Brain

In red the brainstem, in yellow the diencephalon, in beige the telencephalon; attached we see the cerebellum.[1]

The brain is the largest and most specialized portion of the encephalon. Vital organ of the central nervous system, the brain takes its place in the skull, above other nerve structures, also of the encephalon and always very important for life, such as the diencephalon, the brainstem and the cerebellum.

The brain is composed of two almost symmetrical elements, called cerebral hemispheres. Each cerebral hemisphere has a distinct superficial cellular layer, called the cerebral cortex, and a deeper cellular component, loosely called the subcortical component.

An embryological offshoot of the forebrain, the brain presides over the control of emotions and voluntary functions, the control of sensory functions (hearing, smell, sight, touch, and taste), the ability to speak and understand language, the faculty of memory, learning, and the processing of memories.

Together with the endocrine system, the brain is responsible for part of the regulation of vital functions and is the seat of homeostatic regulation and higher brain functions. In humans, the activity of the brain, studied by neuroscience, gives rise to the mind with its higher cognitive functions and more generally to the psyche with its psychic functions, studied in psychiatry and psychology.

Anatomy

Separated from the underlying nerve mass by the tentorium (expansion of the dura mater) and located in the anterior part (or cerebral lodge) of the cranial cavity, the brain is the largest and most important segment of the neuraxis, because it is the motor center of nerve activity and the seat of the intellectual faculties. In current use, the brain is improperly understood as the entire nerve mass contained in the skull.

The shape of the brain is roughly similar to that of an ovoid, more expanded posteriorly, more convex in the upper part (in contact with the vault of the skull) and with the lower surface (which rests on the skull base) flattened. Externally, the brain appears to be formed by two voluminous masses, the cerebral hemispheres, with a surface rich in grooves and scissors that divide them into lobes and circumvolutions; in section it appears formed by gray matter and white matter. The gray substance is arranged peripherally and constitutes the cerebral cortex, the white one centrally, formed by bundles of nerve fibers and has nuclei of other gray substance that are important nerve centers. Covered by the cerebral hemispheres or in the framework of these there are then other formations that have mostly anatomical distinction and autonomous physiological properties, which are also an integral part of the brain.

Considering the evolution of the brain from the primitive embryonic vesicles, it is usually divided into two main parts, the midbrain and the forebrain. The midbrain, derived from the primary middle vesicle, is in turn composed of the cerebral peduncles, the lamina quadrigemina and the aqueduct of Silvio; the forebrain, developed from the anterior primary vesicle, includes the diencephalon (subdivided in turn into hypothalamus and thalamencephalon) with the third ventricle, and the telencephalon, which is formed by the two cerebral hemispheres (containing the lateral ventricles), interconnected by interhemispheric formations (corpus callosum, fornix, septum pellucidum, anterior commissure, lamina terminalis), and by the cerebral mantle or pallium, the rhinencephalon or olfactory lobe, and the corpus striatum. The midbrain, which follows the rhomboencephalon and continues with the diencephalon, is placed on the quadrangular lamina of the sphenoid, from which it is separated by the meninges.

Its shape is roughly similar to that of a parallelepiped, about 1 cm high and almost twice as wide, with longitudinal axis directed upward and forward. It can be distinguished in a ventral portion, thicker and expanded, represented by the cerebral peduncles (which connect it directly with the rhomboencephalon), and a dorsal one, thinner, the lamina quadrigemina, on which there are two pairs of hemispherical reliefs, the quadrigeminal tubercles, superior and inferior; between the two portions is the cerebral aqueduct or Silvio (remnant of the cavity of the primary vesicle), which connects the third with the fourth cerebral ventricle.

Among the mesencephalic nerve nuclei, the red nucleus, the mesencephalic integumental center (consisting of scattered multipolar cells, which are part of the indirect cerebellum-spinal nerve pathway) and the negra substance of Sömmering are of importance. The forebrain is the most voluminous part of the entire encephalon and, according to some anatomists, would alone represent the portion of the neuraxis that should be properly defined as the brain. In it, the diencephalon (equipped with its own cavity, the third ventricle, a median sagittal fissure, communicating with the Silvio aqueduct) is in turn subdivided into: hypothalamus, an area of a few centimeters, resting on the underlying sella turcica, on the lower face of which are evident the tubercles or mammillary bodies, the tuber cinereum, the chiasma of the optic nerves; thalamencephalon, a set of anatomical formations including the two optic thalami, the epithalamus and the metathalamus, consisting of the geniculate bodies.

