THE BIOLOGY BOOK
Medulla: The Vital Brain
c. 530 Million BCE
When thoughts turn to the brain, we undoubtedly think about reasoning, emotions, and, of course, thinking—activities that are controlled at the highest levels. However, far more basic are those vital functions that are essential for survival, and these are regulated by the medulla oblongata, likely the first brain part to have evolved. Some authorities argue that the medulla is the most important brain component.
Animals possessing a bilaterally symmetrical body—bilaterians, the common ancestor of all vertebrates—first appeared some 555–558 million years ago. Among their characteristics was a hollow gut tube that ran from the mouth to the anus and contained a nerve cord, a precursor of the spinal cord. More than 500 million years ago, the first vertebrates, which are thought to have resembled the modern hagfish, emerged. Their anatomy had developed three swellings at the mouth end of their nerve cord: the forebrain, midbrain, and hindbrain.
The medulla, a structure in the hindbrain, developed from the top of the spinal cord. It is the lowermost portion of the brain and the most primitive portion of the vertebrate brain. The medulla regulates those functions upon which life is most dependent and which occur without our voluntary action: control of breathing, heart rate, and blood pressure. Chemoreceptors, located in the medulla, monitor oxygen and carbon dioxide levels in the blood and orchestrate appropriate changes in the rate of breathing. Its destruction causes instant death from respiratory failure. Baroreceptors, located in the aorta and carotid artery, detect changes in arterial blood pressure and, via nerve impulses, transmit messages to the cardiovascular center in the medulla, which in turn trigger changes that restore blood pressure and heart rate to normal levels.
The medulla is also the site of a number of reflex centers that respond without delay, in the absence of cognitive processing, when required to initiate vomiting, coughing, and swallowing. Additionally, it provides a pathway for nerves entering and leaving the brain and transmitting messages between the brain and spinal cord.
SEE ALSO: Fish (c. 530 Million BCE), Blood Pressure (1733), Nervous System Communication (1791), Neuron Doctrine (1891).
The medulla, the most primitive brain structure, controls such essential functions as breathing, heart rate, and blood pressure, as well as such reflex responses as coughing and sneezing. This poster, warning US soldiers to cover their coughs and sneezes in order to prevent the spread of germs, was issued by the Office of War Information during World War II.