A medical apparatus, for artificially stimulating internal tissue of an animal, applies a composite voltage pulse to a pair of electrodes implanted in the animal. The composite voltage pulse is formed by a first segment and a second segment contiguous with the first segment, both of which have generally rectangular shapes. The amplitude of the first segment is significantly greater than, e.g. at least three times, the amplitude of the second segment. However, the second segment has a significantly longer duration than the first segment, e.g. at least three times longer. Preferably the integrals of the first and second segments are substantially equal.