[NEST Users] V_act_NMDA and S_act_NMDA parameter in ht_neuron

Dear all, I am looking for the details of ht_neuron model and referred to DOI: https://doi.org/10.1152/jn.00915.2004 for details. However, I couldn't find information on V_act_NMDA and S_act_NMDA. Can anyone please point me in the specific paragraph/table in the paper or share a one line explanation. I have attached a screenshot of the equation from the example notebook and also the complete notebook for quick reference -- Thanks and Regards *Maryada*