Gyral and Sulcal Microsurgical Anatomy

C H A P T E R

1 Gyral and Sulcal Microsurgical Anatomy Vicent Quilis-Quesada1,2 1

Department of Neurosurgery, Hospital Clinic Universitari de Val
encia, Val
encia, Spain; 2Associate Professor of Neuroanatomy, Department of Human Anatomy and Embryology, Faculty of Medicine, University of Valencia, Val
encia, Spain

INTRODUCTION

2003, Ribas, 2010, T€ ure et al., 1999, Yasargil, 1994, 1996, 1999, Yasargil et al., 2005). New technologies provide high-definition imaging studies, highly accurate pre- and intraoperative functional tests, and exceptional possibilities for mapping and neuronavigation. However, they should not replace the neurosurgeon’s exhaustive neuroanatomical knowledge. Familiarity with brain geography and a well-honed surgical technique are a surgeon’s best tools in the daily battle against conditions affecting the brain.

Knowledge of the microsurgical anatomy of the brain is a basic pillar of neurosurgery. The series of sulci and gyri on the surface of the brain forms a map with which neurosurgeons should be familiar when it comes to planning and carrying out their procedures, more particularly in the case of intrinsic brain lesions. Based on the general organization of the lobes and gyral convolutions of the brain, as defined by sulci and fissures, the preoperative (by means of neuroimaging) and intraoperative identification of the anatomical features of each patient is of critical importance. Knowledge of the microsurgical anatomy of the surface enables us to correctly locate the lesions for which surgery is to be performed, make the appropriate decision regarding the surgical approach, and treat the lesions with Optimal functional results. Each brain function has been shown to have a close relationship with the areas of the cerebral cortex (gray matter) and subcortical elements (white matter), and therefore microanatomical knowledge is the basis for the precise and safe execution of neurosurgical techniques (Qui~ nones-Hinojosa et al.,

Comprehensive Overview of Modern Surgical Approaches to Intrinsic Brain Tumors https://doi.org/10.1016/B978-0-12-811783-5.00001-X

THE CEREBRAL HEMISPHERES The brain is divided into two hemispheres (telencephalon, from the Ancient Greek for “endbrain”), separated by the longitudinal or interhemispheric fissure, connected by the corpus callosum, and further by the diencephalon (from the Ancient Greek for “between the brain”). The anatomically continuous superolateral, basal, and medial surfaces can be identified on each hemisphere, and these are delimited by their superior, lateral, and medial edges (Ludwig

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1. GYRAL AND SULCAL MICROSURGICAL ANATOMY

and Klinger, 1956, Ono et al., 1990, Rhoton, 2002). The complex system of sulci and gyri on the surface defines the external telencephalic morphology. Deep below the surface, axons of white matter, areas of gray matter, and the different elements of the ventricular system complete the intrinsic brain anatomy (Ludwig and Klinger, 1956, Ono et al., 1990, Rhoton, 2002). The arterial and venous networks of the vascular tree around the surface structures (like an exoskeleton) and around the inside of the brain anatomy (similar to an endoskeleton) provide a negative image of the overall anatomy of the intracranial volume.

THE CEREBRAL LOBES The first descriptions of the general organization of the brain sulci and gyri, and their subdivision into what are now known as cerebral lobes, date to the 19th century and the work of French anatomist Louis Pierre Gratiolet. The first divisions of the cerebral lobes were determined by those areas of the brain underlying the different bones of the cranial vault. In this way, Gratiolet was initially able to describe the frontal, parietal, occipital, and temporal lobes. Throughout the history of anatomical nomenclature, the definition of the cerebral lobes has seen the addition of morphological and functional aspects, until the current definition was achieved in Terminologia Anatomica (1998) in which there are six lobes: frontal, parietal, occipital, temporal, insular, and limbic (Rhoton, 2002, Ribas, 2010, Ribas et al., 2006, Yasargil, 1994, Yasargil et al., 2005) This chapter conceptually divides each hemisphere into seven lobes, with the addition of the so-called central lobe to the six described in the 1998 edition of Terminologia Anatomica, based on the microsurgical and functional principles developed by Prof. M. G. Yasargil (Yasargil, 1994, 1996, 1999).

FRONTAL LOBE The frontal lobe is the largest and farthest forward of the seven lobes into which we have divided the brain in this work. Its posterior limit is what is known as the precentral sulcus, the anteriormost of the slightly oblique sulci that can be identified on the lateral surface of the brain. The lateral surface of the frontal lobe comprises three main longitudinal gyri: the superior frontal gyrus (adjacent to the sagittal suture or midline), the middle frontal gyrus (centered on the lateral surface of the lobe), and the inferior frontal gyrus (the most lateral of the frontal gyri, in contact with the anterior cranial fossa and the lateral fissure of the cerebrum, the sylvian fissure), also referred to as F1, F2, and F3,

FIGURE 1.1 Gyral and sulcal basic configuration of the lateral surface of the brain. Red dashed line, superior frontal sulcus; yellow dashed line, inferior frontal sulcus; green dashed line, intraparietal sulcus; green line, intermediate sulcus of Jensen; blue dashed line, sylvian fissure; orange dashed line, superior temporal sulcus; pink dashed line, inferior temporal sulcus; red dotted line, precentral sulcus; black dotted line, central sulcus; green dotted line, postcentral sulcus; pink dotted line, inferior occipital sulcus; orange dotted line, superior occipital sulcus; ag, angular gyrus; ifg, inferior frontal gyrus; iog, inferior occipital gyrus; itg, inferior temporal gyrus; mfg, middle frontal gyrus; mog, middle occipital gyrus; mtg, middle temporal gyrus; pcg, precentral gyrus; pg, postcentral gyrus; po, pars orbitalis; pop, pars opercularis; pt, pars triangularis; sfg, superior frontal gyrus; sg, supramarginal gyrus; sog, superior occipital gyrus; spl, superior parietal lobule; stg, superior temporal gyrus.

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FRONTAL LOBE

FIGURE 1.2 Superolateral view of the right hemisphere. The cranial sutures and the superior temporal line were kept in place. 1, Sagittal suture; 2, bregma; 3, superior sagittal sinus; 4, superior parietal lobule; 5, postcentral gyrus; 6, central sulcus; 7, precentral gyrus; 8, superior frontal gyrus; 9, middle frontal gyrus; 10, the coronal suture; 11, superior frontal sulcus; 12, frontomarginal sulcus; 13, supramarginal gyrus; 14, euryon (parietal tuberosity); 15, superior temporal line; 16, stephanion; 17, angular gyrus; 18, superior temporal sulcus; 19, sylvian fissure (ascending termination of the posterior ramus); 20, pars opercularis; 21, pars orbitalis; 22, squamous suture; 23, sylvian fissure; 24, pterion; 25, greater sphenoid wing; 26, middle temporal gyrus; 27, inferior temporal gyrus; 28, inferior temporal sulcus.

respectively, in a number of works (Yasargil, 1994, 1996). They are delimited by the two longitudinal sulci, the superior frontal sulcus, and the inferior frontal sulcus (Figs. 1.1 and 1.3). The anatomical particularities of these sulci and gyri enable each one to be precisely identified, both in neuroimaging and neurosurgery, and provide references of orientation, location, and surgical access, more particularly for the treatment of intrinsic brain disorders (Rhoton, 2002, Yasargil, 1994, 1996). Each of the frontal gyri bears a relation with elements of the cranial surface. Thus, the superior frontal sulcus is located at the midpoint between the midline and the superior temporal line. The inferior frontal sulcus lies below the front half of the superior temporal line. The inferior frontal gyrus (the frontal

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FIGURE 1.3 Lateral view of the brain (left hemisphere). 1, Frontal pole; 2, superior frontal gyrus; 3, superior frontal sulcus; 4, precentral gyrus; 5, central sulcus; 6, postcentral gyrus; 7, postcentral sulcus; 8, supramarginal gyrus; 9, angular gyrus; 10, roof of the orbit; 11, middle frontal gyrus; 12, inferior frontal sulcus; 13, precentral sulcus; 14, ascending termination of the posterior ramus of the sylvian fissure; 15, pars orbitalis; 16, anterior horizontal ramus of the sylvian fissure; 17, pars triangularis; 18, anterior ascending ramus of the sylvian fissure; 19, pars opercularis; 20, anterior subcentral ramus of sylvian fissure; 21, subcentral gyrus; 22, posterior subcentral ramus of sylvian fissure; 23, lesser sphenoid wing; 24, sylvian fissure; 25, anterior sylvian point; 26, temporal pole; 27, superior temporal gyrus; 28, superior temporal sulcus; 29, middle temporal gyrus.

