In terms of voice production, vocal folds need to close or nearly close to produce voice. Vocal fold vibration is a passive phenomenon in which air from the lungs is driven past the vocal folds. Because the vocal folds or the space between the vocal folds is smaller than the space in the lung, the vocal folds themselves are driven into vibration. That vibration produces a buzzing sound. That buzzing sound is the neutral or schwa vowel and has a similar sound to. I then use my tongue jaw lips mouth and throat muscles to shape that neutral or schwa vowel sound into words. This whole passive phenomena is referred to as the myoelastic aerodynamic theory of vocal fold vibration, and we'll go into this more later within this module. Laryngeal function for voice then requires that we understand the vocal folds structure. Here we have a human neck and the plane cut through here is something called a coronal plane. This is cut through the middle of the larynx and we can see the trachea and the vocal folds structures that we've seen earlier as well as the super glottic vocal tract or the area above the vocal folds. In real life, this close up segment translates to this region right here. Here we have the thyroarytenoid muscle and then the skin or skin surface, mucosal surface, of the human vocal fold. The body-cover theory of vocal fold vibration also promotes the fact that vocal fold vibration is a passive event. Researchers in the mid-70s, demonstrated and it was accepted that vibration was not a neurologic phenomenon but actually a passive phenomena that was due to differential physical properties of the vocal fold cover that allowed it to vibrate separately from the vocal fold body. Let's talk about the vocal fold cover for a few minutes. First of all, mucosa or skin requires that we have an epithelial layer. And underneath this epithelial layer, we have a subepithelial layer. The subepithelial layer helps support the living epithelial tissues. It is responsible for maintaining connection of this epithelial or cellular tissues with the underlying structures. Within the human vocal fold, because of the way we use our voice, this subepithelial tissue differentiates into three specific layers. These layers are determined by the concentration of different proteins within this subepithelium. The first layer is called the superficial layer of the lamina propria. And it is composed of particles of proteins called glycosaminoglycans or glycoproteins. These are responsible for the sponginess or the thickness or thinness also known as viscosity of this layer. The intermediate layer of this region is composed primarily of these black fibers here which are elastin particles. Lastly, the smallest or the deep layer of this region seen in yellow here on this particular stain, is composed of dense-packed collagen fibers. Together, these form the layers responsible for the body-cover theory of vocal fold vibration. The cover is the epithelium plus the superficial layer of the lamina propria. The transition zone also known as the vocal ligament is the intermediate layer of the lamina propria which is rich in elastin, and the deep layer which is rich in collagen. The body is the thyroarytenoid muscle or the vocalis muscle. Because of this rich elastin layer that develops through the way we use our voice, the cover separates from the body during vibration. This allows us to have high pitch and low pitch and everything in-between. No other mammalian species has this arrangement of the subepithelial tissues within their vocal fold. The purpose of the intrinsic laryngeal musculature then is to bring the vocal folds skin, the vibratory portion, into a nearly closed configuration. We then through action of the thyroarytenoid muscle and the vocalis muscle, control tension in the body and the cover separately and independently. This allows us to contract or tense one portion without tensing the other, or tense one portion without tensing the other, allowing us to have different modes of vibration, allowing us again to have low pitch and high pitch and everything in-between. Dogs bark or growl but you rarely hear a dog glissando. Here is the vocal folds seen in coronal section with a cartoon of vibration. What happens here is the air comes up through the trachea, is passed between the two vocal folds in the region called the glottis, and because of the different air properties, aerodynamic properties of this space, it draws the vocal folds together and closes them and then it blows them open again. This creates vocal fold vibration. Here is a clinical view of vocal folds and here's what we see with laryngeal stroboscopy. The vocal folds are vibrating. They open to breath in and then our patient closes them and begins to blow air through them, and you can see how they are entrained or driven into vibration. What are the