brainstem

[breyn-stem]
noun
the portion of the brain that is continuous with the spinal cord and comprises the medulla oblongata, pons, midbrain, and parts of the hypothalamus, functioning in the control of reflexes and such essential internal mechanisms as respiration and heartbeat.
Also, brain stem.


Origin:
1875–80; brain + stem1

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Collins
World English Dictionary
brainstem (ˈbreɪnˌstɛm)
 
n
the stalklike part of the brain consisting of the medulla oblongata, the midbrain, and the pons Varolii

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
brainstem   (brān'stěm')  Pronunciation Key 
The part of the vertebrate brain located at the base of the brain and made up of the medulla oblongata, pons, and midbrain. The brainstem controls and regulates vital body functions, including respiration, heart rate, and blood pressure. See also reticular formation.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

brainstem

area at the base of the brain that lies between the deep structures of the cerebral hemispheres and the cervical spinal cord. It is divided into three sections: midbrain (mesencephalon), pons (metencephalon), and medulla oblongata (myelencephalon). The brainstem houses many of the control centres for vital body functions, such as swallowing, breathing, and vasomotor control. All of the cranial nerve nuclei, except those associated with olfaction and vision, are located in the brainstem, providing motor and sensory function to structures of the cranium, including the facial muscles, tongue, pharynx, and larynx, as well as supplying the senses of taste, equilibrium, and hearing. The brainstem also has nuclei important for sympathetic and parasympathetic autonomic functions. All efferent and afferent pathways between the cerebrum and cerebellum course through the brainstem, and many of them decussate, or cross, within this structure. Because of the important neural structures concentrated in this small portion of the nervous system, even very small lesions of the brainstem may have profound effects. Disorders involving the brainstem include trauma, tumours, strokes, infections, and demyelination (multiple sclerosis). Complete loss of brainstem function is regarded by some experts as equivalent to brain death.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
To be more precise, it's not the brain but the brainstem that regulates the
  heartbeat.
The team observed similar results using rat brainstem slices and cell-culture
  models.
In contrast, the sleepy subjects showed tighter coupling with basic alertness
  networks in the brainstem and thalamus.
The auras began a year or two following surgery for a tumor that was pressing
  on my brainstem.
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