operculum) lies below the bone between the superior temporal line and the temporal squama. The anteriormost part of the sylvian fissure lies below the anteriormost part of the temporal squama (Rhoton, 2002, Ribas, 2010) (Fig. 1.2). The superior frontal sulcus separates the superior frontal gyrus from the middle frontal gyrus. This sulcus is easily identified both through magnetic resonance imaging and in surgery, as it runs longitudinally parallel to the midline until its intersection with the anteriormost of the oblique sulci of the lateral surface of the brain, the precentral sulcus (Figs. 1.13 and 1.14). Medial to the superior frontal sulcus is the superior frontal gyrus (the posterior portion of the superior frontal gyrus, close to the precentral gyrus, corresponds to the

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FIGURE 1.4 Anterior view of the brain. A coronal cut has been performed in the right hemisphere at the level of the foramen of Monro. 1, Superior frontal gyrus; 2, superior frontal sulcus; 3, middle frontal gyrus; 4, cingulate gyrus; 5, corpus callosum; 6, external capsule; 7, caudate nucleus; 8, putamen; 9, lateral ventricle; 10, apex of the insula; 11, frontal operculum; 12, anterior perforated substance; 13, frontal pole; 14, frontomarginal sulcus; 15, frontal sinus; 16, roof of the orbit; 17, planum polare; 18, uncus; 19, middle cerebral artery; 20, optic nerve; 21, anterior cerebral arteries; 22, sylvian fissure; 23, superior temporal gyrus; 24, inferior frontal gyrus. The black arrows show how the sulci of the lateral and basal surface of the brain are oriented toward the ventricular cavities. Blue dotted line, cingulate sulcus; red dotted line, callosal sulcus.

supplementary motor area). The precentral gyrus shows an U-shaped curve around the posteriormost end of the superior frontal sulcus, corresponding to the primary motor cortex controlling the contralateral hand. The intersection between the superior frontal sulcus and the precentral sulcus is an easy-to-identify anatomical landmark that allows the frontal lobe to be distinguished from the central lobe and functionally relevant areas (supplementary motor cortex and primary motor cortex) to be located (Fig. 1.14). The superior frontal gyrus is frequently divided longitudinally by a smaller sulcus known as the medial frontal sulcus. The middle frontal gyrus is the largest of the three frontal gyri on the lateral surface of the frontal lobe. It can be divided longitudinally by

FIGURE 1.5

Superolateral view of the brain (left hemisphere). 1, Superior frontal sulcus; 2, superior frontal gyrus; 3, precentral sulcus; 4, precentral gyrus (motor); 5, central sulcus; 6, postcentral gyrus; 7, postcentral sulcus; 8, superior parietal lobule; 9, intraparietal sulcus; 90 , postcentralintraparietal sulci confluence; 10, occipital lobe; 11, middle frontal gyrus; 12, falx cerebri; 13, supramarginal gyrus; 14, gyrus of Jensen; 15, angular gyrus; 16, inferior frontal gyrus; 17, frontal pole.

FIGURE 1.6 Posterior view of the brain. The cranial sutures and the superior temporal line were kept in place. 1, Sagittal suture; 2, lambda; 3, superior parietal lobule; 4, intraparietal sulcus; 5, inferior parietal lobule; 6, superior temporal line; 7, lambdoid suture; 8, superior sagittal sinus; 9, inion (external occipital protuberance); 10, cuneus; 11, calcarine fissure; 12, lingual gyrus; 13, inferior occipital gyrus; 14, middle occipital gyrus; 15, superior occipital gyrus; 16, superior occipital sulcus; 17, inferior occipital sulcus; 18, superior nucal line; black*, opistocranium; white*, asterion.

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FRONTAL LOBE

FIGURE 1.7 Superior view of both temporal lobes. An axial cut through the third ventricle has been performed to expose both superior surfaces of the temporal lobes. The mediobasal temporal region has been dissected on the right temporal lobe to expose the intraventricular and extraventricular anatomical structures. 1, Anterior clinoid process; 2, optic nerve; 3, planum sphenoidale; 4, optic chiasm; 5, lamina terminalis; 6, optic tract; 7, carotid artery; 8, ophthalmic artery; 9, posterior communicating artery, 10, third nerve; 11, anterior choroidal artery; 12, middle cerebral artery; 13, temporal pole; 14, planum polare; 15, Heschl’s gyrus; 16, middle transverse temporal gyrus; 17, semilunar gyrus; 18, temporal stem; 19, thalamus; 20, hypothalamus; 21, tuber cinereum; 22, mammillary bodies; 23, floor of the third ventricle; 24, mesencephalon; 25, apex of uncus; 26, amygdala; 27, uncal recess; 28, posterior segment of uncus; 29, head of hippocampus; 30, collateral eminence; 31, collateral trigone; 32, choroid plexus; 33, parahippocampal gyrus; 34, posterior cerebral artery; 35, anterior choroidal artery; 36, choroid glomus; 37, splenium of the corpus callosum.

what is known as the intermediate frontal sulcus. A large number of connections between the middle frontal gyrus and the inferior frontal gyrus are the cause of frequent interruptions to the continuity of the inferior frontal sulcus. Likewise, it can connect with the precentral gyrus by interrupting the continuity of the precentral sulcus (Figs. 1.3 and 1.4) (Rhoton, 2002, Ribas, 2010, Ribas et al., 2006, Yasargil, 1994). The morphology of the inferior frontal gyrus is defined by the different arms (“rami”) of the sylvian fissure. Specifically, the anterior horizontal ramus and the anterior ascending ramus delimit, from anterior to posterior, the pars orbitalis, pars triangularis, and pars opercularis.

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FIGURE 1.8 Inferior view of the brain. The posterior segment of the uncus and the middle segment of the parahippocampal gyrus have been dissected to expose the extraventricular representation of the head and body of the hippocampus (left hemisphere). 1, Frontal pole; 2, olfactory bulb; 3, anterior orbital gyrus; 4, lateral orbital gyrus; 5, medial orbital gyrus; 6, gyrus rectus; 7, olfactory tract; 8, temporal pole; 9, rhinal incisura; 10, anterior segment of the parahippocampal gyrus; 11, apex of the uncus; 12, anterior segment of the uncus; 13, uncinate gyrus (extraventricular representation of the head of hippocampus); 14, amygdala; 15, rhinal sulcus; 16, inferior temporal gyrus; 17, occipitotemporal sulcus; 18, fusiform gyrus; 19, collateral sulcus; 20, parahippocampal gyrus (middle segment); 200 , parahippocampal gyrus (posterior segment); 21, pons; 22, dentate gyrus; 23, temporal horn of the lateral ventricle; 24, intralingual sulcus; 25, lingual gyrus; 26, occipital pole; 27, inferior occipital gyrus; Black line, H-shaped orbital sulcus.

