THE STUB

a big toe meets a table leg

a 1.75 m adult · slowed down

0after the knock · real time

You catch your big toe on a table leg. Three kinds of nerve fibre in your foot all fire at once, and race up the leg in the sciatic nerve. The fastest (Aα, and the Aβ touch fibres) tell you about the knock and where your foot is. The thin Aδ fibres carry the sharp pain. The thinnest, the C fibres, have no insulation at all and crawl at about walking pace. Before any of it reaches your brain, your spinal cord has already started to pull the foot back.

SCAN Skin, skeleton and brain are BodyParts3D. The skull is left off so the brain shows.
DRAWN The scan has no peripheral nerves, so the sciatic nerve and its branches are routed by hand along this skeleton's landmarks: the greater sciatic notch, behind the femur, the knee, behind the inner ankle, along the sole.
MODEL Arrival times are this path's length ÷ each fibre's speed, at mid-range speeds. Synapses add about a millisecond each.

EDUCATION ONLY Education only, not medical advice. Drugs and therapies named here are mentioned to explain the science, not as recommendations. For pain that is severe, lasting or unexplained, see a doctor.

sources

Fibre classes, diameters and speeds (Aα 80–120, Aβ 35–75, Aδ 5–30, C 0.5–2 m/s): Purves D. et al., Neuroscience, 6th ed., 2018, ch. 9–10; Kandel E.R. et al., Principles of Neural Science, 6th ed., 2021.
First and second pain: Price D.D., Hu J.W., Dubner R., Gracely R.H. Peripheral suppression of first pain and central summation of second pain evoked by noxious heat pulses. Pain 1977;3:57–68.
Withdrawal and crossed extension: Sherrington C.S. Flexion-reflex of the limb, crossed extension-reflex, and reflex stepping and standing. J Physiol 1910;40:28–121.
Nociceptive flexion (RIII) reflex, ~100 ms: Willer J.C. Comparative study of perceived pain and nociceptive flexion reflex in man. Pain 1977;3:69–80.
Leg motor nerve conduction ~40–60 m/s: Preston D.C., Shapiro B.E. Electromyography and Neuromuscular Disorders, 3rd ed., 2013.
Electromechanical delay: Cavanagh P.R., Komi P.V. Eur J Appl Physiol 1979;42:159–163.
The body map, foot on the medial surface: Penfield W., Boldrey E. Somatic motor and sensory representation in the cerebral cortex of man. Brain 1937;60:389–443.
Saltatory conduction: Huxley A.F., Stämpfli R. Evidence for saltatory conduction in peripheral myelinated nerve fibres. J Physiol 1949;108:315–339.
Speed ≈ 6 × diameter for myelinated fibres: Hursh J.B. Conduction velocity and diameter of nerve fibers. Am J Physiol 1939;127:131–139. Internode ≈ 100 × diameter: Rushton W.A.H. J Physiol 1951;115:101–122.
Sodium channels clustered at nodes: Waxman S.G., Ritchie J.M. Molecular dissection of the myelinated axon. Ann Neurol 1993;33:121–136.
Nav1.7 structure, PDB 6J8G: Shen H., Liu D., Wu K., Lei J., Yan N. Structures of human Nav1.7 channel in complex with auxiliary subunits and animal toxins. Science 2019;363:1303–1308. Coordinates via RCSB PDB (CC0). Topology: UniProt Q15858.
No Nav1.7, no pain: Cox J.J. et al. An SCN9A channelopathy causes congenital inability to experience pain. Nature 2006;444:894–898.
No Nav1.7, no smell: Weiss J. et al. Loss-of-function mutations in sodium channel Nav1.7 cause anosmia. Nature 2011;472:186–190.
Overactive Nav1.7: Yang Y. et al. Mutations in SCN9A cause primary erythermalgia. J Med Genet 2004;41:171–174. Fertleman C.R. et al. SCN9A mutations in paroxysmal extreme pain disorder. Neuron 2006;52:767–774.
Channel gating and inactivation: Hille B. Ion Channels of Excitable Membranes, 3rd ed., 2001.
Gate control: Melzack R., Wall P.D. Pain mechanisms: a new theory. Science 1965;150:971–979. Revisited: Mendell L.M. Constructing and deconstructing the gate theory of pain. Pain 2014;155:210–216.
Descending control and endogenous opioids: Basbaum A.I., Fields H.L. Annu Rev Neurosci 1984;7:309–338. Enkephalins identified: Hughes J. et al. Nature 1975;258:577–580.
Placebo pain relief and naloxone: Levine J.D., Gordon N.C., Fields H.L. The mechanism of placebo analgesia. Lancet 1978;2:654–657.
Phantom pain in 79.9% of 914 amputees: Ephraim P.L. et al. Arch Phys Med Rehabil 2005;86:1910–1919.
Cortical reorganisation: Flor H. et al. Nature 1995;375:482–484. Preserved representation: Makin T.R. et al. Nat Commun 2013;4:1570.
Mirror box: Ramachandran V.S., Rogers-Ramachandran D. Proc R Soc Lond B 1996;263:377–386. Trial: Chan B.L. et al. N Engl J Med 2007;357:2206–2207.
Skin, skeleton and brain: BodyParts3D, © The Database Center for Life Science (DBCLS), CC BY-SA 2.1 Japan; the derived meshes are re-shared under the same licence.

Skin, skeleton and brain: BodyParts3D, © The Database Center for Life Science, licensed CC BY-SA 2.1 Japan; the derived meshes are re-shared under the same licence. Nav1.7: RCSB Protein Data Bank entry 6J8G (CC0). The nerves, spinal cord, cord segment, axon, membrane and ions are drawn.