CALCIUM DYNAMICS OF S1HL NEURONS ENCODE HINDLIMB TRACTION FORCES

Weight-bearing is a fundamental constraint on sensorimotor control, yet how the hind-limb primary somatosensory cortex (S1HL) encodes gradient load remains unclear. We established a head-fixed, treadmill-compatible mouse paradigm that applies parametric traction via shin-mounted micro-weights (0–25% body mass) to the hind limbs, allowing stable locomotion and stationary stances while precisely manipulating axial load. Using two-photon calcium imaging of layer 2/3 CaMKII⁺ neurons in S1HL during quiet stance and belt locomotion, we quantified event rates, event amplitude, event duration and peak value across weights. After the neuronal clustering based on spontaneous firing, we observed three types of activity feature during the recording session: two types of neuron are characterized by non-synchronous firing and independence from hindlimb movement, whereas another type is characterized by synchronous firing and association with hindlimb loading movements. Spatial mapping uncovered heterogeneous load-receptive fields: under the same weight condition, neurons at different locations can respond to distinct hindlimb movement states; under varying weight conditions, the activation sites of neurons partially overlap in certain regions but generally exhibit different activation patterns. Together, these results demonstrate that S1HL is a load-sensitive brain region whose CaMKII⁺ dynamics flexibly reconfigure with hind-limb weight, offering a cortical substrate for adaptive gait and posture under changing bio-mechanical demands.