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Thirteen The Human Nervous slstem: An Anetomical WewoinL Sixth Edition, M\ftay L. Bffi and John A, Kiemil. J.B. Lippincort Company, Philadelphia, @ 1993. 1 t ( ( I l I The large surface area of the human cerebral cortex results in a pattern of gyri and sulci. Some of these convolutions are important anatomical landmarks or functional areas. Five lobes (including the insula) are recognized in each cerebral hemisphere. On the medial surface of the hemisphere, the parieto-occipital sulcus separates the parietal from the occipital lobe. In the occipital lobe, the calcarine sulcus is the site of the primary visual conex. In the parietal lobe, the postcentral gyrus corresponds to the first general sensory area. The supramarginal and angular gyri are parts of the receptive language area, which e>rtends onto the superior temporal gyrus. The central sulcus is between the parietal and frontal lobes, separating the first somesthetic from the primary motor area. ln the frontal lobe, the precentral gyrus corresponds to the primary motor area. The olfactory bulb and tract are applied to the orbital surface of the frontal lobe. The lateral sulcus (syMan fissure) separates the frontal and parietal lobes from the temporal lobe. The insular lobe (insula), in the floor of the lateral sulcus, is a landmark for part of the corpus striatum. The superior surface of the superior temporal gyrus includes the primary auditory area. The parahippocampal gyrus includes the uncus (a primary olfactory area) and the entorhinal area, which has olfactory and limbic functions. The "limbic lobe" includes the parahippocampal and cingulate gyri. It is part of the limbic system, which is involved in memory. 222 ,oinl, J.B. Chapter 13: Topography of the Cerebral Hemispheres tional importance, whereas others have no known significance. They are described and illustrated in the later sections of this chapter. The complicated folding of the surface of the cerebral hemispheres substantially increases the surface area and, therefore, the volume of cerebral cortex. The folds or convolutions are called gyti, and the intervening grooves are called sulci, About two-thirds of the cortex forms the wplls of the sulci and is, therefore, hiddenfrom surface view. Although some gyri are constant features of the cerebral surface, Major Sulci The lateral and parieto-occipital sulci appear early in fetal development and are especially deep in the mature brain. These, together with the central and calcarine sulci, are the boundaries for division of the cerebral hemisphere into the frontal, parietal, temporal, and occipital lobes (Figs. 13-f and 13-2). The lateral sulcus (fissure of Sylvius or sylvian fissure) begins as a deep furrow on the inferior surface of the hemisphere. This is the stem of the sulcus, which extends Iaterally between the frontal and temporal lobes and divides into three rami on reaching the lateral surface. The posterior ramus is the main part of the sulcus on the lateral surface of the hemi- others vary from one brain to another and even between the two hemispheres of the same brain. Subtler depressions in the cerebral cortex are grooves and notches unrelated to the pattern of gyri and sulci. They are made by extracerebral structures such as the bones of the skull and the venous sinuses of the dura mater. A sulcus is a groove on the surface of a cergbral hemisphere, whereas a fissure is a cleft that separates large components of the brain. Despite the different definitions of sulci and fissures, the two terms often are used interchangeably for the deepest sulci. At an early stage in studying human neuro- sphere, whereas the anterior and ascending rami project for only a short distance into the frontal lobe. Al area of cortex called the insular lobe or insula (island of ReiI) lies at the anatlmy, the student should be able to delineate the llbes of the cerebral hemispheres and to recognize the major sulci, fissures, and gyri, which com' bottom of the lateral sulcus and is hidden from monly are referced to as landmarks. Of the smaller sulci and gyri, some are of great func- sur{ace view. This cortex appears to have been bound to the underlying corpus striatum dur- Central su lcus /-{, Figure l3-1. Lobes of the cerebral hemisphere (lateral surface). 