These three anatomical elements, known as the frontal operculum, can be identified in both imaging studies and surgery, and are essential anatomical landmarks for locating the precentral and postcentral gyri (Figs. 1.3 and 1.13). From anterior to posterior, the pars orbitalis is located above the orbital roof in the anterior cranial fossa. The anterior horizontal and the anterior ascending rami of the sylvian fissure define the pars triangularis, which curves back to form a

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FIGURE 1.10

FIGURE 1.9 Lateral view of the right hemisphere. White fiber dissection of the brain and removal of the frontoparietal operculum to expose the surface of the insula. 1, Postcentral sulcus; 2, precentral sulcus; 3, middle frontal gyrus; 4, superior longitudinal fasciculus; 5, planum temporale; 6, sylvian point; 7, posterior long insular gyrus; 70 , postcentral insular sulcus; 8, anterior long insular gyrus; 9, posterior short insular gyrus; 10, precentral insular sulcus; 11, middle short insular gyrus; 12, short insular sulcus; 13, anterior short insular gyrus; 14, accessory insular gyrus; 15, pars orbitalis; 16, anterior horizontal ramus of sylvian fissure; 17, middle transverse temporal gyrus; 18, Heschl’s gyrus, 19, superior temporal gyrus; 20, superior temporal sulcus; 21, middle temporal gyrus; 22, inferior temporal sulcus; 23, inferior temporal gyrus; 24, temporal pole. Yellow dotted line, representation of the lateral ventricle to illustrate how the insula is situated in the C-shaped curve of it; red line, superior limiting sulcus of the insula; green line, anterior limiting sulcus of the insula; blue line, inferior limiting sulcus of the insula; black dotted line, central sulcus of the insula; gray line, central sulcus of the brain; (*), insular apex.

large subarachnoid cistern in the anterior portion of the sylvian fissure, the so-called anterior sylvian point (Fig. 1.3). This cisternal point forms an exceptional surgical landmark with which to plan the microsurgical dissection of the sylvian fissure, both for approaches to the basal cisterns and for the full opening of the sylvian fissure to expose the distal segments of the middle cerebral artery and the surface of the insular lobe. The anterior sylvian point lies slightly posterior, below what is known as the pterion of the skull (region of confluence of the sphenoid, frontal,

Medial surface of the right hemisphere. 1, Superior frontal gyrus; 10 , medial frontal gyrus; 2, paracentral sulcus; 20 , anterior paracentral lobule; 3, posterior paracentral lobule; 4, marginal ramus of the cingulate sulcus; 5, precuneus; 6, parietooccipital sulcus; 7, cuneus; 8, calcarine fissure; 9, occipital pole; 10, frontal pole; 11, gyrus rectus; 12, paraterminal gyrus; 13, paraolfactory gyri; 14, velum interpositum; 15, genu of the corpus callosum; 16, rostrum of the corpus callosum; 17, body of the corpus callosum; 18, cingulate gyrus; 19, callosal sulcus; 20, cingulate sulcus; 21, fornix; 22, anterior commissure; 23, optic chiasm; 24, lamina terminalis; 25, third ventricle; 26, splenium of the corpus callosum; 27, pineal gland; 28, subparietal sulcus; 29, parahippocampocingulate gyrus; 30, parahippocampolingual gyrus; 31, external perpendicular fissure (lateral surface); 32, lingual gyrus; 33, temporal pole; 34, fusiform gyrus; 35, rhinal sulcus; 36, anterior segment of the parahippocampal gyrus; 37, middle segment of the parahippocampal gyrus; 38, collateral sulcus; 39, apex of the uncus; 40, rhinal incisura; 41, uncal notch.

parietal, and temporal bones) (Fig. 1.2). Immediately posterior to the anterior ascending ramus is the pars opercularis, which is characteristically U-shaped and in continuity with the precentral gyrus in its inferiormost part. The U that typifies the pars opercularis surrounds the inferior end of the precentral sulcus and constitutes another anatomical landmark for locating and identifying the different features of the lateral surface of the brain (Figs. 1.3 and 1.13). Finally, the pars opercularis, the posteriormost part of the frontal operculum, is bound by the central lobe through what is known as the anterior subcentral sulcus, occasionally located deep inside the sylvian fissure and therefore not visible from the surface. The Broca area (responsible for the

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FRONTAL LOBE

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FIGURE 1.11

Inferior view of the right hemisphere. 1, Frontal pole; 2, olfactory nerve; 3, gyrus rectus; 4, optic nerve; 5, optic tract; 6, mesencephalon; 7, ambient cistern; 8, quadrigeminal cistern; 9, splenium of the corpus callosum; 10, isthmus of the cingulate gyrus; 11, parahippocampocingulate gyrus; 12, parahippocampolingual gyrus; 13, anterior calcarine sulcus; 14, parietooccipital sulcus; 15, precuneus; 16, cuneus; 17, lingual gyrus; 18, calcarine sulcus; 19, orbital gyri; 20, temporal pole; 21, rhinal incisura; 22, anterior segment of the parahippocampal gyrus; 23, rhinal sulcus; 24, anterior segment of the uncus; 25, apex of the uncus; 26, posterior segment of the uncus; 27, uncal notch; 28, middle segment of the parahippocampal gyrus; 29, collateral sulcus; 30, posterior segment of the parahippocampal gyrus; 31, fusiform gyrus; 32, inferior temporal gyrus; 33, occipitotemporal sulcus; 34, inferior occipital gyrus.

production for the spoken language) of the dominant hemisphere is located in the pars triangularis and pars opercularis of the inferior frontal gyrus (Ono et al., 1990, Rhoton, 2002, Ribas, 2010, Ribas et al., 2006, Yasargil, 1994). At the anterior end of the frontal lobe (frontal pole), the frontomarginal sulcus, parallel to the supraciliary margin, separates the superolateral surface of the frontal lobe from the basal surface (Fig. 1. 4). The basal surface of the frontal lobe is located above the so-called anterior fossa of the skull base. It comprises a continuous and straight gyrus running parallel to the midline, known as the gyrus rectus, and the so-called orbital gyri. The gyrus rectus is a continuation of the superior frontal gyrus on the superolateral and medial surfaces of the frontal lobe. Lateral to the rectus gyrus is the olfactory sulcus, a deep longitudinal and paramedian sulcus in which

FIGURE 1.12 Medial view of the mediobasal temporal region of the right hemisphere. The dissector opens the uncal notch to expose the inferior surface of the posterior segment of the uncus. 1, Entorhinal area; 2, dissector opening the uncal notch; 3, anterior segment of the uncus; 4, apex of the uncus; 5, optic tract; 6, uncinate gyrus; 7, band of Giacomini; 8, intralimbic gyrus; 9, inferior choroidal point; 10, lateral geniculate body; 11, choroidal fissure; 12, fimbria; 13, dentate gyrus; 14, hippocampal sulcus; 15, fimbrodentate sulcus; 16, parahippocampal gyrus; 17, parahippocampocingulate gyrus; 18, parahippocampolingual gyrus; 19, splenium of the corpus callosum.

the olfactory tract is found. The division of the olfactory tract into medial and lateral striae marks the posterior limit of the basal surface of the frontal lobe and the start of what is known as the anterior perforated substance. Most of the basal surface of the frontal lobe is made up of orbital gyri, which are lateral to the olfactory sulcus. The characteristically H-shaped orbital sulcus delimits the four orbital gyri (anterior, medial, posterior, and lateral), which are all connected to the frontal gyri of the superolateral surface (Figs. 1.8 and 1.11). The medial surface of the frontal lobe comprises the interhemispherical face of the superior frontal gyrus. The superior frontal gyrus continues at its anterior end as the rectus gyrus of the basal surface of the frontal lobe. The paracentral sulcus marks the posterior limit of the frontal lobe and the anterior limit of the paracentral lobule. A part of the supplementary motor cortex is found in the posterior and medial portions of

