The act of speaking involves a highly coordinated sequence of actions of the respiratory musculature, larynx, pharynx, palate, tongue, and lips. These structures are innervated by the vagal, hypoglossal, facial, and phrenic nerves, the nuclei of which are controlled by both motor cortices through the corticobulbar tracts. As with all movements, those involved in speaking are subject to extrapyramidal influences from the cerebellum and basal ganglia. The act of speaking requires that air be expired in regulated bursts and each expiration must be maintained long enough (by pressure mainly from the intercostal muscles) to permit the utterance of phrases and sentences. The current of expired air is then finely regulated by the activity of the various muscles engaged in speech.
Phonation, or the production of vocal sounds, is a function of the larynx, more particularly the vocal cords. The pitch of the speaking or singing voice depends upon the length and mass of the membranous parts of the vocal cords and can be varied by changing their tension; this is accomplished by means of the intrinsic laryngeal muscles, before any audible sound emerges. The controlled intratracheal pressure forces air past the glottis and separates the margins of the cords, setting up a series of vibrations and recoils. Sounds thus formed are modulated as they pass through the nasopharynx and mouth, which act as resonators. Articulation consists of contractions of the pharynx, palate, tongue, and lips, which interrupt or alter the vocal sounds. Vowels are of laryngeal origin, as are some consonants, but the latter are formed for the most part during articulation; the consonants m, b, and p are labial, l and t are lingual, and nk and ng are guttural (throat and soft palate).
Defective articulation (dysarthria) and phonation (dysphonia) are recognized at once by listening to the patient speak during ordinary conversation or read aloud. Contrived test phrases such as “Methodist Episcopal” or attempts at rapid repetition of lingual, labial, and guttural consonants (e.g., la-la-la-la, me-me-me-me, or k-k-k-k) bring out the particular abnormality. Disorders of phonation call for a precise analysis of the voice and its apparatus.
In pure dysarthria or its most severe representation, anarthria, there is no abnormality of the cortical language mechanisms. The patient is able to understand perfectly what is heard and has no difficulty in reading and writing, although he may be unable to utter intelligible words. This is the strict meaning of being inarticulate. Defects in articulation may be subdivided into several types: lower motor neuron (neuromuscular); spastic (pseudobulbar); rigid (extrapyramidal); cerebellar-ataxic; and hypoand hyperkinetic dysarthrias, each of which is taken up below.