The cortical node is an implant in the brain of every Borg drone that regulates its other implants, and its failure is fatal to a drone.
The node coordinates the cybernetic systems throughout a drone's body. When Seven of Nine's cortical node began to fail, and it could not simply be replicated, leaving her facing death.
The young former drone Icheb insisted on donating his own node, believing he could survive without it. The Doctor performed the transplant on both of them, and both survived, Icheb having been shown to function without the implant.