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FIGURE 1.13 A. Preoperative sagittal T1-weighted magnetic resonance image of a low-grade astrocytoma in the posterior segment of the middle frontal gyrus. (A) 1, Pars orbitalis; 2, pars triangularis; 3, pars opercularis; 4, subcentral gyrus; 5, precentral gyrus; 6, central sulcus; 7, postcentral gyrus; 8, supramarginal gyrus; 9, superior temporal gyrus; 10, middle temporal gyrus; 11, sylvian fissure; (*), anterior sylvian point. (B) Preoperative sagittal T1-weighted magnetic resonance image of a lowgrade astrocytoma in the anteriormost portion of the supramarginal gyrus. 1, Pars orbitalis; 2, pars triangularis; 3, pars opercularis; 4, subcentral sulcus; 5, central sulcus; 6, postcentral gyrus; 7, postcentral sulcus; 8, supramarginal gyrus; 9, superior temporal gyrus; 10, middle temporal gyrus; 11, middle frontal gyrus; 12, precentral gyrus; (*), anterior sylvian point. (C) Preoperative sagittal T1-weighted magnetic resonance image of a cavernoma located in the confluence of the parietooccipital sulcus and calcarine fissure (white arrow). 1, Superior frontal gyrus; 2, cingulate gyrus; 3, splenium of corpus callosum; 4, paracentral lobule; 5, marginal ramus of cingulate sulcus; 6, cingulate sulcus; 7, precuneus; 8, parietooccipital sulcus; 9, cuneus; 10, calcarine fissure; 11, lingual gyrus. (D) Preoperative coronal T2-weighted magnetic resonance image of a low-grade astrocytoma in the right cingulate gyrus. 1, Parahippocampal gyrus; 2, fusiform gyrus; 3, inferior temporal gyrus; 4, middle temporal gyrus; 5, superior temporal gyrus; 6, inferior frontal gyrus; 7, middle frontal gyrus; 8, superior frontal gyrus; 9, cingulate gyrus (tumor); 10, corpus callosum; 11, thalamus; 12, caudate nucleus. (E) Preoperative coronal T2-weighted magnetic resonance image of a low-grade astrocytoma in the left fusiform gyrus. 1, Atrium of the lateral ventricle; 2, cingulate gyrus; 3, superior parietal lobule; 4, corpus callosum; 5, posterior segment of the parahippocampal gyrus; 6, fusiform gyrus (tumor); 7, inferior temporal gyrus; white arrow, collateral sulcus; black arrow, collateral trigone. (F) Preoperative sagittal T2-weighted

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CENTRAL LOBE

the superior frontal gyrus (Figs. 1.10 and 1.13) (Ono et al., 1990, Rhoton, 2002, Ribas, 2010, Ribas et al., 2006, Yasargil, 1994).

CENTRAL LOBE The concept of the central lobe, introduced by Prof. M. G. Yasargil, is justified because it can be considered in morphological and functional terms to be a distinct area separate from the other cerebral lobes (Yasargil, 1994, 1996). The central lobe comprises the precentral (motor) and postcentral (sensitive) gyri, separated by the central sulcus (frequently continuous and barely serpiginous). Arranged obliquely to the midline and located between the frontal and parietal lobes, it is limited anteriorly by the precentral sulcus and posteriorly by the postcentral sulcus, both commonly discontinuous. Both cerebral gyri run from the sylvian fissure (where they interconnect through what is known as the subcentral gyrus) and reach the medial surface of the brain where they give rise to the paracentral lobule. The brain surface point at which the central sulcus intersects with the interhemispheric fissure, known as the superior rolandic point, is located approximately 5 cm posterior to the bregma (Figs. 1.1e1.3). At the level of the sylvian fissure, the subcentral gyrus is limited anteriorly by the frontal operculum through the anterior subcentral sulcus and

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posteriorly by the parietal operculum through the posterior subcentral sulcus. The posterior half of the subcentral gyrus projects over Heschl’s gyrus (also known as the anterior transverse temporal gyrus) in transition between the planum polare and the planum temporale of the temporal lobe. On the medial surface, the paracentral lobule is limited anteriorly by the superior frontal gyrus by means of the paracentral sulcus and is separated posterobasally from the parietal lobe through the marginal ramus of the cingulate gyrus. The paracentral lobule controls the motor and sensory innervations of the contralateral lower extremity (Figs. 1.10 and 1.13). There is a third connection between the precentral gyrus and the postcentral gyrus, known as the medial frontoparietal pli de passage of Broca, generally located at the level of the posterior end of the superior frontal sulcus. This connection, together with the confluence of the precentral and superior frontal sulci, creates a subarachnoid cistern, previously described, associated with a U-shaped deformity of the precentral gyrus, which is easily identified in imaging studies and in surgery. The U of the precentral gyrus corresponds to the primary motor cortex of the contralateral hand and is a very useful landmark for locating each of the sulci and gyri of the lateral surface of the brain, in particular those associated with the central lobe (Figs. 1.2, 1.5 and 1.13) (Ribas et al., 2006, Yasargil, 1994, 1996).

=magnetic resonance image of a low-grade astrocytoma in the cingulate gyrus and precuneus. 1, Corpus callosum; 2, cingulate gyrus; 3, superior frontal gyrus; 4, precuneus; 5, parietooccipital sulcus; 6, cuneus; 7, calcarine fissure; 8, lingual gyrus; 9, isthmus of the cingulate gyrus; 10, splenium of corpus callosum. (G) Preoperative axial T2-weighted magnetic resonance image of an arteriovenous malformation in the left precentral sulcus. 1, Middle frontal gyrus; 2, superior frontal gyrus; 3, precentral and superior frontal sulci point of confluence; 4, precentral gyrus (U); 5, central sulcus; 6, postcentral gyrus; 7, marginal ramus of cingulate sulcus; 8, postcentral sulcus; 9, superior parietal lobule. (H) Preoperative sagittal T2-weighted magnetic resonance image of a low-grade oligoastrocytoma located in the precuneus. 1, Subcallosal area; 2, gyrus rectus; 3, genu of corpus callosum; 4, superior frontal gyrus; 5, cingulate gyrus; 6, paracentral lobule; 7, marginal ramus of cingulate sulcus; 8, precuneus; 9, cuneus; 10, lingual gyrus; 11, splenium of corpus callosum; 12, thalamus; (*), parietooccipital and calcarine point of confluence. (I) Preoperative sagittal T2-weighted magnetic resonance image of a metastasis located in the depth of the intermediate sulcus of Jensen (notice the vasogenic edema “enhancing” the inferior parietal lobule). 1, Posterior ramus of sylvian fissure; 2, supramarginal gyrus; 3, superior temporal gyrus; 4, angular gyrus; black arrow, intermediate sulcus of Jensen.

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FIGURE 1.14 A 33-year-old left-handed male who had suffered several seizures, at presentation, had no neurological abnormalities. (AeC) Preoperative T2-weighted magnetic resonance images of a low-grade astrocytoma located in the posterior portion of the superior frontal gyrus (supplementary motor area). Three years after surgery, the patient remains asymptomatic. (A) 1, Superior frontal gyrus; 2, superior frontal sulcus; 3, precentral gyrus; 4, postcentral gyrus; 5, central sulcus; (*), precentral and superior frontal sulci point of confluence; black arrow, marginal ramus of the cingulate gyrus. (B) 1, Corpus callosum; 2, cingulate gyrus; 3, superior frontal gyrus; 4, paracentral lobule; white arrow, marginal ramus of the cingulate gyrus. (C) 1, Inferior frontal gyrus; 2, middle frontal gyrus; 3, superior frontal gyrus; 4, cingulate gyrus; 5, corpus callosum. (D) Surgical planning (see-through view). 1, Precentral gyrus; 2, superior frontal gyrus; 3, sagittal suture; 4, tumor; 5, middle frontal gyrus; (*), precentral and superior frontal sulci point of confluence. (E) Cranial surface; (*), sagittal suture (midline). (F) Brain surface after dural opening. 1, Precentral gyrus; 2, superior frontal gyrus (tumor); 3, superior frontal gyrus (anterior to the tumor); (*), precentral and superior frontal sulci point of confluence (enlarged subarachnoid space). (G) Surgical field after radical resection of the tumor. 1, Precentral gyrus; 2, superior frontal gyrus; 3, middle frontal gyrus. (H) Postoperative T2-weighted magnetic resonance image. Radical resection of the tumor. 1, Superior frontal gyrus; 2, precentral gyrus; 3, postcentral gyrus; 4, middle frontal gyrus; 5, brain cavity after tumor removal. I. Postoperative sagittal gadolinium-enhanced T1-weighted magnetic resonance image. 1, Superior frontal gyrus; 2, cingulate gyrus; 3, splenium of corpus callosum; 4, paracentral lobule; 5, precuneus; 6, cuneus; 7, lingual gyrus; 8, brain cavity after tumor removal.

