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It is difficult to consider that the process of speaking does not begin with the mouth, or even the vocal tract, the lungs, or the spinal cord, but rather occurs in all of those places after beginning in the brain. In fact, any utterance originates primarily in cognitive systems within multimodal association cortices of the brain, in which a thought that may initiate speech occurs. Then, in the inferior frontal cortex, the unimodal Broca’s area functions to provide projections to the primary motor cortex regarding speech movements. The corticospinal and corticobulbar tracts then bring the message down to the spinal cord and cranial nerves through fibers that originate in the motor cortex. Both the spinal and cranial nerves are essential for the respiratory, articulatory, resonance, and phonatory functions that together create speech production. The corticospinal tract is especially crucial to respiratory function. Information for respiratory control originates in the precentral gyrus of the motor cortex and travels through the medulla where the fibers decussate to the cervical, thoracic, and lumbar regions of the spinal cord via the corticospinal tract. From here, spinal nerve C4 of the phrenic nerve innervates the diaphragm, which functions as a primary and crucial muscle for inspiration. During speech breathing, the diaphragm is innervated by the phrenic nerve at the same time the external intercostal muscles are innervated by the intercostal nerves of the thoracic region of the spinal cord. The internal intercostals and abdominal wall muscles will also contract in order to stiffen the rib cage and prevent intercostal spaces from being sucked in by the negative air pressure caused by inspiration in the lungs. For conversational speech, inspiration will most likely be short and quick, Stelmach 2 while expiration will be gradual. This is possible because even though the diaphragm and external intercostals contract simultaneously for inspiration, the diaphragm relaxes immediately while the external intercostals remain contracted. When the external intercostals finally relax, the internal intercostals will contract to maintain the desirable 5-8 cm H2O of subglottic pressure in order to continue vocal activity. During expiration, phonation can be explained by the Myoelastic Aerodynamic Theory. This theory requires subglottic pressure, vocal fold elasticity, and the Bernoulli effect in order to produce vocal activity. The vocal folds are able to adduct due to their elasticity, or ability to be manipulated. They are adducted by the lateral cricoarytenoid and interarytenoid muscles, which adduct the arytenoid cartilages by rotating them medially, pulling them forward and downward, and in turn adducts the vocal folds, as they are attached to the arytenoid cartilages at the vocal process and fovea oblonga. This process describes the prephonation phase, in which the vocal folds move to an adducted position. As soon as the vocal folds are adducted, the attack phase begins and vocalization occurs. This is referred to as simultaneous attack. Vocal folds are abducted by the posterior cricoarytenoid muscles, which simply reverse the arytenoid cartilages back to their original position (again, vocal fold elasticity is important). It is important to remember that in the word /fon/, the /f/ phoneme is unvoiced, and therefore the vocal folds remain abducted for the production of its sound. Afterward, though, the /o/ and /n/ are voiced and require adduction. For these sounds to be produced, the vocal folds must adduct and allow subglottic pressure to build up. Phonation cannot occur until subglottic pressure, or the force of air upon the inferior surfaces of the vocal folds, overcomes the medial compression or resistance of the vocal folds to stay shut. The vibration of the vocal folds causing phonation after this is not explained by the arytenoid muscles adducting and abducting the vocal folds repeatedly. Instead, the Bernoulli effect causes a vacuum that increases the velocity of the air through the vocal folds, which have natural recoil that brings them back Stelmach 3 together again. With this complex process of phonation occurring at a frequency of nearly 200 Hz for typical vocal tone, resonance may occur as the air within the vocal tract vibrates within its chambers. Before this begins, considering the goal is to produce the word /fon/, one must produce the unvoiced /f/ phoneme while the vocal folds are still abducted. /f/ is a labiodental fricative, so it is articulated by compressing the upper teeth upon the lower lip, which is the primary articulator for this phoneme. The lip is moved back and compressed onto the upper teeth by its orbicularis oris muscles. Additonally, air is forced through a narrow constriction within the vocal tract during the production of /f/ in order to produce turbulence and create the aperiodic sound known as /f/. Due to coarticulation, while the /f/ is being produced, the tongue is already placed horizontally in the back and vertically in the middle to prepare for the /o/ phoneme. Because /o/ is a vowel, it’s articulation is made solely by this tongue placement and through phonation. This tongue placement is possible by the hyoglossus and anterior fibers of the genioglossus extrinsic tongue muscles. Once the speaker moves from the /f/ to the /o/ phoneme, vocal fold vibration and voicing begins. As voicing continues, the /n/ phoneme is produced by the superior longitudinal intrinsic muscles raising the tongue on the maxillary alveolar ridge. Because /n/ is a nasal consonant, the velopharyngeal port opens, meaning that the velum separates from the posterior pharyngeal wall. All of this motor movement results from the corticobulbar tract sending messages from the Broca’s area and the motor cortex to the motor nuclei in the medulla of the vagus, spinal accessory, facial, glossopharyngeal, trigeminal, and hypoglossal cranial nerves, which in turn innervate each of the muscles necessary for articulation, resonance, and phonation at the vocal folds. Though /fon/ may seem like a simple utterance, it is clear that it is actually an incredible process that proves just how complex and miraculous the abilities of the human body are.

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