The lowest of the three sections of the triune brain is the reptilian brain, also referred to as the basal or primitive brain. Dubbed the “R-complex” by MacLean, this section of the brain includes the cerebellum, brain stem, and all of their associated structures and is the oldest part of the brain itself.

This section of the brain is called the “reptilian brain” because it is the most dominate and controlling in animals, specifically reptiles. The reptilian brain is responsible for obsessive, compulsive, and ritualistic behaviors. If you were to rely on the reptilian brain alone you would repeat the same actions over and over again – even if they were detrimental. You would never learn from your past actions or mistakes.

On a physical level, the reptilian brain is responsible for your body’s autonomic functions. It is responsible for muscle control, balance, breathing, and your heartbeat, amongst many other things. The reptilian brain is always active and working – even when the rest of your body is fast asleep.

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