223 224 Regional Anatomy of the Central Neruous System Central su I cus Parieto-occipital sulcus Solenium \ nortrur I I "":,\ Iobe of corpus Figs callosu m 'l in fr later surfi paft post tral I are ( sign their Stem of lateral sulcus Calcarine sulcus Preoccioital notch rior end of the corpus callosum and follows an arched course to the occipital pole. In some brains, the sulcus continues over the pole for a short distance on the lateral surface. The calcarine sulcus is an important landmark for the visual cortex, most of which lies in the walls of the sulcus. The parieto-occipital sulcus extends from the calcarine sulcus to the superior border of the hemisphere, which it intersects about 4 cm from the occipital pole, The longitudinal and transverse cerebral fissures are external to the hemispheres and the I fron 1 lobr later men1 growth of the sunounding cortex would then produce the deep lateral sulcus. The central sulcus (sulcus of Rolando; rolandic sulcus) is an important Iandmark for the sensorimotor cortex because the general sensory area is immediately behind the sulcus and.the motor area is immediately in front of it,,The central sulcus indents the superior border of the hemisphere about I cm behind the face of the hemisphere begins under the poste- hemisphere (medial and inferior surfaces). ing late embryonic and early fetal develop- midpoint between the frontal and occipital poles. The sulcus slopes downward and. forward at an angle of 70o to the vertical, stopping just short of the lateral sulcus, and there usually are two bends along its course. The central sulcus is about 2 cm deep, and its walls, therefore, constitute much of the sensorimotor cortex, The calcarine sulcus on the medial sur- Figure l3-2. Lobes of the cerebral . are, therefore, in a different category from the two foregoing surface markings. The longitudinal cerebral fissure separates the hemi- the spheres, and a dural partition called the falx cerebri extends into the lissure. The corous callosum, which constitutes the main cerebral commissure, crosses from one hemisphere to the other at the bottom of the longitudinal midr fissure, The transverse cerebral fissure intervenes between the cerebral hemispheres above and the cerebellum, midbrain, and di- encephalon below. The posterior part of this fissure is between the cerebral hemispheres and the'cerebellum; it contains a dural partition known as the tentorium cerebelli. The anterior part of the transverse fissure intervenes between the corpus callosum and the diencephalon. It is triangular in outline, tapering anteriorly, and contains the tela choroidea, which consists of vascular connective tissue derived from the pia mater that covers the brain. The tela choroidea is continuous with the connective tissue core of the choroid plexuses of the lateral ventricles and the third ventricle, and the plexuses are completed by choroid epithelium derived from the ependymal lining of the ventricles. tr tal n sulcr spicr dent porti surfa fronl and T eral r prevl temtr from the notcl the it is setr desct parie the < post( occip T sure trun Cerebral bers the n Each cerebral hemisphere has lateral, medial, and inferior surfaces on which the extent of the sum Hemispheres , stitut the t Chapter 13: Topography of the Cerebral Hemispheres lobes of the hemisphere are now defined (see Figs. l3-r and 13-2). frontal lobe occupies the entire area in front of the central sulcus and above the The lateral sulcus onthe lateral surface. The medial surface of the frontal lobe envelops the anterior part of the corpus callosum and is bounded posteriorly by a line drawn between the central sulcus and the corpus callosum. (Such lines are drawn elsewhere; they have no functional significance and can be ignored after serving their initial purpose.) The inferior surface of the frontal lobe rests on the orbital plate of the tl frontal bone. e ii1S 1l o il I- ' The natural boundaries of the parietal lobe on the lateral surface are the central and lateral sulci. The other boundaries consist of two lines; the first of these is drawn between the parieto-occipital sulcus and the preoccipital hotch, and the second line runs from the middle of the one just established to the lateral sulcus. (The preoccipital notch is an inconspicuous landmark consisting of a shallow indentation of the brain formed by the petrous portion of the temporal bone.) On the medial surface, the parietal lobe is bounded by the frontal lobe , corpus callosum, calcarine sulcus, and parieto-occiPital sulcus' The temporal lobe is outlined on the lateral surface by the lateral sulcus and the lines previously noted. The inferior surface of the temporal lobe extends to the temporal pole from a line drawn between the anterior end of the calcarine sulcus and the preoccipital notch. Most of the occipital lobe appears on the medial surface of the hemisphere, where it is separated from the temporal lobe, as alre ady )- al described, and from the parietal lobe by the parieto-occipital sulcus. On the lateral surface, the occipital lobe consists of the small area posterior to the Iine that joins the parietooccipital sulcus and preoccipital notch, The portion of the great cerebral commissure in and near the midline is known as the trunk of the corpus callosum, and the fibers of the commissure that spread out within the medullary centers of the hemispheres con- il, le stitute the radiations of the corpus callosum. Names are assigned to certain regions of