THE SKIN

the largest organ, about 1.8 m²

where this comes fromsources

Scanned: the body surface and the brain are BodyParts3D (© DBCLS, CC BY-SA 2.1 JP), re-shared under the same licence. Measured: PIEZO2 and TRPV1, every heavy atom, from the Protein Data Bank (CC0). Drawn: the block of fingertip skin and its receptors (to the depths and sizes given in the literature; Merkel cells and free endings ≈ 3× larger), the receptive-field mosaic, the nerve route from hand to brain, the lipids, ions and capsaicin. Modelled: the firing of each receptor, PIEZO2's flattening, and the in-between frames of TRPV1 opening.

PIEZO2 structure, PDB 6KG7: Wang L., Zhou H., Zhang M. et al., Nature 573, 225 (2019), via RCSB PDB (CC0)
TRPV1 closed, PDB 3J5P: Liao M., Cao E., Julius D. & Cheng Y., Nature 504, 107 (2013), via RCSB PDB (CC0)
TRPV1 open, PDB 3J5Q: Cao E., Liao M., Cheng Y. & Julius D., Nature 504, 113 (2013), via RCSB PDB (CC0)
Body surface area: Du Bois D. & Du Bois E.F., Arch Intern Med 17, 863 (1916)
Skin layers and thickness: Kanitakis J., Eur J Dermatol 12, 390 (2002)
Touch units in the hand: 17,000; 241 / cm² at the fingertip, 58 in the palm: Johansson R.S. & Vallbo Å.B., J Physiol 286, 283 (1979)
FA I / SA I / FA II / SA II properties and shares: Johansson R.S. & Flanagan J.R., Nat Rev Neurosci 10, 345 (2009); Abraira V.E. & Ginty D.D., Neuron 79, 618 (2013)
Flutter vs vibration bands: Talbot W.H., Darian-Smith I., Kornhuber H.H. & Mountcastle V.B., J Neurophysiol 31, 301 (1968)
Few Ruffini endings in the human finger pad: Paré M., Behets C. & Cornu O., J Comp Neurol 456, 260 (2003)
C-tactile afferents and gentle stroking: Löken L.S. et al., Nat Neurosci 12, 547 (2009)
Two-point thresholds across the body: Weinstein S., in The Skin Senses, ed. Kenshalo (1968); method caveats: Tong J., Mao O. & Goldreich D., Front Hum Neurosci 7, 579 (2013)
Discovery of PIEZO1 and PIEZO2: Coste B. et al., Science 330, 55 (2010)
PIEZO2 is the touch transducer in mice: Ranade S.S. et al., Nature 516, 121 (2014)
PIEZO2 in Merkel cells: Woo S.-H. et al., Nature 509, 622 (2014); Maksimovic S. et al., Nature 509, 617 (2014)
People without PIEZO2: Chesler A.T. et al., N Engl J Med 375, 1355 (2016)
Membrane dome model: Guo Y.R. & MacKinnon R., eLife 6, e33660 (2017); Haselwandter C.A. & MacKinnon R., eLife 7, e41968 (2018)
PIEZO1 flattened in membranes: Lin Y.-C. et al., Nature 573, 230 (2019); Yang X. et al., Nature 604, 377 (2022)
TRPV1 cloned; heat above ≈ 43 °C and capsaicin: Caterina M.J. et al., Nature 389, 816 (1997)
Capsaicin's binding residues: Jordt S.-E. & Julius D., Cell 108, 421 (2002); Yang F. et al., Nat Chem Biol 11, 518 (2015)
Nobel Prize in Physiology or Medicine 2021: David Julius and Ardem Patapoutian, “for their discoveries of receptors for temperature and touch”
Fibre classes, diameters and speeds: Kandel E.R. et al., Principles of Neural Science, 5th ed. (2013)

Education only, not medical advice. Numbness, tingling or pain that does not go away is worth showing to a doctor.