 Dear students, in this module, we shall discuss the speed of propagation of action potentials. First, we shall compare the speed of propagation of action potential with the speed of electric current. Dear students, you know that action potential is an electrochemical current that is produced because of the flow of ions which are charged particles and flow through the ion channel proteins. This kind of current is comparatively slower as compared to the electric current that is produced because of the flow of electrons. In invertebrates, the axons have a speed of propagation. If the axons are narrow, they have a flow of a few centimeters per second. Whereas in invertebrates, larger axons, like giant axons of squids or mollusks, they have a speed of propagation of 30 meters per second, which is significantly less than speed of light or speed of electric current. In invertebrates, as the axons are malinated, so the speed of conduction of action potential is comparatively high. The speed of action potential in the larger axons of vertebrates is about 120 meter per second. However, in small diameter, non-malinated axons of vertebrates, the speed may be low and it may range about 0.25 meter per second. Dear students, speed of propagation of action potential is related to the length constant of an axon. In this way, if the length constant is higher, then the speed of propagation is higher. That is because of the reason that resistance to electrical current flow is inversely proportional to the cross-sectional area. That is why the animals have a trend to increase the length constant. Wider axons of animals conduct action potentials more rapidly than the narrow axons with lower diameters. Dear students, during the evolution process, the length constant is very important. The reason is that when we look at the development or evolution of animals' nervous system, we see that there had been an evolutionary trend towards increasing the diameter or thickness of the neurons. As a result of this, the length constant is also increased due to increase in thickness. In vertebrates, the axonal diameter increased that actually reduced the internal longitudinal resistance to ionic flow. As a result, the length constant of action potentials increased. In vertebrates, when there is an evolution of vertebrates, the diameter increases in vertebrates and there are two more mechanisms which increase the length constant. First factor in vertebrates that contributes to the increase in the speed of propagation is axon myelination. And the second factor that contributed in this manner was the formation of nerves which have tens of thousands of nerve fibers in a single nerve.