The middle cerebral artery (a. cerebri media) , the largest branch of the internal carotid, runs at first lateralward in the lateral cerebral or Sylvian fissure and then backward and upward on the surface of the insula, where it divides into a number of branches which are distributed to the lateral surface of the cerebral hemisphere.
The middle cerebral artery is divided into 2 segments (M1 and M2) in the terminologia anatomica but surgical and radiological approaches use 4 segments (M1, M2, M3, M4) that are well described by Gibo et al (Microsurgical anatomy of the middle cerebral artery - GiboH et al. - J Neurosurg. 1981 Feb;54(2):151-69.) :
- The M1 segment (Sphenoid part; Horizontal part) began at the origin of the MCA and extended laterally within the depths of the Sylvian fissure. It lay an average of 9.4 mm (range 4.3 to 19.5 mm) posterior to the sphenoid ridge in the sphenoidal compartment of the Sylvian fissure. This segment ter- minated at the site of a 90° turn, the genu, located at the junction of the sphenoidal and operculoinsular compartments of the Sylvian fissure. The horizontal orientation of this segment, roughly parallel to the sphenoid ridge, has led to its being referred to as either the horizontal or the sphenoidal segment. The M1 segment was subdivided into a pre-bifurcation and post- bifurcation part. The pre-bifurcation segment was composed of a single main trunk that extended from the artery's origin to its bifurcation. The post-bifurcation trunks of the M1 segment ran in a nearly parallel course, diverging only minimally, prior to reaching the genu. This bifurcation occurred proximal to the genu in 86% of hemispheres. The small cortical branches arising from the main trunk proximal to the bifurcation are referred to as early branches.
- The M2 segment (Insular part) included the trunks that lay on and supplied the insula. It began at the genu of the MCA where the trunks of the MCA passed over the limen insulae and terminated at the circular sulcus of the insula. The greatest branching of the MCA occurred at the anterior part of the insula, distal to the genu. The branches passing to the anterior cortical areas had a shorter path across the insula than those reaching the posterior cortical areas: the branches to the anterior frontal and anterior temporal areas crossed only the anterior part of the insula, but the branches supplying the posterior cortical areas coursed in a nearly parallel but diverging path across the length of the insula. The frontal branches coursed over the short gyri only be- fore leaving the insular surface, whereas a branch sup- plying the posterior parietal or angular region passed across the short gyri, the central sulcus, and the long gyri of the insula before leaving the insular surface.
- The M3 Segment (Opercular part) began at the circular sulcus of the insula and ended at the surface of the Sylvian fissure. The branches forming the M3 segment were closely adherent to and coursed over the surface of the frontoparietal and temporal opercula to reach the superficial part of the Sylvian fissure. The branches that were directed to the brain above the Sylvian fissure underwent two 180° turns called a "double flexion" by Lazorthes, et al. The first turn was located at the circular sulcus, where the vessels coursing upward over the insular surface turned 180° and passed downward over the medial surface of the frontoparietal operculum. The second 180° turn was located at the external surface of the Sylvian fissure, where the branches completed their passage around the inferior margin of the fronto- parietal operculum and turned in a superior direction on the lateral surface of the frontal and parietal lobes. The arteries supplying the cortical areas below the Sylvian fissure pursued a less tortuous course: these branches, upon reaching the circular sulcus, ran along its inferior circumference before turning upward and laterally on the medial surface of the temporal operculum, thus producing a less acute change in course at the inferior margin of the circular sulcus. Upon reaching the external surface of the Sylvian fissure, these branches turned downward on the surface of the temporal lobe.
- The branches forming the M4 segment (Cortical part) began at the surface of the Sylvian fissure and extended over the cortical surface of the cerebral hemisphere.The more anterior branches turned sharply upward or downward after leaving the Sylvian fissure. The intermediate branches followed a gradual posterior incline away from the fissure, and the posterior branches passed backward in nearly the same direction as the long axis of the fissure.
The branches (rami) of the MCA can be described by the areas that they irrigate.
- Lateral frontobasal (orbitofrontal): This artery branches out anteriorly, superiorly and laterally to vascularize the inferior frontal gyrus. It "competes" in size with the frontal polar branch of the anterior cerebral artery
- Prefrontal arteries: These arteries fan out over the insula and exit to the cortex via the medial surface of the frontal operculum. The arteries fan superiorly over the pars triangularis and vascularize the inferior and middle frontal gyrus. Near the superior frontal gyrus these arteries anastamose with branches from the pericallosal artery of the anterior cerebral artery.
- Pre-Rolandic artery (precentral): The artery extends out on the medial surface of the operculum and supplies the posterior parts of the middle and inferior frontal gyri as well as the lower parts of the pre-central gyrus. This artery branches once or twice and is relatively invariant across anatomies.
- Rolandic arteries (central): The artery extends out and exits from the central portion of the operculum then passes inside the central sulcus. This artery bifurcates in 72% of individuals and irrigates the posterior pre-central gyrus and the inferior portion of the post-central gyrus.
- Anterior parietal: This artery usually originates from the anterior or middle MCA trunk. In some cases it branches from the rolandic artery or from the posterior parietal artery. It extends the length of interparietal sulcus and descends slightly posteriorly.
- Posterior parietal: Emerges from the posterior end of the Sylvian fissure and extends first posteriorly, and then anteriorly along the posterior of the parietal lobe. It also branches to the supramarginal gyrus.
- Angular: The angular artery is a significant terminal branch of the anterior or middle trunk of the MCA. It emerges from the Sylvian fissure and passes over the anterior transverse temporal gyrus and usually divides into two branches. One of the branches supplies the angular gyrus while the other supplies the supramarginal gyrus, posterior superior temporal gyrus, and the parietooccipital arcus (sulcus).
- Temporooccipital: The longest cortical artery, it runs posteriorly opposite to the center of the operculum. Upon its exit from the Sylvian fissure, it runs parallel to the superior temporal sulcus and supplies the superior and inferior occipital gyri. This vessel anastamoses with the posterior cerebral artery and may exist as one or two arteries, 67% or 33% of the time, respectively.
- Temporopolar: The artery extends from the sphenoidal segment of the MCA via the inferior surface of the operculum and supplies the polar and anterior lateral portions of the temporal lobe. The vessel can be identified in 52% of normal angiograms
- Anterior temporal: This artery extends in the similar fashion as the temporopolar artery and vascularizes the same regions.
- Middle temporal: This artery extends from the Sylvian fissure opposite to the inferior frontal gyrus and supplies superior and middle portions of the middle temporal lobe. It can be identified in 79% of angiograms.
- Posterior temporal: This artery extends out and away from the operculum and turns in a step-wise manner first inferiorly then posteriorly into the superior temporal sulcus then to the middle temporal sulcus. This vessel supplies posterior portion of the temporal lobe and is the origin of several perforating arteries that irrigate the insula. It is readily identifiable in most radiograms.