A spinal injury patient can't move the legs but feels a pinprick on the thigh. Motor (descending corticospinal) tracts are damaged but some sensory (ascending spinothalamic) fibers survive — an incomplete cord injury. Dermatome testing maps the level: if sensation stops at the groin crease (L1) but returns nowhere below, the lesion sits around L1. This distinction guides prognosis — incomplete injuries recover more function than complete ones.