In general, the greater the intensity of a stimulus, (whether it be a light stimulus to a photoreceptor, a mechanical stimulus to the skin, or a stretch to a muscle receptor) the greater the number of action potentials elicited. Rather, the frequency or the number of action potentials increases. When the intensity of the stimulus is increased, the size of the action potential does not become larger. Third, nerve cells code the intensity of information by the frequency of action potentials. Second, nerve action potentials are elicited in an all-or-nothing fashion. First, the nerve action potential has a short duration (about 1 msec). The recordings in the figure above illustrate three very important features of nerve action potentials. Action potentials are the basic events the nerve cells use to transmit information from one place to another. These spike-like events are called action potentials, nerve impulses, or sometimes simply spikes. Very dim lights produced no changes in the activity, but brighter lights produced small repetitive spike-like events. (By placing electrodes on the surface of a nerve, it is possible to obtain an indication of the changes in membrane potential that are occurring between the outside and inside of the nerve cell.) Then 1-s duration flashes of light of varied intensities were presented to the eye first dim light, then brighter lights. Electrodes were placed on the surface of an optic nerve. Hartline 80 years ago on electrical signaling in the horseshoe crab Limulus. One example is the pioneering work of H.K. The issue was not resolved until the 1930s with the development of modern electronic amplifiers and recording devices that allowed the electrical signals to be recorded. However, there was debate among scholars whether the electricity was within nerves and muscle or whether the nerves and muscles were simply responding to the harmful electric shock via some intrinsic nonelectric mechanism.

A major paradigm shift occurred with the pioneering work of Luigi Galvani who found in 1794 that nerve and muscle could be activated by charged electrodes and suggested that the nervous system functions via electrical signaling (see this animation of Galvani's experiment). The basic theory held for centuries and was further elaborated by René Descartes (1596 – 1650) who suggested that animal spirits flowed from the brain through nerves and then to muscles to produce movements (See this animation for modern interpretation of such a hydraulic theory for nerve function).

Theories of the encoding and transmission of information in the nervous system go back to the Greek physician Galen (129-210 AD), who suggested a hydraulic mechanism by which muscles contract because fluid flowing into them from hollow nerves.

Tap the colored circles (light stimulus) to activate.

Speed of neurons in the brain