Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Nervous system
Brain
Telencephalon

The telencephalon (cerebrum) consists of the right and left cerebral hemispheres and the fibers connecting them, which form the corpus callosum and other commissures. Beneath the corpus callosum lies the fornix, consisting of two curved bands joined together by a commissure in their middle section. The anterior part of the fornix, directed downwards, forms the columns of the fornix. The posterior, flattened, and diverging part is known as the crura of the fornix. Anterior to the columns of the fornix is a transversely oriented bundle of fibers—the anterior (white) commissure.

Anterior to the fornix, in the sagittal plane, lies the septum pellucidum, which consists of two parallel laminae. Anteriorly and superiorly, these laminae fuse with the anterior part of the corpus callosum. Between the laminae is 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 composed of gray and White matter. The peripheral part of the hemisphere, marked by sulci and gyri, forms the so-called Pallium, which is 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 clusters of gray matter—the subcortical nuclei (Basal Ganglia). The cavities of the cerebral hemispheres are the Lateral ventricles.

Three surfaces are distinguished in each hemisphere: the superolateral (convex) surface, the medial (flat) surface facing the adjacent hemisphere, and the inferior surface, which has a complex relief corresponding to the irregularities of the internal Skull base. Numerous depressions—sulci—and multidirectional ridges between them—gyri—are visible on the surfaces of the hemispheres. The shape, size, and orientation of the sulci and gyri show significant individual Variability. At the same time, the largest sulci and gyri are relatively constant in their orientation. Each hemisphere is divided into five lobes: frontal, parietal, occipital, temporal, and insular (the insula). These lobes are separated from one another by deep sulci. The central sulcus (of Rolando) separates the frontal lobe from the parietal lobe; the lateral sulcus (Sylvian) separates the temporal lobe from the frontal and parietal lobes; and the parieto-occipital sulcus divides the parietal and occipital lobes (Fig. 93). Deep within the lateral sulcus lies the insular lobe. Smaller sulci divide the lobes into gyri.

Superolateral surface of the cerebral hemisphere. In the frontal lobe, anterior and parallel to the central sulcus runs the precentral sulcus, which separates the precentral gyrus. From the precentral sulcus, two sulci run more or less horizontally forward, separating the superior, middle, and inferior frontal gyri. In the parietal lobe, the postcentral sulcus separates the corresponding gyrus. The horizontal intraparietal sulcus divides the superior and inferior parietal lobules. In the occipital lobe, there are several gyri and sulci, the most constant of which is the transverse occipital sulcus. In the temporal lobe, two longitudinal sulci—the superior and inferior temporal sulci—separate three temporal gyri: the superior, middle, and inferior. Deep within the lateral sulcus, the insular lobe is separated from adjacent PARTS OF THE hemisphere by the deep circular sulcus of the insula.

Medial surface of the cerebral hemisphere. All lobes of the cerebral hemisphere, except the temporal and insular lobes, contribute to The formation of its medial surface. The long, arch-shaped sulcus of the corpus callosum (callosal sulcus) separates it from the cingulate gyrus. Above the cingulate gyrus runs the cingulate sulcus, which begins anterior and inferior to the rostrum of the corpus callosum, ascends, and turns posteriorly, running parallel to the sulcus of the corpus callosum. Posteriorly and inferiorly, the cingulate gyrus continues as the parahippocampal gyrus, which extends downward and terminates anteriorly as the uncus; superiorly, the parahippocampal gyrus is bounded by the hippocampal sulcus. The cingulate gyrus, its isthmus, and the parahippocampal gyrus are collectively referred to as the gyrus fornicatus. Deep within the hippocampal sulcus lies the dentate gyrus.

Class="center">

Fig. 93. Superolateral surface of the cerebral hemisphere:

1 — central sulcus, 2 — precentral gyrus, 3 — superior frontal gyrus, 4 — middle frontal gyrus, 5 — inferior frontal gyrus, 6 — lateral sulcus, 7 — superior temporal gyrus, 8 — middle temporal gyrus, 9 — inferior temporal gyrus, 10 — Pons, 11 — Medulla Oblongata, 12 — Cerebellum, 13 — occipital lobe, 14 — inferior parietal lobule, 15 — superior parietal lobule, 16 — subcentral gyrus

Superiorly on the medial surface of the occipital lobe, the parieto-occipital sulcus is visible, separating the parietal lobe from the occipital lobe. The calcarine sulcus runs from the posterior pole of the hemisphere to the ISTHMUS OF THE gyrus fornicatus. Between the parieto-occipital sulcus anteriorly and the calcarine sulcus inferiorly lies the cuneus, with its apex directed anteriorly.

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 (anterior) pole and the inferior surface 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 fissure, runs the olfactory sulcus, to which the olfactory bulb and olfactory tract adhere from below, continuing posteriorly into the olfactory trigone. Between the longitudinal fissure and the olfactory sulcus lies the gyrus rectus. Lateral to the olfactory sulcus are 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 corresponding medial gyrus.

