Vocal folds
Encyclopedia
The vocal folds, also known commonly as vocal cords, are composed of twin infoldings of mucous membrane
stretched horizontally across the larynx
. They vibrate
, modulating the flow of air being expelled from the lungs during phonation
.
Open during inhalation, closed when holding one's breath, and vibrating for speech
or singing
(oscillating
440 times per second when singing A above middle C
), the folds are controlled via the vagus nerve
. They are white because of scant blood circulation.
Birds do not produce vocal sounds by means of vocal folds but by oscillations of membranes in the syrinx.
, generating sound through the rhythmic opening and closing of the vocal folds. To oscillate, the vocal folds are brought near enough together such that air pressure builds up beneath the larynx. The folds are pushed apart by this increased subglottal pressure, with the inferior part of each fold leading the superior part. Under the correct conditions, this oscillation pattern will sustain itself. In essence, sound is generated in the larynx by chopping up a steady flow of air into little puffs of sound waves.
You can see a movie clip showing human vocal folds in action here.
The perceived pitch of a person's voice is determined by a number of different factors, most importantly the fundamental frequency
of the sound generated by the larynx. The fundamental frequency is influenced by the length, size, and tension of the vocal folds. In an adult male, this frequency averages about 125 Hz
, adult females around 210 Hz, in children the frequency is over 300 Hz. Depth-Kymography is an imaging method to visualize the complex horizontal and vertical movements of vocal folds.
The vocal folds generate a sound rich in harmonic
s. The harmonics are produced by collisions of the vocal folds with themselves, by recirculation of some of the air back through the trachea, or both.
Some singers can isolate some of those harmonics in a way that is perceived as singing
in more than one pitch at the same time—a technique called overtone singing
.
at the top of the trachea
. They are attached posteriorly to the arytenoid cartilage
s, and anteriorly to the thyroid cartilage
. Their outer edges (as shown in the illustration) are attached to muscle in the larynx while their inner edges, or margins are free (the hole). They are constructed from epithelium
, but they have a few muscle fibres in them, namely the vocalis muscle which tightens the front part of the ligament near to the thyroid cartilage. They are flat triangular bands and are pearly white in color. Above both sides of the vocal folds (the hole and the ligament itself) are the vestibular folds or false vocal folds which have a small sac between the two folds (not illustrated).
Situated above the larynx, the epiglottis
acts as a flap which closes off the trachea during the act of swallowing to direct food into a separate tube behind the trachea called the esophagus
. If food or liquid does enter the trachea and contacts the vocal folds because of a failure of this safeguard ("going down the wrong pipe"), it causes a cough
reflex to expel the matter in order to prevent choking
.
The female vocal folds are between 12.5 mm and 17.5 mm (approx 0.5" to 0.75") in length.
Folds are pearly white in color - more white in women than in men.
The difference in vocal fold size between men and women causes a difference in vocal pitch. Additionally, genetic factors cause variations between members of the same sex, with men's and women's voices being categorized into types.
of the word "chord". While both spellings have historical precedence, standard American spelling is 'vocal cords'. According to the Oxford English Corpus
, a database of 21st Century texts that contains everything from academic journal articles to unedited writing and blog entries, contemporary writers opt for the nonstandard 'chords' instead of 'cords' 49% of the time. The 'vocal cords' spelling is also standard in the United Kingdom and Australia.
, but are often used to produce deep sonorous tones in Tibetan chant and Tuvan throat singing, as well as in musical screaming
and the death growl
vocal style.
The false folds are also called vestibular fold
s and ventricular folds. They can be seen on the diagram above as ventricular folds.
HA is a bulky, negatively charged glycosaminoglycan, whose strong affinity with water procures HA its viscoelastic and shock absorbing properties essential to vocal biomechanics. Viscosity and elasticity are critical to voice production. Chan, Gray and Titze, quantified the effect of HA on both the viscosity and the elasticity of vocal folds (VF) by comparing the properties of tissues with and without HA. The results showed that removal of HA decreased the stiffness of VF by an average of 35%, but increased their dynamic viscosity by an average of 70% at frequencies higher than 1 Hz. Newborns have been shown to cry an average of 6.7 hours per day during the first 3 months, with a sustained pitch of 400–600 Hz, and a mean duration per day of 2 hours. Similar treatment on adult VF would quickly result in edema, and subsequently aphonia. Schweinfurth and al. presented the hypothesis that hyaluronic acid high content and distribution in newborn VF is directly associated with newborn crying endurance. These differences in newborn vocal fold composition would also be responsible for newborns inability to articulate sounds, besides the fact that their lamina propria is a uniform structure with no vocal ligament. The layered structure necessary for phonation will start to develop during the infancy and until the adolescence.
The fibroblasts in the newborn Reinke’s space are immature, showing an oval shape, and a large nucleus-cytoplasm ratio. The rough endoplasmic reticulum and Golgi apparatus, as shown by electron micrographs, are not well developed, indicating that the cells are in a resting phase. The collagenous and reticular fibers in the newborn VF are fewer than in the adult one, adding to the immaturity of the vocal fold tissue.
In the infant, many fibrous components were seen to extend from the macula flava towards the Reinke’s space. Fibronectin is very abundant in the Reinke’s space of newborn and infant. Fibronectin is a glycoprotein that is believed to act as a template for the oriented deposition of the collagen fibers, stabilizing the collagen fibrils. Fibronectin also acts as a skeleton for the elastic tissue formation. Reticular and collagenous fibers were seen to run along the edges of the VF throughout the entire lamina propria. Fibronectin in the Reinke’s space appeared to guide those fibers and orient the fibril deposition. The elastic fibers remained sparse and immature during infancy, mostly made of microfibrils. The fibroblasts in the infant Reinke’s space were still sparse but spindle-shaped. Their rough endoplasmic reticulum and Golgi apparatus were still not well developed, indicating that despite the change in shape, the fibroblasts still remained mostly in a resting phase. Few newly released materials were seen adjacent to the fibroblasts. The ground substance content in the infant Reinke’s space seemed to decrease over time, as the fibrous component content increased, thus slowly changing the vocal fold structure.
