The central plexus is the complex in which the hive minds of all drones on-board the vessel are transmitted further to the Unicomplex. On board class 4 tactical vessels the central central plexus is protected by multi-regenerative security grids making beaming inside of the plexus virtually impossible.
Central Plexus cut-away diagram.
Central Plexus within a Borg Tactical Cube
The Central Plexus device is shown in the center of the room
Exposed Borg circuitry inside of the Central Plexus device