Kinetic reconstruction of graded membrane excitability in Paramecium

Electrical excitability in the ciliate Paramecium links sensory transduction to behavioral output through voltage-dependent control of ciliary activity. Despite its long history as a model organism, a quantitative framework that can reconstruct graded membrane excitability from overlapping macroscopic current components has remained incomplete. Here, we developed a conductance-based kinetic model for Paramecium multimicronucleatum using controlled voltage-clamp protocols and Hodgkin–Huxley-type analyses. We first characterized the resting leak component and two major late components: a depolarization-activated component (⁠$Idepo$⁠) and a hyperpolarization-activated component (⁠$Ihyper$⁠). Together, these components produced a conductance minimum near the resting potential. Using this baseline framework, we operationally isolated the transient inward Ca²⁺ current (⁠$ICa$⁠). Ca²⁺-dependent inactivation was strongly non-single exponential and was described by a phenomenological parameterization relating channel availability to cumulative Ca²⁺ entry during conditioning. Recovery from inactivation was substantially slower than the onset of inactivation. Finally, we integrated these parameterized components into a single-membrane equation and reconstructed the graded membrane responses observed in current-clamp recordings. The model reproduced the initial depolarizing peak and the subsequent sustained depolarized phase followed by repolarization. Together, these results demonstrate that an integrated kinetic description can capture major features of graded membrane excitability in P. multimicronucleatum under the present recording conditions.