Peristalsis refers to the series of muscle contractions that take place in the gastrointestinal (GI) tract due to which food is propelled through the digestive tract. This movement begins right from the pharynx or throat and ends in the anus. These muscle contractions are innervated and controlled by enteric nervous system (ENS) and are involuntary movements.
The food that is chewed and mixed with saliva is called bolus and goes down the throat. As the muscles of the throat, foodpipe, stomach, small intestine and large intestine, rectum and anus, contract in a sequential and rhythmic manner. As the food passes through each stage, the contents undergo a change in terms of mixing of juices and absorption of glucose and nutrients. Finally it collects in the rectum and is eliminated via the anus.
The movement of the muscles of the GI tract is a set of coordinated movements of two types of muscles. The two types of muscles are longitudinal and circular muscles. The brain releases a hormone called serotonin. This hormones cause the contractions resulting in peristalsis.
The longitudinal muscles contract to narrow down the pipe-like esophagus, which propels the food downwards. The circular muscles right below the longitudinal muscles that had just contracted, expand simultaneously, allowing the pushed food some space. This occurs in an alternate manner, resulting in peristalsis.
When the food reaches the end of esophagus, the gastroesophageal spincter (a flap that opens one-way to allow contents of food-pipe to pass to the stomach) opens up allowing the bolus to pass to the stomach. The bolus is mixed thoroughly by the churning movements of the stomach muscles, along with the acidic stomach juice. This mixture is called Chyme. When these muscles contract finally, peristalsis occurs and the chyme is sent to the intestine via the one-way flap called pyloric sphincter.
The semi-fluid chime is mixed again in the intestine with slower peristaltic movements so that nutrients are absorbed. Chyme reaching the large intestine then moves forward with much slower contractions. Mass action contractions occur about one to three times per day here, propelling the chyme (feces) to the rectum.