brain, memory
腦 centers on the organ of thought and recollection: the brain and its function of memory.
腦 is the traditional form of 脳. It combines ⺼ (flesh/body part) with a phonetic component that also suggests the soft, convoluted mass of the brain. The modern simplified form 脳 reduces the right side.
The left side ⺼ indicates a body part, and the right side looks like a cross-section of the brain's folds and chambers. Together, 腦 is the brain, the seat of memory.
For ノウ, picture a brain with a glowing 'know' light inside: know -> ノウ, and the brain knows and remembers.