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

Nervous System (General Outline of the Structure and Development of the Nervous System)
Central Nervous System
Brain

The Brain (encephalon) is located in the cranial cavity, the contour of which conforms to the shape of the brain.

The brain is covered by three membranes: the pia mater, the arachnoid mater, and the dura mater. The weight of an adult brain ranges from 1,100 to 2,000 g. On average, it is 1,394 g in men and 1,245 g in women. This difference is due to the lower body mass in women.

Based on its origin, structural features, and functional significance, the brain is divided into two major parts: the Brainstem and the Telencephalon, which includes the cerebral hemispheres.

The brainstem is represented by the Medulla Oblongata, Hindbrain (Pons and Cerebellum), Midbrain, and Diencephalon. Being a phylogenetically older formation compared to the structures of the cerebral hemispheres, the brainstem exhibits features characteristic of the Spinal Cord. First, the brainstem contains sequentially arranged cranial nerve nuclei and The Emergence of their roots. Second, it features the topographical continuation of motor, autonomic, and sensory nuclei extending from the spinal cord into the brainstem. In the medial Regions of the brainstem, motor Neurons develop from the basal plate (homologous to the nuclei of the anterior horns of the spinal cord) and group together to form the motor nuclei of the Cranial Nerves. Interneurons develop from the alar plate (homologous to the Cells of the posterior horns of the spinal cord), forming the sensory nuclei of the cranial nerves. Between these two groups of nuclei, neurons (nuclei) of the Autonomic Nervous system are formed.

However, the brainstem has A number of distinguishing features compared to the spinal cord. It no longer shows clear signs of segmentation. The Gray matter in the brainstem is represented by isolated clusters of neurons—the cranial nerve nuclei, from the III to the XII pair inclusive—rather than columns of gray matter (horns in transverse sections), as is the case in the spinal cord. The structures of the brainstem are divided into the base (basis) and the tegmentum. The base is a phylogenetically newer formation. It contains descending pathways connecting the Cerebral Cortex with the spinal cord. The tegmentum contains the cranial nerve nuclei, as well as phylogenetically older ascending and descending Pathways of the brain.

The structures of the Forebrain show no signs of segmentation. Large clusters of Nerve Cells—the subcortical nuclei of the cerebral hemispheres (the immediate subcortex, according to I.P. Pavlov), separated by White matter—are located within the cerebral hemispheres. The cerebral cortex covers the hemispheres externally and has a completely different, layered Structure. The cerebellum is located above the posterior part of the brainstem and is covered superiorly by the posterior part of the cerebral hemispheres (occipital lobes).

The brainstem is covered by the hemispheres from above and the sides and is visible only from the inferior aspect. Both cerebral hemispheres are separated by the deep longitudinal cerebral fissure, at the bottom of which lies the corpus callosum, consisting of transversely oriented nerve fibers connecting the two hemispheres. The Superolateral surface of the brain is convex and formed by the cerebral hemispheres (Fig. 98). The inferior surface (base) is flattened and generally mirrors the relief of the internal cranial base. The roots of 12 pairs of cranial nerves emerge at the Base of the brain (Fig. 99). Most of the brain base is occupied by the frontal (anteriorly) and temporal (laterally) lobes of the cerebral hemispheres, the pons, the medulla oblongata, and the cerebellum (posteriorly).

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Fig. 98. 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 — postcentral gyrus

When examining the base of the brain from anterior to posterior, the following anatomical structures are visible. The olfactory bulbs are located in the olfactory sulci of the frontal lobes (15–20 olfactory nerves—the 1st pair of cranial nerves—converge upon them). Posteriorly, the olfactory bulbs continue into the olfactory tracts and olfactory triangles. Posterior to the triangle, the anterior perforated substance is visible on both sides, through which Blood Vessels penetrate deep into the brain. The optic chiasm (a continuation of the optic nerves—the II pair of cranial nerves) is situated between the olfactory triangles. Directly posterior to the chiasm lies the tuber cinereum, which transitions into the infundibulum connected to the Pituitary Gland. Posterior to the tuber cinereum lie the two mamillary bodies. The optic chiasm, tuber cinereum, and mamillary bodies belong to the Hypothalamus (diencephalon).

Fig. 99. Base of the brain and sites of emergence of the cranial nerve roots:

1 — olfactory bulb; 2 — olfactory tract; 3 — anterior perforated substance; 4 — tuber cinereum; 5 — optic tract; 6 — mamillary bodies; 7 — trigeminal ganglion; 8 — posterior perforated substance; 9 — pons; 10 — cerebellum; 11 — pyramid; 12 — olive; 13 — spinal nerve; 14 — Hypoglossal nerve; 15 — Accessory nerve; 16 — Vagus nerve; 17 — Glossopharyngeal nerve; 18 — Vestibulocochlear nerve; 19 — Facial Nerve; 20 — Abducens nerve; 21 — Trigeminal nerve; 22 — Trochlear nerve; 23 — Oculomotor nerve; 24 — Optic nerve; 25 — olfactory sulcus

Posterior to the mamillary bodies lie the cerebral peduncles (midbrain) and the pons (hindbrain). The cerebral peduncles connect the pons to the cerebral hemispheres. The interpeduncular fossa opens between the cerebral peduncles, the floor of which is perforated, representing the posterior perforated substance. On the Medial surface of each cerebral peduncle, near the anterior border of the pons, the oculomotor nerve (III pair) emerges, while the trochlear nerve (IV pair of cranial nerves) emerges laterally from the peduncle. The middle cerebellar peduncles diverge posteriorly and laterally from the pons, connecting it to the cerebellum. The trigeminal nerve (V pair) emerges from the substance of the middle cerebellar peduncle. Posterior to the pons lies the medulla oblongata. Between the pons and the medulla oblongata, the abducens nerve (VI pair) emerges medially, while the facial (VII pair) and vestibulocochlear (VIII pair of cranial nerves) emerge more laterally. Longitudinal elevations—the pyramids—are visible on either side of the median fissure of the medulla oblongata, with the olive located lateral to each pyramid. The glossopharyngeal (IX), vagus (X), and accessory (XI) nerves emerge sequentially from the postolivary sulcus of the medulla oblongata. The hypoglossal nerve (XII pair of cranial nerves) emerges from the sulcus between the pyramid and the olive.

Anatomically, the brain consists of the following parts: the medulla oblongata, the hindbrain (which includes the pons and cerebellum), the midbrain, the diencephalon, and the telencephalon, all of which are clearly visible in the median section of the brain (Fig. 100).

Fig. 100. Medial surface of the brain in its median section:

1 — hypothalamus; 2 — cavity of the Third ventricle; 3 — anterior (white) commissure; 4 — fornix; 5 — corpus callosum; 6 — interthalamic adhesion; 7 — thalamus; 8 — epithalamus; 9 — midbrain; 10 — pons; 11 — cerebellum; 12 — medulla oblongata