The most striking anatomical feature of nerve cells is that part of the cell is produced into an enormously elongated cylindrical process, the axon. It is this part of the cell with which we shall be concerned in this chapter and the next. The essential function of the axon is the propagation of nerve impulses.

Action potentials in single axons

Let us consider a simple experiment on the giant fibres in the nerve cord of the earthworm. These fibres are anatomically not axons, because they are multicellular units divided by transverse septa in each segment, but physiologically each fibre acts as a single axon. There are three giant fibres, one median and two lateral, which run the length of the worm; the laterals are interconnected at intervals. The experimental arrangement for eliciting and recording impulses in the giant fibres is shown in fig. 4.1. The stimulator produces a square voltage pulse which is applied to the nerve cord at the stimulating electrodes. The recording electrodes pick up the electrical changes in the nerve cord and feed them into the amplifier. Here they are amplified about 1000 times, and then passed to the oscilloscope where they are displayed on the screen of the cathode ray tube. The output of the stimulator is also fed into the oscilloscope so that it is displayed on the second trace on the screen. The timing of the oscilloscope sweep is arranged so that both traces start at the moment that the pulse from the stimulator arrives.