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An appreciation of the normal swallowing process1 and how it may be affected by neurological disease can inform the clinical diagnosis and management of patients who complain of swallowing problems or present with the nutritional and respiratory complications of failure of oral feeding (table 1).
Sherrington originally studied swallowing in decerebrate cats.2 He described the effect of various agents—whiskey, oil, water—which, when dropped onto the area supplied by the superior laryngeal nerve, would elicit a “swallow”. This “swallow” did not involve any preparation of a bolus or any lingual propulsion, and was justifiably considered to be a reflex. I will use the terms reflex or crude swallowing to describe the quintessential components of swallowing, as observed in Sherrington’s cat:
open upper oesophagus
move bolus into oesophagus
The first two are the sine qua non of swallowing, the third a compulsory accompaniment for oral feeding.
In unconscious humans this sort of swallow is an upper airway protective reflex in response to the unexpected arrival of a bolus in the pharynx. When awake, swallowing is a planned manoeuvre; at a chosen time, with or without a suitably prepared bolus, we swallow. I refer to this as voluntary swallowing, a process initiated voluntarily and not necessarily occurring as a response to stimulation by a bolus.
Bolus preparation is a separate process and may involve chewing (cranial nerve V motor and sensory), mouth closure (VII), manipulation and retrieval of the bolus (V, VII, XII), and initial propulsion of the bolus (VII and XII).
In the following the term dysphagia is avoided because it can be used to denote symptoms, clinical signs, radiological signs, and as a putative mechanistic explanation for otherwise unexplained nutritional or respiratory problems. Also, instead of the traditional, bolus based separation …