Microsurgical anatomy of the central core of the brain
By Eduardo Carvalhal Ribas MD, PhD 1 , 3 , Kaan Yağmurlu MD 1 , Evandro de Oliveira MD, PhD 5 , 4 , Guilherme Carvalhal Ribas MD, PhD 6 , 3 and Albert Rhoton Jr. MD, PhD 1
ABSTRACT
OBJECTIVE: The purpose of this study was to describe in detail the cortical and subcortical anatomy of the central core of the brain, defining its limits, with particular attention to the topography and relationships of the thalamus, basal ganglia, and related white matter pathways and vessels.
Figure 1. The central core at the center of the brain. A: An axial cut of the brain at the level of the middle frontal gyrus was made, and the frontal operculum over the insular surface was removed. The central core is exposed and found at the morphological center of the brain, with its medial aspects represented by the caudate nucleus and the thalamus found and with its lateral aspect represented by the insular surface. B: An axial cut was performed through the central core, revealing its composition by basal ganglia and fiber pathways. These fiber pathways are organized inside the central core into capsules and named according to their position in relation to the basal ganglia: the extreme capsule is found between the insular surface and the claustrum, the external capsule between the claustrum and the lentiform nucleus, and the internal capsule (formed by the anterior limb, genu, posterior limb, and retrolenticular and sublenticular portions) between the lentiform nucleus laterally and the caudate nucleus and thalamus medially. C: The insular surface is the most superficial aspect of the central core, has a triangular shape, and is easily seen after the sylvian fissure is split. The insular surface is bounded by the anterior, superior, and inferior limiting sulci, with the limen insula at its anteroinferior vertex. The meeting points of these sulci were used in this study and were defined as follows: anterior insular point (where the anterior and superior limiting sulci meet), posterior insular point (where the superior and inferior limiting sulci meet), frontal limen point (where the limen and the anterior limiting sulcus meet), and temporal limen point (where the limen and the inferior limiting sulcus meet). D: A deep cut made around the insular surface, at the limiting sulci, passes through the cerebral isthmus, reaching the lateral ventricle and detaching the central core from the rest of the cerebral hemisphere. E: Most temporal mesial structures are already separated from the central core by the choroidal fissure, a natural cleft between the fornix and the thalamus. The temporal pole and the amygdala are attached to the central core and can be separated by a line traced from the bifurcation of the internal carotid artery or the proximal segment of M1 to the inferior choroidal point (carotid choroidal line), just lateral to the optic tract. F: The frontal, parietal, and occipital lobes were removed, and the choroidal fissure is seen inside the lateral ventricle. This circular cleft naturally separates the thalamus from the hippocampal formation (fornix and hippocampus), and its opening will reach the roof of the third ventricle (velum interpositum cistern), the pineal region (quadrigeminal cistern), and the parapeduncular region (ambient cistern). A.Ch.A. = anterior choroidal artery; AIP = anterior insular point; ALS = anterior limiting sulcus; Ant. Limb = anterior limb; Caps. = capsule; Caud. Nucl. = caudate nucleus; Chor. Fiss. = choroidal fissure; Chor. Plex. = choroid plexus; FLP = frontal limen point; For. Monro = foramen of Monro; G. = gland; Heschl’s G. = Heschl’s gyrus; I.C.A. = internal carotid artery; ILS = inferior limiting sulcus; Ins. = insula; Insul. Surf. = insular surface; Lat. Ventr. = lateral ventricle; Lenti. Nucl. = lentiform nucleus; LGB = lateral geniculate body; M.C.A. = middle cerebral artery; Opt. Tr. = optic tract; P.C.A. = posterior cerebral artery; PIP = posterior insular point; Post. Limb = posterior limb; Retrolent. = retrolenticular; Sept. Pell. = septum pellucidum; SLS = superior limiting sulcus; Temp. = temporal; TLP = temporal limen point; Vent. = ventricle. Anatomical dissections performed by Eduardo Carvalhal Ribas, MD, at Dr. Rhoton’s laboratory. (Images courtesy of AL Rhoton, Jr.)
METHODS: The authors studied 19 cerebral hemispheres. The vascular systems of all of the specimens were injected with colored silicone, and the specimens were then frozen for at least 1 month to facilitate identification of individual fiber tracts. The dissections were performed in a stepwise manner, locating each gray matter nucleus and white matter pathway at different depths inside the central core. The course of fiber pathways was also noted in relation to the insular limiting sulci.
RESULTS: The insular surface is the most superficial aspect of the central core and is divided by a central sulcus into an anterior portion, usually containing 3 short gyri, and a posterior portion, with 2 long gyri. It is bounded by the anterior limiting sulcus, the superior limiting sulcus, and the inferior limiting sulcus. The extreme capsule is directly underneath the insular surface and is composed of short association fibers that extend toward all the opercula. The claustrum lies deep to the extreme capsule, and the external capsule is found medial to it. Three fiber pathways contribute to form both the extreme and external capsules, and they lie in a sequential anteroposterior disposition: the uncinate fascicle, the inferior fronto-occipital fascicle, and claustrocortical fibers. The putamen and the globus pallidus are between the external capsule, laterally, and the internal capsule, medially. The internal capsule is present medial to almost all insular limiting sulci and most of the insular surface, but not to their most anteroinferior portions. This anteroinferior portion of the central core has a more complex anatomy and is distinguished in this paper as the “anterior perforated substance region.” The caudate nucleus and thalamus lie medial to the internal capsule, as the most medial structures of the central core. While the anterior half of the central core is related to the head of the caudate nucleus, the posterior half is related to the thalamus, and hence to each associated portion of the internal capsule between these structures and the insular surface. The central core stands on top of the brainstem. The brainstem and central core are connected by several white matter pathways and are not separated from each other by any natural division. The authors propose a subdivision of the central core into quadrants and describe each in detail. The functional importance of each structure is highlighted, and surgical approaches are suggested for each quadrant of the central core.
CONCLUSIONS: As a general rule, the internal capsule and its vascularization should be seen as a parasagittal barrier with great functional importance. This is of particular importance in choosing surgical approaches within this region.
INTRODUCTION
The human brain has a complex anatomy, and to favor its understanding, to organize its nomenclature, and for clinical practice, it has been arbitrarily divided into lobes, regions, and compartments. Since these divisions correspond to different concepts, they have been created according to different criteria, and hence they can overlap but are definitely complementary to each other and can be helpful in different ways. The concept of a central core within each cerebral hemisphere stands out as a natural and obvious anatomical delimitation with important clinical and surgical implications.
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