<p>Although the gross anatomy of the brachial plexus is well described, the segmental organization of spinal motoneurons projecting to individual terminal nerves has not been quantitatively defined. We mapped segmental origins and relative contributions of motoneuron pools innervating upper-extremity terminal nerves in a rat model using retrograde labeling of the axillary, musculocutaneous, radial, median, and ulnar nerves combined with selective transection of brachial plexus roots to isolate segment-specific motoneuron pools. Each nerve exhibited a distinct, reproducible motoneuron distribution. The axillary nerve (<i>n</i> = 8) originated predominantly from C5 and C6 (C5: 37.9%, C6: 42.7%) with a consistent C7 contribution (19.4%). The musculocutaneous nerve (<i>n</i> = 9) was centered on C5 and C6 (C5: 50.5%, C6: 43.7%) with minimal C7 input (5.4%). The radial nerve (<i>n</i> = 8) spanned C5 to T1, peaking at C7 (35.8%) and C8 (26.0%), with additional T1 input (19.4%). The median (<i>n</i> = 8) and ulnar (<i>n</i> = 8) nerves were caudally dominated by T1 (59.3% and 68.9%, respectively) with secondary C8 contributions (21.0% and 27.1%). No significant sex-related differences were observed. This quantitative map of root-resolved motoneuron contributions to the terminal brachial plexus nerves provides an anatomical framework for interpreting motor deficits after proximal nerve injury and may inform surgical strategies for peripheral nerve reconstruction and nerve transfer.</p>