Motor cortex somatostatin interneurons adaptively shape the structure of action sequences.

The brain flexibly reorganizes action sequences to optimize behavioral outcomes through reinforcement learning and adaptive motor control. Although the primary motor cortex (M1) is essential for skill learning and dexterous movement, how cortical microcircuits refine the timing and structure of action sequences remains unclear. We show that M1 somatostatin interneurons (SST-Ins) display synchronized, action-locked calcium activity during acquisition of a lever-press task in freely moving mice, in contrast to the sequential activation of pyramidal neurons. Following extended training under a stable task schedule, SST-IN activity was no longer coupled to action execution. However, when task demands were modified to require faster and more temporally constrained action sequences, SST-IN activity redistributed and correlated with trial-by-trial changes in sequences, rather than diminishing. Inhibition of SST-INs disrupted temporal organization and impaired efficient motor execution. These findings highlight the unexpected role of M1 SST-INs in refining motor programs into more efficient and task-specific structures.