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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\u2013Huxley-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.<\/jats:p>","DOI":"10.1085\/jgp.202613977","type":"journal-article","created":{"date-parts":[[2026,7,7]],"date-time":"2026-07-07T13:50:33Z","timestamp":1783432233000},"update-policy":"https:\/\/doi.org\/10.1085\/jgp.crossmarkpolicy","source":"Crossref","is-referenced-by-count":0,"title":["Kinetic reconstruction of graded membrane excitability in\n                    <i>Paramecium<\/i>"],"prefix":"10.1085","volume":"158","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1078-7989","authenticated-orcid":true,"given":"Takashi","family":"Tominaga","sequence":"first","affiliation":[{"id":[{"id":"https:\/\/ror.org\/00smwky98","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Neuroscience, Tokushima Bunri University 1 , Takamatsu,","place":["Japan"]},{"id":[{"id":"https:\/\/ror.org\/00smwky98","id-type":"ROR","asserted-by":"publisher"}],"name":"Kagawa School of Pharmaceutical Sciences, Tokushima Bunri University 2 , Takamatsu,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7899-5405","authenticated-orcid":true,"given":"Yoko","family":"Tominaga","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/00smwky98","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Neuroscience, Tokushima Bunri University 1 , Takamatsu,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"291","published-online":{"date-parts":[[2026,7,7]]},"reference":[{"key":"2026070709500942100_bib1","first-page":"1145","article-title":"Ionic channels of Paramecium: From genetics and electrophysiology to biochemistry","volume":"77","author":"Adoutte","year":"1981","journal-title":"J. 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