Thoracic Innervation of the Intercostal Muscles
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Whereas the inspiratory and expiratory phases of breathing have almost equal durations during quiet respiration, during discourse the length of an expiration is linked to that of a sentence or sentence-like unit. During quiet respiration the aspiratory muscles of the chest, abdomen and diaphragm expand the lungs. They then release and the elastic recoil of the two lung “sacs” provide the motive force – alveolar air pressure starts high and gradually falls as the lungs deflate. Alveolar (lung) air pressure during speech is kept to a planned steady level (about 10 cm H2O). This entails opposing the elastic recoil force with chest and abdominal muscles used to inflate the lungs during quiet respiration. These muscles in effect ‘hold-back” the force generated by the elastic recoil until it reaches the 10 cm H2O level. During normal discourse, speakers tend to inflate their lungs to a greater degree before they produce a long sentence. This also entails setting a hold-back function predicated on the length of the sentence that the speaker intends to utter and constitutes evidence for the psychological reality of sentences. Transient alveolar air pressure peaks, which raise the fundamental frequency of phonation, are produced by speakers to emphasize words or phrases. The diaphragm which has few muscle-spindles that could monitor its activity is immobilized during speech and in singing. Similar planned chest and abdominal muscle activity occurs during singing. Acoustic analyses of the isolation calls of other mammalian species suggest that the regulation of alveolar air pressure during vocalization is similar to that of humans. The cortical-striatal cortical neural pathways controlling muscle activity during speech appear to be similar in humans and other primates. Recent studies of the FOXP2 transcriptional gene suggest that the human neural pathways are more efficient owing to greater synaptic plasticity and connectivity. The muscular commands that are necessary to produce a steady air pressure in the lungs during speech production are complex. The muscles of the chest (intercostals and abdominal muscles) are innervated through spinal nerves that exit the spinal cord in the thoracic region. Modern humans have vertebral canals in the thoracic vertebrae that are large relative to body mass (compared to other primates), reflecting an enlarged cross-sectional area of the spinal cord in the thoracic region. The human spinal cord has a greater proportion of grey matter in this region than do the spinal cords of other primates, reflecting greater motor control of the muscles of the chest, possibly having to do with breathing control for speech production, or control of trunk movement for bipedal locomotion. A single Homo erectus specimen preserving this portion of the vertebral column does not show an enlargement of the thoracic vertebral canal, suggesting that the condition seen in modern humans is of relatively recent evolutionary origin.
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