 The other type of contractions, called phasic contractions, consist of brief periods of both relaxation and contraction, occurring in the posterior stomach and the small intestine, and are carried out by the muscularis externa.Motility may be overactive (hypermotility), leading to diarrhea or vomiting, or underactive (hypomotility), leading to constipation or vomiting; either may cause abdominal pain.The stimulation for these contractions likely originates in modified smooth muscle cells called interstitial cells of Cajal.