Fundamentals of Neurosurgery: A Guide for Clinicians and Medical Students

The aim of this book is to provide clinicians and medical students with basic knowledge of the most common neurosurgical disorders. There is a vast array of signs and symptoms that every clinician should recognize as neurosurgical affectations, allowing them to identify when to refer the patient to a neurosurgeon. In this text, the editors intend to bridge the gap between clinical medicine and neurosurgery, making neurosurgical practice understandable to a wider medical public.

The book provides a smooth transition from neuroanatomy, neurophysiology and neurological examination to neurosurgery, focusing more on the knowledge underlying neurosurgical practice rather than on surgical technique. The core of the book is composed of chapters discussing each of the most important medical conditions that deserve neurosurgical intervention, providing key information on diagnosis, clinical aspects, disease management, surgical procedures and prognosis. Moreover, complementary discussion of the frontiers and advances in neurosurgery are also covered.

In this sense, this book has two main goals and intended audiences. First, and primarily, it is intended for clinicians in a wide array of non-surgical medical specialties (such as general practitioners, neurologists, pediatricians, oncologists and others) aiming to give an overview on important characteristics and initial management of the most prevalent disorders treated by neurosurgeons. Second, and to a lesser degree, it is intended to be used as a practical guide for medical students who are initiating their study in neurosurgical sciences. Fundamentals of Neurosurgery – A Guide for Clinicians and Medical Students intends to be a comprehensive guide for all non-neurosurgeons who want to broaden their knowledge of neurosurgery. 

• Language: English
• Publisher: Springer
• Release date: Jul 12, 2019
• ISBN: 9783030176495

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© Springer Nature Switzerland AG 2019

Andrei Fernandes Joaquim, Enrico Ghizoni, Helder Tedeschi and Mauro Augusto Tostes Ferreira (eds.)Fundamentals of Neurosurgeryhttps://doi.org/10.1007/978-3-030-17649-5_1

1. Neuroanatomy Applied to Clinical Practice

Mauro A. T. Ferreira¹  

(1)

Department of Anatomy and Radiology, University Hospital, Belo Horizonte, Minas Gerais, Brazil

Mauro A. T. Ferreira

Keywords

Microsurgical anatomyNeurosurgeryBrain anatomyNeuroanatomy

Introduction

Unlike other human organs such as the lung, the liver, the pancreas, and so on, where morphological and functional units tend to repeat themselves regardless of its location on a given organ, the brain is composed of multiple, different, complex, and interconnected neural systems and pathways, each one located in different areas of the brain. This is important because lesions occurring on any given area of the brain may cause different neurological deficits, allowing, thus, a more or less precise topographical diagnosis. The central nervous system (CNS) is composed of a very complex network of neurons, with numerous connections, some of those remaining unknown to this date. The cutting edge of this knowledge is knowing how exactly each neuron communicates with other neurons in the brain (connectome), but this is out of the scope of this chapter. The so-called Brodmann areas (after German anatomist Korbinian Brodmann, 1868–1918) are cortical areas with specific cortical cytoarchitectural features of neurons, as well as cell laminar organization, which differs from one area to the other, as they have been studied using Nissl method of cell staining. The brain has been mapped into 52 different areas, according to his original work, published in 1909. Although a matter of debate, the vast majority of neuroanatomy textbooks mention Brodmann areas as cortical functional reference areas. In theory, the different Brodmann areas represent different cortical areas responsible for different brain functions. Brodmann areas don’t apply to the cerebellum. The main Brodmann areas are presented herein (Fig. 1.1).

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Fig. 1.1

Brodmann’ areas ; this picture depicts the different cortical areas involving information processing of the brain. Each one has its function and is composed of different circuitries inside the central nervous system (http://​thebrain.​mcgill.​ca/​flash/​capsules/​outil_​jaune05.​html) (internet picture)

The knowledge of all Brodmann areas is absolutely unnecessary. Besides establishing these cortical areas, Brodmann studied the comparative anatomy of 52 different animal species.