the trunk of the commissure (see Fig. I)-21; these regions are used as reference points further on. The enlarged posterior portion of the trunk is called the spleniurn. The anterior portion, or genu, curves ventrally and thins out to form the rostrurn. This is continuous with the Iamina terrninalis, which limits the third ventricle anteriorly, Gyri and Sulci Some surface markings of the hemisphere are Iandmarks for important l'unctional areas; the central sulcus for the sensorimotor cortex and the calcarine sulcus for the visual cortex are examples. For the most part, the sulci and gyri serve only as a rough frame of reference for cortical areas whose functions may or may not be known. The markings can be identified according to lobes for the Iateral surface, but this is not practicable for the medial and inferior surfaces. The text and illustratilns that fllllw apply to sulci and gyri of varying functional significance. The student may need to refer to this material when studying the localization of functions in the cerebral corta (see Ch. 15), Iateral Surface Frontal Lobe The precentral sulcus (often broken into two or more parts) runs parallel to the central sulcus; these sulci outline the precentral ryrus, which is a landmark for the primary motor area of the cerebral cortex (Fig. 13-3). The remainder of the lateral surface of the frontal lobe is divided into superior, middle, and inferior frontal glriby the superior and inferior frontal sulci. The anterior and ascending rami of the lateral sulcus dMde the inferior fiontal gyrus into opercular, triangular, and orbital portions, In the left hemisphere, the opercular and triangular portions consist of cortex of l3roca's expressive or motor speech area. In the frontal lobe, as in the other lobes of the hemisphere, there are secondary gyri and sulci that contribute to the variable topography of different brains. Parietal Lobe The postcentral sulcus runs parallel to the cen- tral sulcus; these sulci bound the postcentral 225 226 Regional Anatomy of the Central Ne/vous Systefi Precentral sulcus Postcentral sulcus I Superior ntraparietal su tcus frontal Central sulcus su Icus I nf erior temporal sulcus Superior temporal sulcus Figure l3-3. Gyri and sulci on the lateral surface of the right cerebral hemisphere. (A), (B) and (C) indicate the opercular, triangular, and orbital portions of the inferior frontal gyrus, respectively. ( x 0.63) ryrus, which is the landmark for the first general sensory (somesthetic) area of corto( The intraPafietal sulcus extends posteriorly from the postcentral sulcus and dMdes that part of the surface not occupied by the postcentral gyrus into superior and inferior parietal lobules. Those portions of the inferior parietal lobule that surround the upturned ends of the lateral sulcus and superior temporal sulcus are called the supramarginal grrus and the angular ryrus, respectively. In the left hemisphere, these gyri consist of corte>< included in the receptive language area, which is necessary for perception and interpretation of spoken and written language. Temporal Lobe Superior and inferior temporal sulci dMde the lateral surface of the temporal lobe into superior, middle, and inferior temporal ryri. Among variations in the temporal lobe, the inferior temporal sulcus may be discontinuous, making it difficult to identify. The superior temporal gyrus has a large surface that forms the floor of the lateral sulcus. On this surface, transverse tem- poral ryri (also known as Heschl's convolu- tions) extend to the bottom of the lateral sulcus and mark the location of the auditory area of cortex. The posterior part of the left superior temporal gyrus forms part of the receptive lan- Chapter guage area, which oxtends onto the parietal lobe. Occipital Lobe In the brains of primates other than humans and in some human brains, the calcarine sulcus continues for a short distance over the occipital a curved lunate sulcus around the end of the calcarine sulcus. Except pole. There is then for this inconstant marking, the small area of the occipital lobe on the lateral surface has minor grooves and folds of no special significance. lnsular Lobe (lnsula) The regions that conceal the insula are known as the frontal, parietal, and temporal opercula; they must be spread apart or cut awayto e)pose the insula (Fig. 13-4), The insula is outlined by a circular sulcus and is divided into two regions by a central sulcus. Several short gyri lie in front , of the central sulcus, and one or two long gyri lie behind it. The inferior part of the insula in the region of the stem of the lateral sulcus is known as the limen insulae. The insula is an important landmark for certain structures inside the cerebral hemisphere. The lentiform nucleus, a component of the corpus striatum, is separated from the insula bytwo layers of white matter (the extreme and e>cternal capsules) and an intervening