A special way of entry may be the key to encode sensory information in the cerebellum

In our daily activities, we are constantly bombarded by a continuous stream of sensory information. Take a typical morning routine, such as: wake up to a strident alarm, the powerful aroma of freshly brewed coffee, brake lights and rush hour horns. During a single day, we experience thousands of different signals in all directions.

Despite the many chaotic signals we encounter, our brain does a remarkable job badembling and treating them; allowing us to make sense of the world around us. This treatment can take shape in a very visible way, such as our vision and our hearing, but also occurs in a more subtle and unexpected way. For example, when learning a new movement, our brain constantly monitors the sensory cues that surround us. This sensory snapshot helps instruct and guide motor learning so that when we encounter the same context again, we are more likely to perform the movement more effectively.

Although the knowledge gained supports the idea that sensory cues are beneficial to motor learning, the precise circuits and brain mechanisms linking these two elements have been the subject of debate in recent years. To shed new light on this subject, an article recently published in neuron Dr. Jason Christie's lab, a research group leader at the Max Planck Institute of Neuroscience in Florida (MPFI), has revealed that a special pathway into the cerebellum appears to be the key to coding of sensory information.

The cerebellum is a unique structure in the brain that plays a vital role in motor coordination and learning that improves movement. By receiving a lot of information from different regions of the brain, the cerebellum integrates and sends more detailed information via a single type of neuron called a Purkinje cell. Long-term projections called climbing fibers are an important contribution to Purkinje cells.

"Climbing fibers are well known and widely studied in the field." describes Dr. Michael Gaffield, research fellow at Christie Lab and first author of the publication. "These fibers form long-distance connections with the cerebellum and are expected to deliver instructive motor signals and relay sensory information, but in recent years it has been suggested that local circuits in the cerebellum, such as parallel fibers or molecular layer interneurons, could also play a role in coding sensory information ".

To study, the team used two-photon calcium imaging to monitor the activity of Purkinje cells in the mouse cerebellum while presenting various sensory stimuli (auditory, visual, and somatosensory). They then isolated and evaluated changes in Purkinje cell activity that corresponded directly to the timing of each stimulus and the input of upward fibers.

"Within each Purkinje cell, we found a steady increase in activity whenever a sensory stimulus was presented. But the improvement was not the same for all three sensory types, it has varied according to the type of stimulus presented, "explains Gaffield.

MPFI scientists then examined whether other types of cells, besides climbing fibers, directly contributed to the enhanced sensory activity observed in Purkinje cells. Using optogenetic inactivation techniques (suppression of neuronal activity using light) and chemogenetic inhibition (suppression of neuronal activity using drugs), the l & # 39; team was able to inhibit individual cell types in local cerebellar circuits. Despite the change in local activity, the enhanced sensory activity of Purkinje cells has not changed. However, by directly inhibiting the activity of climbing fibers, the improvement was suppressed; indicating that only climbing fibers are responsible for the transmission of sensory information to the cerebellum.

Furthering their research, the Christie laboratory has developed a new technique that allows them to monitor the activity of axon projections of climbing fibers themselves. They discovered that during the presentation of sensory stimuli, gradual changes in the presynaptic activity of climbing fibers were faithfully represented in Purkinje cells; a stronger activity in the climbing fiber was precisely reflected by a stronger activity in the Purkinje cells with which they connect.

"Our results were a bit of a surprise," notes Dr. Christie. "Traditionally, it was thought that sensory cues coming into the cerebellum were integrated and processed by Purkinje cells using local connections.Our results demonstrate that Purkinje cells simply reflect what climbing does. This means that a more distal region of the brain deposits effective treatment of sensory information and its transmission to the cerebellum. "

"Given that Purkinje's cell activity is of paramount importance for motor learning, we are currently studying the activity derived from sensory functions during more complex motor behaviors," explains Dr. Christie. "Let's hope we can discover the neural mechanisms that underlie the unique ability of climbing fiber to transmit sensory information and how learning benefits from this coding scheme."