Coordinating speech and breathing in the brain

People and other mammals that vocalize need to coordinate their vocalization with breathing. The vocal cords need to be closed to make sound, but they have to open when breathing in to allow air into the airway. This is why we must stop talking periodically to take a breath. The neural circuits that coordinate vocalization and breathing remain poorly understood.

An NIH-funded research team, led by Dr. Fan Wang at the Massachusetts Institute of Technology, studied neurons that control ultrasonic vocalizations (USVs) in mice. These are sounds, too high-pitched for people to hear, that male mice make during interactions with females. As with human speech, making these sounds requires closing the vocal cords while breathing out. The study appeared in Science on March 8, 2024.

The team first used a technique to trace which neurons connected to the motor neurons in the larynx. They then determined which of these “premotor” neurons were active during USVs. This identified a set of premotor neurons in an area of the brain stem called the retroambiguus nucleus (RAm). Previous research had identified the RAm as important for vocalization. The team called these premotor RAm neurons RAm^VOC neurons.

Next, the researchers engineered male mice in which RAm^VOC neurons could be activated or silenced by various means. Mice in which the neurons were silenced did not vocalize in the presence of females, unlike control mice. They also didn’t make audible vocalizations when their tails were pinched. But activating RAm^VOC neurons caused the mice to exhale and vocalize. The longer the activation, the longer the vocalization and exhalation.

The researchers observed that during prolonged RAm^VOC activation, the mice would periodically interrupt their vocalizations to take a breath. This suggests that the need to breathe “overrode” the signal to vocalize from RAm^VOC neurons. To find out how, the team mapped which neurons provided inhibitory signals to RAm^VOC neurons.

Most of these were in a part of the brainstem called the preBötzinger complex (preBötC). The preBötC controls the rhythm of inhalation. When the researchers disabled the connections between the preBötC and the RAm^VOC neurons, the mice had difficulty interrupting vocalization to breathe. The breaths the mice did take were much shallower than normal. The mice also made hoarse, asthma-like vocal sounds while inhaling.

The findings suggest that RAm^VOC neurons drive the closure of the vocal cords and sound production. These neurons are periodically interrupted by the preBötC to allow breathing.

“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,” Wang says.

The researchers note that although human and mouse vocalizations differ in many ways, the basic mechanism of generating sound is the same. Thus, the results may lead to insights into human speech production and breathing.

—by Brian Doctrow, Ph.D.