Spatial long-range interactions in squid giant axons

Abstract

Membrane states and their bifurcation characteristics are studied for the squid giant axon as a function of external Ca 2+ concentration, temperature and externally applied current step. It is well known that the membrane states are divided into two states, resting (R) and spontaneous oscillation (O), according to the Ca 2+ concentration contained in the solution surrounding the axon. The present experiments further clarified that each of these states was further subdivided into higher (H) and lower (L) temperature phases, according to temperature. The spatially unclamped axon in the higher-temperature phase, either in the R or O state, can bifurcate to produce limit-cycle oscillations of action potentials. The bifurcation parameters are external Ca 2+ concentrations and externally applied current for the axon in the O and R states, respectively. Both the axon in the lower-temperature phase and the spatially clamped axon in the higher-temperature phase can bifurcate to produce intermittent oscillations of action potentials. The bifurcation characteristics at or between the higher- and lower-temperature phases are closely related to the spatial properties of the preoscillatory fluctuations along the axon, suggesting that a particular spatial interaction is responsible for the periodically oscillatory dynamics and the bifurcation to it. The molecular origin of the spatial interaction possibly originates from the specific distribution of Na channels, which may be regulated by subaxolemmal cytoskeletons. Electron microscopic experiments and other evidence to support this idea are described.