A neural servo was a device, sometimes inserted subdermally in humanoids, in order to excite neural pathways. These devices were often used to control or punish individuals.
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 were the various agonizers used by a number of species.