Muscle spindles provide flexible sensory feedback for movement sequences

Sensory feedback is essential for motor performance and must adapt to task demands. Muscle spindle afferents (MSAs) are a major primary source of feedback about movement. Their responses are readily modulated online by fusimotor neurons and, in the unique case of jaw MSAs whose cell bodies lie within the central nervous system, by central synaptic inputs. They are, therefore, a powerful site for implementing flexible sensorimotor control. We record from MSAs innervating the jaw musculature of mice performing a directed lick sequence task. Jaw MSAs encode complex jaw-tongue kinematics. However, kinematic encoding alone accounts for less than half of jaw MSA spiking variability. Jaw MSA coding of kinematics changes based on sequence progression (beginning, middle, or end of the sequence or reward consumption), suggesting that jaw MSAs are flexibly tuned across the task. Dynamic modulation of feedback signals from jaw MSAs may shape sensorimotor control during complex behavior.