by Researchers at Stanford University School of Medicine have made a significant breakthrough in understanding the neural mechanisms involved in the production and modulation of mammalian vocalizations. The study, published in Nature Neuroscience, focuses on a specific neural circuit and a cluster of genetically defined neurons in the brain that play a crucial role in the production of sound.
Vocalization in mammals, including humans, is achieved by pushing air past the vocal cords of the larynx, which vibrate to produce sound. The volume and pitch of these sounds convey meaning to the listener and are controlled by approximately 40 different muscles. However, despite the importance of vocalization and its relevance to speech disorders, there is still limited knowledge about how the brain produces sound and controls acoustic features.
Previous studies have suggested that the brainstem, the connection between the primary brain structure and the spinal cord, plays a vital role in mammalian sound production. Building on this knowledge, the researchers aimed to identify specific brainstem neurons responsible for supporting vocalizations.
To achieve this, the team searched the Allen Mouse Brain Atlas, a publicly available mouse gene expression database, in order to find gene markers of brainstem vocalization neurons. They focused on a region of the brainstem called the nucleus retroambiguus, which has been associated with sound production.
The analysis revealed that a gene called neurotensin was expressed in a small cluster of approximately 160 neurons within the nucleus retroambiguus. The researchers conducted further experiments, manipulating neurons expressing the neurotensin gene while recording the vocalizations of mice using optogenetic techniques.
The results showed that the neurotensin-expressing neurons were activated during both neonatal and adult vocalizations. Furthermore, the researchers found that these neurons were essential for sound production, as mice became mute when these neurons were removed. Interestingly, activating the neurotensin neurons caused mice to vocalize even under general anesthesia, and increasing the firing rate of these neurons increased the volume, but not the pitch, of the produced sound.
The findings indicate that neurotensin-expressing neurons in the nucleus retroambiguus are not only involved in the production of mouse vocalizations but also control the volume of the sounds produced. This is achieved by co-activating motor neurons that specifically control the vocal cords and abdominal expiratory muscles. By increasing the expiratory force, the volume of the produced sound increases, similar to a wind instrument.
While this research provides new insights into the neural basis of mammalian vocalizations, there is still much to explore. The team hopes to use the neurotensin gene marker identified in this study to search for similar neurons in other vocalizing species, including humans, other mammals, and birds. Additionally, they are interested in identifying gene markers for neurons that control pitch, syllable structure, and syntax, as these acoustic features convey meaning to the listener.
Understanding the intricate mechanisms behind vocalization in mammals could have significant implications for the study of speech disorders and communication in general. Further research could shed light on how different species produce and modulate sounds, deepening our understanding of the complexities of language and communication.
