The Larynx: Structure and Function
(Page 3 of 3)
Mucosal Lining of the Larynx
The vocal apparatus consists of two pairs of mucosal folds: the vestibular folds (‘false vocal cords’) and the true vocal folds.
The vestibular folds are located above both sides of the glottis (the hole and the ligament itself). They are covered by respiratory epithelium, and do not contain muscle. They are created by the mucosa passing over the vestibular ligament. They vibrate somewhat during phonation and especially during vibrato singing. Although they do play a role in resonance, these false folds, unlike the true vocal folds, are not actually responsible for sound production. The false vocal folds also work with the epiglottis to create a seal so that nothing goes down the trachea (windpipe) during swallowing.
‘Vocal fold’ is the current term for ‘vocal cord’. (Most teachers and singers use these two terms interchangeably.) The change in terminology came about as the anatomy of the larynx, including the structure and function of the folds, came to be better understood by the scientific world. The folds are not a band of string suspended in the air that vibrates when it is plucked or struck, as the word ‘cord’ suggests. Instead, they are part of a muscle on the side of the larynx that is covered with special tissues that can vibrate at a high speed. (Only its outer covering actually vibrates.) A vocal fold resembles a lip of tissue much more than a cord, and the term ‘fold’ is, therefore, more accurate and preferable.
The true vocal folds are a pair of pliable shelves of tissue that stretch horizontally across the top of the larynx. They may also be described as twin infoldings covered on the surface by laryngeal mucous membrane (made up of elastic and fatty tissue, or stratified squamous epithelium), which is supported deeper down underneath by the innermost fibres of the thyroarytenoid muscle. They are flat, triangular bands, and are pearly white in colour. Unlike the false vocal folds, the true vocal folds do contain skeletal muscle. In most males, the vocal folds are longer and thicker and have more mass, producing a deeper pitch.
The folds are attached at the back to the vocal process of the arytenoid cartilages and to the thyroid cartilage at the front. They are enclosed within the thyroid cartilage, which is the hard structure that forms the mass in the neck known as the Adam’s apple. Their outer edges are attached to muscle in the larynx and do not move or vibrate, while their inner edges, or margins, are free to vibrate.
Most of the muscles that act to abduct (open) or adduct (close) the vocal folds attach to the muscular process of the arytenoid cartilages. There is only one muscle that has an abductor action on the vocal folds - the posterior cricoarytenoid muscle - although there are several that act to adduct the folds. Abduction of the vocal folds may be accomplished either by externally rotating the arytenoid cartilages on a pivot located at their base or by sliding the two arytenoid cartilages apart slightly. Vocal fold length and tension can be controlled by rocking the thyroid cartilage forward and backward on the cricoid cartilage (either directly by contracting the cricothyroids or indirectly by changing the vertical position of the larynx), by manipulating the tension of the muscles within the vocal folds, and by moving the arytenoids forward or backward.
The vocal folds have a three-layer construction consisting of a cover called the epithelium or mucosa – (a tissue composed of cells that line the cavities and surfaces of structures throughout the body), a vocal ligament, and muscle fibre, which can shorten and bulge the folds by tightening the front part of the ligament near the thyroid cartilage. These three layers are also referred to as the superficial, intermediate and deep layers.
The cover of the folds is composed of the epithelium (mucosa), basal lamina (or basement membrane zone), and the superficial layer of the lamina propria. The soft, gel-like composition of the cover is important for creating the mucosal wave. The transition is composed of the intermediate and deep layers of the lamina propria. The body of the vocal fold is composed of the thyroarytenoid (vocalis) muscle. This layered structure of tissues is very important for vibration of the true vocal folds.
