To see a brief animation of the process described above, click on the icon.
Food is moved along the digestive tract by waves of involuntary muscle contractions. These operate throughout the tract (modified for churning in the stomach), but they are easiest to feel and to understand in the oesophagus.
Swallowing is a voluntary act, but once we initiate it, the swallowing reflex takes over and it continues involuntarily. The oesophagus (and all of the intestines) are surrounded by layers of circular muscles, whose fibres ring the tract. Thus, if a particular band of circular muscles contracts, it will squeeze the digestive tract and make it narrower at that point. The circular muscles of the oesophagus have reflex reactions that cause them to relax if pressure is applied from above, then to contract. Thus, when a bolus of food enters the oesophagus, the first few rings of muscle relax, making the oesophagus wider. The bolus enters the upper part of the oesophagus, propelled by the force of the initial swallowing act. As it moves past the first ring of muscle, however, that muscle contracts. The contraction squeezes the bolus, forcing it further down the oesophagus. This causes additional downstream circular muscles to relax, then contract as the bolus moves by. This wave of relaxation followed by contraction moves the food smoothly from the pharynx to the stomach.
To see a brief animation of the process described above, click on the icon.
Similar waves move the fluid contents of the intestine along, but in a slower and less rigid way. They are (presumably) controlled by feedback that informs the body of the state of the food and the system itself.
The fact that pressure from below signals the muscles to contract accounts for the normal one-way movement of swallowing. The largest circular muscle, the cardiac sphincter closes the opening into the stomach, and will normally not allow food back up. Only in emergencies, such as poisoning or extreme stress, is the normal reflex reversed to cause vomiting.
Most of the time, you are completely unconscious of peristalsis. You really notice it however, when you swallow something too big or too dry or too angular to pass smoothly down the oesophagus. The circular muscles relax as they should, but when they try to contract again they meet resistance from the slow moving object. This sends a message of mild pain to your brain. You may force additional swallows to push it on, or drink something to lubricate the oesophagus. If the object does not move the circular muscles may fatigue and cramp, as they try to contract against an unyielding resistance. This is why you can often "still feel" the pill lodged in your oesophagus even after you know you have dislodged it.
Other signs of peristalsis are the rumblings and "butterlies" you sometimes get in your gut, particularly if you are hungry or nervous.
