As the Fifth Doctor explained, the nervous system was a delicate network of logic junctions, called synapses. (TV: Castrovalva) It consisted of nerves and axons that enabled control of the body.
As a Time Lord, the Eleventh Doctor's nervous system was hyperconductive. (TV: Nightmare in Silver)
The Third Doctor speculated that the Nestene creature in Channing's tank was a kind of brain or nervous system, humanly speaking. (TV: Spearhead from Space)
Sutekh said he could shred the Fourth Doctor's nervous system into fibres and draw the pain out for centuries. (TV: Pyramids of Mars)
Mehendri Solon required the head from a species with a central nervous system to complete Morbius' body. (TV: The Brain of Morbius)
The Hoothi infected other creatures, usually having better control over ones with more complex nervous systems. (PROSE: Love and War) The Sild tapped into the nervous systems of their hosts to control them. (PROSE: Harvest of Time)
After the Boneless killed Forrest, they performed an exploratory dissection of her nervous system and left a two-dimensional print of it on a wall. (TV: Flatline)