Brainstem · Grey Matter

The brainstem is the stalk that joins the cerebrum to the spinal cord and keeps the body alive without thought: it drives breathing, sets heart rate and blood pressure, runs swallowing, coughing and vomiting, and keeps the cortex awake.

Brainstem. The brainstem is the stalk that joins the cerebrum to the spinal cord and keeps the body alive without thought: it drives breathing, sets heart rate and blood pressure, runs swallowing, coughing and vomiting, and keeps the cortex awake.

It has three parts from top to bottom: the midbrain, the pons and the medulla oblongata. It is small (a few percent of the brain's weight) and dense with function. Ten of the twelve pairs of cranial nerves arise from it, carrying the senses and the muscles of the face, mouth, throat, eyes and ears, and every pathway between the brain and the spinal cord runs through it, among them the corticospinal tract, which crosses to the other side at the bottom of the medulla. The medulla holds the respiratory and cardiovascular centres. Through its core runs the reticular formation, whose ascending projections (the reticular activating system, with the thalamus as relay) keep the cortex in the waking state.

Small lesions here can be catastrophic. Damage to the medulla can stop breathing, and a lesion of the upper pons and midbrain tegmentum, where the arousal system runs, can cause coma while the cortex itself is intact.

It also holds the neurons that send the brain's neuromodulators everywhere: noradrenaline from the locus coeruleus, serotonin from the raphe nuclei, dopamine from the midbrain.

Its reflex circuits are stable but still adjustable. The reflex that keeps the eyes steady while the head moves recalibrates with new glasses, through the brainstem and the cerebellum together.

Questions: Why can a spinal injury high in the neck stop breathing when one lower down does not? The rhythm of breathing is generated in the medulla, and its commands travel down the spinal cord to the motor neurons of the phrenic nerve, which leave the cord at the third, fourth and fifth cervical segments and drive the diaphragm. An injury above that level cuts the medulla off from the diaphragm and breathing stops unless a ventilator takes over. An injury below it leaves the diaphragm working, even when the arms or legs are paralysed. Are some brain regions plastic and others fixed? Every region changes with experience; they differ in what changes and how fast. The hippocampus and the neocortex are built to rewire quickly and constantly, storing new episodes and remapping after injury, while the amygdala also learns fast (fear conditioning is long-term potentiation in its lateral nucleus) but holds what it learns stubbornly. Brainstem and spinal reflexes are the most stable, yet still adjustable: the reflex that steadies the eyes during head movement recalibrates within days to new glasses, through the cerebellum and the brainstem's vestibular nuclei. A ranking of whole regions as plastic or rigid is therefore a simplification, useful only as a rough contrast between circuits designed to learn and circuits designed to be reliable. Why can a small lesion in the brainstem cause a coma when a large one in the cortex does not? Wakefulness depends on a system of nuclei in the upper brainstem (the reticular activating system with the neuromodulator nuclei near it) that project through the thalamus and the basal forebrain to the whole cortex. All those fibres funnel through a few cubic centimetres of the upper pons and midbrain, so a small stroke or bleed there removes the drive for the entire cortex at once. A cortical lesion, however large, leaves the rest of the cortex awake; in a series of brainstem strokes studied by Parvizi and Damasio, coma followed lesions of the upper pontine and midbrain tegmentum.