The epithelium, (the surface ‘skin’ of the larynx, which is continuous with the lining of the mouth, pharynx and with the trachea below the larynx), has been described as a thin shell, the purpose of which is to maintain the shape of the vocal fold. This epithelium is between five and twenty-five cells thick, with the most superficial layer consisting of one to three cells that are lost to abrasion of the vocal folds during the closed phase of vibration (the part of the vibratory cycle when the folds are together during phonation). The free edge of the vibratory portion of the vocal fold, the anterior glottis, is covered with stratified squamous epithelium, an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous cell. The posterior glottis is covered with pseudostratified ciliated epithelium, an epitheleum containing simple columnar epithelial cells whose nuclei appear at different heights, and also possessing fine hair-like extensions called cilia that waft unwanted particles out of the body. On the surfaces of the epithelial cells are microridges and microvilli, which help to spread and retain a mucuous coat on the epithelium. Lubrication of the vocal folds through adequate hydration is essential for normal phonation to avoid excessive abrasion. (Surgery of the vocal folds can disturb this layer with scar tissue, which can result in the inability of the epithelium to retain an adequate mucous coat, which will in turn impact lubrication of the vocal folds.)
The basal lamina, or basement membrane zone, is transitional tissue composed of two zones: the lamina lucida and lamina densa. The lamina lucida appears as a low density, clear zone medial to the epithelial basal cells. The lamina densa has a greater density of filaments and is adjacent to the lamina propria. The basal lamina mainly provides physical support to the epithelim through anchoring fibres, and is essential for repair of the epithelium.
The lamina propria has three distinct layers, each with a different consistency: the superficial layer, which has a jelly-like substance and is close to the surface, the intermediate layer, which is made up of an elastic, fibrous substance, and the deep layer, which is a thread-like collagenous fibre layer.
The superficial layer of the lamina propria consists of loose fibrous components and extracellular matrices - networks of non-living tissue (containing proteins, minerals, and certain carbohydrates) that provide support to cells, performing specific functions, depending on the types of cells that they are associated with - that can be compared to soft gelatin. This layer is also known as Reinke’s space but it is not a space at all, although it is a potential space, (which would indicate a problem). The superficial layer of the lamina propria is a structure that vibrates a great deal during phonation, and the viscoelasticity needed to support this vibratory function depends mostly on these extracellular matrices. The primary extracellular matrices of the vocal fold cover are reticular, collagenous and elastic fibres, as well as glycoprotein and glycosaminoglycan. These fibres serve as scaffolds for structural maintenance, providing tensile (tension) strength and resilience so that the vocal folds may vibrate freely but still retain their shape.
The intermediate layer of the lamina propria is primarily made up of elastic fibres while the deep layer of the lamina propria is primarily made up of collagenous fibres. These fibres run roughly parallel to the vocal fold edge, and these two layers of the lamina propria comprise the vocal ligament. This transition layer is primarily structural, giving the vocal fold support as well as providing adhesion between the mucosa (cover), and the body (the thyroarytenoid or vocalis muscle).
Function of the Larynx
Sound is generated in the larynx. Pitch and volume (loudness) are also manipulated at the laryngeal level.
During inhalation, the vocal folds spread apart in order to allow air into the lungs. (The muscles attached to the arytenoid cartilages control the degree of opening.) After inhalation, and just prior to speaking or singing, the folds are brought close together - they are approximated or closed - by adducting the arytenoid cartilages, which causes air pressure to build up beneath them. This pressure beneath the folds is refered to as subglottic (or subglottal) pressure.
This increased subglottic pressure causes the vocal folds to be pushed apart, with the inferior part of each fold leading the superior part. Air pushes through the very small space between them, the glottis, which then causes the covering of the vocal folds, known as the mucosa, to oscillate or vibrate. The vibration of the vocal folds modulates (regulates) the flow of air being expelled from the lungs, chopping up the steady stream of air into little puffs, thus generating sound. Under the correct conditions, this oscillation pattern will sustain itself.
The basic sound of the voice occurs by means of a phenomenon known as the venturi effect. As air passes through a constriction (or venturi), it speeds up and creates a suction in its wake. This suction draws in the pliable mucosa from each vocal fold, which meet in the midline, only to be pushed aside by more air escaping from the lungs. This cycle creates a repeating undulation known as the mucosal wave.
