The heart has a natural pacemaker: a set of cells whose electrical impulses trigger the heart's chambers to contract rhythmically. The stomach and intestine also have a pacemaker, but until recently no one knew where it lay. Now a team of researchers at McMaster University in Hamilton, Ontario, have pinpointed its location. A group of star-shaped cells that underlie the gut muscles generate gentle waves of excitation that control the rate at which the muscles contract and relax, moving food through the intestine.
Without a pacemaker, says physiologist Jan Huizinga, "gut muscles would generate contractions in a haphazard manner, and they could very easily go into a tonic contraction," a tight spasm from which they would be unable to relax. The pacemaker cells prevent that.
Researchers have suspected for some time that these cells--called the interstitial cells of Cajal--form the gut's pacemaker, but until Huizinga's work no one had been able to prove it. He and his colleagues dissected Cajal cells from the small intestines of mice. With tiny electrodes they measured the current across the cells' surfaces and detected a rhythmic flux of positively charged ions.
Huizinga believes that this flux, together with nerves signaling the presence of food, controls the contractions of gut muscles. When the cells' voltage is high, calcium channels in the gut muscles spring open, and the calcium inflow triggers the muscles to contract. When the voltage is low the gut muscles relax. There are 3 contractions per minute in the stomach and 12 per minute in the small intestine, says Huizinga.
The discovery may help people suffering from intestinal diseases like gastroparesis, which results from a failure of the gut muscles to contract properly. People with this disorder must be fed entirely by tube. Huizinga hopes that scientists will now be able to design drugs that can spur the nonfunctioning cells into action, or regenerate them where they have been destroyed by inflammatory bowel disease. "We now have a better understanding of how food moves through the stomach and small intestine," says Huizinga, "which also means we have a target cell for developing drugs."