In the internal structure of the diencephalon are contained some nervous nuclei that represent intercalated stations on the pathway of general sensitivity and on the sensory acoustic, optical and olfactory ones, from which stimulations can be conducted both to the cerebral cortex and to the motor nuclei of the brain, cerebellum and spinal cord. Among them we should mention: the hypothalamic nucleus or body of Luys (dense cluster of yellow-pigmented nerve cells, with afferent fibers from the lenticular nerve bundle and efferent fibers to the encephalic nerve nuclei); the lateral, medial and anterior nuclei of the thalamus, which are centers of tactile, thermal and painful sensitivity, as well as olfactory and acoustic; the nucleus of the abenula of the epithalamus, which receives afferent fibers from the olfactory centers; the nuclei of the upper tubercles (centers of the optical pathways) and lower (acoustic centers).

The mass of greater volume of the forebrain is represented by the telencephalon, formed essentially by the two cerebral hemispheres, containing in their thickness the lateral ventricles, and by some interhemispheric formations already described. The hemispheres are not, as a rule, perfectly symmetrical, neither for volume nor for correspondence of the many grooves and clefts. In them we must consider three main formations: the rhinencephalon, which represents the oldest part, consisting of two equal and symmetrical formations, that of the olfactory lobe or rhinencephalon proper and that of the paraolfactory region; the corpus striatum, set of nuclei of gray substance immersed in the white substance connected to them by means of laminae and trabeculae, which give them a particular striated appearance (caudate nucleus, lenticular nucleus, to which can be added the amygdalic nucleus and the antimural or claustrum); has no direct connection with the cerebral cortex, but with the thalamus and hypothalamus, representing an inhibitory center of motor functions; the cerebral mantle or pallium, the largest and most important part, presents in section a peripheral gray layer of a few millimeters, the cerebral cortex, and a more internal white mass, the medullary body.

Since externally the cerebral hemispheres are marked by grooves, incisions and scissures, the surface of the same appears rich in folds more or less large, the lobes, laps or circumvolutions, arranged in man according to a uniform plan, despite a great individual variability. The cerebral mantle is thus subdivided by the scissures (the most important of which are: the lateral or Silvio scissure, the central Rolando sulcus, the parieto-occipital scissure, the cingulum sulcus and the collateral scissure) into eight lobes: five on the outer surface of the hemispheres (frontal, parietal, occipital, and temporal, whose location corresponds, albeit partially, to the location of the eponymous cranial bones; plus the lobe of the insula), one on the medial surface (limbic lobe), and two on the inferior surface (orbital and temporo-occipital lobes); they are in turn marked by numerous less marked grooves and smaller laps or circumvolutions. This external conformation causes the cerebral cortex to be much larger than would be necessary to simply envelop the brain mass: the average surface of the cortex is in fact around 2100-2450 cm2 in the male and 1980-2300 in the female.

The intimate structure of the cerebral cortex is formed by six successive layers, composed of different cellular elements: molecular or plexiform (rich in neuroglia cells and with few nerve cells); outer (rich in small pyramidal cells and other very small cellular elements, called granules); layer of medium and large pyramidal cells (including also, in the deepest parts, the peripheral end of the bundles of radial fibers that originate in the cortex itself or from afferent and efferent fibers); inner layer (composed mainly of granules); layer of large pyramidal cells; deep layer, characterized by the presence of polymorphic cells.

The cerebral cortex is the place of origin and termination of two groups of fibers: those of projection, which put it in relation with the other parts of the neuraxis; and those of association, which connect different areas of the cortex of one or both cerebral hemispheres. In the cortex, in particular areas (called of projection), are located the various faculties of the brain, psychomotor, sensory (for tactile sensations, visual, olfactory, taste), mnemonic, language, writing, etc.. The medullary body is formed by nerve fibers that connect the cerebral cortex with other parts of the neuraxis and has in its interior nuclei of gray matter. To different horizontal and vertical sections of the hemispheres correspond therefore different anatomical and structural characteristics: the mass of white matter thus appears in its maximum extension (it forms the so-called semioval center of each hemisphere) in a horizontal section that affects flat the corpus callosum, while in a frontal section of the brain, at the level of the insula, the white matter is distinguishable in a more internal part (internal capsule), between the caudate nucleus and the thalamus, and a more external one (external capsule) between the lenticular nucleus and the antimural.

