Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Nervous System (General Plan of Structure and Development)
Central Nervous System
Brain - Telencephalon

The Telencephalon (cerebrum) consists of the right and left cerebral hemispheres and connecting fibers that form the corpus callosum and other commissures. Beneath the corpus callosum lies the fornix, appearing as two curved tracts connected by a commissure in their middle part. The anterior part of the fornix is directed downward and forms the columns of the fornix. The posterior, flattened, and diverging part is termed the crura of the fornix. Anterior to the columns of the fornix lies a transversely oriented bundle of fibers—the anterior (white) commissure. Anterior to the fornix in the sagittal plane is the septum pellucidum, consisting of two parallel lamina. Anteriorly and superiorly, these lamina connect with the anterior part of the corpus callosum. Between the lamina lies a narrow slit-like cavity containing a small amount of fluid. Each lamina forms the medial wall of the anterior horn of the lateral ventricle.

Each cerebral hemisphere is formed by gray and White matter. The peripheral part of the hemisphere forms sulci and gyri covered by a thin layer of Gray matter—the Cerebral Cortex. The surface area of the cortex is approximately 220,000 mm². Beneath the cerebral cortex lies the white matter, deep within which are large accumulations of gray matter—subcortical nuclei (Basal Ganglia). The cavities of the cerebral hemispheres are the Lateral ventricles.

Each hemisphere has three surfaces: the superolateral (convex), the medial (flat), facing the adjacent hemisphere, and the inferior, which has a complex relief corresponding to the irregularities of the internal cranial base. The shape, size, and landmarks of the sulci and gyri exhibit considerable individual Variability. At the same time, the largest sulci and gyri are notable for the constancy of their orientation.

Each hemisphere is divided into lobes. The central sulcus (of Rolando) separates the frontal lobe from the parietal lobe, the lateral sulcus (of Sylvius) separates the temporal lobe from the frontal and parietal lobes, and the parieto-occipital sulcus separates the parietal and occipital lobes. Deep within the lateral sulcus lies the insular lobe. Finer sulci divide the lobes into gyri.

Superolateral surface of the cerebral hemisphere. Frontal lobe. Almost parallel to the central sulcus, anterior to it, runs the precentral sulcus, which separates the precentral gyrus. Extending forward more or less horizontally from the precentral sulcus are two sulci that divide the superior, middle, and inferior frontal gyri.

Parietal lobe. The postcentral sulcus separates the gyrus of the same name. The horizontal intraparietal sulcus divides the superior and inferior parietal lobules.

The occipital lobe is divided into several gyri by sulci, of which the most constant is the transverse occipital sulcus.

Temporal lobe. Two longitudinal sulci—the superior and inferior temporal—separate the superior, middle, and inferior temporal gyri from one another.

The insular lobe (insula) is located deep within the lateral sulcus. The deep circular sulcus of the insula separates it from adjacent Regions of the cerebral hemisphere.

Medial surface of the cerebral hemisphere. All lobes of the cerebral hemisphere, except the insular lobe, take part in forming the medial surface of the cerebral hemisphere. The sulcus of the corpus callosum arches over it superiorly, separating the corpus callosum from the cingulate gyrus; it then courses downward and forward and continues into the hippocampal sulcus. Above the cingulate gyrus runs the cingulate sulcus, which begins anterior and inferior to the rostrum of the corpus callosum, ascends, curves backward, and runs parallel to the sulcus of the corpus callosum. At the level of the splenium of the corpus callosum, its marginal branch diverges upward from the cingulate sulcus, while the sulcus itself continues into the subparietal sulcus. The marginal branch of the cingulate sulcus bounds the paracentral lobule posteriorly and the precuneus anteriorly. Inferiorly and posteriorly, through the isthmus, the cingulate gyrus transitions into the parahippocampal gyrus, which terminates anteriorly in the uncus and is bounded superiorly by the hippocampal sulcus. The cingulate gyrus, isthmus, and parahippocampal gyrus are collectively termed the limbic gyrus. Deep within the hippocampal sulcus lies the dentate gyrus.

The medial surface of the occipital lobe is separated from the parietal lobe by the parieto-occipital sulcus. Extending from the posterior pole of the hemisphere to the ISTHMUS OF THE limbic gyrus is the calcarine sulcus, which bounds the lingual gyrus superiorly. Between the parieto-occipital sulcus anteriorly and the calcarine sulcus inferiorly lies the cuneus, pointing forward with an acute angle.

