[box type=”download”] Phases (voluntary and reflex), control and functional anatomy of the swallowing process Function of the upper and lower oesophageal sphincters and of peristalsis [/box]
The first phase is voluntary and involves the formation of a bolus of food by chewing and tongue movements (backwards and upwards), which push the food into the pharynx.
The remaining phases are reflex responses – stimulation of mechanoreceptors – afferents in the glossopharyngeal (IX) and vagus (X) nerves to the medulla and pons (brain stem); – the ‘swallowing centre’ coordinates to eventually deliver the bolus into the oesophagus.
The soft palate elevates to prevent food from entering the nasal cavity, respiration is inhibited, the larynx is raised, the glottis is closed and the food pushes the tip of the epiglottis over the tracheal opening, preventing food from entering the trachea.
As the bolus enters the oesophagus, these changes reverse, the larynx opens and breathing continues.
A ring of skeletal muscle– the upper oesophageal sphincter closes the pharyngeal end of the oesophagus.
During the oesophageal phase of swallowing, this is relaxed and it closes immediately after food passes down.
Once in the oesophagus, the bolus is propelled to the stomach by peristalsis (a coordinated wave of relaxation in front of the bolus and contraction behind the bolus of the circular and longitudinal muscle layers of the oesophagus), forcing the food into the stomach.
Before the bolus enters the stomach, it passes through the lower oesophageal sphincter (a ring of smooth muscle).
The swallowing centres in the medulla produce a sequence of events via efferent somatic nerves (innervating skeletal muscle) and autonomic nerves (innervating smooth muscle).
This sequence of events is influenced by afferent receptors in the oesophagus wall.
The sphincters and the peristaltic waves are principally controlled by activity in the vagus nerve and aided by the activity within the enteric nerve plexuses within the tract itself.