A neuristor was a piece of technology known to 20th century Earth and to the Second Doctor.
When worn at the back of the neck, it was able to protect a human mind from the hypnotic force produced by the micromonolithic circuits of the Cybermen. Its functioning was related to a depolariser.
During a Cybermen invasion of the Earth, thanks to the suggestion of the Doctor, the UNIT personnel were provided with neuristors to resist the control of the Mondasians. (TV: The Invasion)