Anatomy of the ventral surface and pathways
Medulla oblongata acts as the lower part of the brainstem. Its ventral (anterior) surface has a characteristic relief. Strictly along the midline lie two longitudinal elevations — the pyramids (pyramides). Lateral to each pyramid are oval structures called olives (olivae).
In the anterior sections, ascending sensory and descending motor pathways pass in transit. Within the so-called closed part of the medulla oblongata, these pathways undergo decussation (decussatio). In addition to the conduction apparatus, the nuclei of the lower cranial nerves (CN) are embedded within its bulk. It is important to note that these nuclear clusters are too small and anatomically blend with the surrounding tissue, and therefore they are not differentiated on standard neuroimaging scans.
Vertebrobasilar circulation system
The blood supply to the brainstem structures is provided by the vessels of the vertebrobasilar system. It is formed by two vertebral arteries (VA, arteriae vertebrales). Typically, they are the first branches of the subclavian arteries (which, along with the brachiocephalic trunk and the left common carotid artery, arise from the aortic arch). The left VA is more often dominant and sometimes branches directly from the aortic arch.
Topographically, each VA is divided into four segments:
- V1: the section from the origin to the entry into the transverse foramen of the sixth cervical vertebra (C6).
- V2: runs vertically upward within the transverse foramina of the cervical vertebrae.
- V3: curves around the anterior arch and runs posterior to the lateral mass of the atlas (C1). It then pierces the dura mater (DM) and the arachnoid mater at the level of the foramen magnum.
- V4: intracranial segment. The two VAs join together anterior to the inferior part of the pons (directly above the medulla oblongata), forming a single basilar artery (arteria basilaris, BA).
These vessels give off numerous muscular branches that form a network of anastomoses with ascending arteries.
Anterior circulation system and Circle of Willis
The anterior and posterior systems connect to form the Circle of Willis. On CT angiography, it appears as a symmetrical closed system displaying: the internal carotid (ICA), anterior cerebral (ACA), middle cerebral (MCA), posterior cerebral (PCA), basilar, and vertebral arteries, as well as communicating branches.
The anterior system originates from the internal carotid arteries. The common carotid arteries run within the carotid sheath and divide into the external (ECA) and internal (ICA) carotid arteries at the level of the C4 vertebra. The ECA supplies the extracranial tissues, having rich anastomoses with the intracranial vasculature.
The ICA is divided into segments (C1–C5). In the cavernous segment, the vessel forms a bend (carotid siphon), lying closely adjacent to cranial nerves III and IV. From here, the meningohypophyseal trunk (giving off tentorial and hypophyseal branches) and the inferolateral trunk (supplying the nerves) arise. The exit from the sinus is bounded by the proximal and dural rings. Upon leaving the sinus, the ICA enters the subarachnoid space and, prior to bifurcation, gives off:
- The ophthalmic artery.
- The posterior communicating artery (PCoA), which sometimes forms a fetal-type PCA.
- The anterior choroidal artery, which supplies the optic tract, the posterior limb of the internal capsule, the cerebral peduncle, and the temporal lobe.
Further on, the ICA divides into the ACA and MCA.
Branching of ACA and MCA
The anterior cerebral artery (ACA) consists of segments A1 (horizontal), A2 (vertical), and A3 (distal with cortical branches). The A1 segments of both sides are connected by the anterior communicating artery (ACoA). Perforators arise from A1, including its largest branch—the recurrent artery of Heubner, which loops backward toward the parent trunk. Cortical branches supply the medial surface of the hemisphere. Known anomalies include A1 hypoplasia or fusion of A2 into an azygos ACA.
The middle cerebral artery (MCA) is divided into M1 (horizontal, may have a bifurcation or trifurcation), M2 (insular), M3 (opercular), and M4 (cortical). Medial and lateral lenticulostriate arteries arise from M1, supplying the basal ganglia. Cortical branches of the MCA supply blood to the lateral surface of the hemispheres.
Briefly: answers to topical questions
Which specific cranial nerves have nuclei within the bulk of the medulla oblongata?
Sources indicate that the nuclei of the lower cranial nerves are located within the bulk of the medulla oblongata; however, the only specific one named in the provided materials is the nucleus of the hypoglossal nerve (XII). These nuclei are not visible using standard neuroimaging methods.
Topography on the ventral surface: Pyramids — In the center (medially), Olives — on the Sides (laterally).
Class and exam questions
Is it possible to see the cranial nerve nuclei of the medulla oblongata on an MRI?
No, the nuclei of the lower cranial nerves lie within the tissue of the medulla oblongata, but their size does not allow them to be differentiated using standard neuroimaging methods.
Where exactly does the decussation of pathways occur in the brainstem?
Ascending sensory and descending motor pathways decussate at the midline within the closed part of the medulla oblongata.
How is the basilar artery formed?
The basilar artery is formed by the union of the right and left intracranial segments (V4) of the vertebral arteries anterior to the inferior part of the pons.
Which artery supplies the medial surface of the cerebral hemispheres?
The blood supply to the medial surface of the hemisphere is provided by the cortical branches of the anterior cerebral artery (ACA).
What else to study on this topic
- Anomalies of the Circle of Willis: fetal PCA, azygos ACA, hypoplasia of segment A1.
- Topography of the cavernous sinus and branches of the cavernous segment of the internal carotid artery.
- Segmental structure of the vertebral artery and its course through the transverse foramina of C1–C6.
- Supply territories of the lenticulostriate arteries and the recurrent artery of Heubner.
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