Medulla Oblongata

Medulla Oblongata – Myelencephalon

Ashley Davidoff MD

Definition

The medulla oblongata is more commonly known just as the medulla. It is part of the brainstem and the hindbrain.

Structurally it is the most caudal part of the brain and brainstem, lies above the spinal cord, below the pons, and anterior to the cerebellum. It is tubular in shape. There is a closed part near the top of the spinal cord and an open part connected to the lower portion of the fourth ventricle.

Functionally it helps to relay motor and sensory messages to and from the brain. It assists in the control of autonomic functions including respiration, sympathetic and parasympathetic control of the heart, baroreceptors, and reflex centers of swallowing, coughing, sneezing, and vomiting.

From an evolutionary perspective the medulla is part of the myelencephalon which includes parts of the 4th ventricles and some of the cranial nerves. The myelencephalon represents the oldest part of the brain controlling the most vital functions of life grouped in the functions of the autonomic nervous system . differentiates from the metencephalon in the embryo by approximately 5 weeks of age.

The Medulla Oblongata in Context

The Medulla (dark pink)

Part of the Hindbrain(Rhombencephalon)

Member of the Myelencephalon

The basic and simplest classification of the brain into forebrain midbrain and hindbrain is shown in this diagram and advanced to a more complex tree using the embryological and evolutionary terminologies.

The medulla is part of the hindbrain

The forebrain consists of the cerebrum also called the prosencephalon, which contains the more advanced form of the brain and the thalamic structures which contain more basic structures. The cerebrum (telencephalon) itself consists of two cerebral hemispheres and paired basal ganglial structures. Each cerebral hemisphere will have gray and white matter distributed in the frontal parietal temporal and occipital lobe, with the basal ganglia being part of the gray matter deep in the cerebral hemispheres. The most important thalamic structures arising from the diencephalons include the thalamus itself and the hypothalamus. The midbrain (mesencepaholon) consists of the tectum tegmentum and cerebral peduncles. The hindbrain has two major branch points based on the evolutionary development. The pons and cerebellum(part of the metencephalon) are grouped and the medulla (part of the myelencephalon is the second branch.

Sagittal

Gross anatomy of the Medulla

Mid Sagittal View

The midsagittal section view of brain reveals the distinctive shape position and character of the midline structures of the brain. The distinction between the character of the cerebral cortex which has a creamy color and the white matter exemplified by the corpus callosum (c) and septum pellucidum (sp) which are white, and the midbrain (mb) pons (p) and medulla (m) which are off white as opposed to the color of the cerebellum (c) which is light salmon pink is well demonstrated. The relative sizes of the forebrain, midbrain and hindbrain and their components are well appreciated in this section.

Image Courtesy of Thomas W.Smith, MD; Department of Pathology, University of Massachusetts Medical School. 97805b02

Basic Components of the Medulla as seen in the in the Sagittal Plane

The shapes in fact are not quite ovoid. The pons (lightest pink) has shapes reminiscent of an oversize egg lying in a small bed The anterior eggshaped portion is called the ventral or anterior pons and the posterior portion called the tegmentum which is a continuation of the tegmentum of the midbrain. The medulla oblongata is almost rectangular but has a subtle forward bulging to its anterior surface. The anterior portion is called the ventral or anterior medulla and the posterior portion is called the tegmentum, similar in name and a continuation of the tegmentum of the midbrain and pons. The cerebellum is situated posteriorly, is the largest of the three major components and it consists of two hemispheres and a central vermis better appreciated in the axial plane.

Coronal

In the Coronal Plane

In this T2 weighted MRI image – the medulla (salmon) which part of the hindbrain is demonstrated. It lies below the tenntorium .

The Medulla in Relation to the Pons Cerebellum and Midbrain

In this T2 weighted coronal MRI image the distinction between the supratentorial and infratentorial structures is made apparent by the bright pink tentorium that acts as a roof of the posterior fossa. The forebrain (green) midbrain (orange) and hindbrain (pink salmon and maroon) and the cerebellum (maroon), with other parts of the hindbrain filled in including the pons (light pink) middle cerebellar peduncles (mauve) and medulla (salmon) All the structures above the pink line are supratentorial, and those below are infratentorial. Part of the midbrain is supratentorial and part is infratentorial. The ventricular system is outlined in blue

Axial Projection

The Medulla

The medulla in the axial plane on this T 2 weighted MRI is almost rectangular in shape but is characterized by the two sets of bulges anteriorly. The medulla is divided into the anterior ventral portion portion and the posterior tegmentum. The anterior portion consists of the medial pyramidal tracts and the lateral olives . It is these two structures that account for the symmetrically positioned anterior bulges.

Conceptual Components of the Medulla

The Medulla

4 Ice Cream Cones in a Row

The medulla at its classic location has some subtle shape diferences when compared to the midbrain and pons mostly accentuated by the olives (green)which are the second set of ice cream shaped wedges . The medial set of ice cream cones that are light pink are the medullary pyramids Posteriorly – the salmon pink bilobed section is called the tegmentum

The vertebrobasilar System and the Medulla

The diagram shows the main branches of the blood supply to the brain including the circle of Willis overlaid on coronal MRI image to portray the approximate position of the vessels in the brain. The image on the right shows the combined system all in red, and the image on the right shows the derivation from the vertebrobasilar and carotid systems The carotid system supplies the brain from the internal carotid (salmon pink). We demonstrate its terminal bifurcation into middle cerebral (dark green) and anterior cerebral (bright green). The anterior communicating artery runs between the two anterior cerebrals (bright red) The basilar artery (pink) is formed by the two vertebral arteries and it travels as a single artery over the upper medulla and the pons. Its terminal branch is the posterior cerebral artery (maroon). The first branch off the posterior cerebrals is the posterior communicating which joins the middle cerebral to complete the circle of Willis Each of the carotid and vertebro-basilar systems contributes to the circle of Willis through communicating arteries. The vertebro-basilar system provides the posterior communicating arteries bilaterally from the posterior cerebral and the carotid system provides the anterior communicating arteries via the anterior cerebral arteries.

The Medulla and the RAS

The reticular activating system (aka ascending reticular activating system) is a part of the brainconsidered to be the center of arousal and motivation. Structurally it lies betweent the medulla oblongata and midbrain and is connected to the thalamus. In turn al parts of the brain can be stimulated as a result including the cerbral cortex basal regions of the brain and the medulla Functionally it indirectly relates to our state of conciousness, and is involved with the control of the circadian rhythm, respiration, cardiac rhythms, and sexual function. In the instance of painthe RAS is activated by the C fibers and hence pain can arouse us from sleep through the RAS, can create a sense of urgency, and can cause changes in heart rate or respiration rate.

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Medulla Oblongata

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