Integrated input-output connectomics at single-neuron resolution reveals parallel projection streams in the mouse ventral posterolateral nucleus.

The ventral posterolateral nucleus (VPL) has long been regarded as a linear relay of somatosensory information from the spinal cord to the primary somatosensory cortex (SSp). However, the circuit architecture underlying VPL integration of non-sensory signals, including motor commands and affective states, remains poorly defined; in particular, the relevant circuit mechanisms of canonical excitatory neurons labeled by vesicular glutamate transporter 2 (VGluT2) within this nucleus remain largely unelucidated. By combining monosynaptic rabies tracing with fluorescence micro-optical sectioning tomography (fMOST), we generated a quantitative, single-cell resolution connectome of VPLVGluT2 neurons in mice. We find that these neurons receive substantial convergent inputs from motor, limbic, and basal ganglia circuits, challenging the classical view that VPL is a unimodal sensory relay. Single-neuron reconstructions (n = 149) further reveal four distinct projection subtypes that innervate not only the SSp but also the primary motor cortex (MOp) and higher-order association areas. Notably, projections to MOp preferentially target layer 5, bypassing the canonical sensory hierarchy. These anatomical findings are consistent with the hypothesis that the VPL may serve as a hub for sensorimotor integration, and they provide a critical structural basis for understanding how somatosensation interfaces with action and cognition.