 or swallowing is passage of food from mouth to stomach via esophagus. For swallowing to take place properly, bowler should form in the mouth and tongue should push it to back of mouth. Then, all unnecessary parts need to be blocked. Nasopharynx should be closed, air path should be closed and esophageal path should open. And finally, bowler should pass via esophagus to stomach. These different actions are defined as different phases of swallowing. Oral, pharyngeal and esophageal. Before we go into details of each of the phase, we should know some structural details. The posterior wall of pharynx, it has three different sphincters. Superior, middle and inferior. Plus esophagus, it is mostly of smooth muscle. But the initial one-third portion of esophagus is striated muscle. In oral phase, tongue pushes against the palate like this, separating a part of the bolus from the food in the mouth. So, bolus will reach to the posterior pharyngeal wall. For oral phase to function properly, we want muscles of jaw, facial muscles and tongue muscles to function properly. These muscles are supplied by 5th, 7th and 12th cranial nerves. So, these cranial nerves should be intact for oral phase to function properly. Plus, size of the bolus formed is also important. Now, oral phase is a voluntary phase. Rest of the two phases, the pharyngeal and esophageal phase, are involuntary. As bolus touches the oropharyngeal wall, it initiates pharyngeal phase. That is the second phase. Touching of bolus through the oropharyngeal wall initiates a reflex response, where sensory receptors present at the wall of oropharynx are stimulated. And afferents then go via 9th and 10th cranial nerves to swallowing centre in medulla. From there, a series of motor responses are initiated, which bring about entry of bolus into the esophagus. The series of responses include contraction of superior constrictor of the pharynx and elevation of the soft palate, so as to close the mesopharynx. Then, there is contraction of the laryngeal muscles and larynx is pulled upward and anteriorly and laryngeal opening is closed. Not only that, there is flipping back of epiglottis for closure of the air path with inhibition of respiration. So, to close the air passage, three things are taking place. Larynx is moving anteriorly and upward with closure of glottis. Epiglottis is closing the path and there is inhibition of respiration. This is important because if food enters the air pathway, it can lead to pneumonia. When larynx moves upward and anteriorly, what happens is it stretches this path and widens the opening of the esophagus. As it happens, there is sequential contraction of the other constrictors that is the middle and inferior constrictors of the pharynx also with relaxation of the upper esophageal sphincter for entry of food into the esophagus. All these events, closure of the nasopharynx, closure of the airways and opening of the esophagus occur in less than one second. So, this is a quick reflex response. When food enters the esophagus, there starts a wave of peristalsis. Because of distention of the esophagus caused by the bolus, there are stretched signals from the esophageal wall which cause contraction of the esophageal muscles behind the bolus and relaxation ahead of the bolus. So, there is a wave of contraction and relaxation which keeps on moving the bolus into the esophagus. This is known as primary peristalsis, the peristalsis which is preceded by the oral and pharyngeal phases. But what will happen if this peristalsis is not strong enough? What will get stuck in the esophagus? You might have had an experience of bolus getting stuck. Suppose if the size of the bolus is too large or you were too fast in following it, then there was not proper bolus formation and lubrication of the bolus. This creates another wave of peristalsis due to distention of the esophageal wall caused by the bolus. Now, that peristalsis is known as secondary peristalsis. So, secondary peristalsis is due to distention of the esophageal wall and is not preceded by oral and pharyngeal phases as opposed to primary peristalsis. It's a predictive response so that bolus can be pushed into the stomach. Secondary peristalsis is also initiated in case the contents from the stomach reflects back into the esophagus. They also cause distention of the esophagus and then there is secondary peristalsis. So, it empties the esophagus of the gastric contents. It takes approximately 10 seconds for food to pass from esophagus into the stomach and this peristaltic wave is important for transport of both solids and liquids. So, even we have upright beings and gravity supports this transport from esophagus to stomach. But still, the peristaltic wave is essential for both solids and liquids transferred into the stomach. Now, this esophagus is functionally divided into upper esophageal sphincter, esophageal body and lower esophageal sphincter. Upper esophageal sphincter forms the upper 3 cm of esophagus and lower esophageal sphincter forms the lower 3 cm. We saw that upper esophageal sphincter relaxes in the pharyngeal phase. Similarly, as food bowlers is passing from esophagus to stomach, lower esophageal sphincter relaxes and it is because of this peristaltic wave which is travelling. But normally this LES, lower esophageal sphincter is tonically contracted. That means it is always kept in contracted state. So, this diagram is showing the lower esophageal sphincter. Here, esophagus is making contact with the stomach. So, this is showing the stomach. It is important that it remains tonically contracted so that contents from the stomach should not pass into the esophagus. It is a very delicate place because if acid from stomach moves into the esophagus, then it can corrode the lining of the esophagus and it can also lead to ulcers. Ulcers when healed, it can lead to stenosis and narrowing of the esophagus. So, not only the lower esophageal sphincter but there are other mechanisms also to keep it contracted. And these are the diaphragmatic fibres which form the external sphincter. And there are also these sling fibres of stomach. So, when there is increase in intra-abdominal pressure, these fibres contract and they narrow the opening of esophagus into the stomach. So, whenever there is rise in intra-abdominal pressure such as in cuffing or straining, so it doesn't happen that contents from stomach go into the esophagus. Only an extreme increase in intra-abdominal pressure will this back flow or reflux will happen. So, this is a normal mechanism of swallowing. It consists of three phases, oral, pharyngeal and esophageal.