The regularity of the mucosal wave is essential to the production of good voice. A number of factors can influence the formation and maintenance of the mucosal wave. The two main factors involve vocal fold closure and the integrity of the superficial layer of the lamina propria. Inability of the vocal folds to close sufficiently, as in some cases of vocal fold paralysis (when one or both vocal folds are paralyzed), may make it impossible to create the venturi effect, which underlies mucosal vibration. Tethered or stiff mucosa resulting from a change in the lamina propria, as in a vocal fold scar, will not vibrate well or at all, regardless of the vocal fold closure. Sometimes, a large mass like a cyst, polyp or nodule (vocal node) blocks both the vocal folds from closing and the mucosa from vibrating. Irregularity of mucosal vibration also results in problems like hoarseness.
The frequency of the mucosal wave determines the pitch of the voice. Fine manipulation of the larynx is used to generate a source sound with a particular fundamental frequency, or pitch. Pitch is altered (caused to either rise or fall) by the changing tenseness and length of the vocal folds, which can be controlled by rocking the thyroid cartilage forward and backward on the cricoid cartilage, (a mechanism known as the ‘laryngeal tilt’), and by manipulating the tension of the muscles within the vocal folds. (Increasing vocal fold tension by lengthening the folds and making them more taut produces a higher pitch.) To maintain consistent phonation and healthy vocal fold function, tension must be altered symmetrically (by both folds simultaneously and equally). These changes in pitch occur rapidly and precisely many times during speaking tasks as the speaker inflects his or her voice. Intentional and repeated patterns of pitch give rise to melody.
The volume of sound is principally a result of the pressure of the air that is blown past the vocal folds. A more forceful expulsion of air from the lungs raises this pressure and creates a louder sound. The vocal folds must increase tension to maintain the near-closure that is needed for the venturi effect. If they do not, the increased air pressure will simply blow them aside and interrupt vibration, causing a cessation in sound production. This tensing of the vocal folds usually happens instinctively, without conscious effort. However, people with vocal fold paralysis or other types of vocal fold pathologies and/or weakness are often unable to do this, and frequently complain of an inability to increase the volume of their voices.
This initial source sound generated in the larynx is then altered as it travels through the vocal tract, configured differently based on the position of the tongue, lips, mouth, and pharynx. The process of altering a source sound as it passes through the filter of the vocal tract, is known as articulation, which creates the many different vowel and consonant sounds of the world's languages.
How Puberty and Aging Affect the Singing Voice
The age and physical development of a singer significantly impact the kind and quality of sound that is produced by the larynx, as well as how vocal instruction must be approached. Every age group has its own unique needs, as I will explain in more depth in this section, and it is important for vocal teachers, singers and parents to take into consideration these changes that occur to the voice over time. Understanding the development of the vocal instrument will ensure that expectations remain more realistic, frustration is minimized, success, (even if redefined with every stage of development), is achieved, and optimal vocal health is maintained.
In infants, the lamina propria is composed of only one layer, and there is no vocal ligament. At about four years of age, the vocal ligament begins to be present in children. Between the ages of six and twelve, two layers appear in the lamina propria. By the conclusion of adolescence, the mature three-layered lamina propria is present.
This presence or absence of tissue layers influences a difference in the number of vocal formants between the adult and pediatric populations because vocal fold vibration is a foundation for formants. For example, the voice of an adult female is three tones lower than that of a child, and has five to twelve formants. The child’s voice has only three to six vocal formants.
At birth, the length of the vocal fold is approximately six to eight millimetres. It grows to its adult length of eight to sixteen millimetres during adolescence.
The infant vocal fold is half membranous (or anterior glottis), and half cartilaginous (or posterior glottis), whereas the adult fold is approximately three-fifths membranous and two-fifths cartilaginous.
