Fig. 6. Physiological conditions for SA depletion versus replenishment.
(A) Synaptic currents entering the dendritic spines through NMDAR and AMPAR trigger CICR by activating RyRs at the base. (B) Small calcium inputs with slow time scales through ORAI1 channels located near the SA membrane are insufficient to trigger a CICR at the base. These ions are either absorbed into calcium pumps located in the spine head or replenish the SA calcium reservoir via the SOCE through SERCA pumps. (C) Phase space described by the main axes: calcium injection rate and the distance dSA between the ORAI1 and SERCA channels. Refilling and depletion conditions of calcium in spines are well separated, so that both do not occur at the same time.