The Inferior surface of the cerebral hemisphere has the most complex relief. Anteriorly lies the inferior surface of the frontal lobe; posterior to it are the temporal pole and the inferior surfaces of the temporal and occipital lobes, between which there is no clear boundary.

On the inferior surface of the frontal lobe, parallel to the longitudinal cerebral fissure, runs the olfactory sulcus, inferior to which lie the olfactory bulb and olfactory tract, continuing into the olfactory trigone. Between the longitudinal cerebral fissure and the olfactory sulcus lies the gyrus rectus. Lateral to the olfactory sulcus lie the orbital gyri. On the inferior surface of the temporal lobe, the collateral sulcus separates the medial occipitotemporal gyrus from the parahippocampal gyrus. The occipitotemporal sulcus separates the lateral occipitotemporal gyrus from the medial gyrus of the same name.

Cerebral cortex. Structure OF THE cortex. The cortex is formed by gray matter located at the periphery (surface) of the cerebral hemispheres, constituting the Pallium. The thickness of the cortex in various regions of the hemispheres ranges from 1.3 to 5 mm. Cortical formations are divided into archipallium (allocortex), paleopallium, and neopallium (isocortex). The neocortex constitutes the bulk of the pallium, accounting for 96% in humans. The criterion for distinguishing the neocortex is its six-layered structure. The old cortex (archicortex), preserved on the medial and inferior surfaces of the hemispheres, is similar in structural Organization to the deeper layers of the neocortex.

The paleocortex occupies small areas located on the basal surface of the frontal lobes. It is represented primarily by the olfactory bulbs, the olfactory trigone, the anterior perforated substance, and the septum pellucidum.

In the olfactory bulbs, Neurons are arranged in layers, and the bulbs receive information from neurons of the nasal mucosa. Efferent fibers of the olfactory bulb Cells form the olfactory tract. In the region of the olfactory trigone, the fibers of the olfactory tract divide into medial, intermediate, and lateral olfactory striae. The fibers of the medial stria pass to the neurons of the septal area, the intermediate stria terminates in the region of the anterior perforated substance, and the lateral stria proceeds to the neurons of the amygdaloid body. The archicortex in the human Brain is represented by hippocampal formations and the dentate gyrus, occupying the medial regions of the hemispheres. At the periphery, they are surrounded by transitional cortex, which separates the archicortex from the neocortex. The hippocampus and dentate gyrus are located in the medial part of the temporal lobe and, during development, come to lie deep within the inferior horn of the lateral ventricle.

The neocortex in humans covers a surface area of about 2,200 cm², with only about 30% of the neocortex located On the surface of the gyri. The remainder is hidden deep within the sulci. The cortex contains over 10¹⁰ diverse Nerve Cells. Based on morphological characteristics—taking into account the shape and size of nerve Cell bodies and the branching patterns of dendrites and axons—Two Types of neurons are distinguished in the cerebral cortex: pyramidal and non-pyramidal. Pyramidal cells, which have a cone-shaped body, account for 75% of neurons. Non-pyramidal cells include stellate, fusiform, and a group of transitional neuron forms. Cortical neurons are arranged in six horizontal layers that differ in the size and shape of the neurons (Fig. 101).

Layer I—molecular layer (plexiform layer); it is poor in cellular elements and consists primarily of fibers running parallel to the cortical surface. Functionally, this layer is associated with regulating the excitability level of cortical neurons and also provides interneuronal connections between cells of various layers.

Layer II—external granular layer; it consists predominantly of small pyramidal-shaped cells. The Role of neurons in this layer is the intracortical transmission of nerve impulses.

Layer III—external pyramidal layer; it consists of pyramidal neurons whose sizes increase in the deep direction. The axons of these cells form short and long Association Pathways.

Layer IV—internal granular layer; it is dominated by small-caliber neurons of all varieties. The majority of fibers arriving from the thalamus terminate in this layer.

Layer V—internal pyramidal layer; it is characterized by large, and in some areas giant, pyramidal cells. The dendrites of most of these pyramidal cells reach Layer I. Layer V concentrates the bulk of cortical neurons whose axons form descending Projection Pathways.

Layer VI—multiform (polymorphic) layer; it borders directly on the WHITE MATTER OF the hemisphere. The efferent fibers of these layers form descending projection systems.