Page 131 - Edited - Webster HEAD AND NECK - part 1

HN 109

MASTICATION AND SUCKLING

1. MASTICATION

For most of our lives mastication is a seemingly voluntary act - but so is walking, which involves an
elaborate spinal cord mechanism of reflex arcs controlled by central pattern generators themselves located in the
cord. So too. with mastication, but in this case the pattern generator is located in the hindbrain, and requires the
integrity of the trigeminal, facial and hypoglossal nerves. If we look at the motor limb of the arc first: the
mandible (lower jaw) is moved on the maxilla (upper jaw) by skeletal muscles all supplied by the trigeminal
nerve (cranial nerve V) (HN 99; 100; 108). This nerve also supplies the muscle (the mylohyoid) which forms
the floor of the oral cavity and supports the tongue (HN 72; 74-78). The tongue (skeletal muscle) (HN 221 et.
seq.) moves around under the control of the hypoglossal nerves (cranial nerve XII). The cheeks contain striated
muscle (buccinator HN 72; 73; 219; 220; 259; 260) supplied by the facial nerve (cranial nerve VII), which also
supplies the orbicularis oris muscle of the lips. Somatic (general) sensory feedback is channelled via the
trigeminal nerves (lips, tongue, cheeks, palate, gums (HN 222; 245); and chemoreceptive (taste) via the seventh
and ninth cranial nerves (HN 222 et seq.). Proprioceptive feedback (including from the ligaments binding teeth
into their sockets) travels with the trigeminal nerves - even that from the muscles of the lips. cheeks and tongue.
Biting precedes chewing and involves the lips and more anterior teeth. The rhythmical nature of chewing is
imposed largely by a central pattern generator. Notice:

a) that the mandible is moved by the primary muscles of mastication - most importantly the masseter,
temporalis and pterygoid muscles.

b) that the tongue and buccinator muscles (secondary muscles of mastication) keep food between the
teeth and are essential for effective mastication. The tongue also tosses the food around to mix it with
saliva and distribute dissolved chemicals around the groups of taste buds by which we become aware
of the taste of the food (ascending fibres from solitary nucleus to thalamus and so to cerebral cortex).
Control of the power of the bite is controlled by proprioceptors of the ligaments binding teeth to their
sockets (the periodontal ligaments), as well as by nociceptors i.e. pain receptors (including those in the
oral mucous membrane - nipping your tongue or biting on a nail in your refectory bun instantly puts a
stop to chewing). Descending fibres from the motor cortex are important to impose the “voluntary”
element on mastication, and in that the jaw muscles can be operated to open and close the mouth
independent of chewing - as can the tongue and lips

c) that the orbicularis oris muscle prevents especially liquids (including resting state saliva) from dribbling
out of the mouth.

Disordered mastication

Mastication can be disturbed by paralysis of any of three cranial nerves - the trigeminal, facial, or
hypoglossal - although. except for facial palsy, these are rare. Much more common are difficulties due to intra-
oral pain, e.g. aphthous ulcers (HN 224), dental caries - or absence of teeth! Glossitis (“inflammation of the
tongue”) is just as likely to present as dysphagia (difficulty in swallowing - see below). Glossitis is usually due
to deficiencies of iron and/or the vitamin B complex and therefore accompanied by anaemia. The lingual mucosa
atrophies, and the loss of papillae (HN 226, Fig. B) from its upper surface and sides make the entire tongue appear
smooth and shiny. Carcinoma of the tongue (often associated with pipe smoking, just as carcinoma of the intra-
oral cheek is associated with tobacco chewing - afficianados store the wad in one vestibule) similarly can present
as difficulty in either chewing or swallowing.

2. SUCKLING
I have given suck, and know / How tender ‘tis to love the babe that milks me:
I would, while it was smiling in my face /Have plucked my nipple from his boneless gums ...
(William Shakespeare: Macbeth)

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