The soft tissues lining the larynx are much more complex than originally thought. The mucosa forms the thin, lubricated surface of the vocal folds which makes contact when the two vocal folds are closed. It looks like the mucosa lining the inside of the mouth. However, the vocal fold is not simply muscle covered with mucosa (right).
The thin, lubricates squamous epithelium lines the surface. Immediately beneath it, connected bya complex basement membrane, is the superficial layer of the lamina propria, also known as Reinke’s space which consists of loose, fibrous components and matrix. It tends to accumulate fluid, and it contains very few fibroblasts (cells that cause scar formation). The epithelium is connected to the superficial layer of the lamina propria by a sophisticated basement membrane. The intermediate layer of the lamina propria contains primarily elastic fibers and a moderate number of fibroblasts. The deep layer of the lamina propria is rich in fibroblasts and consists primarily of collagenous fibers. It overlies the thyroarytenoid or vocalis muscle. The various layers have different mechanical properties important in allowing the smooth shearing action necessary for proper vocal fold vibration.
Mechanically, the vocal fold structures act more like three layers consisting of the cover (epithelium and superficial layer of the lamina propria), transition (intermediate and deep layers of the lamina propria), and body (the vocalis muscle).
What happens above the larynx?
The supraglottic vocal tract (above the larynx) includes the pharynx, tongue, palate, oral cavity, nose and other structures. Together, they act aas a resonator and are largely responsible for vocal quality or timbre and the perceived character of all speech sounds. The vocal folds themselves produce only a “buzzing” sound. During the course of vocal training for singing, acting, or healthy speaking, changes occur not only in the larynx, but also in the muscle motion, control and shape of the supraglottic vocal tract.
What happens below the larynx?
The infraglottic (or subglottic) vocal tract (below the larynx) serves as the power source for the voice. Singers and actors refer to the entire power source complex as their “support” or “diaphragm.” Actually, the anatomy of support for phonation is especially complicated and not completely understood; and performers who use the terms “diaphragm” and “support” do not always mean the same thing. Yet, it is quite important because deficiencies in support are frequently responsible for voice dysfunction.
The structure of the vocal fold. From Sataloff, R.T., Chowdhury, F., Portnoy, J., Hawkshaw, M.J., Joglekar S. Surgical Techniques in Otolaryngology – Head and Neck Surgery: Laryngeal Surgery. New Delhi, India: Jaypee Brothers Medical Publishers; 2013 with permission.
The purpose of the support mechanism is to generate a force which directs a controlled airstream between the vocal folds which is necessary for vocalization to occur. Active respiratory muscles work together with passive forces. The principle muscles of inspiration are the diaphragm (a dome-shaped muscle that extends along the bottom of the rib cage), and the external intercostal (rib) muscles. During quiet breathing, expiration is largely passive. The lungs and rib cage generate passive expiratory forces under many common circumstances such as after a full breath.
Many of the muscles used for active expiration are also employed in “support” for phonation. Muscles of active expiration either raise the intra-abdominal pressure forcing the diaphragm upward, or lower the ribs, and sternum (“breast bone”) to decrease the dimensions of the thorax, or both, thereby compressing air in the chest. The primary muscles of expiration are “the abdominal muscles,” but internal intercostals, and other chest and back muscles are also involved. Trauma and surgery that alters the structure or function of these muscles or ribs undermines the power source of the voice as do diseases that impair expiration, such as asthma.
Deficiencies in the support mechanism often result in compensatory efforts which utilize the laryngeal muscles, not designed for power source functions. Such behavior can result in decreased voice function, rapid fatigue, pain and even structural pathology including vocal fold nodules. Currently, expert treatment (voice physical therapy) for such problems focuses on correction of the underlying malfunction. This often cures the problem, avoiding the need for laryngeal surgery.