Neuroanatomy Lecture 6 PDF

Neuroanatomy 2711212 Cerebrum (Part 1&2) Lecture 6 THE CEREBRUM (Latin = brain) It is the largest part of the brain that fills most of the cranial cavity above the floors of anterior and middle cranial fossae and above the tentorium cerebelli of posterior cranial f...

Neuroanatomy 2711212 Cerebrum (Part 1&2) Lecture 6 THE CEREBRUM (Latin = brain) It is the largest part of the brain that fills most of the cranial cavity above the floors of anterior and middle cranial fossae and above the tentorium cerebelli of posterior cranial fossa. Shape: The cerebrum is a convoluted bilobed structure. A deep median cleft, the longitudinal cerebral fissure, incompletely separates the cerebrum into two lateral halves, called cerebral hemispheres. The cleft lodges a sickle-shaped fold of the dura mater, the falx cerebri. A large mass of white fibers joining the two cerebral hemispheres across the median plane called the corpus callosum. Each cerebral hemisphere consists of: (a) an outer layer of grey matter called cerebral cortex, (b) an inner mass of white matter, (c) large masses of grey matter embedded in the basal part of the white matter called basal ganglia/basal nuclei and (d) a cavity within it called lateral ventricle. EXTERNAL FEATURES: The external features of the cerebral hemisphere include: poles, surfaces, borders, sulci, and gyri. Poles: Each cerebral hemisphere presents the following three poles: 1. The frontal pole, the anterior end of the hemisphere. 2. The occipital pole, the posterior end of the hemisphere. 3. The temporal pole between frontal and temporal LOBE.  Superomedial border Surfaces: Each cerebral hemisphere has the following three surfaces: 1. The superolateral surface is most convex and most extensive. It faces upwards and laterally. 2. The medial surface is flat and vertical. 3. The inferior surface is irregular to adopt the floors of anterior and middle cranial fossae. It is divided into two parts by a deep horizontal groove or sulcus, the stem of lateral sulcus: (a) a small anterior part, the orbital surface, and (b) a large posterior part, the tentorial surface Borders: Each cerebral hemisphere presents the following borders: 1. The superomedial border separates the superolateral surface from the medial surface. 2. The superciliary border is at the junction of superolateral and orbital surfaces. 3. The inferolateral border separates the superolateral surface from the tentorial surface. 4. The medial orbital border separates the medial surface from the orbital surface. 5. The medial occipital border separates the medial surface from the tentorial surface. Sulci and Gyri: The cerebral cortex is thrown into folds, called gyri or convolutions. The grooves between the gyri are termed as sulci. The convolutions greatly increase the surface area of the cerebral cortex. Each gyrus consists of a central core of white matter covered by an outer layer of grey matter. Main Cerebral Sulci: Lateral sulcus (of Sylvius) it has a stem and three rami. The stem of the sulcus begins as a deep cleft on the inferior surface of the cerebral hemisphere and extends laterally to reach the superolateral surface. On reaching the superolateral surface it divides into three rami: (a) anterior horizontal, (b) anterior ascending, and (c) posterior. Central sulcus (of Rolando) begins by cutting the superomedial border of the hemisphere about 1 cm behind the midpoint between the frontal and occipital poles, runs downwards and forwards, and ends just above the posterior ramus of the lateral sulcus. Its upper end usually extends into the medial surface. Parieto-occipital sulcus is present on the medial surface of the hemisphere. It extends about 5 cm in front of the occipital pole, and may extend slightly onto the superolateral surface. Lobes: The superolateral surface of each cerebral hemisphere is arbitrarily divided into four lobes, viz. frontal, parietal, temporal, and occipital with the help of: (a) three main sulci: central, lateral and parieto- occipital, and (b) two imaginary lines. The first imaginary line is a vertical line joining the parieto- occipital sulcus to the preoccipital notch, and the second imaginary line is a backward continuation of the horizontal part of the posterior ramus of the lateral sulcus till it joins the first line. The frontal lobe lies anterior to the central sulcus and above the posterior ramus of the