A stalk weighing under three percent runs your breath
The human brainstem is only about two point six percent of brain weight, yet it sets heart rate and breathing, routes cranial nerves, and relays sensation and movement. Midbrain, pons, and medulla stack between cerebrum and spinal cord. Damage here is often life-threatening.
Anatomically it is the stalk at the back of the brain joining the cerebrum to the spinal cord. Its standard parts are the midbrain, pons, and medulla oblongata, though some anatomists add the diencephalon. It runs from just above the tentorial notch down to the first cervical vertebra, below the foramen magnum. Every signal travelling between body and brain must cross it: the corticospinal tract carries motor commands down, the dorsal column–medial lemniscus pathway brings fine touch, vibration, and body position up, and the spinothalamic tract relays pain, temperature, and itch.
The midbrain houses superior and inferior colliculi for vision and hearing, the dopamine-rich substantia nigra implicated in Parkinson’s disease, and periaqueductal gray circuits that dampen pain. Its reticular formation supports consciousness and houses the locus coeruleus, which tunes heightened alertness and autonomic reflexes. The pons bridges cerebrum, medulla, and cerebellum and holds pontine respiratory centers. The medulla keeps cardiac, respiratory, and vasomotor centers that govern heartbeat, breathing, and blood pressure, plus the area postrema that helps trigger vomiting. Blood arrives mainly via basilar and vertebral arteries, whose variant anastomoses can create distinctive brainstem stroke syndromes when flow fails.
Developmentally the midbrain comes from the mesencephalon vesicle; pons and medulla arise from the hindbrain’s metencephalon and myelencephalon. Ten of the twelve cranial nerve pairs, numbers III to XII, leave from here to serve the face, head, neck, and viscera; only the first two spring from the cerebrum. The oculomotor nerve exits the front of the midbrain and the trochlear its back, the trigeminal emerges at mid-pons, and hypoglossal rootlets sprout from grooves beside the medullary pyramids. Beyond those wiring duties it steers arousal, sleep and waking, attention, and pain sensitivity, which is why damage here so often threatens life.
Source: Brainstem