During puberty, which typically occurs between the ages of twelve and seventeen, both the voices of males and those of females undergo change. Voice change is controlled mainly by sex hormones, although the physical growth (and thus size) of the laryngeal structures does play a role, as well. The different hormones present between males and females create different kinds of changes.
Testosterone is the primary and most well-known androgen or androgenic hormone. In males, androgens are essential to male sexuality, as they stimulate or control the development and maintenance of male characteristics, which includes the activity of the accessory male sex organs and development of male secondary sex characteristics (such as facial hair). Androgens are the most important hormones responsible for the passage of the boy-child voice to man voice, and the change is irreversible. When secreted by the testes of males, testosterone will cause changes in the cartilages and musculature of the larynx for males during puberty. The thyroid prominence or laryngeal prominence (Adam’s apple) appears, the vocal folds lengthen and become rounded, and the epithelium thickens with the formation of three distinct layers in the lamina propria. In muscles, they cause a hypertrophy (enlargement) of striated muscles with a reduction in the fat cells in skeletal muscles, and a reduction in the whole body fatty mass, leading to more muscle bulk.
Androgens are also the original anabolic steroids and the precursor of all estrogens, the female sex hormones. In women, androgens are secreted principally by the adrenal cortex and the ovaries, and can have irreversible masculinizing effects if present in excessively high concentrations.
For women, the actions of estrogens and progesterone produce changes in the extravascular spaces by increasing capillary permeability, which allows the passage of intracapillary fluids to the interstitial space as well as modification of glandular secretions.
Estrogens have a hypertrophic (enlarging) and proliferative (increasing or multiplying by cell division) effect on mucosa by reducing the desquamating effect on the superficial layers.
Progesterone has an anti-proliferative (decreasing) effect on mucosa and accelerates desquamation. It causes a menstrual-like cycle in the vocal fold epithelium and a drying out of the mucosa with a reduction in secretions of the glandular epithelium. Progesterone has a diuretic (drying out by inhibiting the body’s ability to re-absorb fluid, which leads to a retention of water in the urine and mild dehydration) effect and decreases capillary permeability, thus trapping the extracellular fluid out of the capillaries and causing tissue congestion. This cyclical hormonal change causing a drying out of the vocal instrument explains why some women notice subtle changes in their voice quality during their menstrual cycles. Some professional female opera singers even schedule performances around their cycles so that they can offer their audiences the best performance possible and so that they don’t risk damaging their voices when there is insufficient moisture or lubrication for the folds to function optimally.
The thyroid hormones also affect dynamic function of the vocal folds. (Hashimoto’s Thyroiditis, for example, affects the fluid balance in the vocal folds).
In females during puberty, the thyroarytenoid muscle thickens slightly, but remains very supple and narrow. The squamous mucosa, (an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous cell), also differentiates into three distinct layers (the lamina propria) on the free edge of the vocal folds. The subglottic (below the glottis) and supraglottic (above the glottis) glandular mucosa become dependent on estrogens and progesterone hormones.
As humans age, there is a steady increase in the elastin (a yellow scleroprotein, the essential constituent of the elastic connective tissue) content of the lamina propria, resulting in a decrease in the ability of the lamina propria to expand caused by cross-branching of the elastin fibres. Among other things, this leads to the mature voice being better suited to the rigors of opera.
In old age, there is a thinning in the superficial layer of the lamina propria. In aging, the vocal fold undergoes considerable sex-specific changes. In the female larynx, the vocal fold cover thickens. The superficial layer of the lamina propria loses density as it becomes more edematous (inflammed or marked by edema). The intermediate layer of the lamina propria tends to atrophy only in men. The deep layer of the lamina propria of the male vocal fold thickens because of increased collagen deposits. The vocalis muscle atrophies in both men and women. However, the majority of elderly patients with voice disorders have diseased processes associated with aging rather than physiologic aging alone.
