Schwann Cell and Axon Electrical Potential Differences : Squid nerve structure and excitable membrane location

In fifty-seven resting nerve fibers of the squid Sepioteuthis sepioidea impaled from outside to inside, three potential difference (PD) levels were recorded. In twelve nerve fibers the position of the tip of the micropipette, when it was recording one of the PD levels, was labeled with carmine, and the minute spot of dye was localized histologically. The first PD level of -10 to -26 mv, was located in the endoneurium cells; the second level, formed by a single PD of -33 to -46 mv, was located in the Schwann cell; and the third level, formed by a single PD of -50 to -65 mv, was located in the axon. In sixteen nerve fibers the independence among these PD levels was demonstrated as follows. One micropipette was inserted inside the axon for recording its PD; a second one was inserted inside one endoneurium cell or inside one Schwann cell for recording the PD; and a third micropipette was inserted inside the axon to supply current to depolarize the axon. When the axon was depolarized no significant change was observed in the PD of either the endoneurium cell or the Schwann cell. In all nerve fibers action potentials were registered from the axon only. This identifies the axolemma (axon plasma membrane) as the excitable membrane.