peristalsis
Summary
Peristalsis is a wave of alternating smooth muscle contraction and relaxation that pushes contents along a tube, such as the esophagus and intestines. It is involuntary and mainly propulsive (one-way), unlike segmentation, which mixes with little net movement, though gastric peristalsis also churns food.
Detail
Meaning: Peristalsis is the coordinated, wavelike movement of smooth muscle (muscularis externa) in the walls of the digestive tract. Circular muscle contracts behind the bolus, narrowing the lumen, while the region ahead relaxes. The wave moves contents toward the anus. In the stomach, waves also push chyme against the closed pylorus, which churns and mixes it. Peristalsis is involuntary, coordinated mainly by the enteric nervous system and modified by autonomic input: parasympathetic activity generally increases motility and sympathetic activity generally decreases it.
On the MCAT: Passages may ask you to tell peristalsis from segmentation, or to predict transit when parasympathetic activity rises or falls. Questions may also ask which tissue is responsible (smooth muscle, though the upper esophagus has some skeletal muscle) or why swallowed food reaches the stomach even when someone is upside down.
Common confusion: Peristalsis is mainly propulsive and one-way. Segmentation is a back-and-forth pattern of local contractions in the small intestine that mixes chyme with enzymes and brings it against the absorptive surface, with little net forward movement. Peristalsis is also not voluntary: swallowing is started voluntarily, but the esophageal wave that follows is reflexive.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Principles of Anatomy and Physiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.