The macroscopic and the surgical microscopic anatomies of the brain have been well studied by several authors, but a monumental work, put together by Dr. Albert L. Rhoton Jr. and his fellows, over a 40-year period at the University of Florida, FL, is worth mentioning, and its reading is indicated for those interested in studying this subject in further detail. This work includes the white matter fiber dissection techniques, as proposed by Klingler in 1902, where specific and individual neural systems are dissected, and its anatomical relationships with other systems are presented. This is key to understanding and planning surgical approaches in order to avoid damage. The so-called tractography, or diffusion tensor imaging (DTI) technique, available to most neurosurgeons, is a magnetic resonance imaging (MRI) technique that shows these pathways, in both normal and pathological situations. This is particularly important when pathological conditions like tumors, arteriovenous malformations, or any neurological condition that may require surgery, is located close to eloquent brain areas. The DTI-MRI may show displacement or involvement of such fibers by the disease, allowing, thus, a safer surgical planning and execution.

Objective

This chapter intends to present the basic neuroanatomy and the location of the so-called eloquent areas. Eloquent areas are responsible for the perception of vision, primary motor and sensory functions, speech, some association areas of the brain, and the dominant side of memory. Lesion to these areas may cause gross neurological deficits, and any given disfunction of these systems should prompt immediate action to elucidate any possible aggression to the brain. After the basic neuroanatomy is presented, a brief comment on the clinical picture of dysfunction of the different regions is presented. One key phenomenon concerning the brain hemispheres, almost as a rule, is the fact that lesion to one hemisphere causes deficit to the opposite side of the body. This is different from dominance, when a given area, like speech and language processing, causes a global deficit.

The Central Nervous System

The central nervous system (CNS) comprises all neural tissue inside the cranial cavity and the vertebral canal. The encephalon locates, and it is completely encased by the cranium, and the spinal cord extends down to the level of the first lumbar vertebrae in adults. The spinal cord is also involved completely by the bony vertebral canal. The cranial nerves have its origins inside the cranial cavity, and the peripheral nerves are, for the most part, extensions of spinal cord neurons that convey impulses that arrive at (afferent) or leave the central nervous system (efferent). Afferent and efferent stimuli are processed in the CNS, at various levels, and involve a variable number of neurons. They may become conscious or unconscious. There are no ganglia at the CNS, and, by definition, a group of neurons (gray matter) inside the CNS is called a nucleus. In the spinal cord and at the brainstem, the gray matter is located inside the white matter, while in the cerebellum and the brain, the gray matter (neurons) is located outside the white matter, the so-called cerebellar and cerebral cortex, respectively.

As a simplification, the encephalon comprises the brain, the midbrain (thalamus and hypothalamus), the cerebellum (or small brain), the brainstem (midbrain, pons, and the medulla), the ventricular system, and the cranial nerves, intracranial arteries, and veins. For its importance, it is completely involved by bone. The cranium is divided into two parts: the neurocranium (comprises the bones related to the encephalon) and the viscerocranium (comprises all other cranial bones including the facial bones or facial skeleton).

All the encephalon and spinal cord (thus, the CNS) are covered by three membranes: (1) the dura mater , located and attached to the inner surface of the neurocranium; (2) the arachnoid membrane , which lies between the dura mater and the pia mater. It has a trabeculated aspect; it contains the cerebrospinal fluid (CSF) that circulates around the CNS, being produced at the choroid plexus of the ventricles, and it is absorbed by the arachnoid villi, along the superior sagittal sinus (Pacchioni villi). The intracranial arteries run in the subarachnoid space; (3) pia mater : a very thin membrane underneath the arachnoid and adherent to the surface of the CNS. It shapes the gyri and the sulci. The cerebral veins run attached to the pia mater, although they may cross the surface of two of more gyri. So, as for intracranial vessels, arteries and veins are located inside different anatomical compartments, or spaces, the subarachnoid space and the pia mater, respectively.

The brain is the largest part of the central nervous system (CNS). It is comprised of the right and the left hemispheres, including the cerebral cortex, the underlying white matter, the deep white matter, the basal ganglia, and the ventricular system. The corpus callosum unites the right and the left hemispheres. The brain, the midbrain, the lateral and third ventricles, and the first and second cranial nerves (optic and olfactory nerves, respectively) are located in the supratentorial space, above the tentorium, and lie in relationship with the cranial vault. The cerebellum, the brainstem, the cranial nerves III to XII, the mesencephalic aqueduct, and the fourth ventricles are located in the infratentorial space, a much smaller space than the supratentorial space, and lie in close relationship with the posterior aspect of the temporal and occipital bones. These structures are mentioned to belong to the posterior cranial fossa. Its clinical importance relies on the fact that the infratentorial space, or the posterior cranial fossa, is more vulnerable to mass effect lesions, and if it occurs, the brainstem may be damaged, eventually leading the patient to death. The encephalon communicates with the cord at the most inferior aspect of the occipital bone, where a large bony opening, the foramen magnum, divides the inferior part of the medulla oblongata from the uppermost aspect of the cervical cord.