The extracellular matrix (ECM) of the VF LP is composed of fibrous proteins such as collagen and elastin, and interstitial molecules such as HA, a non-sulfated glycosaminoglycan. While the SLP is rather poor in elastic and collagenous fibers, the ILP and DLP are mostly composed of it, with the concentration of elastic fibers decreasing and the concentration of collagenous fibers increasing as the vocalis muscle is approached. Fibrous proteins and interstitial molecules play different roles within the ECM. While collagen (mostly type I) provides strength and structural support to the tissue, which are useful to withstanding stress and resisting deformation when subjected to a force, elastin fibers bring elasticity to the tissue, allowing it to return to its original shape after deformation. Interstitial proteins, such as HA, plays important biological and mechanical roles in the VF tissue. In the VF tissue, HA plays a role of shear-thinner, affecting the tissue viscosity, space-filler, shock absorber, as well as wound healing and cell migration promoter. The distribution of those proteins and interstitial molecules has been proven to be affected by both age and gender, and is maintained by the fibroblasts.
Hirano et al. previously found that the newborns did not have a true lamina propria, but instead had cellular regions called maculae flavae, located at the anterior and posterior ends of the loose vocal fold tissue. Boseley and Hartnick examined at the development and maturation of pediatric human vocal fold lamina propria. Hartnick was the first one to define each layer by a change in their cellular concentration. He also found that the lamina propria monolayer at birth and shortly thereafter was hypercellular, thus confirming Hirano’s observations. By 2 months of age, the vocal fold started differentiating into a bilaminar structure of distinct cellular concentration, with the superficial layer being less densely populated than the deeper layer. By 11 months, a three-layered structure starts to be noted in some specimens, again with different cellular population densities. The superficial layer is still hypocellular, followed by an intermediate more hypercellular layer, and a deeper hypercellular layer, just above the vocalis muscle. Even though the VF seem to start organizing, this is not representative of the trilaminar structure seen in adult tissues, where the layer are defined by their differential elastin and collagen fiber compositions. By 7 years of age, all specimens show a three-layered vocal fold structure, based on cellular population densities. At this point, the superficial layer was still hypocellular, the middle layer was the hypercellualr one, with also a greater content of elastin and collagen fibers, and the deeper layer was less cellularly populated. Again, the distinction seen between the layers at this stage is not comparable to that seen in the adult tissue. The maturation of the VF did not appear before 13 years of age, where the layers could be defined by their differential fiber composition rather than by their differential cellular population. The pattern now show a hypocellular superficial layer, followed by a middle layer composed predominantly of elastin fiber, and a deeper layer composed predominantly of collagen fibers. This pattern can be seen in older specimens up to 17 years of age, and above. While this study offers a nice way to see the evolution from immature to mature VF, it still does not explain what is the mechanism behind it.
properties of human vocal fold lamina propria are essential for their vibration, and depend on the composition and structure of their extracellular matrix (ECM). Adult VF have a layered structure which is based on the layers differential in ECM distribution. Newborns on the other hand, do not have this layered structure. Their VF are uniform, and immature, making their viscoelastic properties most likely unsuitable for phonation. HA plays a very important role in the vocal fold biomechanics. In fact, HA has been described as the ECM molecule that not only contributes to the maintenance of an optimal tissue viscosity that allows phonation, but also of an optimal tissue stiffness that allows frequency control. CD44 is a cell surface receptor for HA. Cells such as fibroblasts are responsible for synthesizing ECM molecules. Cell surface matrix receptors in return, feed back to the cells through cell-matrix interaction, allowing the cell to regulate its metabolism.
Sato et al. conducted a very interesting study recently. They carried out a histopathologic investigation of unphonated human VF. Vocal fold mucosae, which were unphonated since birth, of three young adults (17, 24, and 28 years old) were looked at using light and electron microscopy. Interestingly, the results show that the vocal fold mucosae were hypoplastic, and rudimentary, and like newborns, did not have any vocal ligament, Reinke’s space, or layered structure. Like newborns, the lamina propria appeared as a uniform structure. Some stellate cells were present in the macula flava, but started to show some signs of degeneration. The stellate cells synthesized fewer ECM molecules, and the cytoplasmic processes were shown to be short and shrinking, suggesting a decreased activity. Those results confirm the hypothesis that phonation stimulates stellate cells into producing more ECM.
Furthermore, using a specially designed bioreactor, Titze et al. showed that fibroblasts exposed to mechanical stimulation have differing levels of ECM production than fibroblasts that are not exposed to mechanical stimulation. The gene expression levels of ECM constituents such as fibronectin, MMP1, decorin, fibromodulin, HA synthase 2, and CD44 were altered. All those genes are involved in ECM remodeling, thus suggesting that mechanical forces applied to the tissue, alter the expression levels of ECM related genes, which in turn allow the cells present in the tissue to regulate the ECM constituent synthesis, thus affecting the tissue’s composition, structure, and biomechanical properties. In the end, cell-surface receptors close the loop by giving feedback on the surrounding ECM to the cells, affecting also their gene expression level.
