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The initial stages of eating and swallowing are under voluntary control. This means that it is governed by the brain.
We do not have to put food in our mouths just because we see it on a plate (although this is sometimes hard to resist!). Neither do we have to chew food once it is in our mouths. We can spit it out if we wish to. The latter stages of pharyngeal and oesophageal swallowing are primarily involuntary and are controlled by basic biomechanical mechanisms and the autonomic nervous system.
Once food enters the mouth the teeth break it down into smaller and smaller
pieces. This has the dual function of making the food easier to swallow and increasing the
surface area of food on which the saliva can act.
The tongue, lips and cheeks assist the teeth in the process by allowing the food to be "rolled" around the oral cavity.
The mechanical action described above produces a softened bolus of food which is now ready to be swallowed. The correct biological term for swallowing is deglutition.
The picture on the left shows the voluntary
stage of deglutition. Here the bolus is pushed into the upper part of the pharynx
(known as the oropharynx) by the action of the tongue.
The pharyngeal stage of deglutiton is stimulated when the bolus enters the oropharynx. This stage of swallowing is mainly due to a reflex response. Various nerve receptors send messages to the deglutition centre of the brain stem. (see medulla and pons in your notes on the central nervous system).
This
sets off muscular contractions in the pharynx. The soft palate closes off the nasopharynx.
The vocal cords in the larynx are moved up and towards the front of the throat thus
closing it off to the passage of food. This is extremely important in preventing food from
entering the airway.I am sure we have all experienced the unpleasant feeling of food or
drink going the "wrong way"!!
Another effect of the process is to widen the opening of the oesophagus thus making the passage of the bolus along the alimentary canal easier.
As
the bolus pushes it's way into the oesophagus it automatically pushes the epiglottis
downwards further closing off the airway.
The
bolus then enters the oesophagus and the final stage of swallowing begins.
This final stage is known as the oesophageal stage of deglutiton.
This is an animation of the whole process
The oesophagus is a muscular tube that carries food and liquids from the throat to the stomach for digestion after it has been chewed and chemically softened in the mouth.
Food is forced downward to the stomach (or upwards, if one is standing on his head) by powerful waves of muscle contractions passing through the walls of the oesophagus known as peristalsis. The peristaltic motion is stimulated by the vagus nerve and the myenteric nerves (see the autonomic nervous system.)
Because these contractions are so strong in the oropharynx and the oesophagus, we can swallow in any position -- even upside-down! In addition the mucosal lining of the oesophagus produces mucous which aids the passage of the bolus into the stomach.
If the food is "off", or more than we can "stomach," it may travel back by the same force to be thrown out through the mouth, which is called vomiting.
The oesophagus has a ring of muscle at the top and at the bottom. These rings (or sphinctres) close or contract after the food passes through and enters the stomach, where there is an abundance of churning acid waiting to digest the food. Liquids may take as little as 1 to 2 seconds to reach the stomach whereas a "heavy" food bolus may take 2 to 3 minutes. When swallowing a small amount of air is also taken in and this is normally expelled after a while by flatulence (belching).
This
is an endoscope picture of the normal adult oesophagus.
Notice the smooth mucosal lining and the absence of any protruding features which may delay the passage of the bolus along the oesophagus.
This is
another endoscope picture of the oesophagus showing the normal adult oesophagus. This
picture shows the bottom end of the oesophagus where it joins the stomach.
Notice the transition between the oesophageal mucosa (outer ring of tissue)and the gastric mucosa (inner).
If the bottom muscle weakens, stomach contents, along with the stomach acid, may return to the oesophagus and cause an uncomfortable, burning sensation known as "heartburn", although it is not connected with the heart at all.
This endoscope picture shows a hiatal hernia
Hiatal hernia is a condition where the stomach herniates upwards into the oesophagus. It is often associated with a reflux of gastric fluid causing heartburn.
This page last updated Friday, 30 April 1999 10:44 +0100
