On the basis of a model presented in a previous paper (Hook and Hildebrand, 1979) the influence of external cation concentrations [K+]0, [Ca2+]0 and of membrane voltage Vm (i.e. the actual potential difference between the two membrane faces) on the locomotor behavior of Paramecium is theoretically analyzed. In an extended model system we discuss the negative feedback of intraciliary calcium [Ca2+]i on the excitability of the ciliary membrane. While a fast blocking of Ca channels is mediated by increased [Ca2+]i and accounts for the short duration of action potentials, a slow [Ca2+ ]i-dependent “denaturation” of channel molecules is assumed to determine excitability changes of Paramecium on a long time scale.
It is emphasized that the duration of long-lasting ciliary reversal which reflects the excitability is not a direct function of the cation ratio Ju ≡ [K+]0/[Ca2+] 0 1/2 but rather of the membrane potential Vm.
Introduction of negative surface charges can well explain why for a series of different [K+]0, [Ca2+]0 but constant Ja value the excitability is unchanged despite corresponding shifts in measured membrane potentials.
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Hook, C., Hildebrand, E. Excitability of Paramecium and the significance of negative surface charges. J. Math. Biology 9, 347–360 (1980). https://doi.org/10.1007/BF00276498
- Ciliary membrane
- Ciliary reversal
- Surface charge
- Surface potential
- Slow Ca2+ inactivation
- Ca pump