At any given time, our nervous system faces an enormous signal control task. Acting as the command centre for the entire body, it is charged with generating and processing the host of different messages sent through nerve cells that allow us to move, think and respond to any given stimulus. Managing this constant and complex network of signals relies on two basic principles formulated by the 1932 Nobel Laureates in Physiology or Medicine.

Charles Sherrington revealed how nerves co-ordinate movement by looking at simple reflexes; for instance, the way that a sharp tap on the knee causes the leg to jerk automatically and uncontrollably. By painstakingly examining thousands of these reflex pathways in animals, Sherrington showed how nerves inform muscles in limbs to contract or relax. Muscles contract when they are actively excited by signals coming from nerve cells, while muscles relax when nerve cells that control them are inhibited instead. Sherrington concluded that the interplay of these opposite excitatory and inhibitory signals generally coordinates and fine-tunes the postures and movements of our body, with each nerve cell somehow able to sum up the thousands of contradicting messages it receives and integrate them into one definite order issued within milliseconds.

More than two decades after Sherrington’s observations, Edgar Adrian used dramatically improved measurement techniques to decipher the form that this issued order takes in each nerve cell. Adrian dissected the breast muscle of frogs until it contained only one receptor that responds to a stimulus and a single nerve fibre, and he recorded and amplified the electrical impulses running through this fibre in response to touching or stretching the muscle. Amazingly, he discovered that the explosive waves of impulses discharged along the nerve are always the same size, regardless of how strong the stimulus is. As stronger stimulations only increase the number of impulses that are produced every second, Adrian deduced that nerve fibres must have an all-or-nothing response. If the electrical signal it receives reaches a certain threshold level at a particular time, a nerve fibre will respond by firing its impulse. If the threshold is not reached, there will be no firing order given out at all.