The brainstem stacks vertically, and each level handles a distinct cluster of functions. From bottom to top: medulla, pons, midbrain. The vertical organization is not aesthetic — it reflects evolutionary layering, with the oldest functions sitting at the bottom.
The medulla is the bottom floor. It runs respiration and blood pressure regulation through autonomic nuclei that fire constantly, without conscious awareness, to keep your diaphragm contracting and your vascular tone steady. The cardiovascular and respiratory centers here are why brainstem strokes at this level are usually fatal — there is no redundancy for the firing pattern that drives breathing.
The pons sits above the medulla. It serves as the major relay between the cerebrum and the cerebellum — every signal that travels from motor cortex on its way to refining a movement passes through pontine nuclei. The pons also contributes substantially to sleep regulation, including the gating of REM sleep, and it houses the locus coeruleus, which we will meet shortly.
The midbrain is the top floor of the brainstem. Small but consequential. The substantia nigra and the ventral tegmental area — both dopamine-producing — live in the midbrain. The midbrain also handles eye movements through the oculomotor nuclei, controls auditory and visual reflexes through the superior and inferior colliculi, and sits at the gateway between the cerebrum and the rest of the central nervous system.
When you assess a comatose patient, the bedside neurologic exam works systematically through brainstem levels. Pupils react — midbrain intact. Corneal reflex — pons intact. Gag reflex — medulla intact. Each test reveals whether a level is functioning, and the rostrocaudal progression of deficits reveals where the lesion is taking hold.
Hold the stack. The next three concepts will visit the most clinically important nuclei in this vertical column: the substantia nigra, the ventral tegmental area, and the locus coeruleus. All three are tiny. All three project broadly. All three matter at the bedside.