Researchers at MIT have found a brain circuit that controls speaking and ensures that you only talk when breathing out, stopping when you breathe in. This discovery sheds light on how the brain coordinates two essential actions for speech: narrowing the voice box and exhaling air from the lungs.
The study, published in Science, reveals that the newly identified circuit is controlled by a region in the brainstem that regulates breathing rhythm. This ensures that breathing takes priority over speech.
“When you need to breathe in, you have to stop vocalization. We found that the neurons that control vocalization receive direct inhibitory input from the breathing rhythm generator,” explains Fan Wang, the senior author of the study.
Using a mouse model, the researchers traced the connections between neurons and discovered that a group of neurons in the hindbrain, called the retroambiguus nucleus (RAm), plays a crucial role in vocalization. When these neurons were silenced, the mice could not produce any sounds, and their vocal cords did not close. On the other hand, activating these neurons led to vocal cord closure, exhalation, and the production of ultrasonic vocalizations (USVs), which mice use to communicate with each other.
However, if the researchers stimulated the neurons for two seconds or longer, the USVs were interrupted by inhalations, indicating that the process is controlled by the same part of the brain that regulates breathing.
Further investigation revealed that neurons in the pre-Bötzinger complex, which acts as a rhythm generator for inhalation, directly inhibit the vocalization neurons in the RAm. This ensures that breathing remains dominant over speech production, forcing us to pause and breathe while speaking.
Although human speech is more complex than mouse vocalization, the researchers believe that this circuit plays a similar role in both species, as the fundamental process of phonation, which requires vocal cord closure and exhalation, is shared between mice and humans.
The team now plans to investigate how other functions, such as coughing and swallowing food, may be affected by the brain circuits that control breathing and vocalization.
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