The brain stem is the stalk of the brain below the cerebral hemispheres. It is the major route for communication between the forebrain and the spinal cord and peripheral nerves. It also controls various functions including respiration, regulation of heart rhythms, and primary aspects of sound localization.

The lower part of the brain stem is the medulla oblongata, grossly comprising the medullary pyramids and the olivary bodies or olives. The pons is a knob above the medulla.

Differentiation of the brain stem from the cerebrum is complex, both anatomically and taxonomically. Some taxonomies describe the brain stem as the pons, medulla and mesencephalon while others include diencephaletic regions.

The adult human brainstem emerges from parts of all three vesicles in the neural tube. The myelencephalon and metencephelon emerge from the rhombencephalon. The mesencephalon matures to become a vital link for nerve pathways related to movement. The prosencephalon gives rise to the diencephalon and to the telencephalon.