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AccessibilityThe Oesophagus
The oesophagus is a muscular, membranous tube approximately 25 cm long, which lies just in front of the vertebral column, behind the trachea and connects the mouth to the stomach, below the diaphragm. The bolus of food is moved along the length of the oesophagus to the stomach by waves of muscular contractions called peristalsis.
In the undistended state, the mucosa is thrown into longitudinal folds, which disappear when the oesophagus is distended by food.
FIGURE 1 – The lower oesophageal
sphincter (click to enlarge)
At the lower end of the oesophagus is a muscular structure, comprising an inner and outer ring of muscle fibres, called the lower oesophageal sphincter (LOS) or cardiac sphincter (Figure 1). Under normal circumstances, the LOS only relaxes momentarily to allow food and liquids to pass into the stomach. The LOS represents the primary barrier to the reflux of food and stomach contents back into the oesophagus when pressure increases in the abdomen.
The inner mucosa of the oesophagus is lined with a delicate, non-keratinised, stratified, squamous epithelium arranged in longitudinal folds. Several mucous glands in the mucosa and submucosa provide a film of lubricating mucus that may facilitate the passage of food to the stomach. The submucosa also contains blood vessels.
The muscle layer consists wholly of skeletal muscle (under voluntary control) in the upper third of the oesophagus, a combination of smooth and skeletal muscle in the middle third, and entirely of smooth muscle in the lower third. The peristaltic action of the smooth muscles further down the oesophagus is not under voluntary control. This change in muscle control from the proximal to the distal oesophagus is paralleled by a transition from voluntary control in the upper oesophagus to autonomic control in the lower oesophagus, above the LOS.
Peristalsis
FIGURE 2 – Peristalsis in the
oesophagus (click to enlarge)
The upper part of the oesophagus contains many sensory nerve endings, so that the progress of a food bolus or hot drink can be felt for a little way down the tube. The lower part of the oesophagus, like the rest of the gut, has only pain receptors. The food bolus is moved by waves of synchronous muscular contractions and relaxations (peristalsis; Figure 2). It is induced by distension of the oesophageal walls. Immediately behind the distension, the circular muscle contracts. In front of the distension, the circular muscle relaxes and the longitudinal muscle contracts, pushing the contents of the oesophagus forward.
Oesophageal sphincters
At rest, the relaxed oesophagus is closed at both top and bottom by sphincters. The upper sphincter, situated below the pharynx, is a thick band of muscle about 2 cm long. It is known as the cricopharyngeal sphincter because it is situated near to the cricopharyngeal cartilage, which forms the voice box (larynx).The LOS cannot be detected anatomically as a band of muscle, but pressure-recording instruments show that there is a region of increased muscle tone between 2 and 6 cm long just above the point where the oesophagus joins with the stomach.
Both the upper and lower oesophageal sphincters relax during the act of swallowing and the LOS remains relaxed until the wave of peristalsis carrying food down to the stomach passes over it.
