The Old Brain
Sitting where the spinal cord meets the brain are its oldest, most essential structures — the machinery of survival that runs without a thought.
The brainstem is the stalk that keeps you alive. Its medulla controls the automatic vital functions: heartbeat, breathing, blood pressure. Damage here is often fatal. Just above, the pons helps coordinate movement and relays signals between the cerebrum and cerebellum, and helps regulate sleep and arousal. Threaded through the brainstem is the reticular formation, a net of neurons that governs arousal and alertness — it filters the flood of incoming stimuli and decides what wakes you up.
Behind the brainstem sits the cerebellum ("little brain"), a densely folded structure that coordinates voluntary movement, balance, and posture, and fine-tunes the timing of skilled motor sequences. It is why a practiced motion — signing your name, riding a bike — becomes smooth and automatic. Damage to the cerebellum does not paralyze, but it makes movement clumsy, jerky, and poorly timed.
Common pitfall: confusing the cerebellum with the cerebrum. The cerebrum is the large, wrinkled outer brain of higher thought; the cerebellum ("little brain") is the small structure tucked behind and below it that coordinates movement and balance. Similar names, very different jobs.