Human Anatomy, Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovskyi 2008

Special Part
Nervous system, systema nervosum - Special Part
Brain - Brainstem

Class="center">General Overview of the Brain

The brain, encephalon, is the central organ of the human Nervous system; together with the Spinal Cord, it regulates the body's interaction with the environment and controls bodily reactions and Functions. Human intellectual activity (thinking, memory, learning, and creativity) is directly linked to brain function.

The brain is located within the cranial cavity and conforms to the shape of the Skull. Its average weight is 1375 g in adult men and 1275 g in women (with individual variations ranging from 900 to 2000 g). Only the elephant (4000 g) and the whale (2800 g) have a brain weight greater than that of humans.

The brain consists of the cerebral hemispheres, hemispherium cerebrale, and the Brainstem, truncus cerebri. The superolateral surface, facies superlaterala hemispherii, is convex and forms the hemispheres. The cerebral hemispheres are divided into right and left by the longitudinal cerebral fissure, fissura longitudinalis cerebraіis, deep within which lies the commissure connecting them—the corpus callosum, corpus calosum.

Fig. 132. Brain, encephalon. Base of the brain and emergence of Cranial Nerves (inferior surface)

Posterior to the corpus callosum, the longitudinal fissure continues into the transverse cerebral fissure, fissura transversa cerebraiis, which separates the posterior Regions of the hemispheres from the Cerebellum.

The inferior surface, facies inferior, is slightly flattened and formed by the inferior surfaces of the hemispheres and PARTS OF THE brainstem (Fig. 132). The anterior part of the Inferior surface of the brain is formed by the frontal lobes of the hemispheres, where the olfactory bulbs, bulbi olfactorii, are located, receiving the olfactory filaments, filia olfactoria, of the Olfactory nerve. The olfactory bulbs continue into the olfactory tracts, tractus olfactorius, between which lies an elevation, the olfactory trigone, trigonum olfactorium. Posterior to the trigonum olfactorium is the anterior perforated substance, substantia perforata anterior. Between the perforated spaces lies the optic chiasm, chiasma opticum. From the anterior surface of the chiasm, a thin grey plate, the lamina terminalis, extends and penetrates deep into the longitudinal cerebral fissure. Posterior to the optic chiasm is the tuber cinereum, tuber сіnеrеum, the apex of which continues into the infundibulum, infundibulum, from which the Pituitary Gland, hypophysis cerebri, is suspended. Posterior to the tuber cinereum are two spherical white structures—the mammillary bodies, corpora mamilaria. Behind them lies the interpeduncular fossa, fossa interpeduncularis, which is laterally bounded by the cerebral peduncles, pedunculi cerebri. At the bottom of the fossa is the posterior perforated substance, substantia perforata posterior, through the openings of which Blood Vessels penetrate into the brain substance. On the Medial surface of the cerebral peduncles, There is a small groove where the Oculomotor nerve, n. oculomotorius (III), emerges, while the Trochlear nerve, n. trochlearis (IV), which emerges from the DORSAL SIDE OF the medullary velum, passes along the lateral side of the peduncles. Inferior to the cerebral peduncles is the Pons, pons, the lateral parts of which merge into the cerebellum to form the middle cerebellar peduncles, pedunculi cerebellaris medii, from the base of which the Trigeminal nerve, n. trigeminus (V), emerges. Below the pons lies the Medulla Oblongata, medulla oblongata. At the border between the pons and the medulla oblongata, the Abducens nerve, n. abducens (VI), emerges, and lateral to the middle cerebellar peduncles, the Facial Nerve, n. facialis (VII), and the Vestibulocochlear nerve, n. vestibulo-cochlearis (VIII), emerge. On the ventral surface of the medulla oblongata, the Hypoglossal nerve, n. hypoglossus (XII), emerges from the sulcus anterolateralis, while the Glossopharyngeal nerve, n. glossopharyngeus (IX), Vagus nerve, n. vagus (X), and Accessory nerve, n. accesorius (XI), emerge from the sulcus relroolivaris.

The brainstem, truncus cerebri, comprises the medulla oblongata, pons, and Midbrain (Fig. 133).

The brainstem is anatomically and functionally connected to the spinal cord and the cerebral hemispheres, which exert a regulatory and controlling influence over all its divisions and centers. Cranial nerves originate or terminate in the nuclei of the brainstem, with the exception of the olfactory nerve, n. olfactorius, the Optic nerve, n. opticus, and the spinal portion of the accessory nerve, n. accessorius. Alongside the cranial nerve nuclei, the brainstem contains other anatomical structures: nuclei olivares, nuclei pontis, nuclei rubri, stratum griscum colliculi superioris, nuclei colliculi inferioris, formatio reticularis. The reticular formation, localized within the brainstem, is composed of at least four neuronal systems.

Fig. 133. Brainstem, truncus cerebri (ventrolateral surface)

One of its functions is to activate the reticular activating system, or RAS (arousal system), which keeps the brain in a state of alertness. Nerve fiber pathways within the reticular activating system receive incoming sensory information. They send activating signals through the midbrain to the Cerebral Cortex to maintain a state of consciousness. The brainstem houses vital centers that regulate Respiration, Heart rate, vascular tone, autonomic functions, and endocrine gland activity, as well as reflex acts (swallowing and vomiting). Both simple and complex reflex arcs synapse within the brainstem nuclei. The brainstem also controls Sleep, modulates spinal Reflexes, and maintains Muscle tone and posture.