Fig 4. Dynamics of the Ca²⁺ concentration in the intracellular compartment during synaptic activation.

a Sample stimulus (spikes). The astrocytic compartment was stimulated for 200 seconds with a Poisson spike train of 100 Hz. The corresponding glutamate concentration in the extracellular compartment as a function of time was calculated using the Tsodyks-Markram model. b [Ca²⁺]_i for different values of the volume ratio (ratio_ER) between the internal Ca²⁺ store and the intracellular compartments. The upper and lower symbols for ratio_ER>0.06 denote the average height of peaks and troughs of the emerging Ca²⁺ oscillations (in [μM]). For ratio_ER≤0.06 no Ca²⁺ oscillations were present and symbols denote the average concentration of Ca²⁺ over the stimulation period. c Frequency of Ca²⁺ oscillations as a function of ratio_ER.