On the medial and inferior surfaces, several structures belonging to the limbic system are distinguished. These include the olfactory bulb, olfactory tract, olfactory trigone, and anterior perforated substance, which are located on the inferior surface of the frontal lobe and also belong to the peripheral part of the rhinencephalon, as well as the cingulate, parahippocampal (along with the uncus), and dentate gyri.

Structure OF THE cerebral cortex. The cortex is formed by gray matter, which lies on the periphery (surface) of the cerebral hemispheres. The thickness of the cortex in different areas of the hemispheres ranges from 1.3 to 5 mm. The structure and arrangement of Neurons in different areas of the cortex vary, which determines the cytoarchitectonics of the cortex. Cells of more or less similar structure are arranged in separate layers (laminae). In the cerebral cortex, neuronal Cell bodies form six layers. The thickness of the layers, The Nature of their boundaries, cell sizes, and their number vary in different Regions of the cortex. On the outside lies the molecular layer, which contains small multipolar association neurons and numerous fibers—processes of neurons from the underlying layers. The second layer—the external granular layer—is formed by many small multipolar neurons. The third layer—the widest, the external pyramidal layer—contains pyramid-shaped neurons whose cell bodies increase in size from top to bottom. The fourth layer—the internal granular layer—is formed by small stellate neurons. In the fifth layer—the internal pyramidal layer, which is most well-developed in the precentral gyrus—lie large pyramid-shaped cells up to 125 µm in size. In the sixth layer—the polymorphic (multiform) layer—neurons of various SHAPES AND SIZES are located. The number of neurons in the cortex reaches 12–18 billion.

In addition to Nerve Cells, nerve fibers are located in each cellular layer. Their structure and density also vary in different regions of the cortex. The specific distribution of fibers in the cerebral cortex is referred to by the term "myeloarchitectonics".

Basal nuclei and WHITE MATTER OF the telencephalon. Deep within the white matter of each cerebral hemisphere, closer to its base, lie accumulations of gray matter that form separate basal nuclei (Fig. 94). The basal, subcortical nuclei, or ganglia, include the corpus striatum, consisting of the caudate and lentiform nuclei, the claustrum, and the amygdaloid body.

The caudate Nucleus is located laterally and superiorly to the thalamus, separated from it by the stria terminalis. The Nucleus has a HEAD, which forms the lateral wall of the anterior horn of the lateral ventricle, a body lying beneath the parietal lobe, and a tail, which participates in forming the roof of the inferior horn of the lateral ventricle. The lentiform nucleus is located laterally to the caudate nucleus. The lentiform nucleus is divided into an inner part—the globus pallidus—and an outer part—the putamen. These nuclei serve as subcortical motor centers. Between the caudate nucleus and thalamus medially and the lentiform nucleus laterally lies a layer of white matter—the internal capsule. This capsule is formed by the main ascending and descending Pathways of the Brain, which connect the cerebral cortex with the Brainstem and Spinal Cord. Lateral to the lentiform nucleus is a narrow band of white matter—the external capsule, and beyond it lies a thin nucleus—the claustrum. Thus, the claustrum is located in the white matter of the hemisphere lateral to the lentiform nucleus, between it and the insular cortex. The claustrum is separated from the insular cortex by a layer of white matter known as the extreme capsule. The amygdaloid body lies in the white matter of the anterior part of the temporal lobe of the hemisphere, 1.5–2 cm posterior to its temporal pole.

White matter of the cerebral hemispheres. The white matter of the hemispheres includes the external and extreme capsules, which separate the basal nuclei from each other, fiber structures between the cortex and basal nuclei, as well as the corpus callosum, anterior commissure, fornix, and commissure of the fornix, which contain systems of nerve fibers connecting cortical areas and subcortical centers both within one half of the brain and between the centers of the right and left cerebral hemispheres.

Fig. 94. Horizontal section of the brain. Basal nuclei:

1 — cerebral cortex (pallium), 2 — genu of the corpus callosum, 3 — anterior horn of the lateral ventricle, 4 — internal capsule, 5 — external capsule, 6 — claustrum, 7 — extreme capsule, 8 — putamen, 9 — globus pallidus, 10 — III ventricle, 11 — posterior horn of the lateral ventricle, 12 — thalamus, 13 — insular cortex, 14 — head of the caudate nucleus, 15 — cavity of the septum pellucidum

The corpus callosum is formed by transversely oriented (commissural) fibers connecting the right and left hemispheres. The corpus callosum has an anterior part—the genu, which thins out to become the rostrum, continuing downward into the lamina terminalis. Posteriorly, the corpus callosum is thickened, forming the splenium. Beneath the corpus callosum lies the fornix, which also consists of white matter. The fornix has the shape of two longitudinal flat arches connected in their middle part by transverse fibers (commissure of the fornix). The anterior part of the fornix curves downward, continuing as rounded tracts—the columns of the fornix, which enter the Hypothalamus and end in the mammillary bodies. Posteriorly, the fornix becomes the crura of the fornix, which diverge and descend. Each crus continues as the fimbria, which reaches the inferior horn of the lateral ventricle, where it attaches to the hippocampus. Between the inferior surface of the genu and rostrum of the corpus callosum and the columns of the fornix, two laminae of the septum pellucidum are located in the sagittal plane, bounding its narrow cavity on the sides. Beneath the middle part of the fornix, its body, lies the Third ventricle—the cavity of the Diencephalon.

