Here's a bet: you can't breathe in and talk at the same time!

Challenge! Try this: Breathe in while you speak.

Chatting, humming, yelling (?), we make these vocalizations almost every day. Naturally, we take vocalization for granted and "think" that we can make sounds at any time. But a simple little experiment tells us: at least when we inhale, we cannot speak.

Sometimes, you will find some people who are so excited about talking that they can't help but talk non-stop, and it seems that no one can interrupt them. Since breathing and speaking cannot coexist, if they really can't stop talking, they will probably end up "talking themselves to death".

Fortunately, we have never heard of such a humorous and terrifying incident. After all, the human body will always prioritize the most basic need - survival. If you find that you are about to be deprived of oxygen, your brain will force you to stop speaking and breathe quickly. After all, staying alive is the first priority.

Vocalization is closely tied to breathing. We always seem to speak when we exhale and stop speaking when we inhale. This is because vocalization requires the release of air from the lungs, which flows through the larynx and forces the vocal cords to vibrate in order to produce sound. The key is the airflow and the vibration of the vocal cords.

But if you think about it, inhaling obviously also moves air through the throat, so why can't the reverse process trigger the vocal cords to vibrate? Why can't we talk while inhaling, and have the ability to chatter all the time? Why don't we "chat to death"?

Delicate and complex

To understand the first two questions, we need to understand the process of vocalization in more detail. Although the complexity of the vocalization system varies from species to species, the basic process of producing sound is similar. As mentioned above, vocalization requires airflow and vibration of the vocal cords, which is closely related to the larynx.

Image source: Wikipedia

The larynx is an ancient organ. When fish crawled onto land from the ocean and evolved into various animals, an important problem encountered in this process was the need to separate the air we breathe from the food we eat. The larynx functions like the "anteroom" of the trachea, and inside it there is a layer of cartilage called the epiglottis, which prevents food or liquid from falling into the trachea and causing suffocation. Below the epiglottis, mammals have evolved additional tissue folds, which are the vocal cords necessary for our vocalization.

In order to make the vocal cords vibrate, the throat usually needs to be contracted and the vocal cords are retracted so that the airflow can arouse the vibration of the folds. If you consciously feel it, you may find that when you squeeze your throat, the tone of the sound is usually high, and when you try to widen your throat, you can produce a low tone. This process is actually tightening or relaxing the vocal cords, thereby adjusting the frequency of vocal cord vibration.

However, when we try to inhale, in order to ensure efficient inhalation, the throat needs to be opened, that is, the vocal cords are abducted, and naturally the vocal cords cannot vibrate and produce sound. Of course, this is under the premise of natural relaxation, and we will not feel any stagnation when inhaling at this time. But if you consciously tighten your throat and inhale at the same time, you can actually make some exclamations like "Shock me", but you will feel difficulty in inhaling.

Image source: Wikipedia

The complex and delicate coordination of vocal cords and breathing enables animals to make sounds and communicate with each other. But scientists are still curious about why vocalization must give way to inhalation when life is threatened? How to ensure that breathing takes priority over vocalization?

The manipulator of dominant behavior

No matter which behavior, it is regulated by neural circuits. For example, vocal cord closure or abduction is controlled by laryngeal motor neurons, while breathing movements are controlled by complex breathing circuits. There are obviously neural circuits between laryngeal movement and breathing that can seamlessly and smoothly regulate the flexible switching between the two and ensure the priority of the breathing circuit.

To explore the "manipulators" behind this dominant behavior, a research team from the Massachusetts Institute of Technology began using mouse models to try to identify the neurons that control vocal cord adduction and explore how these neurons interact with the respiratory circuit.

Mice also need to exhale to allow air to flow through the nearly closed vocal cords. The adduction of the vocal cords leaves a very small hole in the middle. When air passes through the hole, it is like whistling, allowing mice to emit ultrasonic waves to communicate with each other. This process is also called ultrasonic vocalization (USV).

The researchers knew that vocal cord adduction is controlled by laryngeal motor neurons, so they used neural tracers to map the synaptic connections between neurons and began tracing back to find the neurons that innervate them. After observation, the researchers found that a group of motor neurons located in the retroambiguus nucleus (RAm) in the hindbrain region, which previous studies have linked to vocalization, were strongly activated during mouse USVs. Eventually, the researchers targeted a subset of vocalization-specific neurons in RAm, called RAmVOC.

Image source: original paper

When the researchers blocked RAmVOC neurons, the mice could no longer make USVs or any other type of sound, their vocal cords would not close, and their abdominal muscles would not contract. Conversely, when RAmVOC neurons were activated, the mice's vocal cords were able to close, produce USVs, and exhale at the same time. Moreover, the longer the activation time, the longer the exhalation and vocalization time.

But if the RAmVOC neurons were stimulated for two seconds or longer, the USVs were interrupted by inhalation. During prolonged RAmVOC activation, the mice would periodically interrupt their vocalizations to inhale. The need to breathe apparently overwhelmed the researchers' stimulation of the RAmVOC neurons.

To find out the real culprit, the research team mapped the neurons that provide inhibitory signals to RAmVOC neurons. From this, they found that most of the inhibitory signals come from a part of the brainstem that controls respiratory rhythm, called the preBötC complex.

Image source: original paper

When the researchers blocked the connection between preBötC and RAmVOC, the mice had a harder time interrupting vocalizations to breathe. Their breathing became much shallower than normal, and they made hoarse, asthma-like sounds when they inhaled.

The study showed that RAmVOC neurons can control the adduction of the vocal cords to produce sound, but are regularly inhibited by preBötC to ensure smooth breathing. This study, which revealed the neural circuits behind the coordinated movements of breathing and vocalization, was published in Science in March this year.

Looking back at human evolution, the shape of our vocal organs changed after we diverged from our early ape ancestors. Our mouths became smaller and less protruding, and our tongues moved downward, pulling our larynx lower and giving us longer necks. These changes allowed humans to control a variety of tiny muscles with incredible precision, producing complex sounds that other animals cannot achieve.

But opportunities always come with risks. As the larynx is lowered, all the food we eat must pass through the larynx, bypass the trachea, and then enter the esophagus. The danger is that once the food goes to the wrong place, choking will occur. It seems that these structures that enhance our vocal ability make us suffocate to death "more effectively".

In order to avoid this kind of "efficiency", please also pay attention to the following: avoid eating and drinking eagerly while talking, which will greatly increase the risk of choking; or continue talking for too long, which may also cause throat fatigue and cause choking.

References

[1]https://www.science.org/doi/10.1126/science.adi8081

[2]https://news.mit.edu/2024/how-brain-coordinates-speaking-and-breathing-0307

[3]https://www.nih.gov/news-events/nih-research-matters/coordinating-speech-breathing-brain

[4]https://www.smithsonianmag.com/smart-news/scientists-discover-how-some-whales-can-sing-while-holding-their-breath-underwater-180983836/

[5]https://www.npr.org/2010/08/11/129083762/from-grunting-to-gabbing-why-humans-can-talk

[6]https://www.nature.com/articles/s41586-024-07080-1

Planning and production

Source: Global Science (id: huanqiukexue)

Author | Bu Zhou

Editor: Yang Yaping

Proofread by Xu Lailinlin