Cerebellar interneuron activity is triggered by reach endpoint during learning of a complex locomotor task

Locomotion in complex environments depends on the precise timing and active control of single paw movements in order to adapt steps to surface structure and coordinate paws. Such motor control crucially depends on the cerebellum, which is thought to support moment-to-moment prediction and correction of paw trajectories. Although cerebellar activity has been linked to limb kinematics on flat surfaces, it remains unknown how cerebellar cortical neurons encode paw movements when gait becomes irregular and the requirements for precise motor control vary dynamically. To address this question, we developed LocoReach: a new task which combines continuous and discrete aspects of motor control by requiring mice to walk on a runged treadmill, where each step involves reaching for the next rung. Over several days of learning, mice became increasingly proficient at LocoReach, so that they made fewer, longer strides with faster swings and fewer missteps. Through real-time optogenetic disruption of cerebellar processing, we shortened the swing of the perturbed paw highlighting the online contribution of lobule simplex to precise limb control the task. We next investigated the role of the cerebellar lobule simplex during LocoReach learning using electrophysiological recordings, with particular focus on molecular layer interneurons (MLIs) that shape the timing and gain of Purkinje cell (PC) output. When analyzing behaviorally-evoked responses in MLIs and PCs, we found sharp changes in activity around paw-specific transitions from swing to stance and vice versa. Cells in lobule simplex showed clear behavioral specificity: most neurons were tuned to swing-stance transitions of the ipsilateral paw, a large proportion encoded transitions of other or even multiple paws. Specifically MLIs exhibited larger amplitude firing-rate changes during longer strides acquired through learning, indicating increased engagement during higher-demand steps. These results show that cerebellar activity is tightly aligned to defined events in the step cycle, providing a mechanism through which cerebellar cortical circuits can contribute to the precise control of paw placement during adaptive locomotion.