Synaptic weight updates when considering Ca ²⁺ in terms of ions or concentration. (a–d) Synaptic weight updates for each stochastic idealized and real geometry simulation when synaptic weight calculations are in terms of ions (a and b) and concentration (c and d). We plotted the synaptic weight changes against the spine volume for calculations using ions (b) and concentration (d). We fitted the trends using a linear function of volume; r² = 0.4635 for the ion fit and r² = 0.1229 for the concentration fit.