Mechanisms underlying the propagation of intercellular Ca²⁺ waves. Ca²⁺ waves can be propagated by the intercellular transfer of signaling molecules, such as IP₃, through gap junctions. The extracellular pathway involves the release of different molecules, primarily ATP, via hemichannels, a vesicular pathway, or channels such as the P₂X₇ receptor. ATP diffuses to neighboring cells where it activates GPCRs or Ca²⁺-entry channels, thereby increasing [Ca²⁺]_i. Regenerative mechanisms (activation of PLC or stimulation of ATP release mechanisms) are indicated by the dashed arrows.