Morphological Aspects of the Encephalon or Telencephalon

The Brain

The cerebral hemispheres will be referred to as the brain. The brain has six lobes according to Ribas. This division is in accordance with the Terminologia Anatomica , as established in 1989. It is divided into frontal, temporal, parietal, occipital, insular, and limbic lobes. For the sake of simplification, the central lobe will be included in the convexity of the frontal and parietal lobes, respectively. The reader has referred to the work of Ribas and Frigeri who have described the anatomy of the central lobe, but this is rather more of a functional division of the brain than a morphological one. For the same reason, the limbic lobe will be included in the other lobes, since the limbic lobe correlates with the limbic system; however, the limbic system extends far beyond the limits of the limbic lobe. The limbic system is a complex matter that deserves special attention elsewhere.

When looking at the lateral surface of the brain, one can observe the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. The recognition of the two most important morphological accidents, the identification of the lateral sulcus and the central sulcus, is key to understanding how to identify the other sulci and gyri. The lateral sulcus is present in the embryo at week 17, and the central sulcus is present at week 21. They are the earliest sulci that become apparent in the lateral surface of the brain, and they induce the formation of the adjacent sulci and gyri. The lateral sulcus runs anteroposterior and separates the frontal and temporal lobes, above it and below it, respectively. Furthermore, the lateral sulcus is always continuous. The direction of the sulci and the gyri in the frontal and the temporal lobe is grossly horizontal, from anterior to posterior. For a complete study of the human brain’s sulci patterns, we refer the reader to the book by Ono, Kubik, and Abernathey. The intraoperative recognition of all brain sulci and gyri is impossible, but their main morphology patterns are mandatory.

The Frontal Lobe

Lateral surface: the frontal lobe has three gyri and two sulci that tend to run horizontally in an antero-posterior direction. There is, however, a change in the direction of the gyri and sulci as we proceed posteriorly. They tend to be more vertically oriented (Fig. 1.2). Indeed, the superior frontal gyrus ends at a point where it causes an impression on the precentral sulcus, causing a posterior curve on the precentral sulcus and the precentral gyrus. The middle frontal sulcus always communicates (via a bridging gyri) with the precentral gyrus, thus interrupting the precentral gyrus. By definition, the precentral sulcus is interrupted in 100% of times (arrowhead Fig. 1.2). This is an important information because the precentral sulcus, the precentral gyrus, the central sulcus, the postcentral gyrus, the postcentral sulcus, and the postcentral gyrus have a somewhat similar morphology. Between the precentral sulcus and the central sulcus lies the precentral gyrus, an eloquent area that is responsible for contralateral body movement (initiation of conscious movement; Brodmann area 4). Although the morphology of the precentral sulcus and the central sulcus is similar, they can be safely differentiated by the following tips:

1.

The precentral sulcus is interrupted in 100% of the cases.

2.

The central sulcus is continuous in almost 100% of the cases (92%).

3.

The central sulcus almost never reaches the lateral sulcus inferiorly, and it ends at an inferior frontoparietal gyrus (Rolando’s).

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Fig. 1.2

Lateral surface of the brain. The frontal lobe is limited by the central sulcus (yellow paper), by the superomedial margin of the brain, by the anterior infero-lateral margin (black arch), and by the lateral sulcus. Notice the horizontal direction of the frontal and the temporal sulci and gyri. The middle frontal sulcus always interrupts the precentral gyrus. Note: F1 – superior frontal gyrus; 1 – superior frontal sulcus; F2 – middle frontal gyrus; 2 – inferior frontal sulcus; F3 – inferior frontal gyrus; T1 – superior temporal gyrus; 3 – superior temporal sulcus; T2 – middle temporal gyrus; 4 – inferior temporal sulcus; T3 – inferior temporal gyrus; ../images/433792_1_En_1_Chapter/433792_1_En_1_Figa_HTML.gif : communication between middle frontal; gyrus and precentral gyrus; ★: precentral gyrus; Yellow paper: central sulcus; ●: postcentral sulcus

