Pain receptors were receptive parts of a nervous system responsible for sensing pain and sending the information to the brain.

When Ux-Mal criminals took over the bodies of several crewmembers of the USS Enterprise-D in 2368, Commander William T. Riker was spared because he had broken an arm and the firing pain receptors made him immune to the hostile takeover. (TNG: "Power Play")

In 2369, Doctor Beverly Crusher adjusted the optical transducer in Geordi La Forge's VISOR to block some of the pain receptors in his brain after he had been shot by a terrorist at Arkaria Base, although the effect was only partial. (TNG: "Starship Mine")

In 2370, Data, under the influence of Lore, tried to neutralize La Forge's pain receptors as a precursor to an experimental procedure in which the chief engineer's cognitive functions would have been replaced by an artificial neural net. (TNG: "Descent, Part II")

Judging by Data's neutralization procedure, Geordi's pain receptors were apparently directly tied into his VISOR implants.
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