Spontaneous activity is a special feature of smooth muscle which can be observed in some vascular (VSM: mesenteric arteries, portal vein) and gastrointestinal tissues (GSM: gastric antrum, duodenum). Basically, the pattern of rhythmicity is more uniform in gastric tissues than in vascular smooth muscle which is in accordance with a different magnitude of electrical coupling. It is well accepted, that spontaneous activity in gastric smooth muscle is linked to the activity of interstitial cells of Cajal (ICC).

Using four-dimensional correlative network analysis we could demonstrate in portal vein and gastric antrum, that hormones and neurotransmitters increase cell to cell coupling and total excitability. Furthermore, in the ganglia and the plexus myentericus of GSM cell clusters, stronger or weaker phasic calcium activity was observed. Additionally, smaller spots of high calcium activity were detected in portal vein and gastric antrum. The distances between such points ranged between 20 and 100 mm. Most of them exhibited the same phase like the smooth muscle tissue. In about 30% of these activity spots in GSM, additional cells were observed which hat a 180° phase shift of the calcium signal in respect to that of the smooth muscle. In VSM such signal inverting cells (SICs) were also observed but with a lesser extend (8%). Application of Verapamil inhibited the calcium oscillations in SICs and non-SICs.

Conclusion: 

Signal inverting cells play a functional role in the coordination and generation of spontaneous phasic activity in vascular and gastric smooth muscle.