The base of the frontal lobe corresponds to an area that overlies the optic plates of the frontal bones. It has a superiorly directed convexity, and it has two sulci and five gyri. The orbital sulcus is an H-shaped sulcus and limits four gyri: the anterior orbital gyrus, the posterior orbital gyrus, the lateral orbital gyrus, and the medial orbital gyrus. The gyrus rectus locates between the olfactory sulci (covered by the olfactory tract) (Fig. 1.3). On the medial surface: the frontal lobe has its representation at the medial surface of the brain as well. It extends from the gyrus rectus, in the floor or the interhemispheric fissure, to an area below the rostrum of the corpus callosum, anteriorly and superiorly toward the superomedial border, and then, posteriorly, outside the cingulate gyrus (Fig. 1.4).

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Fig. 1.3

The basal view of the right frontal lobe: the base of the frontal lobe sits on the orbital plate of the frontal lobe. It has the frontal sulcus with an H morphology. It limits the lateral frontal gyrus, the medial frontal gyrus, the anterior frontal gyrus, and the posterior frontal gyrus. The olfactory sulcus divides the medial frontal sulcus from the gyrus rectus. Its view is partially obstructed by the olfactory tract. ●: orbital sulcus; LOG: lateral orbital girus; MOS: medial orbital gyrus; AOS: anterior orbital gyrus; POS: posterior orbital gyrus; T, olfactory tract; GR, gyrus rectus; TP, temporal pole

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Fig. 1.4

The medial aspect of the hemisphere is exposed. The corpus callosum has an inverted c shape, as does the sulcus of the corpus callosum, the cingulate gyrus, and the cingulate sulcus. Even the superior frontal sulcus presents a similar display. Notice that the marginal ramus of the cingulate gyrus describes a small notch pointing forward to the central sulcus, highlighted by the yellow markings. The paracentral lobule lies within the paracentral and the marginal ramus of the cingulate gyrus, related to the splenium and the body of the corpus callosum, respectively. CC corpus callosum: 1 – rostrum, 2 – genu, 3 – body, 4 – splenium; ● – sulcus of the corpus callosum; CG – cingulate gyrus; ★ – cingulate sulcus; arrowheads – paracentral ramus of the cingulate sulcus; green paper – marginal ramus of cingulate gyrus; SFS – superior frontal sulcus

The frontal lobe is the largest lobe, both from the lateral and the medial surface of the brain. The superior frontal gyrus extends medially, in the superomedial border, and will remain the superior frontal gyrus in the medial surface of the brain. On the medial aspect of the frontal lobes, a central lobe is present, and it encompasses the sensory and motor areas (Brodmann areas 4 and 5 and 1, 2, and 3). This lobe is located between two rami of the cingulate gyrus. The corpus callosum, the floor of the interhemispheric fissure, has an inverted C-shaped morphology that determines the morphology of the sulci and the gyri on the medial surface. The corpus callosum has a rostrum, a knee, a body, and a splenium. The cingulate gyrus encircles the corpus callosum, and it is located between the callosal sulcus and the cingulate sulcus. The callosal sulcus is continuous in 100% of the times. The cingulate gyrus branches off into various rami, two of which must be identified. The first is the marginal ramus of the cingulate gyrus, which reaches the superomedial border of the hemisphere and causes an impression directed anteriorly. Its importance relies on the fact that it points to the central sulcus located anterior to its impression on the superomedial margin of the hemisphere. To identify with certainty, the ending of this sulcus is in line with the splenium of the corpus callosum. On the other hand, the paracentral sulcus arises in the midportion of the body of the corpus callosum. The central lobule on the medial surface contains motor and sensorial cortex, and it is limited anteriorly by the paracentral sulcus, posteriorly by the marginal ramus of the cingulate sulcus, and inferiorly by the cingulate sulcus (Fig. 1.4). The cingulate gyrus ends as the isthmus of the cingulum, just below the splenium of the corpus callosum. Posterior to the isthmus, the anterior portion of the parieto-occipital sulcus may be identified, and as an anterior continuation of the cingulate belt, the parahippocampal gyrus arises.

The anterior most aspect of the lateral ventricles, the frontal horns, and the anterior part of the body of the lateral ventricles lies deep in relation to the frontal lobe, and it is separated from its counterpart by the septum pellucidum.

