The medulla oblongata is a cone-shaped neuronal mass in the brain located in the brain stem, directly below the pons and anterior to the cerebellum. Apart from being what makes alligators so ornery all the time, the medulla oblongata is implicated in a number of autonomic (involuntary) physiological processes such as breathing, sneezing, vomiting, regulation of blood pressure, and heartbeat.
Ultimately, the medulla oblongata is responsible for some of the most basic physiological functions: respiration, digestion, and vasomotor control.
The discovery of the medulla oblongata as an anatomically important region of the brain was in 1806 by French physician Julien Jean-Cesar Legallois. Legallois noticed that when he removed both the cortex and cerebellum from rabbits, they continued to breathe. However, when he removed a section of the medullary region, the rabbits ceased to breathe. Legallois surmised that the medulla oblongata contained a “respiratory center”; that is, a region responsible for respiration. Subsequent work designated the medulla oblongata as the site of “vital” functions (functions needed for survival). Although we have much more specific anatomical knowledge nowadays, Legallois’ hypothesis that the medulla oblongata is responsible for “vital functions” remains largely correct.
The medulla oblongata also serves as a sort of link between the higher regions of the brain and the brain stem. Signals coming from and going to the body pass through the medulla oblongata and are directed to the correct location. Because it controls so many physiologically basic functions, injury to the medulla oblongata can be fatal, as it may compromise an individual’s ability to breathe, digest, or regulate their blood pressure properly.
Anatomy & Development Of The Medulla Oblongata
Located in the brain stem, the medulla oblongata is composed primarily of neuronal ganglia. The medulla oblongata is divided into two main anatomical regions; the upper “open” part that makes contact with one of the brain’s ventricles, and the lower “closed” part where the brain’s fourth ventricle narrows to become the main canal of the spinal cord.
Near the top of the medulla are two rounded masses known as the pyramidal tracts that serve to connect the region of the brain to the pons above and the spinal cord below. Located directly next to the pyramidal tracts are the medullar “olives”—rounded oval structures that connect the medulla to the anterior cerebellum.
In vitro, the medulla oblongata is completely developed at about 20 weeks. The organ develops from the myelencephalon, the most posterior region of the embryonic neural tube, the embryonic structure that develops into the brain. The medullary region of the neural tube develops into two main kinds of neurons; first-order and second-order. The first order-neurons include those that directly receive sensory information from the body and modulate the body accordingly, where the second-order neurons modulate the responses of first-order neurons. Defects in embryonic development can result in the improper differentiation of these cells, which can cause disorders where a person is unable to maintain internal homeostasis.
Evolution &Function Of The Medulla Oblongata
The medulla oblongata is responsible for a number of autonomic physiological processes such as breathing, digestion, vasoconstriction, and reflex responses like sneezing, gagging, and vomiting. These processes occur unconsciously and are necessary for survival. It is thought that the medulla oblongata is an old anatomical region of the human brain, (sometimes called the “reptile brain”) as opposed to the relatively recently developed cerebrum. The presence of a fully formed medulla oblongata in some species of fish suggests that the anatomical region first emerged around 505 million years ago.
The medulla oblongata performs its regulatory functions via chemoreceptor pathways. For example, one of the functions of the medulla oblongata is to regulate respiration, i.e. the process by which the body gets energy from oxygen. The less oxygen blood has, the more acidic it becomes. The medulla oblongata detects the rising levels of acidity and increases the contraction rate of phrenical and intercostal muscle tissue, increasing oxygenation of the blood. In a similar fashion, the medulla oblongata also regulates heart rate; by detecting rising levels of acidity in the blood and stimulating cardiovascular tissue contraction.
Since the medulla regulates both respiration and digestion, it is also responsible for autonomic reflexive responses in those systems, i.e. vomiting and coughing/sneezing. Neurons in the spinal cord carry sensory information to the medullary region. The sensory information affects motor neurons in the medulla oblongata, and these motor neurons send out an electrical signal to trigger the desired physiological response. For example, particulate matter in the air can irritate olfactory cells, which release histamine in response. The medulla oblongata detects the release of histamine and sends electrical signals to the various muscles involved in sneezing.
Common depictions paint the medulla oblongata as the source of anger and aggression in human beings. This is not entirely true though; the more complex emotion of anger primarily derives from activity in the hypothalamus and amygdala. However, the medulla oblongata controls physiological process like heart rate and blood pressure, physiological responses that are tightly associated with anger and aggression. Damage to the medulla oblongata can compromise the regulation of heartbeat and blood pressure, which could make a person more susceptible to aggressive behavior.
Medullary pathologies are very serious as the medulla oblongata controls basic functions absolutely essential for life. Opiate overdose, for example, can slow the physiological functioning of the medullary regions, resulting in asphyxiation. Traumatic injuries to the medulla can result in various symptoms. For example, ischemia in the lateral region of the medulla, known as Wallenberg’s syndrome, can result in difficulty swallowing, lack of pain sensitivity and difficulty walking/maintaining balance (ataxia). Improper embryonic development of the medulla oblongata can result in homeostatic pathologies, where an individual cannot regulate their body appropriately. Incidentally, various surgical anesthesias work by suppressing the functioning of the medulla oblongata, resulting in decreased respiration and unconsciousness.