Common to the two cerebral hemispheres are finally a group of formations called interhemispheric, consisting mainly of nerve fibers, and therefore whitish in color. These are the corpus callosum, the fornix or cerebral trigone, the septum pellucidum, the anterior commissure (large bundle of fibers that connect transversally the two hemispheres at the level of the perforated substance), the lamina terminalis. Also connecting the two hemispheres is the commessura of the hippocampus, a portion of the horn of Ammon. The blood supply to the brain is provided by superficial and deep cerebral arteries from four large trunks, the two internal carotids and the two vertebral arteries; the waste blood is conveyed from the superficial and deep cerebral veins to the internal and external jugular veins.

Physiology

From a physiological point of view, the brain is the main center of somatic, relational and intellectual life, to which (and more precisely to its cerebral cortex) stimulations come. These stimulations, transmitted from the periphery through sensory nerves, are transformed into conscious impulses and sensations. From the cortex start the voluntary responses that reach the muscles and peripheral organs, through the motor nerves. In particular areas of the cortex (projection areas) are located the various faculties of the brain (motor, general sensitivity and sense organs), although the concept of localization can not be rigidly defined because each brain area is integrated with the others in a framework of general coordination.

The two hemispheres do not perform the same physiological functions; the left one is in fact the seat of particularly important motor centers of articulated language, writing, memory of known words, association between verbal expression and images or ideas, the right one, instead, is involved in non-linguistic activities and has above all the ability to grasp visual messages as a whole, taking into account emotional values. A bridge of nerve fibers, which connect the two halves of the brain, allows the exchange of information. Finally, as far as intellectual activity is concerned, at the current state of knowledge, it is believed that there is no specific area where ideas arise or where memory is located; rather, these capacities are believed to be spread throughout the cerebral cortex and realized through the association between different higher nervous centers.

Pathology

The typical diseases affecting the brain are: traumatic syndromes that, usually considered under the aspect of concussion, contusion and laceration, include disturbances of consciousness (coma, stupor, confusional state and automatism), cerebral hemorrhages, hyper- or hypotensive endocranial syndromes; circulatory syndromes, mainly caused by atherosclerosis, thromboangiitis obliterans, periarteritis nodosa, vascular anomalies, etc.. The clinical forms are mainly represented by: vagal syncope; hypertensive crises (sudden and transient paralysis, monoplegia, hemianopsia or hemiparesis, etc.. ); apoplexy; acute vascular cerebropathies (bulbar thrombosis with violent vertigo, dysphagia, dysarthria, nausea, ataxia, while consciousness remains intact); vascular cerebropathies with slowly progressive nerve disorders (atherosclerotic muscle rigidity, pseudobulbar palsy, internal carotid artery thrombosis) and, depending on the case, crises of disorientation in time and space, progressive psychic and functional impotence, possible dementia of progressive type.

Phlogistic syndromes include forms of infectious origin and probably allergic type (cerebrospinal fever, benign lymphocytic meningitis, tubercular meningitis, acute anterior poliomyelitis, encephalitis). Degenerative syndromes, mostly congenital, hereditary, rarely sporadic, characterized by a progressive course until the total destruction of the entire area or brain system affected, are: congenital diseases, late childhood and youth diseases (tuberous sclerosis, lobar sclerosis); adult diseases (amyotrophic lateral sclerosis); forms of the involutional period (Pick’s disease, Parkinson’s disease, senile dementia).

Tumor syndromes are manifested by signs of local focal irritation (which may also be missing) and a syndrome of endocranial hypertension, which in many cases appears early and often represents the only symptom of the tumor. With regard, however, to the aging of brain cells until the last years of the twentieth century neuroscientists have argued that the responsibility of cognitive deficits of the elderly was due to a gradual thinning of all brain neurons. Following studies carried out on monkeys, at the University of California, it was found that memory loss in the elderly does not result from the disappearance of cortical neurons, more durable and resistant than assumed, but from the death and, above all, atrophy of neurons in the hippocampus and some subcortical nuclei.

References

  1. Images generated by Life Science Databases (LSDB) – from Anatomography website maintained by Life Science Databases (LSDB). CC BY-SA 2.1 jp, https://commons.wikimedia.org/w/index.php?curid=9809653

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