The frontal lobe is extremely important for planning and executing contralateral movements. It is, indeed, considered a motor lobe, since it can be divided into a primary motor area (the precentral gyrus), the premotor area (the cortex located immediately anterior to the primary motor area), and the supplementary motor area (cortical areas located in the medial surface of the hemisphere, immediately anterior to the primary motor cortex). The conscious movement is initiated at the motor cortex, but they have to be planned. The premotor area (Brodmann 6) is responsible for planning and executing movements, especially its sequence. The prefrontal area (Brodmann 6, 8 and 9–12, 32, 45, 47) is located in the medial surface of the hemisphere, and it is an important association area, and one of its functions is the so-called executive function, which means the ability to practice and monitor a series of actions in order to achieve a certain goal. It is related to planning and organization, as well as motivation. It is also related to personality flexibility, problem solving and rewarding. Another important function of the prefrontal area refers to the frontal ocular fields, located in the middle frontal gyrus (area 6). It controls movement of the eyes toward the contralateral side. Extensive lesions to this area cause the eyes to deviate to the same side of the lesion. Epilepsy causes the opposite effect.

The anterior portion of the inferior frontal gyrus on the left side is also particularly important (mainly the pars triangularis and the pars opercularis), since it represents the cortical area responsible for uttering words (the so-called Broca’s area; areas 44 and 45) (Fig. 1.5). Almost as a rule, with a few exceptions, the left hemisphere is responsible for most language and communication processes, both for right-handed individuals and for left-handed ones. Rarely, lesion to the right anterior inferior frontal gyrus may cause dysphasia. Extensive lesions to the frontal lobes may cause major behavioral changes. It may cause humor instability, compulsive crying, laughter, or fury. There may be irritability and euphoria, along with lack of initiative and spontaneity. There is also apathy, indifference, and decreased spontaneous speech and social interaction.

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Fig. 1.5

Lateral view of the left hemisphere . The lateral or Sylvian fissure has three main rami; a posterior, which usually ends in a bifurcation; a superior; and an anterior. The superior and the anterior rami divide the inferior frontal gyrus into an orbital part, a triangular part, and an opercular part. The triangular and the opercular portions are responsible for the expression of words and naming objects, the so-called Broca’s area, on the left side. The center for perceiving language is located in the left posterior temporal lobe, on the left side (Wernicke’s area). Red sphere – stem of the lateral fissure; Yellow line – superior ramus of lateral fissure; White line – anterior ramus of lateral fissure; Dotted red line – bifurcation of posterior ramus of lateral fissure; Black circle – Broca’s area; Green line – Wernicke’s area; 1 – pars orbitalis; 2 – pars triangularis; 4 – pars opercularis; 41 and 42 – primary auditory cortex

The Temporal Lobe

Lateral surface: the temporal lobe has two sulci and three gyri (Fig. 1.3). Its tip dives into the temporal fossa on the middle fossa floor. The superior temporal gyrus lies between the lateral sulcus and the superior temporal sulcus. The inferior temporal gyrus lies below the inferior temporal sulcus, and it turns medially to continue as the inferior temporal sulcus in the basal surface of the brain. These gyri and sulci extend posteriorly and it limits posteriorly to the lateral temporoparietal line, an artificial line drawn from the preoccipital notch, in the infero-lateral border of the hemisphere, to the impression of the parieto-occipital sulcus (well seen in the medial surface) in the superomedial border of the hemisphere.

Basal surface: the temporal lobe has a large representation on the basal surface of the brain. When viewed from below, the inferior temporal gyrus is the most lateral structure, until it meets the occipitotemporal sulcus. The occipitotemporal gyrus lies between the occipitotemporal sulcus and the collateral sulcus (this gyrus is also known as the fusiform gyrus). Medial to the collateral sulcus, the parahippocampal gyrus is found, and it folds over itself anteriorly to form the uncus and extends posteriorly until the isthmus of the cingulate gyrus, below the splenium of the corpus callosum (Fig. 1.6). All these sulci and gyri are separated from the occipital lobe by the basal temporoparietal line that unites the preoccipital notch to the impression of the parieto-occipital sulcus as seen from an inferior perspective.

