A bolus of food leaving the mouth is very quickly transported to the stomach in a process that we refer to as deglutition (swallowing). Although the oesophagus is the major structure mediating this process it also involves the pharynx and the larynx.
Deglutition
Swallowing is initiated by a conscious decision to do so. Once our bolus of food has been prepared by the mouth and is ready to start its journey we put on the back of our tongue, elevate the tip of our tongue and push it towards the cavity behind the mouth known as the oropharynx.
This voluntary action results in the bolus of food stimulating mechanoreceptors that then trigger a reflex that eventually results in it being deposited in the stomach. This reflex is controlled by the swallowing centre located in the brainstem and is characterised by the following sequence of events:
1. Elevation of the soft palate: This blocks the entry to nasopharynx and prevents the food from going into the back of your nose.
2. Elevation of the larynx: This has the effect of positioning the epiglottis over the entrance to the trachea and prevents food falling into you respiratory airways. You can confirm this yourself by placing a finger on your laryngeal prominence (Adam’s apple) and then swallowing.
3. Cessation of breathing: This also helps prevent food entering your airways and is known as deglutition apnoea.
4. Relaxation of upper oesophageal sphincter: This allows the food to move into your oesophagus.
5. Peristaltic contractions: These move the bolus through the oesophagus.
6. Relaxation of the lower oesophageal sphincter: Allowing food to fall into the stomach.
This reflex action is very efficient and can transport food from the mouth to the stomach in a remarkably quick time. A bolus of food can reach the stomach within 4 seconds of leaving the mouth and fluids are transported even quicker.
Once this whole sequence of events has been initiated there is nothing we can do to stop it. All the steps are initiated by the swallowing centre and because it is a reflex it is virtually impossible to override these.
However head injuries that affect the swallowing centres, damage to the nerves that innervate the pharynx or larynx and movement disorders (such as Parkinson's disease) can lead to seriously compromised deglutition (dysphagia) and can have life-threatening consequences if food enters and blocks the airways.
Emesis
The digestive tract is of course designed for the movement of food along it length from mouth towards anus. However as anyone who has ever over-indulged in alcoholic beverages or eaten something that didn’t agree with them will know, there is a remarkably efficient mechanism for the movement of one’s stomach contents in the reverse direction. You may know it as 'throwing up' or 'tossing your cookies' but in physiological terms it is referred to as emesis (vomiting).
Emesis is initiated by the presence of stimuli that are detrimental to our health (alcohol, bacterial toxins, spicy foods and drugs) and, like deglutition, is largely a reflex controlled by the emetic centre in the brainstem. These stimuli activate sensory neurones that are responsible for the loss of appetite (anorexia) and feeling of nausea that often precedes emesis. The signals also trigger a number of physiological responses that eventually results in expulsion of food from the stomach (and sometimes even the duodenum):
1. Contraction of the pyloric sphincter: This ensures the contents of the stomach are pushed towards the mouth.
2. Contraction of the diaphragm and abdominal muscles: This causes a dramatic pressure increase inside the stomach.
3. Relaxation of the lower and upper oesophageal sphincters: This permits movement of the gastric contents out of the stomach through the oesophagus.
4. The larynx is raised and breathing is inhibited: Raising the larynx positions theepiglottis over the entrance to the trachea and this together with the cessation of breathing prevents material (vomitus) entering the airways.
5. The soft palate is raised: This prevents vomitus entering the nasopharynx.
With all avenues of escaped blocked off, the vomitus has no choice but make a spectacular exit through the mouth.
Although vomiting is generally considered a protective reflex designed to remove potentially toxic substances from our bodies, the emetic centre also appears to receive neural input from the cerebral cortex and semicircular canals (associated with the inner ear). The functional significance of these projections isn’t clear but it is likely that they are responsible for vomiting associated with seeing something unpleasant (e.g. blood) and travel sickness, respectively.