requirements for laryngeal vibration? We mentioned earlier the myoelastic aerodynamic theory of vocal fold vibration. Specifically, the glottis needs to adopt an aerodynamic configuration. The intrinsic laryngeal muscles are responsible for placing the vocal folds into a nearly closed configuration so that we can blow air through them in entrained vibration. This requires intact muscle and nerve function as well as a normal or intact mucosal surface. If this mucosal surface is absent or derange, it's going to affect our overall aerodynamic glottic space. When I prepare to speak, my brain tells my vocal folds to become nearly close to adopt this aerodynamic configuration. My brain sends signals to my vocal folds to place them into the appropriate position. Studies of laryngeal muscle contraction demonstrate that this electrical activity within the muscle occurs microseconds prior to the onset of sound. This is referred to as the pre-phonatory burst. My brain sets my vocal folds into the appropriate position for phonation to begin. Other research has demonstrated that the ideal gap between the vocal folds or the pre-phonatory whip is between 0.6 and one millimeter. When I bring my vocal folds into that position, I can entrain vibration of the vocal fold mucosa with the least amount of effort with the most efficiency. The Bernoulli principle is actually the conservation of energy in flowing fluids, and air is a fluid. It essentially states that as air speed increases, the pressure will decrease. We have the trachea with a large column of air coming up. It needs to pass between the narrowed vocal folds or glottic space. So that the air does not back up in the trachea, each individual air particle needs to increase its speed. As it increases its speed, pressure around it will drop. The pliable or myoelastic vocal fold tissue will then be drawn inward. So, as the pressure drops, the pliable rima glottic tissue is drawn inward toward closure. This produces vocal fold vibration. The cover is specifically defined as the epithelium plus the superficial layer of the lamina propria. This tissue needs to be well-hydrated and supple so that in the body-cover theory of vocal fold vibration, it will be drawn inward or together. We can control the tension in this tissue through contraction of the thyroarytenoid muscle or the cricothyroid muscle which puts direct tension on the vocal ligament and the cover region of the vocal fold. The body-cover theory of vocal fold vibration then relies on the myoelastic nature of the cover, the deformability or suppleness of this region. Remember, it wasn't until the middle 1970s that this theory was widely accepted. The cover as we said, is the epithelium plus the superficial layer of the lamina propria. The transition zone is the vocal ligament. It is the intermediate and deep layer. The intermediate layer is rich in elastin, and those elastin particles allow an uncoupling of the cover from the body during vibration so that we can have different modes of vibration. Just to give you an idea of this side, the entire lamina propria or subepithelial tissue is only a millimeter and one half in width. It's roughly five to seven millimeters in height. So I always tell my patients, they're speaking with tissue the size of a dime or a nickel. Here we see clinical vocal fold vibration under stroboscopy, and we'll talk about stroboscobic principles in module three. But we can see how we can stretch out the vocal folds to give low pitched or higher pitched vibration. This relies on the ability to close the vocal folds. We have to have intact nerve and muscles, and we have to know how to stretch them, to tense them, and how to relax them, to take them off tension to allow lower pitched vibrations. This concludes module one. In summary of module one, I stated at the very beginning that, it's my belief that benign vocal fold lesions are a response to trauma. This is not trauma like you'd get from a car wreck or a choking episode. This is vocal trauma from the way we speak, which we'll talk more about in modules two and three. Vocal trauma is the most common source of that injury. Surgery for vocal fold lesions then is only indicated after failure to have vocal improvements from voice therapy which addresses the way we produce voice, hopefully reducing the amount of vocal trauma. Specialized muscles both inside, intrinsic to the larynx, and outside, extrinsic to the larynx, place the vocal folds in an appropriate position and stabilize the larynx within the neck to allow voice to be produced. The body-cover theory established by researchers in the mid-1970s, explains current concepts of vocal fold vibration. Vocal fold vibration is a passive event. It occurs due to differential tension and mass between the cover and the body of the vocal folds. I look forward to presenting module two to you.
