Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Brain
Forebrain

The Diencephalon (Figs. 3.14, 3.15) is largely concealed by the cerebral hemispheres. It comprises the following structures: the paired thalami (thalamus), the lateral and medial geniculate bodies, and the hypothalamic (Hypothalamus) and epithalamic (epithalamus) regions. The cavity of the diencephalon is the Third ventricle.

The thalamus (thalamus opticus) is a large ovoid body (Fig. 3.22). Inferiorly, it merges with the hypothalamus, from which it is separated by the hypothalamic sulcus. Laterally, the thalamus borders the Basal Ganglia of the cerebral hemispheres, separated from them by the terminal sulcus (Fig. 3.30). The Medial surface of the thalami forms the lateral wall of the III ventricle. This surface is separated from the superior surface by the stria medullaris, which widens caudally to form the habenular trigone. The medial surfaces of the two thalami are joined by the interthalamic adhesion. On the anterior part of the superior surface of the thalamus, the anterior tubercle is clearly visible, while its posterior portion features a prominent Swelling called the pulvinar. Ventral to the pulvinar lie the medial and lateral geniculate bodies (Fig. 3.27).

The basal plate of the embryonic neural tube terminates in the Midbrain, and the thalamus (along with the entire forebrain) develops from the alar plate, which contains no motor Neurons.

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Fig. 3.30. Frontal section of the brain through the cerebral peduncles:

1 — fornix; 2 — choroid plexus in the central part of the lateral ventricle; 3 — tail of the caudate Nucleus; 4 — medial and 5 — lateral nuclei of the thalamus; 6 — internal capsule; 7 — lentiform nucleus; 8 — external capsule; 9 — claustrum; 10 — red nucleus; 11 — substantia nigra; 12 — choroid plexus of the inferior horn of the lateral ventricle (13); 14 — fimbria of the fornix; 15 — hippocampus; 16 — hippocampal sulcus; 17 — corticospinal tract; 18 — Pons; 19 — parahippocampal gyrus; 20 — crus cerebri; 21 — optic tract; 22 — ventral Nucleus of the thalamus; 23 — insula; 24 — lateral sulcus; 25 — corpus callosum

The thalamus consists of Gray matter organized into nuclei (Fig. 3.30). There are five main groups of thalamic nuclei: anterior, intralaminar, medial, lateral, and posterior (Atl. Fig. 120). Each of these groups contains 5–6 or more nuclei. The nuclear groups are separated by a V-shaped internal medullary lamina (laminae medullares), which also contains clusters of neurons known as the intralaminar nuclei. The mammillothalamic tract, which carries multicomponent impulses from the hypothalamus, terminates in the anterior nucleus. At the caudal pole of the thalamus, in the pulvinar, some fibers of the optic tract terminate (Atl. Fig. 122).

Along with the morphological Classification (by Location), There is a Structure/81.html">Functional Classification of thalamic nuclei. Thalamic nuclei whose neurons project axons to the Cerebral Cortex are classified as projection or Relay (specific) nuclei. The axons of ascending Sensory Systems, except for the olfactory system, synapse on their neurons. For example, the lower part of the thalamus contains the primary nuclei of the somatosensory and musculoskeletal sensitivity systems—the ventrolateral and ventromedial nuclei, where the spinothalamic tract, medial lemniscus, trigeminal lemniscus, fibers of the superior cerebellar peduncles, and other pathways carrying impulses from the Skin and proprioceptors terminate. The lateral and medial geniculate bodies also belong to this group. They relay impulses of the visual and auditory sensory systems, respectively. Destruction of projection nuclei leads

to a complete loss of the corresponding type of sensation, indicating that there is no alternative pathway for sensory information to reach the cortex.

Projection nuclei exhibit topographic Organization. This means that each point of the receptive field of a given sensory system projects to a specific group of neurons within these nuclei. The higher the density of receptor endings in a given surface area, the larger the number of thalamic neurons receiving impulses from these receptors. A surface area with a lower density of receptor endings has a smaller representation. The same pattern is repeated in the cerebral cortex. In addition to transmitting sensory signals to the cortex, these nuclei perform complex Processing of sensory information.

Among the projection nuclei, some receive afferent impulses from the Cerebellum and basal ganglia and transmit efferent signals to the motor areas of the cortex (Atl. Figs. 121, 122). In addition, afferent input to these nuclei may come from other subcortical structures and project to the limbic Regions of the cortex.

Other nuclei, such as the lateral nuclei and most of the pulvinar, transmit excitation to the inter-analyzer or association areas of the cortex; these are classified as association nuclei. They are characterized by receiving multiple sources of afferent input. Information reaches these nuclei from the geniculate bodies, other thalamic nuclei, the amygdaloid complex, etc. Damage to the neurons of the association nuclei and their connections with the parieto-occipital areas of the cortex (the thalamo-parietal system) leads to speech recognition deficits and attention disorders. These nuclei also project to the frontal areas of the cortex (the thalamo-frontal system), which are involved in regulating emotional states, spatial and temporal perception, etc. The association nuclei of the thalamus are considered phylogenetically recent, as they develop and differentiate in conjunction with The formation of the association areas of the neocortex.