Vocal fold phonatory functions are known to change from birth to old age. The most significant changes occur in development between birth and puberty, and in old age. Hirano et al. previously described several structural changes associated with aging, in the vocal fold tissue. Some of those changes are: a shortening of the membranous vocal fold in males, a thickening of the vocal fold mucosa and cover in females, and a development of edema in the superficial lamina propria layer in both sexes. Hammond et al. observed that the HA content in the vocal fold lamina propria was significantly higher in males than in females. Although all those studies did show that there are clear structural and functional changes seen in the human VF which are associated with gender and age, none really fully elucidated the underlying cause of those changes. In fact, only a few recent studies started to look at the presence and role of hormone receptors in the VF. Newman et al. found that hormone receptors are indeed present in the VF, and show a statistical distribution difference with respect to age and gender. They have identified the presence of androgen
, estrogen
, and progesterone
receptors in epithelial cells, granular cells and fibroblasts of VF, suggesting that some of the structural changes seen in the VF could be due to hormonal influences. In this specific study, androgen and progesterone receptors were found more commonly in males than in females. In others studies, it has been suggested that the estrogen/androgen ratio be partly responsible for the voice changes observed at menopause. As previously said, Hammond et al. showed than the HA content was higher in male than in female VF. Bentley et al. demonstrated that sex skin swelling seen in monkey was due to an increase in HA content, which was in fact mediated by estrogen receptors in dermal fibroblasts. An increase in collagen biosynthesis mediated by the estrogen receptors of dermal fibroblasts was also observed. A connection between hormone levels, and ECM distribution in VF depending on age and gender could be made. More particularly a connection between higher hormone levels and higher HA content in males could exist in the human vocal fold tissue. Although a relationship between hormone levels and ECM biosynthesis in vocal fold can be established, the details of this relationship, and the mechanisms of the influence has not been elucidated yet.
Any injury to human vocal focal elicits a wound healing process characterized by disorganized collagen deposition and, eventually, formation of scar tissue. Verdolini and her group sought to detect and describe acute tissue response of injured rabbit VF model. They quantified the expression of two biochemical markers: Interleukin-1 and prostaglandine-E2, which are associated with acute wound healing. They found the secretions of these inflammatory mediators were significantly elevated when collected from injured VF versus normal VF. This result was consistent with their previous study about the function of IL-1 and PGE-2 in wound healing. Investigation about the temporal and magnitude of inflammatory response in VFs may benefit for elucidating subsequent pathological events in vocal fold wounding, which is good for clinician to develop therapeutic targets to minimize scar formation. In the proliferative phase of VFs wound healing, if the production of HA and collagen is not balanced, which means the HA level is lower than normal, the fibrosis of collagen cannot be regulated. Consequently, regenerative-type wound healing turns to be the formation of scar. Scarring may lead to the deformity of vocal fold edge, the disruption of LPs viscosity and stiffness. Patients suffering from vocal fold scar always complain about increased phonatory effort, vocal fatigue, breathlessness, dysphonia as well. Vocal fold scar is one of the most challenging problems for otolaryngologists because it’s hard to be diagnosed at germinal stage and the function necessity of VF is delicate.
which is bordered by pseudostratified ciliated epithelium. The luminal surface of this squamous epithelium is covered by a layer of mucus (mucociliary blanket) which is composed of two layers: a mucinous layer and serous layer. Both mucus layers provide viscous and watery environment for cilia beating posteriorally and superiorly. The mucociliary blanket which keeps the vocal fold moist and lubricated is indispensable in vocal health.
The epidermis layer is secured to the deeper connective tissue by basement membrane. Due to the primarily amorphous fibrous and nonfibrous proteins in lamina propria (LPs), the basement membrane applies strong anchoring filaments like collagenⅣ and Ⅶ to secure the hemidesmosome of basal cell to LPs. These attachments are strong enough to sustain beating and stretch to which VFs are subjected. The population density of some of the anchoring fibers in basal membrane, collagen Ⅶ, for instance, is genetically determined, which points out that genetics may influence vocal fold health and pathogenesis.
The next three layers comprise lamina LPs, which is stratified by their histological composition of elastin and collagen fibers, with fibroblast, myofibroblast and macrophages interspersed sparsely. The superficial layer LPs (SLLPs), also known as Reinke’s space, is composed of amorphous substance and microfibrils which allows this cover layer to “slide” over the deep layer easily. The vibratory and viscoelastic characteristics of human VFs are mainly attributed to the molecular composition of SLLPs. In normal vocal fold, the jelly-like “Reinke’s space” is very loose and abundant with interstitial proteins such as hyaluronic acid, fibronectin, proteoglycan like fibromodulin, decorin and versican. All these ECM components together regulate the water content of vocal fold and render the viscous shear property for it. The squamous epithelium and superficial lamina propria form the vocal mucosa which serves as vibratory component in phonation. The mucosa layer vibrates at a frequency range of 100–1000 Hz and displacement at 1mm approximately. The intermediate layer of LPs consists primarily of elastic fiber while the deep layer LP consists of fewer elastin and more collagen fibers. These two layers have poor differentiated boundary but are increasingly stiffer than SLLPs. The intermediate and deep layers of LPs compose the vocal ligament which is responsible for strain in phonation. Within the ECM community of vocal ligament, fibrous proteins such as elastin and collagen are pivotal in maintaining the proper elastic biomechanical property of vocal fold. Elastin fibers impart the flexibility and elasticity of VFs and, collagen is responsible for the resistance and resiliece to tensile strength. The normal strain level of vocal ligament ranges from 0-15% during phonation These fibrous proteins exhibit distribution variations spatially and temporally due to fibroblast turnover during tissue maturation and aging.
