Answer
As one swallows , mechanical impulses are picked up by receptors in the pharynx. Theses impulses travel to the medulla oblongata and back by the vagus nerve (CN X) to the stomach ; the effect is to cause the stomach to relax and be receptive of the bolus of food
Stomach response.
After an initial resistance to relation, the stomach muscles relax in preparation to receive the swallowed food. This is followed by rhythmic peristaltic contractions down the walls of the stomach--this wavelike motion is under the control of pacemaker cells in the muscularis externa.
The fundus of the stomach does not participate in this action. Each peristaltic wave starts as a contraction just below mid-stomach; they then moves caudally, sequentially, at the rate of about three waves per minute, getting stronger as they move towards the antrum. The strength of the contractions become greater after 30 mins of action; the increased vigor facilitates the mixing of the food with the gastric juice and this promotes digestion.
Work Step by Step
The capacity of the antrum is only about 30 ml, so it receives the food in batches. When the food particles are of small enough dimensions ( 1-5 mm dia.) small amounts -- about 3 ml at a time-- are let into the duodenum. It is easier for the duodenum to neutralize the acidity and digest the compound substances , small quantity at a time. If the duodenum becomes overloaded, a negative feedback mechanism slows down gastric motility and the rate of passage of chyme from stomach to duodenum.
Generally, complete digestion of a normal meal takes about four hours. However, the time need varies with the composition of the meal: a meal that is largely liquid requires less time for digestion; if the acidity of the meal is high, digestion time is prolonged; finally, if a meal is high in fats, time needed for complete digestion may be as along as six hours.
