brainstem [AS, bragen + stemm] , the portion of the brain comprising the medulla oblongata, the pons, and the mesencephalon. It performs motor, sensory, and reflex functions and contains the corticospinal and reticulospinal tracts. The 12 pairs of cranial nerves from the brain arise mostly from the brainstem. Compare medulla oblongata, mesencephalon, pons.