Like the Brainstem, the thalamus contains structures functionally similar to the reticular formation. These include the midline and intralaminar nuclei, as well as certain nuclei of the lateral group (Atl. Fig. 120). They exert a non-specific influence on the cerebral cortex, producing not only excitatory but also distinct inhibitory effects, and are therefore termed non-specific. Similar to the brainstem reticular formation, the non-specific thalamic nuclei do not perform any specific higher integrative Functions, but to some extent participate in modulating various afferent signals traveling along the ascending pathways of sensory systems. This group of nuclei receives afferent input from A large number of subcortical structures (the red nucleus, substantia nigra, cerebellar nuclei, hippocampus, amygdala, and other subcortical nuclei); they lack clearly organized projections to the cortex. These nuclei are involved in regulating Sleep-wake cycles, emotional states, etc.

The processes of thalamic neurons form the thalamic radiation (corona radiata). Its fibers project to the cerebral hemispheres, where they terminate primarily in the cortex, as well as on the Cells of the basal ganglia.

The lateral geniculate body (corpus geniculatum laterale) (Fig. 3.27; Atl. Fig. 150) is located near the pulvinar of the thalamus and the optic tract, whose fibers terminate within it. The lateral geniculate body consists of dorsal and ventral nuclei. The dorsal nucleus has a laminated structure and receives afferent fibers from the lateral ROOT of the optic tract and the brachia of the superior colliculus. Efferent fibers project to areas 17, 18, and 19 of the occipital lobe of the cortex. The ventral nucleus receives afferents from the superior colliculus, cerebellum, and optic tract, while its efferent fibers project to the intralaminar nuclei of the thalamus, superior colliculus, tegmentum of the cerebral peduncles, and midbrain reticular formation. This nucleus has no direct projections to the cortex.

The medial geniculate body (corpus geniculatum mediale) lies at the level of the transverse groove of the corpora quadrigemina (Fig. 3.27; Atl. Fig. 154). It is formed by several nuclei. Afferent fibers arrive via the lateral lemniscus and the brachia of the inferior colliculus. Efferent fibers travel within the auditory radiation to areas 41 and 42 of the cerebral cortex.

The hypothalamus, or subthalamic region, is a part of the diencephalon consisting of the mammillary bodies, tuber cinereum, infundibulum, and optic chiasm. The hypothalamus lies below the thalamus and is visible on the Base of the brain between the cerebral peduncles. The posterior part of the hypothalamus is formed by two mammillary bodies (corpora mamillaria) (Figs. 3.14, 3.31). The columns of the fornix—pathways from the archicortex of the Telencephalon—terminate in them. Efferent fibers of the mammillary bodies form the mammillothalamic tract, through which impulses reach the anterior nuclei of the thalamus (Atl. Fig. 125). Recently, the mammillary bodies, like the anterior thalamic nuclei, have been associated with the Limbic System and the organization of behavioral responses.

Fig. 3.31. Diencephalon and midbrain (inferior view):

1 — chiasm; 2 — olfactory triangle; 3 — tuber cinereum; 4 — optic tract; 5 — mammillary bodies; 6 — posterior perforated substance; 7 — cerebral aqueduct; 8 — corpora quadrigemina; 9 — red nucleus; 10 — lateral and 11 — medial geniculate bodies; 12 — cerebral peduncles; 13 — infundibulum; 14 — anterior perforated substance; 15 — lateral olfactory stria; 16 — Optic nerve; 17 — medial olfactory stria; 18 — olfactory tract

Anterior to the mammillary bodies lies the tuber cinereum. Tapering downward, it becomes the infundibulum, which penetrates the hypophyseal fossa of the sella turcica through its diaphragma sellae. The Pituitary Gland is suspended, as it were, from the infundibulum. In front of the tuber cinereum, the optic nerves (cranial nerve II) cross to form the optic chiasm (chiasma opticum), beyond which they are referred to as the optic tracts. After winding around the cerebral peduncles, the optic tracts divide into medial and lateral roots. The medial root projects to the superior colliculus and the thalamus, while the larger lateral root goes to the lateral geniculate body.

The hypothalamus is divided into anterior, intermediate, posterior, and dorsal regions (Atl. Fig. 123). These regions contain clusters of nuclei (32 pairs) whose neurons participate in regulating the Autonomic Nervous system, thereby helping to maintain Homeostasis and adapt the body's internal environment to external conditions (Atl. Fig. 124). They send impulses to the corresponding nuclei of the midbrain, Medulla Oblongata, and Spinal Cord, regulating their activity.