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PARIETAL LOBE

PARIETAL LOBE The parietal lobe is characterized by curvilinear gyri of short length and by its delimitation by long sulci. It is largely divided into two parietal lobules (superior and inferior) by the longitudinal intraparietal sulcus, which is parallel to the midline and continues along the length of the occipital lobe in the form of the superior occipital sulcus. It is limited anteriorly by the postcentral gyrus by means of the postcentral sulcus, which generally connects with the intraparietal sulcus to give rise to a large subarachnoid cistern (Fig. 1.5) (Ribas et al., 2006). This space facilitates identification of the different elements of the parietal lobe in imaging tests and their dissection in transsulcal approaches. The posterior limit of the parietal lobe is arbitrarily defined by a line that joins the protrusion on the superolateral surface of the parietooccipital sulcus (medial surface of the hemisphere) with what is known as the preoccipital notch (a laterobasal indentation of the brain that separates the temporal lobe from the occipital lobe, typically located about 5 cm anterior to the occipital pole). The intraparietal sulcus lies approximately below the posterior half of the superior temporal line on the cranial surface. The superior parietal lobule is located medial to superior temporal line, whereas the inferior parietal lobule is located below the cranial surface between the superior temporal line and the temporal squama (Ono et al., 1990, Rhoton, 2002, Ribas, 2010, Ribas et al., 2006, Yasargil, 1994, Yasargil, 1996) (Figs. 1.2 and 1.6). In turn, the inferior parietal lobule is divided into two main gyri: the supramarginal gyrus, around the distal end of the posterior ramus of the sylvian fissure, and the angular gyrus, around the posterior end of the superior temporal sulcus. Both gyri are separated by what is known as the intermediate sulcus of Jensen, a ramus of the intraparietal sulcus, superior temporal sulcus, or both. The supramarginal gyrus is connected anteriorly to the postcentral gyrus

13

surrounding the inferior end of the postcentral sulcus. The continuity of the posteroinferior portion of the supramarginal gyrus as the superior temporal gyrus, where the Wernicke area (speech comprehension) is found in the dominant hemisphere, is of great relevance. Like the supramarginal gyrus, the angular gyrus is located around the posterior end of the superior temporal sulcus, continuing from the middle temporal gyrus. The superior part of the supramarginal gyrus lies underneath the so-called euryon (craniometric point corresponding to the parietal tuberosity) (Wen et al., 2009) (Figs. 1.1, 1.2, 1.5 and 1.13). The superior parietal lobe is frequently connected at its anterior end with the postcentral gyrus, creating the discontinuity of the superior portion of the postcentral sulcus. It is limited laterally by the intraparietal sulcus, which continues in the form of the superior occipital sulcus. The superior parietal lobule also continues as the superior occipital gyrus, after passing what is known as the external perpendicular fissure (superior end of the parietooccipital sulcus on the superolateral surface of the brain). The external perpendicular fissure, identifiable in imaging tests, is a landmark that delimits the superior parietal lobule (anterior) from the superior occipital gyrus (posterior) on the superolateral surface. Likewise, the parietooccipital sulcus (on the medial surface of the hemisphere) allows the precuneus (anterior and part of the superior parietal lobule) to be separated from the cuneus (posterior and part of the occipital lobe) (Rhoton, 2002, Ribas, 2010, Ribas et al., 2006) (Figs 1.10 and 1.13). The superior parietal lobule, supramarginal gyrus, and angular gyrus are also referred to as the P1, P2, and P3, respectively, by a number of authors (Yasargil, 1994, 1996). On the medial surface of each hemisphere, the superior parietal lobule continues as the precuneus, limited anteriorly by the marginal ramus of the cingulate gyrus, posteriorly by the

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1. GYRAL AND SULCAL MICROSURGICAL ANATOMY

parietooccipital sulcus, and inferiorly by the subparietal sulcus. The precuneus connects inferiorly with elements of the limbic lobe, such as the isthmus of the cingulate gyrus and the parahippocampal gyrus (Fig. 1.10).

OCCIPITAL LOBE While the occipital lobe is anatomically the most inconsistent of the cerebral lobes, it is organized according to the general pattern of three longitudinal gyri (superior, middle, and inferior occipital gyri, also known as O1, O2, and O3, respectively) separated by two main sulci (superior and inferior) that converge at the occipital pole. The superior occipital sulcus and the inferior occipital sulcus have a great many ramifications, which together with a large number of anastomotic bridges between the different gyri produce a variable and irregular superolateral surface (Ribas et al., 2006) (Fig. 1.1). The medial surface of the occipital lobe is more defined than the lateral surface. The parietooccipital sulcus forms its anterior limit, while another continuous sulcus, the calcarine fissure, divides it into the cuneus (anterior) and lingual gyrus (posteroinferior). The superior end of the parietooccipital sulcus, projected onto the lateral surface as the external perpendicular fissure, lies underneath the so-called lambda (craniometric point of the skull midline surface). The calcarine fissure is divided into an anterior portion and a posterior portion by the point where the parietooccipital sulcus begins. The precuneus is located over the anterior portion; the cuneus is found over the posterior portion; and finally, what is known as the lingual gyrus is found beneath the entire calcarine fissure. The lingual gyrus, which continues anteriorly as the parahippocampal gyrus (part of the mediobasal temporal region and limbic lobe), forms the mediobasal surface of the occipital lobe. The anteriormost part of the calcarine fissure is projected downward at the level of the medial wall of the ventricular atrium of the brain, in

the form of the calcar avis. The primary visual cortex is found in the posterior part of the calcarine fissure, on both the superior (cuneus) and inferior (lingual gyrus) surfaces (Rhoton, 2002) (Figs. 1.10 and 1.11). The inferior surface continues as the basal posterior surface of the temporal lobe, from which it is separated by an imaginary line that joins the preoccipital notch with the starting point of the parietooccipital sulcus from the calcarine fissure. The lingual gyrus is limited laterobasally by the so-called collateral sulcus, lateral to which is the fusiform gyrus, which runs along the basal surface of both temporal and occipital lobes. Lateral to the fusiform gyrus, what is known as the occipitotemporal sulcus runs from the temporal pole to the occipital pole along the basal surface of the brain. The inferior temporal gyrus, continuing from the inferior occipital gyrus, forms a part of both the lateral surface and basal surface of the brain, and both are located lateral to the occipitotemporal sulcus (Ono et al., 1990, Rhoton, 2002, Ribas et al., 2006) (Figs. 1.8 and 1.11).

TEMPORAL LOBE The limits of the temporal lobe are relatively artificial (defined descriptively but without any clear anatomical definition), particularly with regard to its posterior and basal limits. Viewed laterally, it is separated from the frontal, central, and parietal lobes by the posterior ramus of the sylvian fissure. It is delineated posteriorly by the occipital lobe by means of the imaginary lateral parietotemporal line, which runs between the parietooccipital sulcus on the lateral surface of the brain (external perpendicular fissure) to the preoccipital notch. It is separated from the parietal lobe by the temporooccipital line, which runs from the posterior end of the posterior ramus of the sylvian fissure to the midpoint of the lateral parietotemporal line. It is separated from the occipital lobe on its basal

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15

INSULAR LOBE

surface by the basal parietotemporal line, which connects the preoccipital notch with the inferior limit of the parietooccipital fissure. In anatomical terms, the temporal lobe continues into the occipital lobe without any clear delineation. The parahippocampal gyrus of the mediobasal temporal region (actually part of the limbic lobe) continues anatomically as the lingual gyrus of the occipital lobe, but functionally, the parahippocampal gyrus continues posterosuperiorly with the cingulate gyrus through the isthmus of the cingulate gyrus. The temporal lobe is connected superiorly to the insular lobe by means of the so-called temporal stem. It is connected anteriorly and medially with the globus pallidus through the amygdala and with the basal surface of the frontal lobe through the limen insulae. Finally, the medial part of the temporal lobe posterior to the uncus is separated from the thalamus by what is known as the choroidal fissure. The temporal lobe has four surfaces: (1) basal surface, (2) lateral surface, (3) superior or opercular surface, and (4) medial surface (Rhoton, 2002). The basal surface comprises, from lateral to medial, the inferior temporal gyrus (starting from the lateral surface), occipitotemporal sulcus, fusiform gyrus, collateral sulcus, and finally, the parahippocampal gyrus (limbic lobe) (Figs. 1.8 and 1.11). The lateral surface comprises the superior, middle, and inferior temporal gyri, separated by the superior and inferior temporal sulci. As described in the previous sections of this chapter, the superior temporal gyrus continues as the posteroinferior portion of the supramarginal gyrus. Likewise, the middle temporal gyrus continues as the angular gyrus of the inferior parietal lobule (located surrounding the distal end of the superior temporal sulcus) (Figs. 1.1 and 1.3). The superior surface is also known as the opercular surface, as it forms the temporal operculum of the sylvian fissure. There are three clearly differentiated portions from anterior to posterior: the planum polare, Heschl’s gyrus,

and the planum temporale. Heschl’s gyrus is of special interest given that it is located beneath the postcentral gyrus and its main longitudinal axis runs toward the atrium of the lateral ventricle. The primary auditory cortex is located at the level of Heschl’s gyrus and the planum temporale (Fig. 1.7). The anteriormost medial surface comprises the uncus and, posterior to this, from basal to superior, the subiculum (medial part of the parahippocampal gyrus), dentate gyrus, fimbria, and choroidal fissure. These elements will be described in detail in the section of this chapter dealing with the limbic lobe, of which they are a part (Rhoton, 2002, Ribas et al., 2006, Wen et al., 1999, 2009).

