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The brain stem is the lowest part of the brain, and is a structural continuation of the spinal cord. The brain stem is the oldest part of the brain, structurally analogous to parts of the brain found in all vertebrates, ostensibly even the very oldest vertebrates, such as the jawless fish Haikouichthys, whose fossils are dated to 530 million years ago. The brain stem includes the hindbrain and the midbrain, two of the three major regions that the neural tube differentiates into during embryogenesis. The third region differentiates into the rest of the brain.
Arguably the most primitive part of the brain, but also among the most essential, the brain stem primarily regulates autonomic functions along with the parts of the Central Nervous System outside the brain. These functions include cardiovascular control (heart beat and blood pressure), pain sensitivity control, alertness, respiratory control (breathing), and, most broadly, consciousness (though this word is notoriously slippery to define). If your brain stem is damaged, death is probable, whereas if, say, part of the cortex (outer layer) of the brain is damaged, survival is much more likely.
Besides regulating the most basic functions of life, the brain stem plays the role as a sort of neuroanatomical switchboard but the rest of the brain, as all information that passes in and out of the brain must pass through the brain stem. Its third primary role is to serve as the nucleus of the cranial nerves, nerves that emerge from the brain stem rather than the spinal cord, as most nerves do. Humans have 12 cranial nerves, which serve the motor and sensory systems of the head and neck.
Components of the brain stem include the medulla oblongata, the pons, and the midbrain. The medulla contains the brain regions that control breathing, the heart, swallowing, vomiting, and defecation. The cardiac center in the medulla, specifically, is what controls the heart. The medulla is the lowest part of the brain stem. Just above it is the pons, which is the characteristic bulge of the brain stem. Its main roles are to relay sensory information between the cerebellum ("little brain") and the cerebrum (the bulk of the brain), also helps regulate respiration, and controls arousal -- not in the sexual sense, but in the sense of an animal sleeping or being awake, being calm or agitated.