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, Commander William T. Riker was spared because he had broken a leg and the firing pain receptors made him immune to the hostile takeover. (TNG: "Power Play")

Data, under the influence of Lore, once tried to neutralize Geordi 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pain receptors as a precursor to an experimental procedure in which the chief engineer's cognitive functions would have been replaced by an artificial neural net. Judging by Data's neutralization procedure, Geordi's pain receptors were apparently directly tied into his VISOR implants. (TNG: "Descent, Part II")