INSULAR LOBE The insular lobe, also known as the insula, is concealed deep inside the sylvian fissure and is covered by the frontal, central, parietal, and temporal lobes. It is traditionally described as a pyramid with a triangular base, with an anterior surface (covered by the frontoorbital operculum), a superior surface covered by the frontoparietal operculum, and an inferior surface covered by the temporal operculum (Wen et al., 1999, Wen et al., 2009) The insular lobe is divided by the central sulcus (obliquely arranged, like the central sulcus of the central lobe) in an anterior portion formed by small insular gyri and a posterior portion of long insular gyri parallel to the central sulcus. On the surface, the central sulcus of the insular lobe is covered by the subcentral gyrus (which connects the precentral and postcentral gyri). As a rule, three insular gyri originating at the point defining the apex of the pyramid of the insular lobe comprise the anterosuperior portion. The so-called transverse and accessory gyri, located on the anteroinferior face of the pyramid, comprise what is known as the insular pole, which is connected to the posterior and medial orbital gyri. The insular apex protrudes under

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the anterior sylvian point, which in turn is defined by the tip of the so-called pars triangularis of the frontal operculum. The smaller surface of the posterior portion comprises two large oblique gyri parallel to the central sulcus, known as the anterior and posterior long gyri of the insula (Fig. 1.9) (Ribas and de Oliveira, Mar 2007, T€ ure et al., 1999, Wen et al., 2009). The insular lobe is surrounded by the circular sulcus of Reil, which is also divided into superior, inferior, and anterior limiting sulci. The superior limiting sulcus forms the transition between the superolateral surface of the insula and the frontoparietal operculum. The superior limiting sulcus continues along the surface as the anterior horizontal ramus of the sylvian fissure. The inferior limiting sulcus defines the transition between the inferolateral surface of the insula and the temporal operculum. Finally, the anterior limiting sulcus of the insular lobe is a deep sulcus between the anterior surface of the insula and the posterobasal surface of the frontal lobe (frontoorbital operculum). The anterior limiting sulcus continues along the surface as the anterior ascending ramus of the sylvian fissure (Fig. 1.9) (Wen et al., 2009). The surface of the insular lobe forms an envelope for a set of elements that a number of authors refer to as the central core of the brain. This cortical gray matter covers a heterogeneous collection of elements, among which are the thalamus, basal nuclei, and internal capsule. The entire central core is surrounded by the lateral ventricle, which means that the superior limiting sulcus protrudes over the body of the lateral ventricle and the inferior limiting sulcus over the temporal horn. The posterior apex of the circular sulcus, the point of confluence for the superior and inferior limiting sulci of the insula, comprises the sylvian point in the anterior angiographic projection, and is located in proximity to the atrium of the lateral ventricle (Ribas and de Oliveira, Mar 2007, Ribas et al., 2006, T€ ure et al., 1999).

LIMBIC LOBE The term limbic, coined by the French anatomist Broca from the Latin limbus, meaning edge or border, describes the arc-shaped formation of a number of elements of brain anatomy forming the edge of the diencephalon. The most recent edition of Terminologia Anatomica, from 1998, designated the limbic lobe as another cerebral subdivision. The cingulate gyrus and the parahippocampal gyrus largely make up what is referred to as the limbic lobe. Functional studies have shown how the different elements making up the limbic lobe are greatly interconnected with other diencephalic and telencephalic structures (Ribas et al., 2006). The hippocampal formation (generally related to memory circuits) and the amygdala (involved with emotion circuits) are the main anatomical and functional elements of limbic lobe. The cingulate gyrus and the parahippocampal gyrus border the diencephalon in an arc, hence the name of this lobe (Fig. 1.10). The cingulate gyrus is located above the corpus callosum, running parallel to it, and is limited inferiorly by the callosal sulcus and superiorly by the cingulate sulcus (Rhoton, 2002, Ribas et al., 2006). The cingulate gyrus starts below the rostrum of the corpus callosum and surrounds it completely, providing connections with the frontal lobe (medial part of the superior frontal gyrus), central lobe (paracentral lobule), and parietal lobe (precuneus). Its rear portion, at the level of the splenium of the corpus callosum, tapers to form the isthmus of the cingulate gyrus and then continues as the parahippocampal gyrus. It is at the isthmus of the cingulate gyrus that the anterior portion of the calcarine fissure originates. The inferior portion of the limbic lobe arc comprises the parahippocampal gyrus, a part of the so-called mediobasal temporal region (Fig. 1.10) (Wen, 1999). The parahippocampal gyrus occupies the transitional area between the basal and purely

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LIMBIC LOBE

medial surfaces of the temporal lobe. It extends longitudinally and deviates at its anteriormost portion, running medially, and folding over to form the uncus. Posteriorly, just at the level of the splenium of the corpus callosum, is the anteriormost portion of the calcarine fissure, which divides the posterior part of the parahippocampal gyrus into the parahippocampocingulate gyrus, superiorly, and the parahippocampolingual gyrus, inferiorly, which then continues as the lingual gyrus of the occipital lobe (Fig. 1.11) (Yasargil, 1994, 1996, Yasargil et al., 2005). The parahippocampal gyrus follows the free edge of the tentorium and surrounds the brain stem. Superiorly the parahippocampal gyrus is separated from the dentate gyrus by the hippocampal sulcus. The posterior portion of the parahippocampal gyrus is limited laterally by the collateral sulcus, which indents the floor of the temporal horn of the lateral ventricle (collateral eminence). The anteriormost portion of the parahippocampal gyrus is limited laterally by the rhinal sulcus. The rhinal sulcus is the lateral limit of the entorhinal cortex of the parahippocampal gyrus. It is anterosuperiorly related to the uncus through the uncal notch, which separates the posterosuperior part of the uncus from the parahippocampal gyrus. The parahippocampal gyrus is medially related to the contents of the ambient cistern. The parahippocampal gyrus comprises the subiculum (rounded medial edge), presubiculum, parasubiculum, and the entorhinal cortex (Fig. 1.11) (Wen et al., 1999). The term “uncus” means a hook. It is the name given to the anteriormost portion of the mediobasal temporal region. The inferior anatomical limit of the uncus is the uncal notch. The uncus continues anteriorly without clear delineation as the anterior portion of the parahippocampal gyrus; it continues superiorly to the globus pallidus (basal ganglia), and basally, it is separated laterally from the rest of the temporal lobe by the rhinal sulcus. Finally, the most medial and basal part of the uncus is