The hypothalamus links the body's nervous and endocrine systems. Axons from the supraoptic and paraventricular nuclei travel along the hypothalamo-hypophyseal tract within the pituitary stalk to the posterior pituitary (neurohypophysis), where they make contact with Blood capillaries. Hormones are released from these axons via exocytosis and carried throughout the body by the bloodstream. Neurons of other nuclei project their axons to the anterior pituitary (adenohypophysis) and secrete releasing hormones. The latter regulate the Synthesis and Secretion of hormones by the endocrine cells of the adenohypophysis.

The hypothalamus is connected to various brain regions via numerous pathways (Atl. Fig. 125), establishing an interface between the central and autonomic nervous systems. For example, the fibers connecting the hypothalamus to the lateral geniculate bodies convey signals from the retina to the hypothalamic nuclei, which is essential for regulating circadian rhythms. The hypothalamus is also linked to the structures of the limbic system by numerous tracts. Hypothalamic projections to the cortex are mediated by the thalamic nuclei. The medial forebrain bundle runs through the entire hypothalamus, reaching the brainstem reticular formation.

The epithalamus, or epithalamic region, forms part of the walls of the third ventricle and consists of the striae medullares, which widen posteriorly into the habenular triangles (Fig. 3.22). From these triangles, white bands—the habenulae—extend backward, connecting them to the Pineal Gland (epiphysis), an endocrine gland. The Inferior surface of the pineal gland rests against the posterior commissure, which curves backward. Descending fibers from the habenular nuclei, located within the corresponding triangles, project to the midbrain nuclei. These nuclei are also directly connected to the thalamus and hypothalamus. The epithalamus is also linked to the olfactory system.

The third ventricle (ventriculus tertius) has the appearance of a vertical slit (Fig. 3.22; Atl. Fig. 131). Its lateral walls are formed by the medial surfaces of the thalami and hypothalami. The latter also bounds the ventricle from below. Its posterior wall includes the posterior commissure, below which the opening of the cerebral aqueduct is located. The anterior wall of the ventricle is formed by the columns of the fornix and the anterior commissure passing in front of them, which belong to the telencephalon. Between the columns of the fornix and the thalami are the interventricular foramina, connecting the cavity of the third ventricle with the Lateral ventricles of the cerebral hemispheres. The superior wall of the third ventricle consists of an epithelial lamina (a remnant of the cerebral vesicle wall) and the pia mater, forming the choroid plexus of the third ventricle.

Cerebral hemispheres. The diencephalon, along with the brainstem, is covered from above and from the sides by the cerebral hemispheres—the telencephalon. The hemispheres consist of subcortical nuclei (basal ganglia) and White matter, and contain cavities called the lateral ventricles. Externally, the hemispheres are covered by the cortex (Pallium).

The basal ganglia, or subcortical nuclei (nuclei basales), are phylogenetically older structures than the cortex. The basal ganglia derive their name from the fact that they lie at the base of the cerebral hemispheres, in their basal region. They include the caudate and lentiform nuclei, which are collectively referred to as the corpus striatum (striatum), the claustrum, and the amygdaloid body (Atl. Fig. 134).

The caudate nucleus (nucleus caudatus) is elongated in the sagittal plane and highly curved (Figs. 3.22; 3.32; 3.33). Its anterior, enlarged portion—the HEAD—is located in front of the thalamus, in the lateral wall of the anterior horn of the lateral ventricle; posteriorly, it gradually tapers and becomes the tail. The caudate nucleus arches over the thalamus anteriorly, superiorly, and laterally.

Fig. 3.32. The brain — horizontal section through the lateral ventricles:

1 — corpus callosum; 2 — insula; 3 — cortex; 4 — tail of the caudate nucleus; 5 — fornix; 6 — posterior horn of the lateral ventricle; 7 — hippocampus; 8 — choroid plexus; 9 — interventricular foramen; 10 — septum pellucidum; 11 — head of the caudate nucleus; 12 — anterior horn of the lateral ventricle

Fig. 3.33. Horizontal section of the cerebral hemispheres at the level of the basal ganglia:

1 — corpus callosum; 2 — fornix; 3 — anterior horn of the lateral ventricle; 4 — head of the caudate nucleus; 5 — internal capsule; 6 — putamen; 7 — globus pallidus; 8 — external capsule; 9 — claustrum; 10 — thalamus; 11 — pineal gland; 12 — tail of the caudate nucleus; 13 — choroid plexus of the lateral ventricle; 14 — posterior horn of the lateral ventricle; 15 — cerebellar vermis; 16 — corpora quadrigemina; 17 — posterior commissure; 18 — cavity of the third ventricle; 19 — lateral sulcus fossa; 20 — insula; 21 — anterior commissure

The lentiform nucleus (nucleus lentiformis) is located lateral to the thalamus, at the level of the insula. The shape of The Nucleus resembles a three-sided pyramid with its base facing outward. The nucleus is clearly divided by white matter laminae into a darker lateral portion—the putamen—and a medial portion—the globus pallidus, which consists of two segments: internal and external (Figs. 3.33; 3.34).

