The cranial unit housed the positronic brain of an android, and the brain was protected by multiple levels of shielding that made it virtually invulnerable to most kinds of radiation as well as a high-intensity charge of electricity or a heavy impact blow. The neural network, part of the positronic brain, was also housed in the cranial unit. Both the brain and the neural nets were completely self-contained within an android's head so that the head was able to be removed and the android could maintain all intellectual functions. The cranial unit was detached from the body by depressing a small actuator located directly behind the right ear.
After the USS Enterprise-D was struck and disabled by a quantum filament in 2368, Data allowed his body to serve as an electrical conductor to enable himself and Commander Riker to move through a series of Jefferies tubes to reach engineering. Data instructed Riker to remove his cranial unit and carry it with him to engineering so that the android could assist him. There, Data was capable of accessing the terminals in engineering and performing adjustments and repairs, even preventing a warp core breach after Riker made a direct connection between the Enterprise computer and a port located in the occipital-parietal area of Data's cranial unit. (TNG: "Disaster")