How to Sing- The Vocal Folds

To innervate is to supply or stimulate. In the human body, nerves originate in the brain and branch out to the spinal cord, connecting to the muscles of the body. Signals from the nerves activate and control our muscles, and also give us feedback. This two-way process is called innervation. 

Two nerves innervate the larynx: the recurrent laryngeal nerve and the superior laryngeal nerve, both branching out of the vagus nerve. The recurrent laryngeal nerve wraps around the aorta, the main artery leading out of the heart, and then comes back up and attaches to the larynx.

STRUCTURE OF THE VOCAL FOLDS

Men have larger vocal folds and a correspondingly larger and more prominent larynx than women and children do. The length of the vocal folds is about 16 mm or .62 inch for an adult male and about 10 mm or .39 inch for an adult female.

The male larynx is more prominent due to the 90-degree angle of formation; the female larynx has a softer 120-degree angle and is less prominent. Children’s vocal folds and larynxes are continually growing in size as they mature.

In a very simplified model, the folds can be compared to two side-by-side rubber bands. Air passes between them to initiate sound by creating a vibrational wave.

The vocal folds are very flexible, assuming shapes ranging from short and thick to thinner and longer, while vibrating in complex patterns from bottom to top and front to back as they close and open rapidly. The number of times they complete a cycle of open/closed in a second corresponds to frequency, or pitch, expressed in Hz (hertz).

Like rubber bands, they become longer, thinner, and more taut for higher pitches and they shorten, contract, and become less taut for lower pitches. When the folds are open, the space between them is known as the glottis.

Events that occur underneath the glottis are known as subglottal or subglottic, and events that occur above the glottis are referred to as supraglottic.

SECTIONS OF THE VOCAL FOLDS

The vocal folds are comprised of three sections arranged in layers.

The epithelium and superficial lamina propria make up the outer section known as the mucosa.

The outermost structure of the vocal folds contains the epithelium, which is similar to the scale-like loose skin on the back of the hand. The thin and lubricated epithelium is the covering of the vocal folds and is the area of contact between the two folds. It acts as a capsule, forming the folds into shape.

The next layer is the superficial lamina propria, also known as Reinke’s space, which provides the flexibility of the fold. It is made up of elastin fibers that allow for stretching and elongation, similar to the function of a rubber band. This layer is about 0.5 mm thick.

The mucosal tissue is made of collagen fibers arranged in layers; the outermost layer is quite floppy. Each layer gives varying degrees of strength, flexibility, and stiffness for vibrational activity. The degree of stiffness of the mucosa depends on the state of contraction of the laryngeal muscles.

Mucosa is the loose structure most involved in vocal fold vibration; in upper register phonation, it is the mucosa that vibrates rather than the entire vocal fold. Mucosa is soft, wet tissue similar to the tissue that lines the mouth and throat.

This type of tissue needs to be moist to be healthy; that is why singers should drink lots of water- at least 100 ounces per day!

The next section is known as the vocal ligament.

Vocal fold elasticity and tension are dependent upon the health of the vocal ligament. The ligament functions somewhat like a purse string, bunching the folds or lengthening to allow them to stretch.

The ligament is made up of two layers: the intermediate lamina propria and the deep lamina propria.

The intermediate lamina propria is comprised of both elastin and collagen fibers. The deep lamina propria is composed primarily of collagen, a protein structure. The thickness of the intermediate and deep lamina propria together is about 1-2 mm.

The innermost section is known as the body

The body is comprised of muscles- the thyroarytenoid (TA) and vocalis muscles. The body of the vocal fold is 7-8mm (0.31 in) thick. The fold body helps to close the glottis and regulate tension in the vocal folds during speaking or singing.

The layered structure of the vocal folds is not present at birth, but begins to develop at the age of seven or eight years and is not complete until the end of adolescence. This was once considered a reason to not give children vocal training until after puberty.

However, a good vocal technique that teaches balanced registration can be taught at any age, with care given to not allow over-singing. Shouting and screaming on the playground are far more likely to cause vocal trauma than is correct vocal training.

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