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Fig. 1.6

The basal and medial temporal lobes are illustrated. At the base of the brain, the temporal lobe has the major representation among all lobes. Notice that the parietal lobe doesn’t have any representation. The sulci and gyri are disposed as follows, from lateral to medial: Inferior temporal gyrus, Occipitotemporal sulcus, Occipitotemporal or fusiform gyrus, Collateral sulcus, Uncus (triangular shape), Parahippocampal gyrus

Medial surface: the temporal lobe is represented by the parahippocampal gyrus and the uncus anteriorly. The uncus has a triangular shape with its apex directed medially, toward the anterior part of the midbrain (Fig. 1.6). The hippocampus lies in the uncus, and the amygdaloid nucleus is located just above the hippocampus. The uncus separates itself from the lateral and basal temporal lobe by the rhinal sulcus. An anterior, inferior, and lateral extension of the ventricular system, the temporal horn, lies within the temporal lobe.

The temporal lobe has important wholes in an array of mental functions. The primary auditory cortex (areas 41 and 42) is located in the superior temporal lobe, precisely at the anterior temporal transverse gyrus (Heschl’s) (Fig. 1.6). Although hearing has bilateral representation, some degree of differentiation and sound discrimination may be observed in unilateral lesions. Stimulus to peri-cortical areas may elicit hearing, visual, and olfactory hallucinations. Particularly important for musicians, temporal lobe lesions may affect the ability to perceive music pace and rhythm. The temporal lobe has a cavity that is an anterior extension of the atrium of the lateral ventricles. Surrounding the anterior part of the temporal horn, the Meyer’s loop (a bundle of visual fibers from the corpus callosum that turns anterolateral, anterior to the ventricular cavity, toward the lateral wall of the temporal horn; it arises at the lateral geniculate body in route to the calcarine sulcus) is found, and limiting the temporal horn laterally lies the inferior fibers of the geniculocalcarine tract. Lesion to these fibers may cause a visual deficit located in the contralateral superior external visual field. The hippocampus is also located at the uncus of the temporal lobe and is responsible for learning and memory (areas 26, 27, and 28). Acute lesion to the dominant hippocampus may cause severe memory deficits, including retention of any new information and loss of acquired memory. Memento, 2000, starring Guy Pearce, is an interesting and striking movie depicting the clinical picture of anterograde memory loss.

The posterior and superior temporal lobe bears the cortical area that interprets words, especially on the left side, and whose lesion may cause a severe neurological deficit called sensory dysphasia (areas 42 and 22). The patient is unable to discriminate words, as if he hears a completely different language, with its consequent lack of meaning and understanding of spoken words (Wernicke’s area) (Fig. 1.5).

One of the most impressive experimental findings ever, as far as behavior is concerned, the Klüver-Bucy syndrome , has been found in monkeys submitted to bilateral temporal lobe resection. Its full developed symptoms and signs are rare in humans. The syndrome is characterized by apathy, hypersexuality, tendency to recognize objects by putting them in the mouth, memory loss, bulimia, and visual agnosia. Interestingly, Hreniuc reported on a case of a 71-year-old man who had an ischemic stroke on the right side and developed a Klüver-Bucy-like syndrome. The amygdala, the hippocampus, and the medial temporal lobe play an important whole in the limbic system, thus in behavior and emotions. Neurobehavioral research is underway to try to elucidate the limbic system circuitry and its clinical manifestations of how emotions and behavior manifest in a normal and abnormal range of mental status.

The Parietal Lobe

The parietal lobe is located posterior to the frontal lobe, superior to the temporal lobe, and anterior to the occipital lobe. Its limits are superiorly the superomedial border of the hemisphere, anteriorly the central sulcus, inferiorly the temporo-occipital line, and posteriorly the lateral temporoparietal line that limits the occipital lobe located posteriorly (Fig. 1.7). The parietal lobe has the following features:

It has the primary sensory cortex (the postcentral gyrus, areas 1, 2, and 3), where various sensory stimuli are processed and become conscious.

It is divided into two parietal lobules, superior and inferior, respectively, in relation to the intraparietal sulcus. The inferior intraparietal sulcus encompasses the supramarginal and the angular gyri. They correspond externally to the most prominent part of the parietal bone: the parietal prominence. Both the superior and the inferior parietal lobules are association cortical areas.

