Neural servo
From Memory Alpha, the free Star Trek reference.
A neural servo is a device, sometimes inserted subdermally in humanoids, in order to excite neural pathways. These devices were often used to control or punish individuals.
In 2370, Arctus Baran used neural servos, which were implanted in each member of his crew, as a means of maintaining obedience. (TNG: "Gambit, Part I", "Gambit, Part II")
In 2268, the instrument of obedience was used by the Oracle of Yonada to punish sacrilegious or contumely behavior. It appeared to operate in a manner similar to a neural servo. (TOS: "For the World is Hollow and I Have Touched the Sky")
Other examples of such devices are the various agonizers used by a number of species.
