Page 89 - Edited - Webster HEAD AND NECK - part 2-Merge PDF

HN282
edge of the bevel in front of the bolus which rolls backwards and downwards , actively propelled by the peristalsis of the
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pharyngeal walls, through the laryngo-pharynx (hypopharynx) (sensory supply, cranial nerve X) and towards the
oesophagus.

Phase 4: The oesophagus begins behind the cricoid cartilage where it is closed by a skeletal muscle sphincter - the cricopharyngeus
muscle (the most inferior part of the inferior pharyngeal constrictor – HN 286). The central pattern generator must programme this
sphincter to relax as the skeletal muscle peristaltic, wave it is coordinating reaches the 1 lower pharynx. If this superior sphincter does not
relax, pressure in the hypopharynx rises abnormally, and the mucous membrane of the defective posterior wall (the dehiscence of Killian
between the inferior constrictor proper and crico-pharyngeus muscle – HN 280) evaginates backwards. If this event is repeated with every
swallow, a permanent pharyngeal diverticulum or pouch is eventually created, within which food collects. Normal relaxation of the
sphincter allows food to enter the oesophagus.

Phase 5: The muscle of the upper third of the oesophagus is skeletal (cranial nerve X, nucleus ambiguus); the lower third smooth (visceral-
enteric nerve plexus modulated by axons from the parasympathetic motor nucleus of cranial nerve X); the intermediate third is mixed
smooth and skeletal muscle (SI 145). Oesophageal peristalsis carries the bolus to the stomach: that for the upper oesophagus (skeletal
muscle) is organised by the central pattern generator, and for the lower (smooth muscle) by the enteric autonomic plexus, i.e. by autonomic
neurons within the oesophageal wall itself, independent of extrinsic nerve supply.

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Peristaltic waves pass down the oesophagus at ~5 cms sec : it takes ~5 secs for a bolus to travel its length. At thelower end of
the oesophagus there is another sphincter, the most important component of which is smooth muscle. (The skeletal muscle of the surrounding
diaphragm plays a secondary rôle.) This sphincter is normally closed to prevent reflux of acidic gastric contents into the oesophagus, and normally
relaxes a second or so after a bolus reaches it. Failure of relaxation is achalasia (pron: “akalazia”) and is due to local disorder of the enteric plexus:
food accummulates in the oesophagus, which dilates.

Swallowing liquids is slightly different, in that they bypass the larynx: the vertical epiglottis divides the stream into right and
left, which flow each to one side of the ary-epiglottic folds. These two lateral “river beds” or gullies - the piriform fossae (HN 277) - are
sheltered from the clearing mechanism of coughing and irritating particles of food stuck in them can often be dislodged by drinking.

Notice that the mechanism of swallowing is active and works in any posture, independent of gravity, and sometimes against it.
However, drinking is difficult in most positions other than the upright, simply because gravitational effects on the fluid are great:
pharyngeal peristalsis cannot grip liquids, which therefore run about willy-nilly and easily get into the glottis and nasal cavity when the
sphincteric mechanisms have relaxed.

It is also worth noting that although swallowing is possible with the mouth open, it is difficult and uncomfortable because the
anterior part of the tongue cannot be brought up to the palate. The progress of the sequence of the reflex is thereby disrupted. This is why
dental practitioners stick a sucker in your mouth: they are usually pouring in water (from their nasty drills) and stimulating salivation (have
you ever tasted a dentist?). In my experience, the sucker does not eliminate the sense of imminent death by drowning, and the sense that,
as you sneak a surreptitious swallow, your larynx is about to pop into your mouth.

Dysphagia (difficulty in swallowing)

The commonest cause of dysphagia is pain accompanying pharyngitis, palatine tonsillitis and even laryngitis if the inlet is
involved, and especially if the problem is tuberculosis. Neuromuscular disorders cause dysphagia by affecting the skeletal muscles
involved in the process, and include myesthenia gravis, disease of the hindbrain (e.g. motor neuron disease, multiple sclerosis) and
peripheral neuropathies, especially associated with (the now very rare) infectious disease diphtheria. Achalasia is a special disorder of the
enteric nerve plexus of the lower oesophagus (SII 104). Oesophageal disorders are intrinsic (carcinoma, traumatic stenosis, reflux
oesophagitis - see SI 144; and also SII 104) or extrinsic, i.e. due to external pressure from disorder of organs anatomically related to the
oesophagus, for example, an enlarged thyroid gland, mediastinal lymphadenopathy, bronchial carcinoma or even pericardial effusion
(inflammatory increase in pericardial fluid). Pressure from a food-filled pharyngeal diverticulum (dehiscence of Killian) can likewise
cause dysphagia.

2. CHOKING.

Food, and especially liquids, can easily slip past the ary-epiglottic folds into the upper larynx, and even through the glottis. The
sensory feedback (cranial nerve X) sets off a coughing reflex. Food or foreign bodies (including dental detritus) lodged in the hypopharynx
or laryngeal inlet is very unpleasant and can lead to glottal spasm (which is life-threatening).

3 Note: The bolus rolls over the epiglottis, which bends passively over the laryngeal incline and later flips back into the vertical (it is
elastic cartilage). The epiglottis is not crucial for closing the larynx: swallowing occurs normally after its removal.
\NewCMedPhysSc\20 HN SwallowVomit.