Central Core · Hindbrain / Midbrain
The brain stem connects the brain to the spinal cord and controls the most fundamental life-sustaining functions. It regulates breathing, heart rate, blood pressure, and sleep cycles, and serves as a conduit for signals between the brain and the rest of the body.
The brain stem forms the stalk at the base of the brain, connecting the cerebral hemispheres above to the spinal cord below. It's made up of three parts, stacked in sequence: the midbrain (topmost, closest to the thalamus), the pons (in the middle), and the medulla oblongata (lowest, tapering into the spinal cord). Nearly every signal travelling between the brain and the rest of the body passes through this narrow structure.
The brain stem is the brain's life-support core. The medulla contains the control centres for breathing rate, heart rate, and blood pressure - functions that continue automatically whether you're awake, asleep, or unconscious. The pons helps regulate the transitions between sleep stages and contributes to functions like facial sensation and eye movement. The midbrain plays a role in vision- and hearing-related reflexes (such as reflexively turning your head toward a sudden sound) and contains the substantia nigra, a structure that produces dopamine and is central to the smooth initiation of movement.
The brain stem also houses the origin points of ten of the twelve cranial nerves, which control functions ranging from eye movement and facial expression to swallowing and vocal function, and it contains the reticular activating system, a network of neurons essential for maintaining wakefulness and consciousness itself.
Because the brain stem controls basic survival functions and is evolutionarily ancient - present in some form across nearly all vertebrates - it's sometimes informally called the "reptilian brain," a term popularised by physician Paul MacLean's mid-20th-century "triune brain" model. Modern neuroscience has moved away from that model as an accurate account of brain evolution, but the underlying observation still holds: the brain stem's core survival functions are remarkably conserved across species, while the elaborate cerebral cortex is a much more recent evolutionary development, especially pronounced in humans.
Because so many vital functions and nerve pathways pass through such a small area, even minor brain stem damage - from traumatic brain injury, stroke, or tumour - can have outsized and serious effects, including impaired breathing, heart rate abnormalities, or loss of consciousness. The substantia nigra within the midbrain is the structure most directly affected in Parkinson's disease, where the progressive loss of its dopamine-producing neurons underlies the condition's characteristic movement symptoms.
The brain stem is discussed in connection with the following conditions on this site:
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