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Fig. 1.7

The parietal lobe: the parietal lobe is located above the temporal lobe and anterior to the occipital lobe (dashed black lines). It is divided into a superior and inferior parietal lobules , by the intraparietal sulcus, an arched sulcus (arched curved black line) that tend to meet the postcentral sulcus (straight red line). The inferior parietal lobule is an eloquent area and is formed by two gyri. The first, the supramarginal gyrus (rounded red dashed area, approximately), is located by identifying and following the superior temporal sulcus (arrows). The angular gyrus is located by following the superior temporal sulcus that tends to bifurcate or trifurcate. The angular gyrus (rounded dashed yellow area, approximately) has an inverted m shape or a ʊ shape. SupmG – supramarginal gyrus; AG – angular gyrus

The postcentral gyrus is limited anteriorly by the central sulcus and posteriorly by the postcentral sulcus. The intraparietal sulcus initiates at the intraparietal point, located 7.0 cm anterior to the lambda and 5.0 cm off the midline, usually at the midpoint of the postcentral sulcus, and may be continuous posteriorly as the intraparietal sulcus, or it may have a separate origin. The supramarginal gyrus (area 40) and the angular gyrus (area 41) may have different morphologies, but its identification is relatively simple. The supramarginal gyrus is a natural continuation of the superior temporal gyrus. It encircles the posterior part of the lateral fissure that usually ends in a superior and an inferior ramus, the first being more prominent, thus, the name supramarginal or the gyrus that follows the ending, especially superiorly, of the lateral sulcus. The angular gyrus is found by following the superior temporal sulcus. It ends at the inferior parietal lobule as a bifurcation or as a trifurcation. Thus, the angular gyrus has an omega shape, or an M-letter shape, clearly different from the supramarginal gyrus (Fig. 1.7). The intraparietal sulcus is arched superiorly, and posteriorly, it usually ends at the occipital lobe. The inferior parietal lobule is usually larger than the superior (Fig. 1.7).

There is no representation of the parietal lobe at the base of the brain.

On the medial surface of the brain, the parietal lobe is represented mainly by a quadrangular lobe called the precuneus . Its superior limit is the superomedial margin of the hemisphere; anteriorly, by the marginal ramus of the cingulate gyrus; posteriorly by the parieto-occipital sulcus; and inferiorly, by the cingulate gyrus. It usually has a sulcus, the subparietal sulcus, an H-like sulcus that does not divide the precuneus into other sulci and gyri. Just anterior to the marginal ramus of the cingulate sulcus, an impression of the central sulcus directed posteriorly is seen. Between these sulci, the postcentral gyrus is located (Fig. 1.8).

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Fig. 1.8

Representation of the parietal lobe in the medial brain surface (precuneus). The precuneus is limited by the marginal ramus of the cingulate gyrus anteriorly, the cingulate sulcus inferiorly, and the parieto-occipital sulcus posteriorly. Its superior limit is the superior-medial border of the hemisphere. Just posterior and inferior to the precuneus lies the cuneus, an edged shaped region of the medial part of the occipital lobe. Its main structure is the calcarine sulcus, where the primary visual cortex is located (Brodmann 17)

The parietal lobes are primarily association lobes. They locate close to occipital and the posterior temporal lobe. They also receive important input from the thalamus. The main functions of the parietal lobes are sensory reception, correlation, analysis, synthesis, integration, interpretation, and elaboration of primary sensory impulses from the thalamus. The clinical compromise to the left parietal lobule may cause the so-called Gerstmann’s syndrome: (1) acalculia; (2) agraphia; (3) digital agnosia, or the difficulty to name the fingers of the hand; and (4) disorientations to right and left sides. On the right hemisphere, an inferior parietal lobule lesion may cause various forms and intensities of apraxia, hemineglect, and anosognosia, or failure to recognize deficits or limiting condition. Usually, patients do not recognize contralateral paralysis. This condition may be accompanied by asomatognosia , a form of hemineglect in which patients deny ownership to their limbs.

The Occipital Lobes

The occipital lobes are located posterior to the parietal and the temporal lobes. They have a somewhat pyramidal shape with its apex forming the occipital pole. The occipital lobe is basically related to vision. We herein present the different surfaces of the occipital lobe with emphasis on its medial surface, where the visual area is located (Fig. 1.8). The nuances of the occipital lobe sulci and gyri in the convexity are well discussed elsewhere.

On the