First proposed in the 1960s, Paul MacLean conceptualizes the brain as three functional layers. The reptilian complex, the limbic system and the neocortex. Although simple by today’s standards, the Triune Brain model offers an excellent overview. Especially if you’re new to neuroscience.
Layer 1 - The Reptilian Complex
Found at the base, this is the most ancient part of our brain. It is automatic in function and controls our regulatory systems. If we’re expending more energy, the reptilian complex senses it and commands our lungs to breathe deeper, our hearts to beat faster. If body temperature rises, it activates the nervous system to stimulate our glands so that we perspire.
Layer 2 - The Limbic System
Evolved from the earliest mammals, the limbic system mediates our emotions. Our unconscious mind if you would. It tells layer 1 to produce a craving for comfort food when we’re feeling lonely and sentimental. It tells layer 1 to send shivers down our spine when a jump scare pops up on screen.
Layer 3 - The Neocortex
Wrapped around as the outermost layer, it is the most recently evolved. The neocortex is the rational, deliberate and thinking part of our brain. Hear the hissing of a snake and it signals layer 2 to make us feel frightened, prompting layer 1 to pump blood quicker through our body. See a gorgeous man or woman on the street and it influences layers 2 and 1.
Again, the Triune Brain theory is a zoomed out representation of our brain. It serves as a good introductory model for better understanding. Our brains consist of many different parts, subparts, sub-subparts, sub-sub-subparts and well you get my point. So it shouldn't be viewed as an accurate depiction.