THE SPEECH MECHANISM: A 4-PART SERIES Part 2: Our Voice, the Larynx
By Bryana Williams M.S. CCC-SLP
http://image.slidesharecdn.com/anatomyoflarynx-130917075237-phpapp01/95/anatomy-of-larynx-5-638.jpg?cb=1379404592In part one of this series, we learned about the driving force for air pressure and control: our lungs. Today we are exploring the larynx, which is the next part of the intricate speech mechanism. Colloquially known as the “voice box”, this small but complex organ packs a punch in the sound production department!
Sitting at the top of our trachea, the larynx plays a key role in the production of speech because it contains the first valve that can interfere with the airstream provided by the lungs. It is composed of cartilages and muscles that serve two main purposes: protecting the airway and allowing us to create voice.
Thyroid cartilage, anterior view.The first and easiest cartilage to identify is the thyroid, a v-shaped structure with two sides or walls called laminae that is the biggest of the group. It is an attachment point for many muscles and tendons, and is what forms the “Adam’s apple” in men-the point of their laryngeal prominence (i.e. the point of the V where the two laminae meet) is a more acute 90-degree angle compared to 120-degrees in women.
The other two large cartilages are the cricoid and the epiglottis. The cricoid forms a ring at the top of the trachea and primarily serves as an anchor point-muscles and tendons connected to it can contract to move other aspects of the larynx. The epiglottis is leaf-shaped and attaches to the thyroid cartilage. Its main job is to fold down over the opening to the larynx when we swallow, helping to direct food into our esophagus and keep it out of our larynx, trachea, and lungs.
Lastly, we have 3 pairs of smaller cartilages, the arytenoids, corniculates, and cuneiforms. The arytenoids are one of the most important because they attach to several intrinsic (i.e. inner) muscles of the larynx along with the vocal folds themselves. Their movement gives us much of our control over things like pitch and intensity-though other aspects of the larynx and throat can contribute to this as well.
Now that we know some of our attachment points, we can focus on the vocal folds themselves, which are just two elongated muscles (the thyroarytenoid or TA muscles) covered in a layer of mucosal membrane. They stretch across the top of the trachea, and are usually relaxed as we breath to allow air to pass through the V-shaped space between them called the glottis. When we swallow food or liquid, they close completely to keep foreign material out of the lungs. Sometimes things do “go down the wrong tube” though, so they can also help us eject it by closing tightly together, allowing pressure to build as our abs and internal intercostals contract, and then pop open to produce a cough.
However, as opposed to just slamming shut to protect or clear the airway, they can also approximate (i.e. come into light contact with one another) as the air stream flows from the lungs. This causes the folds to vibrate and produces the sound that we refer to as voice! The many external and internal muscles that surround and control the movement of the laryngeal cartilages and the vocal folds are incredibly skilled and work together to create a few main aspects of voice.
First, they can change pitch (how high or low a voice sounds) by tightening or loosening the folds. For example, one muscle called the cricothyroid muscle pulls the front of the thyroid cartilage down and forward, which lengthens the vocal folds and, thus helps increase pitch. The vocal folds themselves can also contract to shorten themselves (resulting in lower pitch) or to create more tension, which raises pitch.
Secondly, they can switch quickly between approximated and open positions for sounds that require voice (ex. /Z/) and those that do not (ex. voiceless sounds like /s/). This is achieved through the complimentary actions of the lateral cricoarytenoid muscles or LCA which closes the folds and the posterior cricoarytenoid muscle or PCA which opens them. You can actually feel this at home-place your hand gently on your throat and produce both sounds. Notice how the “Z” creates a buzz in your throat while the “S” doesn’t!
If we bring this back to nonspeaking people, you can see once again why the process in just this one area of the speech mechanism can cause a hold up. Speech requires us to make regular switches between voiceless sounds and voiced ones-the word “sit”, for instance, goes from voiceless /s/, to the voiced vowel /I/, and back to voiceless /t/. Difficulty controlling those very tiny muscles to navigate that switch can mean a person may produce the entire word with voiced sounds instead (i.e. “Zid”) or with an unexpected mix of voiced/voiceless sounds (i.e. “Zit” or “Sid”). Secondly, vocal quality itself can be affected in terms of how harsh, loud, soft, etc. the voice sounds-someone’s intentional speech may be whispered if their vocal folds remain mostly open, or very growly if the folds are tightly shutting together. That can have a major effect on clearly and efficiently relaying your message!
When it comes to pitch and intonation, we garner a lot of information from variations in a person’s spoken message. For example, statements tend to be spoken with a flat intonation (i.e. little change in pitch) whereas questions have a rising intonation (i.e. rise in pitch) at the end. Try reading “The donut shop is closed” vs. “The donut shop is closed?” to hear this difference. Now imagine if your purposeful speech always had you speaking in a flat intonation pattern or made it difficult to switch between high and low pitch-how difficult might it be to communicate a question vs. statement vs. command? Many nonspeaking people also experience unexpected pitch changes between their automatic vs. intentional speech-purposeful utterances may always come out in a very high or low pitch, or always in the same intonation pattern. An example may be someone who’s intentional speech is always in an up/down pattern between pitches (Ex. I WANT to EAT the CAKE).
These are important facets to highlight because garnering more control over voice takes significant energy-singers, for instance, spend years building their skills for pitch control and change! For an apraxic person, the process of achieving intentional voice control can require just as much effort, and deserves just as much recognition.
In parts 3 and 4 of this series, we’ll move beyond breathe support and voice to some even more complex aspects of speech: the resonators and articulators!
Living in Atlanta and supporting non-speakers in Georgia and Nashville, Tennessee, Bryana Williams is a Speech-Language Pathologist and registered Spelling to Communicate Practitioner. She owns her own private practice and has, for the past three years, working as part of the I-ASC S2C Professional Training Course as a mentor, Cohort Captain, and continues to support I-ASC’s practitioner and CRP training initiatives through the development and improvement of training protocols.
Resources:
http://marina-teachingandtranslating.blogspot.com/2013/06/the-speech-mechanism-how-are-sounds.html
http://my.ilstu.edu/~jsawyer/respiration3/respiration2_print.html
https://emedicine.medscape.com/article/1949369-overview
https://dysphonia.org/your-journey/how-voice-works/
The mission of I-ASC is to advance communication access for nonspeaking individuals globally through training, education, advocacy, and research. I-ASC supports all forms of augmentative and alternative communication (AAC) with a focus on methods of spelling and typing. I-ASC currently offers Practitioner training in Spelling to Communicate (S2C)with the hope that other methods of AAC using spelling or typing will join our association
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