layer of gray mat- ter (the claustrum). i3: Topography oJ the Cerebral Hemispheres Medial and lnferior Surfaces The cingulate ryrus begins beneath the genu of the corpus callosum and continues above the corpus callosum as far back as the splenium (Fig. 13-5). The gyrus is separated from the corpus callosum by the sulcus of the corpus callosum (callosal sulcus).'The superior surface of the corpus callosurn is covered by a very thin layer of cortical gray rnatter known as the indusium griseum. The cingulate sulcus intervenes between the cingulate gyrus and the medial frontal grrus, which is continuous with the superior frontal gyrus on the lateral surface of the hemisphere. The cingulate sulcus gives off a paracentral sulcus and then divides into marginal and subparietal sulci in the parietal lobe. The region bounded by the paracentral and marginal sulci, which surrounds the indentation made by the central sulcus on the superior border, is called the paracentral lobule. The anterior and posterior parts of the paracentral lobule are, respectively, extensions of the precentral and postcentral gyri of the lateral surface of the hemisphere, The area above the subparietal sulcus is called the precuneus and is continuous with the superior parietal lobule on the lateral surface. The parieto-occipital and calcarine sulci bound the cuneus of the occipital lobe. On the medial suuface of the frontal lobe, underneath the rostrum of the corDus calCircular sulcus f Figure ll-4. The insula (island of Reil) of the left cerebral hemisphere, by cutting away the frontal, parietal, and temporal opercula. e<posed ( x 1.8) \\! Long gyri ,. 227 Central sulcus Paracentral sulcus Marginal sulcus Cingulate sulcus Subparietal sulcus Parieto- occipltal su tcus los the pal lim (Fir fro pol sisl the ba< re€ Th of sh( Pat enl Oht por anl latr ito die Rhinal su I cus cip wit Collateral su tcus SUI Figure.l3-5. Gyri and sulci on the medial and inferior surfaces of the right cerebral hemisphere. (A) uncus. (B) lsthmus (retrosplenial corte><) connecting the cingulate and parahippocampal gyri. ( x 0.63) orl co re( lan Occipitotemporal sulcus Olfactory Olfactory f;iJ ""0 surcus Figure-13-6. Gyri and sulci on the inferior surface of the right cerebral hemisphere. (A) Uncus. (x 0.63) 228 229 Chapter 13: Topography of the Cerebral Hemispheres losum, is the subcallosal glrus, also known as the parolfactory area. This is not corto(, but it is part of the septal area, a component of the limbic system (see Ch. 18). On the inferior surface of the hemisphere (Figs. 13-5 and 13-6), a convolution extends from the occipital pole almost to the temporal pole. The posterior part of the convolution consists of the lingual glrus. The anterior part forms the parahippocampal gtms, which hooks sharply baclavard on its medial aspect as the uncus, a region in which fibers of the olfactory tract end. The collateral sulcus defines the lateral margin of the lingual and parahippocampal gyti. The short rhinal sulcus, at the lateral edge of the parahippocampal gyrus anteriorly, delimits the entorhinal area, which belongs to the olfactory and limbic systems. The medial occipitotem' poral ryrus, which is inconstant in morphology Lnd broken up by irregular sulci, lies along the lateral side of the collateral sulcus, The occip' itotempbral sulcus intervenes between the medial occipitotemporal gyrus and the lateral oc' cipitotemporal ryrus. The latter is continuous wittr the inferior temporal gyrus on the lateral surface of the hemisPhere. The olfactory bulb and olfactory tract on the orbitalsurface of the frontal lobe (see Fig. 13-6) conceal most of the olfactory sulcus' The ryrus rectus is medial to the olfactory sulcus, and the large area lateral to the olfactory sulcus consists of irregular orbital ryri. The cingulate and para- hippocampal gyri are connected by a narrow isthmus (retrosplenial corto<) beneath the splenium of the corpus callosum and formthe "lim' bic lobe" of the cerebral hemisphere, which also includes the hippocampus. The limbic lobe is part of the limbic system of the brain, which incorporates several additional structures, most ' prominently the dentate gyrus and amygdaloid body (both in the temporallobe), hypothalamus (especially the mamillary bodies), septal area, and anterior and some other nuclei of the thalamus. The limbic system, which is involved in memory and certain asPects of behavior, is described in Chapter 18. SUGGESTED READING Bisaria KK: Grooves on the occipital lobe of Indian brains, J Anat 139t779-582, 1984 Haines'DE: Neuroanatomy. An Atlas of Structures, Sections and Systems, lrd ed. Baltimore, Urban & Schwarzenberg, 1991 Montemuno DG, Bruni JE: The Human Brain in Dissection, 2nd ed. New York, Oxford University hess, 1988 Nieuwenhuys R, Voogd J, van Huijzen C: The Human Central Nervous System. A Synopsis and Atlas, 3rd ed. Berlin, Springer-Verlag, f 988