Normal speech involves a highly coordinated sequence of contractions of the respiratory musculature, larynx, pharynx, tongue, and lips. These muscles are innervated by the facial, vagal, hypoglossal (reviewed in Chapters 1.1 , 13 , and 14 ), and phrenic nerves. The nuclei of these nerves are controlled by the motor cortices through the corticobulbar tracts. Coordination of the movements of the articulatory structures requires simultaneous and coordinated activation of these cranial nerves, not only by direct cortical motor activation but also by indirect involvement of the basal ganglia and cerebellum. The basal ganglia and the cerebellum are also important participants in producing the prosodic aspects of speech, including pitch or frequency, stress or the differential emphasis placed on syllables, and rhythm or timing.
Thus, pathology affecting speech could be located at several sites within the motor systems. Speech is impaired when any of the following structures are damaged or dysfunctional: motor cortex and its outflow tracts, cerebellum, basal ganglia, brain stem, peripheral portions of cranial nerves controlling the speech apparatus, and target speech musculature, including facial muscles, pharynx, palate, tongue, vocal cords, diaphragm, and intercostal muscles.
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