This video presents a detailed 3D rendering of a mouse brain, highlighting the effects of injecting a retrovirus expressing ChR2(H134R)-tdTomato into the cerebral component of the spinal cord. The visualization uncovers significant inputs from the M1, M2, and S2 cortical areas, emphasizing their crucial roles in the network of the forelimb motor brain. Designed to enhance both educational and research understanding, the video provides deep insights into the intricate neuronal pathways within the cortex.