Summary: Researchers studying marmosets discovered that Purkinje cells in the cerebellum help regulate tongue movements and signal that the tongue should stop when it approaches its target. These cells were strongly activated during precision tasks, such as inserting the tongue into narrow tubes, but not during normal tongue preparation.
When the researchers suppressed these cells, the tongue either overshot its target or delayed its return, demonstrating the importance of Purkinje cell activity for coordination. The results shed light on how the cerebellum contributes to fine motor control and could inspire treatments for speech and swallowing disorders.
Important facts:
- Engine accuracy: Purkinje cells in the cerebellar vermis send signals to prevent the tongue from approaching the target.
- Functional change: Suppression of these cells results in excessive tongue protrusion or slow retraction.
- Medical relevance: Knowledge can help treat speech, swallowing, and voice disorders.
Source: PLOS
By studying the clever movements of marmosets’ tongues, researchers discovered that Purkinje cells (P cells) in a brain region called the cerebellum send signals to stop the tongue from spreading as it approaches its target, according to a study published April 10 in the open-access journal PLOS Biology by Reza Shadmehr of the School of Johns Hopkins Medicine, MeHo.
We use our tongues to shape air and produce sounds for communication. They also help us gauge and move food through the oral cavity when we eat.
These skills require the coordination of more than 100 muscles, which produce movements fundamental to our existence. Lesions in the cerebellum disrupt these movements, resulting in abnormal patterns of muscle activation. However, how the cerebellum controls tongue movements is unclear.
To answer this question, Shad Mehr and his colleagues used an animal model with a long tongue that can deftly hit small targets. Marmosets have a 21-millimeter tongue, which they use to dig into small holes and extract insects and sap. Moreover, they have an unusual ability to control their tongues and vocalize to recognize other marmosets during two-way communication.
Researchers observed that marmosets can naturally twist their tongues and insert them into small tubes, even when the tubes are at a sharp angle to their mouths.
To investigate how the cerebellum contributes to the control of language-related movements, researchers monitored the activity of Purkinje (P) cells within a specific cerebellar region known as the vermis. They found that when a single P cell was suppressed in response to a stimulus, the movement pathway associated with language production became hypermetric—meaning it overshot its intended target. This suggests that P cells play a crucial role in fine-tuning motor commands to ensure precise articulation. Additionally, during the retraction phase of tongue movement, external pressure on the tongue caused a noticeable delay in its return to the mouth, highlighting a disruption in the normal timing and force regulation associated with speech motor control.
These disruptions were significantly amplified when two P cells were suppressed simultaneously, pointing to a cumulative or synergistic effect in cerebellar output modulation. Specifically, the suppression interfered with the normal inhibitory forces that typically act to decelerate the tongue as it approaches its target during movement. Without this modulation, movements became exaggerated and poorly timed—critical impairments in the context of speech, where precise coordination is essential. The findings underscore the cerebellar vermis’s role in governing the subtle dynamics of tongue motion and offer compelling evidence that Purkinje cells are integral to the control of complex, goal-directed motor behaviors like those involved in language.
The results suggest that when the tongue approaches the target, P cells send signals to the anterior structures to stop the movement. This strong P cell activation was observed when the tongue was directed towards a small tube (a movement that requires precision), but not when the tongue was used to prepare the face.
According to the authors, to treat or cure symptoms related to cerebellar dysfunction, such as vocal muscle spasms, difficulty swallowing, or speech disorders, a better understanding of how the cerebellum contributes to the control of language movement and learning is needed.
Because marmosets are exceptionally good at shaping and rotating their tongues, using them almost like fingers, they are an attractive animal model for studying the neural control of a body part that is essential to our existence.
The authors explain that during periods of intense licking, a climbing fiber input triggers a compression of Purkinje cells located in the lingual vermis of the cerebellum. This neural activity appears to suppress the forces that would typically cause the tongue to retract. As a result, there is a noticeable exaggeration in the forward extension of the tongue—referred to as hypermetropia—along with a slowing down of its backward retraction. This suggests a finely tuned regulatory mechanism in which cerebellar output dynamically modulates tongue movement by inhibiting opposing muscular forces, thereby allowing for greater extension during repetitive licking behaviors.
Furthermore, the study highlights that the directionality of these modulated forces corresponds closely with the direction of movement that is driven by olivary input. This pattern of neural activity is not unique to the lingual region; a similar recruitment and modulation of Purkinje cells have been observed in the oculomotor region of the cerebellum, where they influence eye movements. Taken together, these findings point to a broader functional principle: Purkinje cells are selectively recruited to fine-tune and coordinate specific motor actions, playing a central role in the cerebellum’s general strategy for controlling precise and goal-directed movements.
Funding: This work was supported by grants from the National Institutes of Health (R01-EB028156 to RS, R37-NS128416 to RS) and the National Science Foundation (CNS-1714623 to RS). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
About this neuroscience research news
Author: Claire Turner
Source: PLOS
Contact: Claire Turner – PLOS
Image: The image is credited to StackZone Neuro
Original Research: Open access.
“Purkinje cells of the cerebellum control deceleration of tongue movements” by Reza Shadmehr et al. PLOS Biology
Abstract
Purkinje cells in the cerebellum control the latency of tongue movements.
We use our tongues like our hands: to interact with objects and move them. For example, we use our hands to learn about the properties of objects and move them to a nearby location, and our tongues are used to understand the properties of food and move it through the oral cavity.
But what role does the cerebellum play in controlling tongue movements?
Here we trained fixed-headed marmosets to collect food from small tubes placed at a sharp angle in front of their mouths, which are moved by tongue dexterity.
We identified the lingual regions of the cerebellar vermis and then measured the contribution of each Purkinje cell (P cell) to language control, based on the brief but complete suppression they experienced following inferior olive input.
When the P cell was depressed during protraction, the tongue path became hypermetric. When depressed during retraction, there was a delay in the tongue returning to the mouth.
Both effects were enhanced by simultaneous suppression of two P cells. Furthermore, these effects occurred even when the pause was not due to climbing fiber input. Thus, suppression of P cells at the root of the tongue disrupts the forces that normally slow the tongue down as it approaches the target.
Specifically, a peak population activity occurred near the onset of the delay, when the movement required precision (aiming at a tube), but not when the movement was aimed at grooming the animal.
Thus, P cells appear to signal when to stop spreading when the tongue approaches the target.
