As seen in the previous section, one of the great leaps forward in neurophysiology was the basic understanding of the molecular mechanisms underlying the generation of nerve action potentials, or "spikes." Most of the experiments leading to the understanding of nerve spike generation and propagation were done on the giant, unmyelinated nerve axons of the squid, Loligo sp. Squid giant axons were attractive because their large size (~0.5 mm diameter) made it easier to replace their (internal) axoplasm with an artificial ionic media of any desired composition. Of course, the composition of their external bathing solution could be made up in any desired manner, as well.
The cell membrane of the squid axon was found to have a relatively high distributed capacitance of ~1 ^F/cm2. The membrane is studded with protein molecules that penetrate it between the inside and outside of the axon; these proteins offer selective passage for major ionic species to enter or exit the axon interior, depending on the potential energy gradient across the membrane for a given ionic species. For example, the inside of a squid axon has a resting potential from 60 to 70 mV, inside negative (Bullock and Horridge, 1965, Ch. 3). The resting or equilibrium concentration of sodium ions is higher outside than inside the axon. Thus, both an electrical field and a concentration gradient provide thermodynamic potential energy, which tries to force Na+ inward through the membrane. The net inward potential energy in electron volts (eV) for Na+ ions is the sum of the electrical potential, Vm, and the Nernst potential for sodium ions. This potential energy is given by the well-known relation (Katz, 1966):
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