Circuit organization of the excitatory sensorimotor loop through hand/forelimb S1 and M1

Abstract

Sensory-guided limb control relies on communication across sensorimotor loops. For active touch with the hand, the longest loop is the transcortical continuation of ascending pathways, particularly the lemnisco-cortical and corticocortical pathways carrying tactile signals via the cuneate nucleus, ventral posterior lateral (VPL) thalamus, and primary somatosensory (S1) and motor (M1) cortices to reach corticospinal neurons and influence descending activity. We characterized excitatory connectivity along this pathway in the mouse. In the lemnisco-cortical leg, disynaptic cuneate→VPL→S1 connections excited mainly layer (L) 4 neurons. In the corticocortical leg, S1→M1 connections from L2/3 and L5A neurons mainly excited downstream L2/3 neurons, which excite corticospinal neurons. The findings provide a detailed new wiring diagram for the hand/forelimb-related transcortical circuit, delineating a basic but complex set of cell-type-specific feedforward excitatory connections that selectively and extensively engage diverse intratelencephalic projection neurons, thereby polysynaptically linking subcortical somatosensory input to cortical motor output to spinal cord.

Layer 1 of somatosensory cortex: An important site for input to a tiny cortical compartment

“…Earlier work has shown that in addition to targeting L1, long-range input from cortical areas like M1 targets infragranular layers (Geng et al, 2021;Kinnischtzke et al, 2016;Yamawaki et al, 2021;Zagha et al, 2013). This raises the question whether neurons that project to L1 from other brain areas are presynaptic to the L5 pyramidal neurons?…”