1 · Ascending pain — three neurons, one crossing, and the crossing is in the cordFirst-order neuron. ① A nociceptor in the right sole fires → ② the tibial and then the sciatic nerve carries it up the leg → its cell body sits in the ③ S1 dorsal root ganglion, inside the sacral canal — a ganglion, not a relay, so nothing synapses here → ④ the axon climbs ~200 mm of cauda equina to reach the L4–S1 cord segments, then runs 1–2 segments within Lissauer’s tract.
First synapse → second-order neuron. ⑤ Right dorsal horn — specifically a lamina I / lamina V projection neuron. (Lamina II, the substantia gelatinosa, is mostly interneurons; it modulates, it does not project.)
Crossing. That axon crosses the ⑥ anterior white commissure within 1–2 segments of where it entered and joins the ⑦ left anterolateral system — spinothalamic together with spinoreticular fibres, which is exactly what the PAM50 label contains.
The long climb. It ascends the cord, medulla, pons and midbrain without another synapse. Collaterals peel off to the reticular formation and periaqueductal grey on the way (not drawn).
Second synapse → third-order neuron. ⑧ Left VPL nucleus of the thalamus. VPL is body and limb; VPM is face and is never a stop on this route.
Destination. ⑨a Left paracentral lobule — the foot’s S1 territory, on the medial surface of the hemisphere. That is why the foot maps to the top of the brain tucked against the midline, not onto the lateral convexity.
Parallel branch (medial pain system, opt-in). ⑨b intralaminar nuclei → ⑨c anterior cingulate + ⑨d insula — how unpleasant it is, running alongside the VPL→S1 route that carries where and how much.