The Brainstem

Medulla, pons and midbrain: the compact stalk that keeps you breathing, keeps your heart beating, wakes you up and carries every message between brain and body.

The brainstem is a stalk about the length of a thumb connecting the forebrain to the spinal cord. It has three parts, from bottom to top: the medulla oblongata, the pons and the midbrain.

The medulla: vital functions

The medulla contains centres that control breathing, heart rate and blood pressure, as well as reflexes such as swallowing, coughing, sneezing and vomiting. Here the major motor pathways from the cortex cross to the opposite side (the pyramidal decussation) — which is why the left side of the brain controls the right side of the body.

The pons: a bridge

The pons ("bridge") relays signals between the cerebrum and the cerebellum, helps control breathing rhythm, and contains nuclei for facial sensation and movement, hearing and balance. The locus coeruleus, the brain's main source of noradrenaline, sits here, and regions in the pons help generate REM sleep and its muscle paralysis.

The midbrain

The midbrain contains reflex centres for vision and hearing (the superior and inferior colliculi, which turn your eyes and head toward sudden lights and sounds), the substantia nigra and ventral tegmental area (sources of dopamine), and the periaqueductal grey, which controls defensive behaviour and helps suppress pain.

The reticular formation: staying awake

Running through the core of the brainstem is the reticular formation, a network of neurons whose ascending projections (the "ascending reticular activating system") keep the cortex awake and alert. Damage here can cause coma.

Cranial nerves and serious damage

Ten of the twelve pairs of cranial nerves connect to the brainstem, controlling eye movements, facial expression, hearing, swallowing and speech. Because so much passes through a small space, brainstem strokes can be devastating. In locked-in syndrome, damage to the front of the pons paralyses nearly the whole body while consciousness is intact; patients may communicate only with eye movements.