Figure 1.

Optogenetic and pharmacogenetic dissection of motor cortical plasticity underlying recovery after SCI. A, Cross-section through cervical spinal cord in a Thy1-ChR2 mouse immunostained for PKCγ (red), YFP (green), and BDA (pink) with euthanasia 2 weeks after BDA injections into sensorimotor cortex. B, Close-up of the dorsal column showing BDA+ corticospinal axons comprising the dCST. C, Close-up of the dorsolateral funiculus showing BDA+ corticospinal axons comprising the dlCST and their branches into gray matter. D, Close-up of the ventral column showing sparse BDA+ immunoreactivity. E, Schematic of descending motor corticofugal pathways involved in distal limb control. The motor cortex projects directly to the spinal cord via the dCST (∼96%), dlCST (∼3%), and vCST (∼1%), but also provides major excitatory input to the red nucleus (RN) and reticular formation (RF), which descend the rubrospinal tract and reticulospinal tract, respectively. After C3/C4 SCI, the dCST is bilaterally interrupted, but other motor corticofugal pathways remain intact. F, Horizontal ladder task of sensorimotor dysfunction and recovery after C3/C4 dorsal column SCI. Arrows denote errors of the forelimb and/or hindlimb. G, Longitudinal optogenetic motor mapping through intact skull before and after SCI. Anesthetized, head-fixed mice are placed with their left FL and HL suspended from the ground to allow free movement. Laser motion sensors direct at targets on the limbs record movements evoked by optogenetic cortical stimulation. Mapping is repeated in the same animal at multiple time points after SCI. A chronic cranial window preparation allows optical access through completely intact skull. The window is directed over the right cortical hemisphere and centered around bregma (yellow square) rostro-caudally. A 12 × 14 array of cortical points with 300 μm spacing is stimulated in semirandom order by a fixed ∼100-μm-diameter 473 nm laser (10 ms pulse). After three repetitions of stimulation, a map of average evoked forelimb and hindlimb movements are assembled and scaled based on amplitude. Inset brain atlas was adapted from the Allen Brain Atlas and indicates approximate location of motor mapping area within the red outline. Bright green area is an approximate area of primary motor cortex. Scale bars: A, 500 μm; B–D, 50 μm.