A physical model of nerve axon. II: Action potential and excitation currents. Voltage-clamp studies of chemical driving forces of Na+ and K+ in squid giant axon.

An adsorption model of nerve axon has been extended to account for the origin of membrane currents observed under voltage-clamp conditions. Differing from the Hodgkin-Huxley model, which attributes excitation solely to a change of ionic conductances of the membrane, the present model proposes that a layer of axoplasm attached to the membrane (axon cortex) can undergo conformational changes and hence modulate selectivity for mobile ions. To test the model, a two-step voltage-clamp study was made of the chemical driving forces of Na+ and K+ ions in squid giant axon. The forces were measured by determining the instantaneous current-voltage relation when membrane current is carried by Na+ only or K+ only. The data indicate that the chemical driving force varies as a function of time and does not agree with the Nernst relation during the early phase of excitation. Implications of the observations are discussed.