Mucous membrane
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...
stretched horizontally across the larynx
Larynx
The larynx , commonly called the voice box, is an organ in the neck of amphibians, reptiles and mammals involved in breathing, sound production, and protecting the trachea against food aspiration. It manipulates pitch and volume...
. They vibrate
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
, modulating the flow of air being expelled from the lungs during phonation
Phonation
Phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, phonation is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the definition used among those who study laryngeal anatomy and physiology...
.
Open during inhalation, closed when holding one's breath, and vibrating for speech
Speech
Speech is the human faculty of speaking.It may also refer to:* Public speaking, the process of speaking to a group of people* Manner of articulation, how the body parts involved in making speech are manipulated...
or singing
Singing
Singing is the act of producing musical sounds with the voice, and augments regular speech by the use of both tonality and rhythm. One who sings is called a singer or vocalist. Singers perform music known as songs that can be sung either with or without accompaniment by musical instruments...
(oscillating
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
440 times per second when singing A above middle C
Middle C
C or Do is the first note of the fixed-Do solfège scale. Its enharmonic is B.-Middle C:Middle C is designated C4 in scientific pitch notation because of the note's position as the fourth C key on a standard 88-key piano keyboard...
), the folds are controlled via the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
. They are white because of scant blood circulation.
Birds do not produce vocal sounds by means of vocal folds but by oscillations of membranes in the syrinx.
Vocal fold oscillation
The larynx is a major (but not the only) source of sound in speechSpeech
Speech is the human faculty of speaking.It may also refer to:* Public speaking, the process of speaking to a group of people* Manner of articulation, how the body parts involved in making speech are manipulated...
, generating sound through the rhythmic opening and closing of the vocal folds. To oscillate, the vocal folds are brought near enough together such that air pressure builds up beneath the larynx. The folds are pushed apart by this increased subglottal pressure, with the inferior part of each fold leading the superior part. Under the correct conditions, this oscillation pattern will sustain itself. In essence, sound is generated in the larynx by chopping up a steady flow of air into little puffs of sound waves.
You can see a movie clip showing human vocal folds in action here.
The perceived pitch of a person's voice is determined by a number of different factors, most importantly the fundamental frequency
Fundamental frequency
The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the lowest frequency of a periodic waveform. In terms of a superposition of sinusoids The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the...
of the sound generated by the larynx. The fundamental frequency is influenced by the length, size, and tension of the vocal folds. In an adult male, this frequency averages about 125 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, adult females around 210 Hz, in children the frequency is over 300 Hz. Depth-Kymography is an imaging method to visualize the complex horizontal and vertical movements of vocal folds.
The vocal folds generate a sound rich in harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
s. The harmonics are produced by collisions of the vocal folds with themselves, by recirculation of some of the air back through the trachea, or both.
Some singers can isolate some of those harmonics in a way that is perceived as singing
Singing
Singing is the act of producing musical sounds with the voice, and augments regular speech by the use of both tonality and rhythm. One who sings is called a singer or vocalist. Singers perform music known as songs that can be sung either with or without accompaniment by musical instruments...
in more than one pitch at the same time—a technique called overtone singing
Overtone singing
Overtone singing, also known as overtone chanting, or harmonic singing, is a type of singing in which the singer manipulates the resonances created as air travels from the lungs, past the vocal folds, and out the lips to produce a melody.The partials of a sound wave made by the human voice can be...
.
Location
Vocal folds are located within the larynxLarynx
The larynx , commonly called the voice box, is an organ in the neck of amphibians, reptiles and mammals involved in breathing, sound production, and protecting the trachea against food aspiration. It manipulates pitch and volume...
at the top of the trachea
Vertebrate trachea
In tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...
. They are attached posteriorly to the arytenoid cartilage
Arytenoid cartilage
The arytenoid cartilages are a pair of small three-sided pyramids which form part of the larynx, to which the vocal folds are attached...
s, and anteriorly to the thyroid cartilage
Thyroid cartilage
The thyroid cartilage is the largest of the nine cartilages that make up the laryngeal skeleton, the cartilage structure in and around the trachea that contains the larynx....
. Their outer edges (as shown in the illustration) are attached to muscle in the larynx while their inner edges, or margins are free (the hole). They are constructed from epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
, but they have a few muscle fibres in them, namely the vocalis muscle which tightens the front part of the ligament near to the thyroid cartilage. They are flat triangular bands and are pearly white in color. Above both sides of the vocal folds (the hole and the ligament itself) are the vestibular folds or false vocal folds which have a small sac between the two folds (not illustrated).
Situated above the larynx, the epiglottis
Epiglottis
The epiglottis is a flap that is made of elastic cartilage tissue covered with a mucous membrane, attached to the entrance of the larynx. It projects obliquely upwards behind the tongue and the hyoid bone, pointing dorsally. The term, like tonsils, is often incorrectly used to refer to the uvula...
acts as a flap which closes off the trachea during the act of swallowing to direct food into a separate tube behind the trachea called the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...
. If food or liquid does enter the trachea and contacts the vocal folds because of a failure of this safeguard ("going down the wrong pipe"), it causes a cough
Cough
A cough is a sudden and often repetitively occurring reflex which helps to clear the large breathing passages from secretions, irritants, foreign particles and microbes...
reflex to expel the matter in order to prevent choking
Choking
Choking is the mechanical obstruction of the flow of air from the environment into the lungs. Choking prevents breathing, and can be partial or complete, with partial choking allowing some, although inadequate, flow of air into the lungs. Prolonged or complete choking results in asphyxia which...
.
