The psyn increases gastric motility and secretions and causes the production of watery saliva. The syn has the opposite effect, creating dry mouth. Motility is the muscular contractions of the stomach and intestine that mix the food and propel it through the digestive tract. Surrounding the digestive tract are smooth muscle cells called interstitial cells that act like pacemaker cells of the heart. These cells can spontaneously alter their membrane potentials, producing basal electrical rhythms/slow waves that travel down the digestive tract. These membrane potentials do not cause the muscles to contract because they do not reach threshold for the most part. These slow waves travel from smooth muscle cell to smooth muscle cell through gap junctions, much like in the heart. The frequency of these bers varies stomach it is 3 waves/min, duodenum it is 12 waves/min. If the peak of the slow wave reaches the threshold, the smooth muscles will contract.