Chatting, humming, shouting—these vocal behaviors intermittently fill our daily lives. We conventionally consider vocalization a normal activity, almost disregarding its physiological limitations. In fact, a simple experiment reveals that we cannot speak while inhaling.
Occasionally, we encounter people who become loquacious when discussing their interests, as if their words are endless. However, considering that inhaling and vocalizing cannot occur simultaneously, if they truly spoke incessantly, it might lead to a tragedy—talking themselves into suffocation. Fortunately, this has never happened because physiologically, the human body prioritizes the most basic need—survival. If an oxygen shortage is detected, the brain will force us to stop talking and prioritize breathing. Ultimately, survival is always the top priority.
The mechanisms of breathing and vocalization are closely intertwined; we usually speak while exhaling and pause while inhaling. Vocalization requires pushing airflow from the lungs through the throat to then vibrate the vocal cords to produce sound. The key is to combine airflow with vocal cord vibration. So the question arises, why can’t we use the reverse airflow during inhalation to stimulate the vocal cords and have the ability to talk non-stop?
To understand this issue, we need to delve into the process of vocalization. Although the complexity of vocal systems varies among species, the basic principle is similar. As mentioned, air passes through the throat and causes the vocal cords to vibrate; the throat is the key organ in this task. The larynx is an ancient organ that initially helped marine creatures adapt to terrestrial life, acting as a diverter for air and food. Its internal epiglottis can prevent food or liquid from entering the trachea. The vocal cords of mammals, located below the epiglottis, are indispensable in the vocalization process.
To drive vocal cord vibration, airflow needs to be properly pressurized while passing through the larynx. The larynx must contract to bring the vocal cords closer to generate sufficient vibration as the air passes. If you consciously experience it, you may find that compressing your throat produces a higher pitch, while relaxing it produces a lower tone. This is actually done by adjusting the tension of the vocal cords to change the vibration frequency.
However, when we inhale, the larynx should be relaxed and open to efficiently draw in air, and at this time the vocal cords are abducted and therefore sound cannot be produced. This is the natural inhalation when relaxed. Nevertheless, if you deliberately constrict your throat while inhaling, you can still produce sound, but it would make inhalation extremely difficult.
The coordination of the vocal cords and breathing is a complex and delicate physiological activity, allowing animals to communicate through sound. But to ensure efficient airflow and prevent suffocation from excessive speech, natural laws and our physiological structure have established a series of coordinating mechanisms.
Scientists have always been curious about why vocal behavior must give way to breathing when life is threatened. What mechanism ensures that the priority of breathing activities is above vocalization? All behaviors are regulated by neural circuits. For example, closing or opening of the vocal cords is controlled by laryngeal motor neurons, while breathing is controlled by a more complex respiratory neural circuit. Clearly, there exists a neural circuit between laryngeal motion and breathing that can smoothly switch between the two, while ensuring the priority of the respiratory circuit.
In order to explore the main “manipulators” behind these behaviors, a research team from the Massachusetts Institute of Technology in the USA employed a mouse model, aiming to identify the neurons that control the adduction of vocal cords, as well as how these neurons interact with the respiratory circuit. When mice emit sounds, they also do so by exhalation, forcing air through the nearly closed vocal cords. When the vocal cords close, leaving only a very small hole, the passing air generates ultrasonic waves, much like whistling, and the mice communicate through these ultrasonic vocalizations (USV). It is known that the adduction of the vocal cords is controlled by laryngeal motor neurons. Thus, researchers used neural tracing dyes to map synaptic connections between neurons and traced back to find the origin of these neurons.
Upon closer examination, researchers discovered a group of motor neurons related to vocalization in the retroambiguus nucleus (RAm) located in the posterior region of the mouse brain, which were strongly activated during ultrasonic vocalization by the mice. The researchers then identified a subset of neurons in the RAm dedicated to vocalization, naming them RAmVOC.
When the function of RAmVOC neurons was blocked in experiments, the mice were no longer able to emit USVs or any other type of sound; their vocal cords could not close, and their abdominal muscles did not contract. Conversely, when the RAmVOC neurons were activated, mice could close their vocal cords and produce USVs while exhaling. The longer the activation time, the longer the duration of exhalation and vocalization. However, if the RAmVOC neurons were continuously activated for two seconds or more, USVs were interrupted by the inhalation process. During prolonged RAmVOC activation, mice would periodically stop vocalizing to inhale, with the need for breathing significantly “overriding” the continuous stimulation of RAmVOC neurons.
To reveal the “culprit” behind this, the research team further mapped the network of neurons that send inhibitory signals to the RAmVOC neurons. Results showed that the inhibitory signals mainly originated from an area in the brainstem that controls the rhythm of breathing, called the preBötzinger complex (preBötC). When researchers severed the connection between the preBötC and RAmVOC, the mice had difficulty stopping their vocalization to breathe while emitting sound. Compared to normal conditions, their breathing appeared shallower. Moreover, during inhalation, they would emit a hoarse, asthma-like sound. These findings suggest that the RAmVOC neurons control the contraction of the vocal cords to produce sound and are periodically interrupted by inhibitory signals from the preBötC to ensure smooth breathing.
The latest research reveals the neural circuits that control the coordination of breathing and vocalization, which are the neural foundation of our unique human linguistic abilities. These findings were published in the March issue of a scientific journal this year, providing deep insights into our understanding of how humans evolved complex means of language communication.
One of the markers of human divergence from our early primate ancestors during the long journey of evolution is the significant changes our vocal apparatus has undergone. Our mouths became smaller, the prominence of our faces decreased, our tongues moved downward, and our larynxes also descended, resulting in a longer neck. These evolutionary changes in anatomical structure have endowed humans with the extraordinary ability to finely control a range of small muscles, producing complex sounds other animals cannot make.
However, this also comes with certain risks. As the position of the larynx descends, humans require food to bypass the trachea and enter the esophagus. If not managed properly, this process might lead to food entering the respiratory tract and subsequent choking incidents. Therefore, to retain the survival advantages brought by vocal structures, humans must precisely balance the actions of breathing and eating.
For safety reasons, people are advised to avoid engaging in vigorous conversation while eating to reduce the risk of choking; furthermore, prolonged talking can also fatigue the throat, potentially leading to unnecessary coughing fits.
