Medulla Oblongata: Gross Anatomy
Sensory and motor impulses continuously travel back and forth between the peripheral and central nervous system. Once sensory information enters the spinal cord – or when motor sensation leaves the cerebral cortex – they can access to the higher centres of the brain (or axial, peripheral and smooth muscles) by first travelling through the brain stem. The most inferior part of the brain stem, the medulla oblongata, not only acts as a pathway for these nerve fibers to traverse but it also contains several reflexes (whose absence is incompatible with life). Additionally, it contains the nuclei associated with all cranial nerves except CN I (olfactory) and CN II (optic). This article will cover the gross external anatomy of the medulla oblongata as well as its blood supply and some associated pathologies.
The medulla oblongata is a funnel shaped structure that constitutes the terminal portion of the brainstem. It resides in the posterior cranial fossa, below the tentorium cerebelli. The medulla is a continuation of the spinal cord that commences at the foramen magnum. The point of origin can also be identified by the presence of the roots of the first cervical spine, which originate here. It continues superiorly until it transitions at the pontomedullary junction (groove between the pons and the base of the medulla), where the fibers of CN VI (abducent), CN VII (facial) and nervus intermedius or intermediate nerve of Wrisberg, and CN VIII (vestibulocochlear) emerge anteriorly. This region of the brainstem is anteroinferior to the cerebellum and the fourth ventricle. It is important to note that the central canal of the spinal cord is a caudal continuation of the fourth ventricle that also courses through the medulla carrying cerebrospinal fluid (CSF). The fourth ventricle ends at the obex (inferior apex) at the foramen of Magendie. From there, the central canal commences an oblique course anteriorly and inferiorly towards the center of the medulla oblongata, where it continues its caudal course through the spinal cord. Anatomically, the inferior cerebellar peduncle (one of three ‘foot processes’ of the cerebellum) is considered to be a part of the medulla as it provides a pathway for fibers to access the cerebellar lobes from the medulla.
The anterior and posterior surfaces of the medulla oblongata have unique protuberances that are formed from the presence of different nuclei and their associated tracts. Along the midline of the anterior surface of the medulla is the anterior median fissure. This is a continuation of the anterior median fissure of the spinal cord. On either side of the anterior median fissure is a vertical protuberance known as the pyramids. These areas contain efferent corticospinal fibers from the precentral gyrus (Brodmann area 4) that are responsible for muscle movement. Owing to the fact that a substantial amount of these fibers cross the midline with its counterpart on the opposite side – forming the pyramidal decussations – fibers from the precentral gyrus on one side innervate muscles of the contralateral (opposite) side. Just lateral and posterior to each pyramid is another excrescent structure known as the olive. This bulge can be attributed to the presence of the inferior olivary nucleus.
The anterior surface of the medulla is also littered by the protrusion of cranial nerve fibers emerging from the brainstem. The rootlets of the CN XII (hypoglossal) emerge posterolateral to the pyramid and anteromedial to the olive, after which they unite to form the single nerve. Similarly, posterolateral to the olive, but anteromedial to the inferior cerebellar peduncle are the rootlets of CN IX (glossopharyngeal) and CN X (vagus). The respective rootlets fuse with each other to form their resultant nerve.
The raised areas of the posterior medulla are not only arranged differently from those of the anterior surface, but they also arise as a result of different underlying structures. As was observed on the anterior surface, the dorsal median sulcus of the spinal cord continues cranially and passes through the obex (inferior apex of the fourth ventricle). Bilateral to the dorsal median sulcus are vertical raised areas known as the fasciculus gracilis. This is a continuation of the fasciculus gracilis of the dorsal column of the spinal cord. The cranial part of the fasciculus gracilis becomes enlarged and is referred to as the gracile tubercle, which is formed by the gracile nucleus. Fasciculus cuneatus runs laterally to the fasciculus gracilis on either side of the medulla and also transitions into the larger cranial portion, the cuneate tubercle, formed also by its own nuclei. Posterolateral to the gracile tubercle is the trigeminal tubercle (formed by the spinal nucleus of the trigeminal nerve). The lateral funiculus (lateral white matter bundle of fibers) also continues cranially in the same order it courses in the spinal cord.
The posterior superior part of the medulla overlaps with the inferior compartment of the rhomboid fossa (fourth ventricle). The dorsal median sulcus continues through the midline of the fossa to divide it into halves. The facial colliculus is the largest midline protrusion of the floor of the fourth ventricle (caudal end of the medial eminence). It is located superiorly to the horizontally coursing striae medullaris fibers, the hypoglossal trigone and the vagal trigone (in that order). The superior and inferior fovea forms the respective limits of the sulcus limitans that is laterally related to the facial colliculus. The sulcus limitans is the embryological border between the alar and basal plates of the foetal neural tube.
The vertebrobasilar system is responsible for supplying the medulla oblongata with arterial blood. The posterior inferior cerebellar artery (PICA) is a branch of the vertebral artery before it gives its contribution to the anterior spinal artery. It provides direct branches that supply the dorsolateral region of the medulla oblongata. The anterior inferior cerebellar artery (AICA) branches from the basilar artery, which is formed by the union of the two vertebral arteries. It provides slender twigs to the upper medulla. The anterior spinal artery, which is formed by two anastomosing branches of each vertebral artery, supplies the medial medulla. There are several main venous drainage routes for the medulla oblongata. The petrosal vein is one such vessel; it will end up draining to the superior petrosal sinus. Both the anterior and anterolateral medullary veins drain to corresponding spinal veins inferiorly. There are also anterior and posterior median medullary veins that travel along their respective sulci to drain the medulla.
The major pathological concerns of the medulla oblongata have to do with the compression or destruction of the associated tracts that traverse the conduit and the nuclei that reside in its substance. Several conditions, such as tumors, hydrocephalus, vascular accidents and trauma can affect the contents of the caudal brainstem. One example of tumors of the medulla oblongata is a medulloblastoma. This is a primitive neuroectodermal tumor (PNET) accounting for about 20% of paediatric brain neoplasms. Other than disrupting the integrity of the tracts and nuclei present, this is a highly metastatic tumor that fortunately responds well to radiotherapy. Another chief concern with these masses is the likelihood of obstructing the foramina of the fourth ventricle, which would result in hydrocephalus (increased water in the brain) and consequently increased intracranial pressure (ICP).
Vascular insults to the arterial supply of the medulla can result in a Lateral Medullary Syndrome of Wallenberg. In this situation, occlusion of PICA of the vertebral artery can result in the following clinical vignette:
- Random eye movements, nausea, vomiting and dizziness
- Loss of pain and temperature sense to the ipsilateral face
- Lack of voluntary coordination of movement with respect to gait and movement on the ipsilateral side
- And an inability to swallow and speak