The contraction mechanisms of GI smooth muscle can be differentiated with the aid of blockers of the voltage-controlled calcium channel (e.g. nifedipine). On the one hand, nifedipine-sensitive processes produce predominantly phasic-rhythmical contractions which can merge to sustained tonic activation, called 'tetanic tone', and which are combined with spike discharges and calcium influx. On the other hand, nifedipine-resistant and electrically silent processes produce a 'specific tone'. The cooperation of both processes in one and the same cell leads to a great diversity of patterns of smooth muscle activity. 'Specific tone' dominates in regions with reservoir function and contributes significantly to the contractions of GI sphincters, with great differences between sphincter types, the various species, and, in man, also between individuals.