17

herniated over the free edge of the tentorium (Figs. 1.7 and 1.11) (Wen et al., 1999). The uncus comprises five small gyri and a small portion of the entorhinal cortex, which occupies the anterior part of the anteromedial surface of the uncus. The actual three-dimensional structure of the uncus is given by the relationship between extraventricular cortical structures and their correlation with intraventricular elements. Familiarity with the intra- and extraventricular elements allows for their optimum surgical management. The uncus is divided into anterior and posterior segments, both divided by a medial prolongation known as the apex. The anterior or anteromedial segment of the uncus belongs to the parahippocampal gyrus and has two small gyri, the semilunar and ambient gyri. The semilunar gyrus occupies the superior portion of this surface and is inferiorly surrounded by the annular sulcus; the ambient gyrus is located medially and inferiorly to the semilunar gyrus. The anteroinferior portion of this anterior surface of the uncus forms the entorhinal cortex and continues anteriorly and inferiorly as the parahippocampal gyrus. The anteromedial surface is related to the proximal portion of the sylvian fissure and the carotid cistern and forms the posterolateral limit of the anterior perforated substance (Figs. 1.7 and 1.12) (Wen et al., 1999, Yasargil, 1994, 1996). The posterior segment of the uncus is closely related to the hippocampus and comprises two surfaces, a posteromedial surface and an inferior surface. The posteromedial surface contains three small gyri, from anterior to posterior: uncinate gyrus, band of Giacomini, and intralimbic gyrus. The superior and inferior surfaces of the posterior segment of the uncus are related to the crural and ambient cisterns, respectively. Posterior and superior to the uncus is the inferior choroidal point, where the anteroinferiormost portion of the choroid plexus of the temporal horn of the lateral ventricle attaches to the

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choroidal fissure. The anterior choroidal artery becomes intraventricular after its cisternal segment at the inferior choroidal point, where the inferior ventricular veins leave the temporal horn to join the basal vein of Rosenthal at the perimesencephalic cisterns. The inferior choroidal point is an important anatomical landmark for microsurgical procedures at this level (Fig. 1.12) (Wen et al., 1999). The inferior surface of the posterior segment of the uncus is located deep inside the uncal notch. So-called external digitations (two or three small lobules separated by sagittal sulci) are found on the anteriormost portion of this surface and go on to form the uncinate gyrus proper. They are extraventricular projections of the hippocampal allocortex that at intraventricular level go on to form the digitations of the head of hippocampus (corresponding to the CA1 sector of the hippocampal formation). The uncinate gyrus is limited posteriorly by the band of Giacomini through the anterior hippocampal sulcus. The band of Giacomini is also known as the tail of the dentate gyrus, which loses its digitations after emerging from the fimbrodentate sulcus to reach the roof of the uncal notch. The intralimbic gyrus contains sectors CA3 and CA4 of hippocampal formation and forms the posteriormost part of the uncus. It is where the fimbria attaches after widening as it approaches the surface of the intralimbic gyrus (Fig. 1.12) (Wen et al., 1999). Superior and anterior views show that the anteromedial portion of the uncus continues superiorly into the globus pallidus through the semilunar gyrus (surface representation of the temporal amygdala). The limen insulae is found lateral and anterior to the semilunar gyrus (Fig. 1.7) (Wen et al., 1999, 2009). The dentate gyrus owes its name to the toothlike aligned elevations that are more evident at the anterior and medial levels of its anatomy. The dentate gyrus continues anteriorly as the band of Giacomini and posteriorly as the fasciolar gyrus (gray matter at the level of the splenium

of the corpus callosum) and indusium griseum (superior surface of the body of the corpus callosum) to finish at the paraterminal gyri. The hippocampus is one of the main intraventricular elements of the mediobasal temporal region. It forms the medial portion of the floor of the temporal horn of the lateral ventricle and is divided into three parts: head, body, and tail. The hippocampal head is the anteriormost and largest part of the hippocampus. It is directed anteriorly and medially and is the only part of the hippocampus not in contact with the choroid plexus. The posterior part of the hippocampal head coincides with the start of the fimbria and the choroidal fissure. It is characterized by three or four digitations that roughly give it the shape of a cat’s paw (Wen et al., 1999, Yasargil et al., 2005). The hippocampal head is directed and located at the level of the posterior segment of the uncus. The hippocampal head is related anteriorly with the uncal recess of the temporal horn. It is related superiorly to the inferoposterior portion of the temporal amygdala, which protrudes at the level of the uncal recess of the temporal horn and is generally in contact with the more medial portion of the hippocampal head (Fig. 1.7). The origin of the choroid plexus, the fimbria, and therefore the choroidal fissure marks the start of the hippocampal body and the end of the head. The hippocampal body has an anteroposterior and inferosuperior orientation in the medial portion of the floor of the temporal horn. The fimbria of the fornix is found in the medial portion of the hippocampal body. The hippocampal body is laterally related to the collateral eminence, an intraventricular indentation of the collateral sulcus. At the level of the hippocampal body, the medial wall of the temporal horn of the lateral ventricle comprises the choroidal fissure adjacent to the ambient cistern (Rhoton, 2002, Wen et al., 1999). The hippocampus changes its direction at the level of the atrium of the lateral ventricle and again takes an orientation that is transversal to the longitudinal axis to form the hippocampal

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LIMBIC LOBE

tail, which finishes macroscopically at the medial portion of the anterior face of the atrium (histologically continuing as the subsplenial gyrus, which covers the inferior surface of the splenium of the corpus callosum) (Yasargil, 1994, Yasargil et al., 2005). The ventricular surface of the hippocampus is covered by the white matter known as the alveus, which becomes more evident along the medial edge of the hippocampus as it goes to form the fimbria. The alveus represents the white subcortical matter of the temporal lobe and comprises the subcortical white matter of the hippocampal allocortex. At the level of the temporal lobe, the fimbria is separated from the dentate gyrus by the fimbrodentate sulcus. The fimbria is the initial portion of the fornix and runs in a posterior direction until it forms the crura of the fornix. Both crura of the fornix are joined by the hippocampal commissure and continue in an anterior direction to form the body of the fornix at the medial level of the floor of the body of the lateral ventricles. At the level of the foramina of Monro, the body of the fornix continues with the columns of the fornix that make up the superior and anterior portions of the foramina before crossing the postcommissural fibers (posterior to the anterior commissure) to reach the mammillary bodies on the floor of the third ventricle. Together with the hippocampus, the amygdala is at the epicenter of the limbic system. The amygdala is currently divided into the temporal or main portion (located at the level of the temporal lobe as previously described) and the extratemporal or extended amygdala (forming part of the more caudal portion of the basal ganglia) (Fig. 1.7). The temporal amygdala comprises nuclei of gray matter classified as basolateral, corticomedial, and central nuclei. The amygdala merges superiorly with the globus pallidus without clear differentiation. It protrudes inferiorly into the uncal recess of the temporal horn of the lateral ventricle, forming

19

part of the roof at this level and pointing toward the hippocampal head. It is medially related to both the anterior and posterior segments of the uncus and in close relation with the semilunar gyrus on the anteromedial surface (Figs. 1.7 and 1.8) (Wen et al., 1999). The entorhinal cortex is a histologic concept that is particularly relevant in neurosurgery, more specifically with regard to epilepsy. It occupies the anterior third of the parahippocampal gyrus and the anteroinferior portion of the anterior segment of the uncus. It is limited laterally by the rhinal sulcus in its anteriormost portion and by the collateral sulcus in its posterior portion. Histologically, it consists of mesocortex (six cell layers) and plays an important role of communication, both as afferent and efferent connections, between the hippocampus and the isocortical association areas in the temporal, parietal, and frontal lobes (Figs. 1.8 and 1.11) (Yasargil et al., 2005). In surgical terms, the mediobasal temporal region is considered to be the anatomical entity that forms the most basal portion of the limbic lobe arc, which is located medial to the collateral and rhinal sulci of the basal surface of the temporal lobe (Fig. 1.11) (Wen et al., 1999). From the neurosurgical perspective, the mediobasal temporal region is divided into three segments: anterior, middle, and posterior. Each segment contains cisternal and ventricular elements that define their surgical particularities. The anterior segment extends posteriorly from the anteriormost end of the rhinal sulcus to the coronal plane that passes through the posterior end of the uncus. The middle segment stretches from the posterior edge of the uncus to the coronal plane that passes through the tectum. The posterior segment runs from the tectum to the coronal plane that passes through the join between the parietooccipital sulcus and the calcarine fissure (calcarine point) (Ribas et al., 2006, Wen et al., 2009). The frontobasal medial region forms part of the limbic lobe from both an anatomical and a

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1. GYRAL AND SULCAL MICROSURGICAL ANATOMY

functional perspective. This region mainly comprises the paraterminal gyrus and paraolfactory gyri. The paraterminal gyrus is found in the portion adjacent to the lamina terminalis, on the medial surface of both hemispheres, and is limited anteriorly by the small and vertical posterior olfactory sulcus. The paraterminal gyrus extends superiorly via induseum griseum and inferiorly toward the olfactory stria. Anterior to the paraterminal gyrus are the paraolfactory gyri (or subcallosal area), which are vertical and separated from each other by the anterior olfactory sulcus. The paraolfactory gyri harbor the septal nuclei, which receive afferent signals from the precomissural fornix and from the hippocampus and subiculum of the parahippocampal gyrus via indusium griseum, and send out efferent signals to the brain stem and hypothalamus (Fig. 1.10) (Yasargil, 1994, 1996, Yasargil et al., 2005). In conclusion, the limbic lobe is morphologically defined as the set of gyri that connects the mediobasal frontal region with the anterior end of the mediobasal temporal region while bordering the diencephalic structures by means of an arc comprising the cingulate gyrus and the parahippocampal gyrus, which can be identified on the medial surface of each of the brain hemispheres (Fig. 1.10). From the functional perspective, the concept of the limbic system covers a series of much wider and more complex, cortical, subcortical, and nuclear structures whose specific circuits and connections are yet to be determined.

