swallowing

swallowing In preparation for swallowing, a softened or liquid food bolus is moved through the mouth by the action of the tongue. The bolus lies in a longitudinal midline furrow on the tongue, and the floor of this furrow is progressively raised from before backwards, squeezing the bolus back against the hard palate. The kinetic energy imparted to the bolus then moves it into and through the pharynx, the cavity of which continues on from the mouth. In the pharynx, contractions of circularly-arranged muscles complete the movement of the bolus down into the oesophagus and thence to the stomach.

The whole process is complicated by the fact that, in the adult human, the pharynx also forms part of the airway leading from the nose to the larynx. The opening into the larynx (the glottis) is sited about halfway down the front of the pharynx. As a consequence, swallowing and breathing cannot safely occur at the same time. In contrast, in the human new-born and generally in other mammals (both infant and adult), the larynx occupies a higher position relative to the pharynx so that its opening is usually above the soft palate, which extends around it. In this situation there is a degree of anatomical separation of the respiratory tract and the alimentary tract (and in many animals the high larynx divides the pharynx into two passages, which pass laterally either side of the larynx and then rejoin lower down in the pharynx). The timed separation of swallowing and breathing is consequently less critical in this situation than it is in adult man.

The anatomical differences also produce differences in the way that the swallow is executed. The important point with the high larynx is that if the larynx, with the epiglottis that protects its opening, contacts the posterior edge of the soft palate, a space is formed, which is bounded above by the soft palate, behind by the anterior surface of the larynx, and in front and below by the top of the tongue. This space temporarily accumulates food, prior to its onward passage via pharynx and oesophagus. This storage area includes the valleculae (pockets formed between the larynx and the surface of the back of the tongue) and will be referred to as the vallecular space.

Growth in length of the human pharynx (starting a few months after birth) is associated with a descent of the larynx so that its contact with the soft palate is lost. There is consequently no longer an enclosed space in which food can be stored or accumulated, and the airway is no longer anatomically separated from the food passage. A variety of measures operate to protect the airway during swallowing in this situation. They include interruption of breathing, closure of the glottis, tipping the larynx forward so that the back of the tongue bulges over it during swallowing, plus bending of the epiglottis back and down over the laryngeal opening. Because of the low position of the glottis, the pattern of swallowing in the mature human is the exception to the general pattern in mammals. All the early studies of swallowing were carried out on human adults so that the traditional ideas and terminology of swallowing all reflect that origin. Thus swallowing of food is described as being divided into three phases (usually oral, pharyngeal, and oesophageal). In man, approximately 600 swallows occur every 24 hours, but only about 150 of these are concerned with food and drink; the rest simply clear saliva from the mouth.

When cineradiographs of mammalian (non-human) feeding are examined, it becomes clear that there are two separate processes that first fill, and then periodically empty the vallecular space so that the contents pass directly down the oesophagus. Adequate filling of the space appears to be the trigger for emptying. Unless one includes all of the tongue and jaw movements involved in suckling, lapping, or chewing, the true swallow consists only of emptying the vallecular space and the subsequent movement of the bolus down the oesophagus. In contrast, in the human adult, only one transport cycle occurs as the two processes of vallecular filling and of vallecular emptying coalesce within a single cycle of jaw and tongue movement. This occurs because emptying is usually initiated immediately the first trace of food material enters the vallecular region. The question then becomes one of how vallecular emptying is triggered so readily in the adult human, when (unlike other mammals) only a trace of food or liquid may have reached the region. In adult man, unlike other mammals, the movement of a bolus backwards within the mouth (intra-oral transport) is consequently described as the first phase of a swallow, because of its continuity with vallecular emptying.

The neural mechanisms involved in swallowing involve a number of nerves supplying the mucous membrane that lines the structures forming the vallecular space. The most important are the ninth and tenth pairs of cranial nerves (glossopharyngeal and vagus). A branch of the vagus nerve carries important sensory input from the larynx, the epiglottis, and particularly from the vallecular storage area that is present in infants and in all other non-human mammals, i.e. in all those with a high glottis. In these cases, swallowing can be elicited reflexly by fluid in the vallecular space even when there are no connections from higher parts of the brain above the brainstem (e.g. in decerebrate animals and in infants with anencephaly, where the cerebral hemispheres are congenitally absent). It can therefore be assumed that all the necessary neural components for swallowing are present below the level of the midbrain and that sensory input from the surface of the palate, epiglottis, and tongue (the walls of the vallecular space) is alone sufficient to provide the activation necessary to elicit a swallow.

The same argument applies to swallowing in the fetus and in the new-born human with an immature central nervous system. However, in the adult human there is no longer an enclosed vallecular space. Consequently, the level of sensory input must be less than that which would arise when all the mucosal surfaces surrounding that space were stimulated by its filling.

The generally accepted view is that the sensory input from the back of the mouth activates a set of neural circuits within the brain stem that collectively produce the pattern of motor activity constituting a swallow. These circuits constitute a pattern generator for the activity involving the thirty or so muscles that take part in a swallow. The relevant network of brain stem neurons receives sensory input from nerves innervating the mouth, and it also receives excitatory fibres descending from the cerebral cortex.

To explain the situation in adult man, it is proposed that the activity in the nerve fibres descending from the cortex is sufficient to lower the threshold for reflex emptying of the valleculae so that only a trace of material has to reach this region to elicit emptying. A conscious swallow therefore seems to differ from other voluntary movements. One can test this oneself by repeatedly swallowing to eliminate saliva from the mouth; swallowing becomes progressively more difficult to perform and it eventually becomes impossible even to initiate the movement; i.e. there is nothing left to elicit the reflex. The corollary is that, in the presence of excitation from the cortex (a conscious desire to swallow), sensory inputs can elicit vallecular emptying very easily, even though only a trace of material has entered the vallecular region. Vallecular emptying and pharyngeal transit are then followed immediately by oesophageal peristalsis (a moving wave of contraction), so that these events follow seamlessly upon the first phase of intra-oral transport, giving rise to the classical appearance of the three-stage human swallow. It is also necessary to relax the sphincters (rings of muscle fibres) at the top and the bottom of the oesophagus so as to allow the passage of the bolus into the oesophagus and then into the stomach respectively.

‘Dysphagia’ is a word used to describe difficulty or discomfort in swallowing. Clearly a cyst or tumour restricting the width of the pharynx or oesophagus could give rise to such a state. A number of other types of disorder affect swallowing. These include muscle weakness, inability to relax a sphincter, peripheral nerve lesions, and central nervous system damage: a lesion in the medulla can directly damage the neurons making up the swallowing centre. More commonly, swallowing becomes disordered when the motor nerve fibres descending from the cerebral cortex are interrupted, as in a stroke. The malfunction occurs presumably because an important source of excitation to the relevant cells in the medulla is removed, so raising the threshold for reflex emptying of the valleculae. Failure to maintain a competent sphincter at the lower end of the oesophagus (which can occur in diaphragmatic hernia, when part of the stomach protrudes upwards through the diaphragm into the chest) permits regurgitation of the acid contents of the stomach; this can cause discomfort when swallowing and is sometimes loosely classified as dysphagia.

Allan Thexton

Bibliography

Thexton, A. J. and and Crompton, A. W. (1999). ‘Control of Swallowing’ in Scientific Basis of Eating (Frontiers of Oral Biology. Vol 9) Ed. R. W. A. Linden, Karger, Basel, p. 168–222.

See alimentary system.See also epiglottis; larynx; pharynx; tongue.