Spinal cord
From Memory Alpha, the free Star Trek reference.
The Spinal cord is an important part of a living being's nervous system that runs down the spine, and transmits the motor messages from the brain to the rest of the body.
In 2367, Lieutenant Worf severely injured his spinal cord when a cargo container fell on his back, and was partially paralyzed. Doctor Toby Russell, a specialist, came aboard to offer assistance. She and Doctor Crusher replaced his spinal cord with a replicated one made using a genetronic replicator. (TNG: "Ethics")
Captain Picard ripped the Borg Queen's metallic spinal cord apart after all her organic substances had been dissolved by plasma coolant in 2373. (Star Trek: First Contact)
