huxley OM PRS (22 November 1917 – 30 May 2012) was an English physiologist and biophysicist. He was born into the prominent Huxley family. After graduating from Westminster School in Central London, from where he won a scholarship to Trinity College, Cambridge, he joined Alan Lloyd Hodgkin to study nerve impulses. Their eventual discovery of the basis for propagation of nerve impulses (called an action potential) earned them the Nobel Prize in Physiology or Medicine in 1963. They made their discovery from the giant axon of the Atlantic squid. Soon after the outbreak of the Second World War, Huxley was recruited by the British Anti-Aircraft Command and later transferred to the Admiralty. After the war he resumed research at The University of Cambridge, where he developed interference microscopy that would be suitable for studying muscle fibres. In 1952, he was joined by a German physiologist Rolf Niedergerke. Together they discovered in 1954 the mechanism of muscle contraction, popularly called the "sliding filament theory", which is the foundation of our modern understanding of muscle mechanics. In 1960 he became head of the Department of Physiology at University College London. He was elected a Fellow of the Royal Society in 1955, and President in 1980. The Royal Society awarded him the Copley Medal in 1973 for his collective contributions to the understanding of nerve impulses and muscle contraction. He was conferred a Knight Bachelor by Queen Elizabeth II in 1974, and was appointed to the Order of Merit in 1983. He was a fellow of Trinity College, Cambridge, until his death. Huxley was born in Hampstead, London, England, on 22 November 1917. He was the youngest son of the writer and editor Leonard Huxley by Leonard Huxley's second wife Rosalind Bruce, and hence half-brother of the writer Aldous Huxley and fellow biologist Julian Huxley, and grandson of the biologist T. H. Huxley. When he was about 12, Andrew and his brother David were given a lathe by their parents. Andrew soon became proficient at designing, making and assembling mechanical objects of all kinds, from wooden candle sticks to a working internal combustion engine. He used these practical skills throughout his career, building much of the specialized equipment he needed for his research. It was also in his early teens that he formed his lifelong interest in microscopy. He was educated at University College School and Westminster School in Central London, where he was a King's Scholar. He graduated and won a scholarship to Trinity College, Cambridge, to read natural sciences. He had intended to become an engineer but switched to physiology after taking the subject to fulfill an elective. Having entered Cambridge in 1935, Huxley graduated with a bachelor's degree in 1938. In 1939, Alan Lloyd Hodgkin returned from the US to take up a fellowship at Trinity College, and Huxley became one of his postgraduate students. Hodgkin was interested in the transmission of electrical signals along nerve fibres. Beginning in 1935 in Cambridge, he had made preliminary measurements on frog sciatic nerves suggesting that the accepted view of the nerve as a simple, elongated battery was flawed. Hodgkin invited Huxley to join him researching the problem. The work was experimentally challenging. One major problem was that the small size of most neurons made it extremely difficult to study them using the techniques of the time. They overcame this by working at the Marine Biological Association laboratory in Plymouth using the giant axon of the longfin inshore squid (Doryteuthis (formerly Loligo) pealeii), which have the largest neurons known. The experiments were still extremely challenging as the nerve impulses only last a fraction of a millisecond, during which time they needed to measure the changing electrical potential at different points along the nerve. Using equipment largely of their own construction and design, including one of the earliest applications of a technique of electrophysiology known as the voltage clamp, they were able to record ionic currents. In 1939, they jointly published a short paper in Nature reporting on the work done in Plymouth and announcing their achievement of recording action potentials from inside a nerve fibre. Then World War II broke out, and their research was abandoned. Huxley was recruited by the British Anti-Aircraft Command, where he worked on radar control of anti-aircraft guns. Later he was transferred to the Admiralty to do work on naval gunnery, and worked in a team led by Patrick Blackett. Hodgkin, meanwhile, was working on the development of radar at the Air Ministry. When he had a problem concerning a new type of gun sight, he contacted Huxley for advice. Huxley did a few sketches, borrowed a lathe and produced the necessary parts. Huxley was elected to a research fellowship at Trinity College, Cambridge, in 1941. In 1946, with the war ended, he was able to take this up and to resume his collaboration with Hodgkin on understanding how nerves transmit signals. Continuing their work in Plymouth, they were, within six years, able to solve the problem using equipment they built themselves. The solution was that nerve impulses, or action potentials, do not travel down the core of the fiber, but rather along the outer membrane of the fiber as cascading waves of sodium ions diffusing inward on a rising pulse and potassium ions diffusing out on a falling edge of a pulse. In 1952, they published their theory of how action potentials are transmitted in a joint paper, in which they also describe one of the earliest computational models in biochemistry. This model forms the basis of most of the models used in neurobiology during the following four decades. In 1952, having completed work on action potentials, Huxley was teaching physiology at Cambridge and became interested in another difficult, unsolved problem: how does muscle contract? To make progress on understanding the function of muscle, new ways of observing how the network of filaments behave during contraction were needed. Prior to the war, he had been working on a preliminary design for interference microscopy, which at the time he believed to be original, though it turned out to have been tried 50 years before and abandoned. He, however, was able to make interference microscopy work and to apply it to the problem of muscle contraction with great effect. He was able to view muscle contraction with greater precision than conventional microscopes, and to distinguish types of fiber more easily. By 1953, with the assistance of Rolf Niedergerke, he began to find the features of muscle movement. Around that time, Hugh Huxley and Jean Hanson came to a similar observation. Authored in pairs, their papers were simultaneously published in the 22 May 1954 issue of Nature.Thus the four people introduced what is called the sliding filament theory of muscle contractions. Huxley synthesized his findings, and the work of colleagues, into a detailed description of muscle structure and how muscle contraction occurs and generates force that he published in 1957. In 1966 his team provided the proof of the theory, and has remained the basis of modern understanding of muscle physiology.