Impulse traveling in a giant unmyelinated axon

In the lower graph (voltage-vs-space) of this movie, one sees an action potential (AP) traveling from left to right along a length (1 cm) of the squid giant axon. The AP is conducting at high speed. Four color-coded recording electrodes, spaced along the axon as indicated, record the AP as a function of time (color-coded recordings shown in upper graph). A stimulating electrode at the axon's left end triggers the AP with a short sharp shock.

Over this 10,000 um length of axon, the voltage is nearly uniform. Thus, one sees the voltage in space rising and falling as an almost flat wave, while the voltage-vs-time plots show the AP arriving almost simultaneously at the 4 recording electrodes. The speed is, of course, due to the axon's huge diameter: this axon has more membrane area and less internal resistance per unit length than do smaller axons, enabling a large transmembrane current to surge inward and forward.