Further research has determined calcium's role in long-term depression induction. While other mechanisms of long-term depression are being investigated, calcium's role in LTD is a defined and well understood mechanism by scientists. High calcium concentrations in the post-synaptic Purkinje cells is a necessity for the induction of long-term depression. There are several sources of calcium signaling that elicit LTD: climbing fibres and parallel fibres which converge onto Purkinje cells. Calcium signaling in the post-synaptic cell involved both spatial and temporal overlap of climbing fibre induced calcium release into dendrites as well as parallel fibre induced mGluRs and IP3 mediated calcium release. In the climbing fibres, AMPAR-mediated depolarization induces a regenerative action potential that spreads to the dendrites, which is generated by voltage-gated calcium channels. Paired with PF-mediated mGluR1 activation results in LTD induction. In the parallel fibres, GluRs are activated by constant activation of the parallel fibres which indirectly induces the IP3 to bind to its receptor (IP3) and activate calcium release from intracellular storage. In calcium induction, there is a positive feedback loop to regenerate calcium for long-term depression. Climbing and parallel fibres must be activated together to depolarize the Purkinje cells while activating mGlur1s. Timing is a critical component to CF and PF as well, a better calcium release involves PF activation a few hundred milliseconds before CF activity.