Sex differences
Men and women have different vocal fold sizes. Adult male voices are usually lower pitched and have larger folds. The male vocal folds are between 17.5 mm and 25 mm (approx 0.75" to 1.0") in length.The female vocal folds are between 12.5 mm and 17.5 mm (approx 0.5" to 0.75") in length.
Folds are pearly white in color - more white in women than in men.
The difference in vocal fold size between men and women causes a difference in vocal pitch. Additionally, genetic factors cause variations between members of the same sex, with men's and women's voices being categorized into types.
Spelling
Vocal cords, a term commonly used to refer to the vocal folds, is also spelled 'vocal chords', possibly due to the musical connotations or to confusion with the geometrical definitionChord (geometry)
A chord of a circle is a geometric line segment whose endpoints both lie on the circumference of the circle.A secant or a secant line is the line extension of a chord. More generally, a chord is a line segment joining two points on any curve, such as but not limited to an ellipse...
of the word "chord". While both spellings have historical precedence, standard American spelling is 'vocal cords'. According to the Oxford English Corpus
Oxford English Corpus
The Oxford English Corpus is a text corpus of English language used by the makers of the Oxford English Dictionary and by Oxford University Press's language research programme. It is the largest corpus of its kind, containing over two billion words...
, a database of 21st Century texts that contains everything from academic journal articles to unedited writing and blog entries, contemporary writers opt for the nonstandard 'chords' instead of 'cords' 49% of the time. The 'vocal cords' spelling is also standard in the United Kingdom and Australia.
False vocal folds
The vocal folds discussed above are sometimes called 'true vocal folds' to distinguish them from the false vocal folds. These are a pair of thick folds of mucous membrane that protect and sit slightly superior to the more delicate true folds. They have a minimal role in normal phonationPhonation
Phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, phonation is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the definition used among those who study laryngeal anatomy and physiology...
, but are often used to produce deep sonorous tones in Tibetan chant and Tuvan throat singing, as well as in musical screaming
Screaming (music)
Screaming is a vocal technique that is most popular in subgenres of heavy metal, punk and hard rock, including metalcore, deathcore, post-hardcore, groove metal, black metal, and grindcore...
and the death growl
Death growl
A death growl, also known as death metal vocals, guttural vocals, death grunts, and harsh vocals among other names, is a vocalisation style usually employed by vocalists of the death metal and black metal music genre, but also used in a variety of heavy metal and hardcore punk subgenres.Death...
vocal style.
The false folds are also called vestibular fold
Vestibular fold
The vestibular fold is one of two thick folds of mucous membrane, each enclosing a narrow band of fibrous tissue, the ventricular ligament, which is attached in front to the angle of the thyroid cartilage immediately below the attachment of the epiglottis, and behind to the antero-lateral surface...
s and ventricular folds. They can be seen on the diagram above as ventricular folds.
Newborn vocal folds
Newborns have a uniform monolayered lamina propria, which appears loose with no vocal ligament. The monolayered lamina propria is composed of ground substances such as hyaluronic acid and fibronectin, fibroblasts, elastic fibers, and collagenous fibers. While the fibrous components are sparse, making the lamina propria structure loose, the hyaluronic acid (HA) content is high.HA is a bulky, negatively charged glycosaminoglycan, whose strong affinity with water procures HA its viscoelastic and shock absorbing properties essential to vocal biomechanics. Viscosity and elasticity are critical to voice production. Chan, Gray and Titze, quantified the effect of HA on both the viscosity and the elasticity of vocal folds (VF) by comparing the properties of tissues with and without HA. The results showed that removal of HA decreased the stiffness of VF by an average of 35%, but increased their dynamic viscosity by an average of 70% at frequencies higher than 1 Hz. Newborns have been shown to cry an average of 6.7 hours per day during the first 3 months, with a sustained pitch of 400–600 Hz, and a mean duration per day of 2 hours. Similar treatment on adult VF would quickly result in edema, and subsequently aphonia. Schweinfurth and al. presented the hypothesis that hyaluronic acid high content and distribution in newborn VF is directly associated with newborn crying endurance. These differences in newborn vocal fold composition would also be responsible for newborns inability to articulate sounds, besides the fact that their lamina propria is a uniform structure with no vocal ligament. The layered structure necessary for phonation will start to develop during the infancy and until the adolescence.
The fibroblasts in the newborn Reinke’s space are immature, showing an oval shape, and a large nucleus-cytoplasm ratio. The rough endoplasmic reticulum and Golgi apparatus, as shown by electron micrographs, are not well developed, indicating that the cells are in a resting phase. The collagenous and reticular fibers in the newborn VF are fewer than in the adult one, adding to the immaturity of the vocal fold tissue.
In the infant, many fibrous components were seen to extend from the macula flava towards the Reinke’s space. Fibronectin is very abundant in the Reinke’s space of newborn and infant. Fibronectin is a glycoprotein that is believed to act as a template for the oriented deposition of the collagen fibers, stabilizing the collagen fibrils. Fibronectin also acts as a skeleton for the elastic tissue formation. Reticular and collagenous fibers were seen to run along the edges of the VF throughout the entire lamina propria. Fibronectin in the Reinke’s space appeared to guide those fibers and orient the fibril deposition. The elastic fibers remained sparse and immature during infancy, mostly made of microfibrils. The fibroblasts in the infant Reinke’s space were still sparse but spindle-shaped. Their rough endoplasmic reticulum and Golgi apparatus were still not well developed, indicating that despite the change in shape, the fibroblasts still remained mostly in a resting phase. Few newly released materials were seen adjacent to the fibroblasts. The ground substance content in the infant Reinke’s space seemed to decrease over time, as the fibrous component content increased, thus slowly changing the vocal fold structure.
