Foundations and Professional Practice · 7 min read
Anatomy and Physiology of Speech and Swallowing
Respiration: The Power Source
Inspiration follows Boyle's law: the diaphragm, the principal muscle of inspiration (phrenic nerve, C3–C5), contracts with the external intercostals, thoracic volume rises, alveolar pressure falls, and air flows in. Accessory muscles (sternocleidomastoid, scalenes) add lift in deep inspiration. The lungs (three lobes on the right, two on the left) follow the chest wall through pleural linkage, the negative pressure between the pleurae. Forced and speech expiration recruit the internal intercostals and the abdominal muscles (rectus abdominis, external and internal obliques, transversus abdominis).
| Feature | Quiet (tidal) breathing | Speech breathing |
|---|---|---|
| Inspiration time | About 40% of the cycle | About 10% of the cycle (quick inspiration, long expiration) |
| Volume used | About 10% of vital capacity | About 20–25% of vital capacity, in the midrange |
| Expiration | Passive: elastic recoil, gravity | Active: inspiratory muscles brake the recoil (checking action), then expiratory muscles add force |
- Tidal volume: air moved in one quiet breath (about 500 mL in adults). Residual volume: air that cannot be exhaled.
- Vital capacity = inspiratory reserve + tidal + expiratory reserve volumes. Total lung capacity = vital capacity + residual volume.
- Subglottal pressure: about 3–5 cm H_{2}O is the minimum to start phonation; conversational speech uses roughly 5–10 cm H_{2}O. Holding 5 cm H_{2}O for 5 seconds is a common benchmark of adequate support for speech.
Phonation: Laryngeal Framework and Muscles
The adult larynx lies at about C3–C6, suspended from the hyoid bone; it protects the airway, fixes the thorax for effort, and produces voice. Its unpaired cartilages are the thyroid (largest; the vocal folds attach to its inner anterior surface), the cricoid (the only complete ring), and the epiglottis. The paired arytenoids sit on the cricoid: the vocal process anchors the vocal ligament, and the muscular process anchors the PCA and LCA.
| Muscle | Action on the vocal folds | Innervation (CN X) |
|---|---|---|
| Posterior cricoarytenoid (PCA) | Abducts; the only abductor | Recurrent laryngeal nerve (RLN) |
| Lateral cricoarytenoid (LCA) | Adducts (medial compression) | RLN |
| Interarytenoids (transverse, oblique) | Adduct; close the posterior glottis | RLN |
| Thyroarytenoid (medial part = vocalis) | Shortens and thickens the folds, lowering pitch | RLN |
| Cricothyroid | Lengthens and tenses the folds, raising pitch | Superior laryngeal nerve (SLN), external branch |
The left RLN loops under the aortic arch, a longer course that is more exposed to surgical injury. The internal branch of the SLN carries sensation from above the folds. Suprahyoid muscles raise the whole larynx; infrahyoids lower it.
Phonation: Vocal Fold Vibration
In the myoelastic–aerodynamic theory, muscles only set fold position and tension. Subglottal pressure builds beneath the adducted folds and blows them apart; elastic recoil and the pressure drop of fast-moving air (Bernoulli effect) close them, and the cycle repeats at the fundamental frequency.
| Parameter | Mechanism |
|---|---|
| Pitch | Cricothyroid lengthens the folds (more tension, less mass per unit length): higher pitch. Thyroarytenoid shortens and thickens them: lower pitch |
| Loudness | Higher subglottal pressure with firmer medial compression; the folds stay closed for more of each cycle |
| Register | Pulse (glottal fry): lowest. Modal: everyday speech. Falsetto (loft): highest; thin, stretched folds vibrate at their edges |
Resonance and the Velopharyngeal Mechanism
The pharynx has three divisions: nasopharynx (skull base to velum), oropharynx (velum to hyoid level), and laryngopharynx or hypopharynx (hyoid level to the esophageal inlet, behind the larynx). The velopharyngeal port closes for every English phoneme except /m, n, ŋ/: the velum lifts and retracts toward the posterior pharyngeal wall while the lateral walls close in.
| Muscle | Action | Nerve |
|---|---|---|
| Levator veli palatini | Primary elevator; lifts and retracts the velum | CN X |
| Tensor veli palatini | Tenses the velum; opens the Eustachian tube | CN V |
| Musculus uvulae | Shortens the velum and adds bulk to its nasal surface | CN X |
| Palatoglossus; palatopharyngeus (anterior and posterior faucial pillars) | Lower the velum; palatoglossus also raises the back of the tongue, and palatopharyngeus narrows the pharynx | CN X |
Closure patterns are coronal (velum to posterior wall; the most common), sagittal, circular, and circular with Passavant's ridge. Inadequate closure causes hypernasality and nasal emission: insufficiency is structural (cleft palate), incompetence is neuromotor, and mislearning is learned (see Chapter 2.6).
Articulation
The articulators are the tongue (the most important), lips, teeth, alveolar ridge, hard palate, velum, and mandible. CN XII supplies every tongue muscle except the palatoglossus (CN X); CN VII supplies the orbicularis oris (lips) and buccinator (cheeks); CN V supplies the muscles of mastication (masseter, temporalis, and the pterygoids). There are 20 primary and 32 permanent teeth.
| Class | First-molar relationship | Presentation |
|---|---|---|
| I (neutroclusion) | Normal molar relationship | Normal jaw relationship; individual teeth may be misaligned |
| II (distoclusion) | Mandibular first molar posterior to normal | Retruded mandible; excessive overjet (horizontal overlap) |
| III (mesioclusion) | Mandibular first molar anterior to normal | Prognathic mandible or small maxilla; underbite |
Swallowing: Structures and the Four Phases
A central pattern generator in the medulla (nucleus tractus solitarius for sensory input, nucleus ambiguus for motor output) organizes the swallow under cortical modulation. Residue collects in the valleculae (between tongue base and epiglottis) and the pyriform sinuses (beside the larynx, above the esophageal inlet).
| Event | Mechanism and purpose |
|---|---|
| Velar elevation | The velum lifts and retracts, sealing the nasopharynx |
| Hyolaryngeal excursion | Suprahyoids and thyrohyoid pull the hyoid and larynx up and forward, which helps open the UES |
| Airway closure at three levels | True folds adduct, then false folds and arytenoids close, then the epiglottis inverts; breathing pauses (swallow apnea) |
| Pharyngeal constriction | The tongue base retracts to the pharyngeal wall and the constrictors contract top to bottom, driving the bolus |
| UES opening | The cricopharyngeus relaxes; hyolaryngeal traction and bolus pressure open the segment |
Sensory triggering travels mainly in CN IX and X; motor output uses CN V, VII, IX, X, and XII. Infants differ anatomically: the larynx sits high with the epiglottis close to the velum, the tongue fills a small oral cavity, and buccal fat (sucking) pads stabilize the cheeks, supporting a suck–swallow–breathe ratio near 1:1:1.
- Speech breathing: inspiration about 10% of the cycle (40% at rest), with long, controlled expiration.
- PCA is the only abductor. Cricothyroid (SLN, external branch) raises pitch; the RLN supplies all other intrinsic muscles.
- Levator veli palatini elevates the velum; tensor veli palatini (CN V) opens the Eustachian tube.
- Pharyngeal phase: velar elevation, hyolaryngeal excursion, three-level airway closure, constriction, UES opening.
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