Imagine the brain as a tall, ornate building, several stories high, with the most ancient machinery at the bottom and the most evolutionarily recent suites at the top. We are going to enter at the basement and walk up. The basement is the brainstem.
The brainstem is the engine room of the brain. It keeps you alive. It runs your heart rate, your breathing, your blood pressure, your sleep-wake cycle, and your level of consciousness, all without asking your permission. If the brainstem stops working, you die, and you die quickly.
This is why brain death, in the legal and medical sense, is determined by brainstem function, not by cortical activity. The cortex can die while the engine room keeps running, producing the persistent vegetative state — wakefulness cycles, autonomic regulation, sometimes years of biological survival without conscious content. The reverse — engine room dead, cortex alive — is impossible, because nothing supplies the cortex.
The brainstem has three parts, stacked vertically: the medulla at the bottom, the pons in the middle, and the midbrain at the top. The medulla runs respiration and blood pressure. The pons coordinates between cerebrum and cerebellum and contributes to sleep regulation. The midbrain handles eye movements, auditory and visual reflexes, and contains some of the most clinically important nuclei in the brain.
When you next encounter a patient on a ventilator after a brainstem stroke, picture the building's basement going dark. The lights upstairs may still flicker, but without the engine room, the building cannot maintain itself.
The brainstem also houses the three modulatory nuclei that supply dopamine, norepinephrine, and serotonin to the rest of the brain. We will meet each of those in the next few concepts. The pattern to notice: this small structure, this basement, is not just life support. It is also the weather system for the entire building above.