Adult vocal folds
Human VF are paired structures located in the larynx, just above the trachea, which vibrate and are brought in contact during phonation. The human VF are roughly 12 - 24 mm in length, and 3–5 mm thick. Histologically, the human VF are a laminated structure composed of five different layers. The vocalis muscle, main body of the VF, is covered by the mucosa, which consists of the epithelium and the lamina propria. The latter is a pliable layer of connective tissue subdivided into three layers: the superficial layer (SLP), the intermediate layer (ILP), and the deep layer (DLP). Layer distinction is either made looking at differential in cell content or ECM content. The most common way being to look at the ECM content. The SLP has fewer elastic and collagenous fibers than the two other layers, and thus is looser and more pliable. The ILP is mostly composed of elastic fibers, while the DLP has fewer elastic fibers, and more collagenous fibers. In those two layers, which form what is known as the vocalis ligament, the elastic and collagenous fibers are densely packed as bundles that run almost parallel to the edge of the vocal fold.The extracellular matrix (ECM) of the VF LP is composed of fibrous proteins such as collagen and elastin, and interstitial molecules such as HA, a non-sulfated glycosaminoglycan. While the SLP is rather poor in elastic and collagenous fibers, the ILP and DLP are mostly composed of it, with the concentration of elastic fibers decreasing and the concentration of collagenous fibers increasing as the vocalis muscle is approached. Fibrous proteins and interstitial molecules play different roles within the ECM. While collagen (mostly type I) provides strength and structural support to the tissue, which are useful to withstanding stress and resisting deformation when subjected to a force, elastin fibers bring elasticity to the tissue, allowing it to return to its original shape after deformation. Interstitial proteins, such as HA, plays important biological and mechanical roles in the VF tissue. In the VF tissue, HA plays a role of shear-thinner, affecting the tissue viscosity, space-filler, shock absorber, as well as wound healing and cell migration promoter. The distribution of those proteins and interstitial molecules has been proven to be affected by both age and gender, and is maintained by the fibroblasts.
Vocal fold maturation
Vocal fold structure in adults is quite different from that in newborns. Exactly how the VF mature from an immature monolayer in newborns to a mature three layer tissue in adults is still unknown, however a few studies have investigated the subjects and brought some answers.Hirano et al. previously found that the newborns did not have a true lamina propria, but instead had cellular regions called maculae flavae, located at the anterior and posterior ends of the loose vocal fold tissue. Boseley and Hartnick examined at the development and maturation of pediatric human vocal fold lamina propria. Hartnick was the first one to define each layer by a change in their cellular concentration. He also found that the lamina propria monolayer at birth and shortly thereafter was hypercellular, thus confirming Hirano’s observations. By 2 months of age, the vocal fold started differentiating into a bilaminar structure of distinct cellular concentration, with the superficial layer being less densely populated than the deeper layer. By 11 months, a three-layered structure starts to be noted in some specimens, again with different cellular population densities. The superficial layer is still hypocellular, followed by an intermediate more hypercellular layer, and a deeper hypercellular layer, just above the vocalis muscle. Even though the VF seem to start organizing, this is not representative of the trilaminar structure seen in adult tissues, where the layer are defined by their differential elastin and collagen fiber compositions. By 7 years of age, all specimens show a three-layered vocal fold structure, based on cellular population densities. At this point, the superficial layer was still hypocellular, the middle layer was the hypercellualr one, with also a greater content of elastin and collagen fibers, and the deeper layer was less cellularly populated. Again, the distinction seen between the layers at this stage is not comparable to that seen in the adult tissue. The maturation of the VF did not appear before 13 years of age, where the layers could be defined by their differential fiber composition rather than by their differential cellular population. The pattern now show a hypocellular superficial layer, followed by a middle layer composed predominantly of elastin fiber, and a deeper layer composed predominantly of collagen fibers. This pattern can be seen in older specimens up to 17 years of age, and above. While this study offers a nice way to see the evolution from immature to mature VF, it still does not explain what is the mechanism behind it.
Macula flavae
Maculae flavae are located at the anterior and posterior ends of the membranous parts of the VF. The histological structure of the macula flava is unique, and Sato and Hirano speculated that it could play an important role in growth, development and aging of VF. The macula flava is composed of fibroblasts, ground substances, elastic and collagenous fibers. Fibroblasts were numerous and spindle or stellate-shaped. The fibroblasts have been observed to be in active phase, with some newly released amorphous materials present at their surface. From a biomechanical point of view, the role of the macula flava is very important. Hirano and Sato studies suggested that the macula flava is responsible for the synthesis of the fibrous components of the VF. Fibroblasts have been found mostly aligned in the direction of the vocal ligament, along bundles of fibers. It then was suggested that the mechanical stresses during phonation were stimulating the fibroblasts to synthesize those fibers.Impact of phonation
The viscoelasticViscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...
properties of human vocal fold lamina propria are essential for their vibration, and depend on the composition and structure of their extracellular matrix (ECM). Adult VF have a layered structure which is based on the layers differential in ECM distribution. Newborns on the other hand, do not have this layered structure. Their VF are uniform, and immature, making their viscoelastic properties most likely unsuitable for phonation. HA plays a very important role in the vocal fold biomechanics. In fact, HA has been described as the ECM molecule that not only contributes to the maintenance of an optimal tissue viscosity that allows phonation, but also of an optimal tissue stiffness that allows frequency control. CD44 is a cell surface receptor for HA. Cells such as fibroblasts are responsible for synthesizing ECM molecules. Cell surface matrix receptors in return, feed back to the cells through cell-matrix interaction, allowing the cell to regulate its metabolism.
