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The specificity and universality of intracellular Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow><mrow><mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula> signals rely on the variety of spatio-temporal patterns that the Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula> concentration can display. Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula> release into the cytosol through inositol 1,4,5-trisphosphate receptors (IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>Rs) is key for this variety. The opening probability of IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>Rs depends on the cytosolic Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula> concentration. All of the dynamics are then well described by an excitable system in which the signal propagation depends on the ability of the Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula> released through one IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>R to induce the opening of other IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>Rs. In most cell types, IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>Rs are organized in clusters, i.e., the cytosol is a &#8220;patchy&#8221; excitable system in which the signals can remain localized (i.e., involving the release through one or more IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>Rs in a cluster), or become global depending on the efficiency of the Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula>-mediated coupling between clusters. The spatial range over which the signals propagate determines the responses that the cell eventually produces. This points to the importance of understanding the mechanisms that make the propagation possible. Our previous qualitative comparison between experiments and numerical simulations seemed to indicate that Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula> release not only occurs within the close vicinity of the clearly identifiable release sites (IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>R clusters) but that there are also functional IP<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>3</mn> </msub> </semantics> </math> </inline-formula>Rs in between them. In this paper, we present a quantitative comparison between experiments and models that corroborate this preliminary conclusion. This result has implications on how the Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula>-mediated coupling between clusters works and how it can eventually be disrupted by the different Ca<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mrow> <mn>2</mn> <mo>+</mo> </mrow> </msup> </semantics> </math> </inline-formula> trapping mechanisms.