Lateral ventricles. The cavities of the cerebral hemispheres are the lateral ventricles (First and Second), located deep within the white matter beneath the corpus callosum (see Fig. 94). Each ventricle consists of four parts corresponding to the four main lobes of the cerebral hemisphere. The anterior horn lies in the frontal lobe, the central part in the parietal lobe, the posterior horn in the occipital lobe, and the inferior horn in the temporal lobe. The anterior horns of both ventricles are separated from each other by two laminae of the septum pellucidum. The central part of the lateral ventricle arches over the thalamus and continues posteriorly into the posterior horn and downward into the inferior horn. The medial wall of the inferior horn is the hippocampus, which corresponds to the deep hippocampal sulcus of the same name on the medial surface of the hemisphere. On the medial side, the fimbria, which is a continuation of the crus of the fornix, lies adjacent to the hippocampus. On the medial wall of the posterior horn of the lateral ventricle, there is an elevation—the calcar avis, corresponding to the calcarine sulcus on the medial surface of the hemisphere. The choroid plexus projects into the central part and the inferior horn of the lateral ventricle, connecting through the interventricular foramen with the choroid plexus of the third ventricle.

Age-related Features of the brain

In a newborn, the brain is relatively large, with an average mass of 390 g (340–430 g) in boys and 355 g (330–370 g) in girls, which accounts for 12–13% of body weight (in adults, it is approximately 2.5%). The ratio of brain mass to body weight in a newborn is 1:8 (in an adult, this ratio is 1:40). By the end of the first year of life, the brain mass doubles, and by 3–4 years, it triples. Subsequently (after 7 years), the brain mass increases slowly, reaching its maximum value by 20–29 years (1355 g in men and 1220 g in women). In subsequent age periods, up to 60 years in men and 55 years in women, the brain mass does not change significantly, and after 55–60 years, a slight decrease is observed.

In a newborn, the phylogenetically older parts of the brain are better developed. The mass of the brainstem is 10.0–10.5 g, which is approximately 2.7% of body weight (about 2% in adults), and the cerebellum is 20 g (5.4% of body weight). By 5 months of life, the mass of the cerebellum increases threefold, and by 9 months, fourfold (the child can stand and begins to walk). The cerebellar hemispheres develop most intensively. The diencephalon is also relatively well-developed in the newborn. The frontal lobe of the cerebrum is highly convex and relatively small. The temporal lobe is high. The insular lobe (insula) is located deep. Up to 4 years of life, the child's brain grows uniformly in height, length, and width; subsequently, growth in height predominates. The frontal and parietal lobes grow most rapidly.

In newborns, The surface of the cerebral hemispheres already features sulci and gyri. The primary sulci (such as the central and lateral sulci) are well-defined, whereas the Branches of the primary sulci and the minor gyri are poorly demarcated. As the child grows, the sulci become deeper and the intervening gyri more prominent. Myelination of nerve fibers in phylogenetically older regions (such as the brainstem) begins and ends earlier than in newer regions. In the cerebral cortex, nerve fibers conducting various types of general sensation, as well as those connecting with subcortical nuclei, undergo myelination first. Myelination of afferent fibers begins at approximately 2 months and is completed by 4—5 years, while that of efferent fibers starts slightly later, spanning from 4—5 months to 7—8 years.

The spatial relationship of the sulci and gyri to the bones and sutures of the cranial vault in newborns differs somewhat from that in adults. The central sulcus is located at the level of the Parietal bone. The inferolateral portion of this sulcus lies 1,0—1,5 cm cranial to the squamous suture. The parieto-occipital sulcus lies 12 mm anterior to the lambdoid suture. The topographical relationships between the sulci, gyri, and cranial sutures characteristic of adults are established in children aged 6—8 years.

In newborns, the corpus callosum is thin and short. It grows concurrently with the development and expansion of the cerebral hemispheres, primarily in the cranial and caudal directions, and is situated above the cavity of the diencephalon (above the III ventricle). As the cerebral hemispheres develop, the thickness of the trunk of the corpus callosum (up to 1 cm in adults) and the splenium of the corpus callosum (up to 2 cm) increases, which is driven by the growing number of commissural nerve fibers.