Sato et al. conducted a very interesting study recently. They carried out a histopathologic investigation of unphonated human VF. Vocal fold mucosae, which were unphonated since birth, of three young adults (17, 24, and 28 years old) were looked at using light and electron microscopy. Interestingly, the results show that the vocal fold mucosae were hypoplastic, and rudimentary, and like newborns, did not have any vocal ligament, Reinke’s space, or layered structure. Like newborns, the lamina propria appeared as a uniform structure. Some stellate cells were present in the macula flava, but started to show some signs of degeneration. The stellate cells synthesized fewer ECM molecules, and the cytoplasmic processes were shown to be short and shrinking, suggesting a decreased activity. Those results confirm the hypothesis that phonation stimulates stellate cells into producing more ECM.
Furthermore, using a specially designed bioreactor, Titze et al. showed that fibroblasts exposed to mechanical stimulation have differing levels of ECM production than fibroblasts that are not exposed to mechanical stimulation. The gene expression levels of ECM constituents such as fibronectin, MMP1, decorin, fibromodulin, HA synthase 2, and CD44 were altered. All those genes are involved in ECM remodeling, thus suggesting that mechanical forces applied to the tissue, alter the expression levels of ECM related genes, which in turn allow the cells present in the tissue to regulate the ECM constituent synthesis, thus affecting the tissue’s composition, structure, and biomechanical properties. In the end, cell-surface receptors close the loop by giving feedback on the surrounding ECM to the cells, affecting also their gene expression level.
Impact of hormones
Other studies suggest that hormones play also an important role in vocal fold maturation. Hormones are molecules secreted into the blood stream to be delivered at different targeted sites. They usually promote growth, differentiation and functionality in different organs or tissues. Their effect is due to their ability to bind to intracellular receptors, modulating the gene expression, and subsequently regulating protein synthesis. The interaction between the endocrine system and tissues such as breast, brain, testicles, heart, bones, etc, is being extensively studied. It has clearly been seen that the larynx is somewhat affected by hormonal changes, but surprisingly, very few studies are working on elucidating this relationship. The effect of hormonal changes in voice is clearly seen when hearing male and female voices, or when listening to a teenage voice changing during puberty. Actually, it is believed that the number of hormonal receptors in the pre-pubertal phase is higher than in any other age. Menstruation has also been seen to influence the voice. In fact, singers are encouraged by their instructors not to perform during their pre-menstrual period, because of a drop in their voice quality.Vocal fold phonatory functions are known to change from birth to old age. The most significant changes occur in development between birth and puberty, and in old age. Hirano et al. previously described several structural changes associated with aging, in the vocal fold tissue. Some of those changes are: a shortening of the membranous vocal fold in males, a thickening of the vocal fold mucosa and cover in females, and a development of edema in the superficial lamina propria layer in both sexes. Hammond et al. observed that the HA content in the vocal fold lamina propria was significantly higher in males than in females. Although all those studies did show that there are clear structural and functional changes seen in the human VF which are associated with gender and age, none really fully elucidated the underlying cause of those changes. In fact, only a few recent studies started to look at the presence and role of hormone receptors in the VF. Newman et al. found that hormone receptors are indeed present in the VF, and show a statistical distribution difference with respect to age and gender. They have identified the presence of androgen
Androgen
Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...
, estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
, and progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
receptors in epithelial cells, granular cells and fibroblasts of VF, suggesting that some of the structural changes seen in the VF could be due to hormonal influences. In this specific study, androgen and progesterone receptors were found more commonly in males than in females. In others studies, it has been suggested that the estrogen/androgen ratio be partly responsible for the voice changes observed at menopause. As previously said, Hammond et al. showed than the HA content was higher in male than in female VF. Bentley et al. demonstrated that sex skin swelling seen in monkey was due to an increase in HA content, which was in fact mediated by estrogen receptors in dermal fibroblasts. An increase in collagen biosynthesis mediated by the estrogen receptors of dermal fibroblasts was also observed. A connection between hormone levels, and ECM distribution in VF depending on age and gender could be made. More particularly a connection between higher hormone levels and higher HA content in males could exist in the human vocal fold tissue. Although a relationship between hormone levels and ECM biosynthesis in vocal fold can be established, the details of this relationship, and the mechanisms of the influence has not been elucidated yet.
Vocal fold wound healing
Wound healing process is a natural regeneration process of dermal and epidermal tissue in which a series of biochemical events occurs in an organized sequence to restore the injuries. These events are complex and can be categorized into three stages: inflammation, proliferation and tissue remodeling. With respect to human vocal fold particularly, study about human vocal fold wound healing is not that extensive as investigations on animal models due to limited availability of human vocal fold. Vocal fold injuries can be caused by a variety of factors: chronic overuse of vocal, chemical, thermal and mechanical trauma such as over-smoking, laryngeal cancer and surgical operation. What’s more, some benign pathological phenomena like polyps, nodules and edema is another category of lesion which will definitely introduce disordered phonation.Any injury to human vocal focal elicits a wound healing process characterized by disorganized collagen deposition and, eventually, formation of scar tissue. Verdolini and her group sought to detect and describe acute tissue response of injured rabbit VF model. They quantified the expression of two biochemical markers: Interleukin-1 and prostaglandine-E2, which are associated with acute wound healing. They found the secretions of these inflammatory mediators were significantly elevated when collected from injured VF versus normal VF. This result was consistent with their previous study about the function of IL-1 and PGE-2 in wound healing. Investigation about the temporal and magnitude of inflammatory response in VFs may benefit for elucidating subsequent pathological events in vocal fold wounding, which is good for clinician to develop therapeutic targets to minimize scar formation. In the proliferative phase of VFs wound healing, if the production of HA and collagen is not balanced, which means the HA level is lower than normal, the fibrosis of collagen cannot be regulated. Consequently, regenerative-type wound healing turns to be the formation of scar. Scarring may lead to the deformity of vocal fold edge, the disruption of LPs viscosity and stiffness. Patients suffering from vocal fold scar always complain about increased phonatory effort, vocal fatigue, breathlessness, dysphonia as well. Vocal fold scar is one of the most challenging problems for otolaryngologists because it’s hard to be diagnosed at germinal stage and the function necessity of VF is delicate.
