Fig. 4. Activation of STOCs and STICs by Ca²⁺ sparks in smooth muscle. The Ca²⁺ spark originating from RyRs can activate BK channels, C^- channels or both (producing a STOIC). Ca²⁺ entry via L-type VOCCs does not recruit such Ca²⁺ spark sites but leads to global Ca²⁺ increases that trigger contraction.