In this work a physics-based spice model for the Nb2O5 volatile thermally-activated micro-scale threshold switching memristor is presented. The dynamic behavior of the spice model is adjusted by the adoption of a Pearson-Anson oscillator. In that way the value of the internal thermal capacitance of the memristors conducting filament can be attained. This value is not directly accessible by other physical measurements. The model might be used as the base for modeling a large variety of memristor devices exhibiting threshold switching and thermally induced abrupt memory switching.