Level 3 Unit 2 Part 28: Motor control of speaking, swallowing, and chewing

The motor control of enunciating clearly when speaking, and of swallowing, have overlapping involvement of lower motor neuron innervation for muscles controlling: the lips via the facial nerve; the tongue via the hypoglossal nerve; and the soft palate, pharynx (throat), and larynx (voicebox) via the vagus and glossopharyngeal nerves. These lower motor neurons are all in the lower brainstem: the facial nucleus for the facial nerve; the hypoglossal nucleus for the hypoglossal nerve; and the ambiguus nucleus for the vagus and glossopharyngeal nerves. These lower motor neurons are usually innervated by axons of bilateral corticobulbar tracts, except for those controlling lower facial muscles.

Despite this, speech and swallow appear to require such fine muscle coordination that unilateral corticobulbar tract dysfunction often causes dysarthria (abnormal enunciation) and dysphagia (abnormal swallowing). Dysphonia (abnormal voice quality), such as a hoarse voice, may occur with lower motor neuron dysfunction to the larynx, but usually not with unilateral corticobulbar dysfunction.

When something solid, other than food, contacts the pharynx on one side, the muscles of both sides of the pharynx usually contract involuntarily, which is called the pharyngeal or gag reflex. The afferent part of this reflex consists of information from mechanoreceptors in the pharynx travelling into the brainstem via the ipsilateral glossopharyngeal nerve. The efferent part of this reflex consists mainly of lower motor neurons in bilateral vagus nerves causing bilateral pharyngeal muscle contraction.

The muscles of mastication that move the jaw for chewing are innervated by axons in the trigeminal nerve. These are from lower motor neurons in the motor trigeminal nucleus, which is about midway up the brainstem; this usually receives bilateral corticobulbar tract innervation. The upper motor neurons involved in motor control of speech, swallow, and chewing are in the lateral primary motor cortex, close to the area responsible for motor control of the face.


Level 3 Unit 2 Part 29: Extraocular muscles

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