Page 88 - Edited - Webster HEAD AND NECK - part 2-Merge PDF

HN 281

SWALLOWING AND VOMITING

Mastication (HN 109), salivation (HN 229) and swallowing are all closely-linked mechanisms – mastication solid foods
precedes swallowing, and mastication is difficult and swallowing almost impossible unless such food is moistened with saliva. Also,
mastication and the early phases of swallowing involve complex, repetitive movements carried out by skeletal muscles: they therefore
require central pattern generators operating through peripheral nerves connecting brain and muscles. It is also important to remember
that all movements involving skeletal muscles require, for their proper coordination, sensory feedback both from the surfaces (epithelia)
involved (this is exteroception) and from the muscles, tendons and any ligaments involved (proprioception).

SWALLOWING (DEGLUTITION); CHOKING; GAGGING AND VOMITING

1. SWALLOWING (HN 285; 286)

We swallow about 600 times in each 24 hours, but only one third of these events is associated with eating and drinking. The
remainder are attributable to the production of resting state saliva (even during sleep we swallow ~ 10 times hour ) and sometimes
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personality and force of habit. Some people swallow a great deal of air (aerophagia) - much to their discomfort and that of their
companions who subsequently endure the belching (eructation) more or less stoically.

The principal problem to be solved in the action of swallowing arises because of the relative positions of the nasal and oral
cavities on the one hand, and of the oesophagus (gullet) and trachea (airway) on the other: swallowed food must cross the airway in the
pharynx, and it is essential to prevent food entering the larynx (HN 254; 285 et seq). Remember that the motor supply to the skeletal
muscles of the larynx, pharynx and soft palate is from the paired medullary nucleus ambiguus through the vagus nerve of each side. It is
also important to realise that swallowing is a sequential reflex, its initial phases, involving skeletal muscles (HN 285; 286), controlled by
a central pattern generator, i.e. its constituent parts follow one another in a preset order, one triggering off the next. It can be divided
into five phases, the first of which is voluntary and can be reversed at will; the next three phases involve passage through the pharynx
and require the pattern generator and the vagus nerves; the final phase requires no pattern generator or extrinsic nerves, since it is
controlled by the enteric autonomic nerve plexus of the lower oesophagus. The pattern generator becomes fully operational only after the
28th week of gestation: for the last trimester of intrauterine life, the foetus regularly swallows amniotic fluid; a foetus delivered before
the 28th week cannot swallow, and has to be fed by intragastric tube. In the following, refer to HN 286.

Phase I: Well-chewed and saliva-moistened food is rolled into a ball (bolus) by the tongue, which is then approximated to the hard
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palate from anterior to posterior to roll the bolus towards the opening of the oro-pharynx . This voluntary movement implies a cortically-
controlled, but hindbrain co-ordinated, mechanism involving trigeminal nerve afferents and hypoglossal nerve efferents (tongue muscles),
as well as the trigeminal nerve supply of the mylohyoid diaphragm which supports the tongue. (Feel your mylohyoid muscles while
swallowing: it contracts to prevent the floor of the oral cavity bulging downwards as the tongue presses against the hard palate.)

Phase 2: Once the bolus passes the isthmus and enters the oropharynx, the mechanism becomes entirely reflex: the afferent nerve
becomes the glossopharyngeal (but discharging into the hindbrain general sensory nucleus centrally). The pharyngeal tongue is raised by
contraction of the pharyngeal palatoglossus and styloglossus muscles (HN 215 Fig. B; 230; 232; 263; 264, Fig. A). As the bolus is
squeezed into the posterior part of the oro-pharynx, the soft palate is tightened and raised by the muscles from the cranial base (HN 215;
216; 264, Fig. B) to shut off the naso-pharynx above. Gentle stimulation of this part of the oropharyngeal mucous membrane ( cranial
nerve IX) by the bolus triggers the pattern generator to produce peristalsis in the pharyngeal constrictors (HN 255 et seq.; 280; 286) to
push the bolus downwards.

Phase 3: At the same time the two ary-epiglottic folds (forming the sides of the larynx between the arytenoid cartilages and
the epiglottis, and guarding the laryngeal inlet – HN 271) are approximated, thus shutting off the upper larynx from the
hypopharynx (HN 262, Fig. A; 263, Fig. D; 264; 286). This means that (a) at this time respiration must temporarily cease
(involving inhibition of respiratory centres - SI 77 et seq).; and (b) seen from the side, the closed entrance to the larynx
has a bevelled profile, the longest and vertical edge of the bevel (the epiglottis) in front, the sloping edge (the
approximated aryepiglottic folds) behind, and presenting an inclined slope plunging downwards and backwards towards the lower
pharynx HN 286, Phase 3; see also 277). Longitudinal pharyngeal muscles (HN 263) pull the larynx upwards , to insert the sharp
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1 Note: The oro-pharyngeal isthmus, demarcated by the soft palate above, tongue below and a pair of palatoglossal folds or anterior pillars of the
fauces on either side (HN 221 ). The palatoglossal folds are formed by longitudinal pharyngeal muscles. Like all pharyngeal muscles, they are
skeletal, and supplied by the nucleus ambiguus of the medulla, via the vagus nerve.
2 Note: Feel your larynx – “Adam's apple” - and swallow. It leaps upwards. The muscles are the palato- and stylopharyngeus, and the
salpingopharyngeus. Contraction of this last (attached to the cartilaginous part of the auditory tube, like the tensor of the palate, which is also
contracting, opens the normally closed auditory tubes which is why your ears sometimes pop when you swallow. “Salpinx” means “tube”.)

\NewCMedPhysSc\20 HN SwallowVomit.