Human vocal fold anatomy and its biomechanical consideration
Mature human VFs are composed of layered structures which are quite different at the histological level. The top most layer comprises stratified squamous epitheliumStratified squamous epithelium
A stratified squamous epithelium consists of squamous epithelial cells arranged in layers upon a basement membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity...
which is bordered by pseudostratified ciliated epithelium. The luminal surface of this squamous epithelium is covered by a layer of mucus (mucociliary blanket) which is composed of two layers: a mucinous layer and serous layer. Both mucus layers provide viscous and watery environment for cilia beating posteriorally and superiorly. The mucociliary blanket which keeps the vocal fold moist and lubricated is indispensable in vocal health.
The epidermis layer is secured to the deeper connective tissue by basement membrane. Due to the primarily amorphous fibrous and nonfibrous proteins in lamina propria (LPs), the basement membrane applies strong anchoring filaments like collagenⅣ and Ⅶ to secure the hemidesmosome of basal cell to LPs. These attachments are strong enough to sustain beating and stretch to which VFs are subjected. The population density of some of the anchoring fibers in basal membrane, collagen Ⅶ, for instance, is genetically determined, which points out that genetics may influence vocal fold health and pathogenesis.
The next three layers comprise lamina LPs, which is stratified by their histological composition of elastin and collagen fibers, with fibroblast, myofibroblast and macrophages interspersed sparsely. The superficial layer LPs (SLLPs), also known as Reinke’s space, is composed of amorphous substance and microfibrils which allows this cover layer to “slide” over the deep layer easily. The vibratory and viscoelastic characteristics of human VFs are mainly attributed to the molecular composition of SLLPs. In normal vocal fold, the jelly-like “Reinke’s space” is very loose and abundant with interstitial proteins such as hyaluronic acid, fibronectin, proteoglycan like fibromodulin, decorin and versican. All these ECM components together regulate the water content of vocal fold and render the viscous shear property for it. The squamous epithelium and superficial lamina propria form the vocal mucosa which serves as vibratory component in phonation. The mucosa layer vibrates at a frequency range of 100–1000 Hz and displacement at 1mm approximately. The intermediate layer of LPs consists primarily of elastic fiber while the deep layer LP consists of fewer elastin and more collagen fibers. These two layers have poor differentiated boundary but are increasingly stiffer than SLLPs. The intermediate and deep layers of LPs compose the vocal ligament which is responsible for strain in phonation. Within the ECM community of vocal ligament, fibrous proteins such as elastin and collagen are pivotal in maintaining the proper elastic biomechanical property of vocal fold. Elastin fibers impart the flexibility and elasticity of VFs and, collagen is responsible for the resistance and resiliece to tensile strength. The normal strain level of vocal ligament ranges from 0-15% during phonation These fibrous proteins exhibit distribution variations spatially and temporally due to fibroblast turnover during tissue maturation and aging.
See also
- Human voiceHuman voiceThe human voice consists of sound made by a human being using the vocal folds for talking, singing, laughing, crying, screaming, etc. Its frequency ranges from about 60 to 7000 Hz. The human voice is specifically that part of human sound production in which the vocal folds are the primary...
- Vocal fold noduleVocal fold noduleA vocal cord nodule is a mass of tissue that grows on the vocal folds . Typically, this mass will appear on the junction of the anterior and middle two-thirds of the vocal fold, where contact is most forceful....
- Adam's appleAdam's appleThe laryngeal prominence—commonly known as the Adam's Apple—is a feature of the human neck. This lump, or protrusion, is formed by the angle of the thyroid cartilage surrounding the larynx...
- FalsettoFalsettoFalsetto is the vocal register occupying the frequency range just above the modal voice register and overlapping with it by approximately one octave. It is produced by the vibration of the ligamentous edges of the vocal folds, in whole or in part...
- PhonationPhonationPhonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, phonation is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the definition used among those who study laryngeal anatomy and physiology...
- VocologyVocologyVocology is the science of enabling or endowing the human voice with greater ability or fitness.. Its concerns include the nature of speech and language pathology, the defects of the vocal tract , the remediation of speech therapy and the voice training and voice pedagogy of song and speech for...
- Vocal cord dysfunctionVocal cord dysfunctionVocal cord dysfunction is a condition that affects the vocal folds, commonly referred to as the vocal cords, which is characterized by full or partial vocal fold closure that usually occurs during inhalation for short periods of time; however, can occur during both inhalation and exhalation....
- ElectroglottographElectroglottographThe electroglottograph, or EGG, is a device for the noninvasive measurement of the time variation of the degree of contact between the vibrating vocal folds during voice production...
- Histology of the Vocal FoldsHistology of the Vocal FoldsHistology is the study of the minute structure, composition, and function of tissues. The histology of the vocal folds is the reason for vocal fold vibration.-Histoanatomy of the Glottis:...