The voltage triggered release of Ca²⁺ from internal stores in the green alga Chara reveals the hallmarks of an excitable system. The threshold-like dependence of Ca²⁺ mobilisation on electrical stimulation can be simulated by the combination of two models comprised of :i. the voltage dependent synthesis/breakdown of the second messenger inositol 1, 4, 5-trisphosphate (IP₃) and ii. the concerted action of IP₃ and Ca²⁺ on the gating of the receptor channels, which conduct Ca²⁺ release from internal stores. The combined model predicts a complex behaviour of Ca²⁺ mobilisation under periodic stimulation including higher-order phase locking and irregular responses upon increased stimulation frequency. Comparable dependencies on stimulation frequencies were observed experimentally for the voltage stimulated elevation of Ca²⁺ in the cytoplasm and for activation of action potentials. This shows that the all-or-none type activation of the action potential is in reality only the consequence of the preceding all-or-none type mobilisation of volatge stimulated Ca²⁺ from internal stores.