DISCUSSION The discipline of microsurgery, as it is today, is the result of the development and improvement of the solid foundations laid down by Prof. M. G. Yasargil in the late 20th century. His pioneering vision of neurosurgery, his revolutionary applications for the operating microscope in

neurosurgery, and the profound anatomical grounding of his knowledge have made him the father of neurosurgical excellence, which is defined by the effectiveness of the technique and safety of the procedure (Yasargil, 1999). The excellent methodology for the study of microsurgical anatomy developed by Prof. Albert L. Rhoton Jr. at the end of the 20th century is considered a milestone in modern neurosurgery and the basic tool for the training of future neurosurgeons in microneurosurgery (Rhoton, 2002). Disciples of both work philosophies, among whom is the outstanding figure of Prof. Evandro de Oliveira, have devoted their careers to the application, development, improvement, and dissemination of the principles set out by their mentors. This work, led by Profs. Alfredo Qui~ ones-Hinojosa and Kaisorn Chaichana, began n with the premise that comprehensive knowledge of microsurgical anatomy of the brain is the solution to the everyday challenges that neurological conditions and their surgical treatments pose. The applied teaching of these fundamental principles is an endeavor to consolidate the theoretical foundations of the international neurosurgery community and to foster the technical excellence of neurosurgical procedures, so that countless numbers of patients will be able to benefit from treatment. In keeping with the surgical philosophy of our mentors, microsurgical neuroanatomy is our main work tool. There should be a comprehensive knowledge of anatomy that is able to integrate all structures from the cranial surface to the innermost regions of the brain. The study of the different craniometric points, exhaustively described by Prof. G. C. Ribas (Ribas, 2010, Ribas et al., 2006), is an excellent example of how a neurosurgeon can systematize the study of anatomy to develop the skill of visualizing the different cerebral sulci and gyri through structures on the surface and deduce the location of structures deep inside the brain. The close relationship between cranial landmarks and the

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REFERENCES

surface of the brain enables neurosurgical approaches to be studied, planned, and accurately executed. The double check provided by modern neuroimaging and neuronavigation techniques offers improvements in the quality and safety of procedures but always on a solid grounding of anatomical knowledge. Knowing the sulci and gyri of the surface of the brain means becoming familiar with what is the neurosurgeon’s “homeland” and “battlefield.” The surface of the brain should be seen as a continuous whole, and its sulci and gyri should not be considered separate elements but ones that are anatomically and functionally interrelated. The large number of cerebral gyri has allowed the surface of the brain to expand during evolution while maintaining the same intracranial volume; expanding the subarachnoidal space in the form of sulci and fissures. The continuity, length, depth, and complexity of the sulci allow a general organization to be defined, that is closely related to the neurological functions. The conditions for decoding this general organization of the brain surface anatomy are key to the custom interpretation of imaging studies for each patient. The more defined the anatomy of a cerebral gyrus and the surrounding sulci, the more constant is its relationship with the different functional areas. The correct interpretation of the excellent imaging techniques that are currently available can only be possible through a solid grounding in anatomy and precise surgical principles. The identification of the anatomical coordinates at which the disorder is located, and their location in relation with functional structures, enable surgeons to plan and execute their procedures under optimum conditions. Only this way is it possible to achieve excellence in the indication, planning, and microsurgical resection of intrinsic brain lesions. Cerebral sulci and gyri are not only elements for spatial and functional location but also are themselves instruments at the service of the microsurgical technique. The understanding, delimitation, and radical resection of glial

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tumors should not be carried out unless it is based on an in-depth knowledge of the cerebral sulci and gyri and of the subcortical white matter(Yasargil, 1999). The dissection of the main fissures allows us to expose anatomically hidden or deep territories. The main sulci of the lateral and basal surfaces of the brain all reach great depths given their orientation toward the ventricular cavities (Fig. 1.4). Thus, transsulcal approaches are important anatomical access routes to reach deep-seated and even ventricular lesions. Sulci are anatomical paths to intrinsic lesions, anatomical barriers to their infiltration, and at the same time define the gyral and subcortical organization of natural routes for disease to spread. The complex interconnection between different cerebral gyri has anatomically conditioned the natural history of different diseases and conditions (Yasargil, 1994, 1996). In conclusion, the comprehensive knowledge of gyral and sulcal microsurgical anatomy of the brain, together with a well-honed microsurgical technique, is the essential requirement to achieve the effective and safe treatment of intrinsic brain lesions.

References Ludwig, E., & Klinger, J. (1956). Atlas cerebri humani. Basel: S. Karger. Ono, M., Kubik, S., & Abernathey, C. D. (1990). Atlas of cerebral sulci. Stuttgart: Thieme. Qui~ nones-Hinojosa, A., Ojemann, S. G., Sanai, N., Dillon, W. P., & Berger, M. S. (2003). Pre-operative correlation of intraoperative cortical mapping with magnetic resonance imaging landmarks to predict localization of the Broca area. Journal of Neurosurgery, 99, 311e318. Rhoton, A. L., Jr. (2002). The cerebrum. Neurosurgery, 51(Suppl. 4), S1eS51. Ribas, G. C. (2010). The cerebral sulci and gyri. Neurosurgical Focus, 28(2), E2. Ribas, G. C., & de Oliveira, E. P. (March 2007). The insula and the central core concept. Arquivos de Neuro-Psiquiatria, 65(1), 92e100. Ribas, G. C., Yasuda, A., Ribas, E. C., Nishikuni, K., & Rodrigues, A. J., Jr. (2006). Surgical anatomy of microneurosurgical sulcal key-points. Neurosurgery, 59(ONS Suppl. 4), ONS177eONS209.

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T€ ure, U., Yasargil, D. H., Al-Mefty, O., & Yasargil, M. G. (1999). Topographic anatomy of the insular region. Journal of Neurosurgery, 90, 730e733. Wen, H. T., Rhoton, A. L., Jr., de Oliveira, E. P., Cardoso, A. C., Tedeschi, H., Baccanelli, M., & Marino, R., Jr. (1999). Microsurgical anatomy of the temporal lobe: Part I: Mesial temporal lobe anatomy and its vascular relationships and applied to amygdalohippocampectomy. Neurosurgery, 45, 549e592. Wen, H. T., Rhoton, A. L., Jr., de Oliveira, E., Castro, L. H., Figueiredo, E. G., & Teixeira, M. J. (2009). Microsurgical

anatomy of the temporal lobe: Part 2-sylvian fissure region and its clinical application. Neurosurgery, 65, 1e35. Yasargil, M. G. (1994). Microneurosurgery (vol. IVa). Stuttgart: Georg Thieme. Yasargil, M. G. (1996). Microneurosurgery (vol. IVb). Stuttgart: Georg Thieme. Yasargil, M. G. (1999). A legacy of microneurosurgery: Memoirs, lessons, and axioms. Neurosurgery, 45, 1025e1092. Yasargil, M. G., T€ ure, U., & Yasargil, D. C. (2005). Surgical anatomy of supratentorial midline lesions. Neurosurgical Focus, 18(6B), E1.

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