lateral sulcus. The parietal lobe lies behind the central sulcus and in front of the upper part of the first imaginary line. Below it is bounded by the posterior ramus of the lateral sulcus and the second imaginary line. The temporal lobe lies below the posterior ramus of lateral sulcus and second imaginary line. It is separated from the occipital lobe by the lower part of the first imaginary line. The occipital lobe lies behind the vertical line joining the parieto- occipital sulcus and preoccipital notch. Lobes of Cerebrum Insula/Island of Reil (Also Called Central Lobe) The insula is the submerged (hidden) portion of the cerebral cortex in the floor of the lateral sulcus. It is triangular in shape and surrounded all around by a sulcus, the circular sulcus, except anteroinferiorly at its apex called limen insulae. The insula is hidden from the surface view by the overgrown cortical areas of frontal, parietal, and temporal lobes. These areas are termed frontal, frontoparietal, and temporal opercula (operculum = lid). Sulci and Gyri on Superolateral Surface: In the frontal lobe 1. The precentral sulcus, runs downwards and forwards parallel and little anterior to the central sulcus. The area between the central and precentral sulci is called precentral gyrus. 2. Superior and inferior frontal sulci which run horizontally anterior to the precentral sulcus. These sulci divide the region of frontal lobe in front of precentral sulcus into superior, middle, and inferior frontal gyri. In the parietal lobe 1. The postcentral sulcus runs downwards and forwards, a little behind and parallel to the central sulcus. The area between these two sulci is called postcentral gyrus. 2. Intraparietal sulcus: divides the rest of the parietal lobe into superior and inferior parietal lobules. Sulci and Gyri on Superolateral Surface In the temporal lobe 1. Superior temporal sulci 2. Inferior temporal sulci The two sulci in this lobe that run parallel to the posterior ramus of the lateral sulcus. They divide the temporal lobe into superior, middle, and inferior temporal gyri. The superior surface of superior temporal gyrus presents Heschl’s gyrus. In the occipital lobe The occipital lobe possesses rather three short sulci, lateral, and transverse occipital sulci, and lunate sulcus. Sulci and Gyri on Medial Surface 1. Callosal sulcus: It lies above corpus callosum. 2. Cingulate sulcus: It is a curved course about 1 cm above and parallel to the upper margin of corpus callosum. 3. Cingulate gyrus :area between the cingulate and callosal sulcus. 4. Paracentral lobule: a small part around the upper part of the central sulcus, 5. Medial frontal gyrus: infront of the central sulcus.. Sulci and Gyri on Medial Surface 6. Calcarine sulcus: extends below the splenium of corpus callosum backwards towards the occipital pole. 7. Isthmus: a small region between the splenium and calcarine sulcus. 8. Parieto-occipital sulcus 9. Cuneus: triangular area between the posterior part of the calcarine sulcus and the parieto-occipital sulcus is called. 10. precuneus: quadrangular area between the parietooccipital sulcus and paracentral lobule. Sulci and Gyri on Inferior Surface On the orbital part of inferior surface (orbital surface) 1. Olfactory sulcus: It is a straight sulcus close to the medial border of the orbital surface. It is called olfactory sulcus because it lodges the olfactory bulb and tract. The area medial to this sulcus is called gyrus rectus. 2. Orbital sulcus: It is an irregular H-shaped sulcus and divides the rest of the orbital surface into anterior, posterior, medial, and lateral orbital gyri.. Sulci and Gyri on Inferior Surface On the tentorial part of inferior surface (tentorial surface) 1. Collateral sulcus: present on the tentorial part lateral and parallel to the calcarine sulcus. 2. Rhinal sulcus: present at the anterior end of the collateral sulcus. 3. Occipitotemporal sulcus: present on the tentorial surface lateral and parallel to the collateral sulcus. 4. Lingual gyrus: the area between collateral sulcus and calcarine sulcus posteriorly. 5. Parahippocampal gyrus: medial to collateral sulcus. 6. Uncus: hook-like anterior end of parahippocampal gyrus and is limited laterally by a short rhinal sulcus. 7. Medial Occipito-temporal gyrus: lies between the collateral sulcus and the occipitotemporal sulcus. 8. Lateral occipitotemporal gyrus: lies lateral to occipitotemporal sulcus.