The putamen is genetically, structurally, and functionally similar to the caudate nucleus. Both of these structures have a more complex organization than the globus pallidus. They receive fibers mainly from the CEREBRAL CORTEX AND the thalamus (Fig. 3.35).

Fig. 3.34. Coronal section through the cerebral hemispheres at the level of the basal ganglia:

1 — corpus callosum; 2 — lateral ventricle; 3 — caudate nucleus (head); 4 — internal capsule; 5 — lentiform nucleus; 6 — lateral sulcus; 7 — temporal lobe; 8 — claustrum; 9 — insula; 10 — external capsule; 11 — septum pellucidum; 12 — radiation of the corpus callosum; 13 — cerebral cortex

The globus pallidus is primarily involved in transmitting impulses along numerous descending pathways to lower brain structures, such as the red nucleus, substantia nigra, and others. Fibers from the neurons of the globus pallidus project to the same thalamic nuclei that are connected to the cerebellum. From these nuclei, numerous pathways project to the cerebral cortex. The globus pallidus receives inputs from the caudate nucleus and the putamen.

The corpus striatum (striatum), which combines the caudate and lentiform nuclei, belongs to the efferent extrapyramidal system. The dendrites of striatal neurons are covered with numerous dendritic spines. Fibers from neurons of the cortex, thalamus, and substantia nigra terminate on them (Fig. 3.35). In turn, striatal neurons send axons to the intralaminar, anterior, and lateral nuclei of the thalamus. From these nuclei, fibers project to the cortex, thereby closing the feedback loop between cortical neurons and the striatum.

Fig. 3.35. Afferent and efferent connections of the basal ganglia:

1 — precentral gyrus; 2 — putamen; 3 — external and internal segments of the globus pallidus; 4 — ansa lenticularis; 5 — reticular formation; 6 — reticulospinal tract; 7 — rubrospinal tract; 8 — cerebellothalamic tract (from the dentate nucleus of the cerebellum); 9 — red nucleus; 10 — substantia nigra; 11 — subthalamic nucleus; 12 — Zona incerta; 13 — hypothalamus; 14 — ventrolateral, 15 — intralaminar and centromedian nuclei of the thalamus; 16 — III ventricle; 17 — caudate nucleus

During phylogenesis, these nuclei developed on top of the midbrain nuclei. Receiving impulses from the thalamus, the corpus striatum participates in the execution of complex automatic movements such as walking, climbing, and running. The reflex arcs of highly complex unconditioned (i.e., innate) Reflexes close within the nuclei of the corpus striatum. The extrapyramidal system is phylogenetically older than the pyramidal system. In newborns, the latter is not yet sufficiently developed, and impulses to the Muscles are delivered from the subcortical ganglia via the extrapyramidal system. Consequently, a child's movements in the first months of life are characterized by being generalized and undifferentiated. As the cerebral cortex develops, the axons of its cells grow down to the basal ganglia, and The activity of the latter begins to be regulated by the cortex. Subcortical ganglia are associated not only with motor responses but also with autonomic functions—they are the higher subcortical centers of the autonomic nervous system.

The caudate nucleus, the lentiform nucleus, and the thalamus are separated from each other by white matter—the internal capsule.

The claustrum (claustrum) is a thin sheet of gray matter adjacent laterally to the putamen, from which it is separated by a thin layer of white matter—the external capsule (Figs. 3.30, 3.33). The claustrum is separated from the insular cortex by an equally thin layer of white matter.

The basal ganglia play an important role in The regulation of movement and sensorimotor coordination. Clinical data have shown that damage to the putamen and globus pallidus leads to slow, stereotypical movements, whereas degeneration of striatal neurons results in abrupt, involuntary movements, etc.

The amygdaloid body (corpus amygdaloideum) (amygdala) is a cluster of nuclei located within the WHITE MATTER OF the temporal lobe. It is connected to the contralateral amygdala via the anterior commissure. The amygdaloid body receives inputs from various afferent systems, including the olfactory system, and is involved in emotional responses (Fig. 3.36).

The surface of the hemispheres. In humans, the cerebral cortex (pallium) is represented by a sheet of gray matter separated from the ventricular cavity by white matter. This is primarily the neocortex, which almost completely replaces the cortex of lower vertebrates; the latter persists in humans only as small areas of the paleocortex and archicortex. Progressively developing in mammals, the neocortex reaches immense proportions and a complex structure in humans, dominating the functions of all underlying brain regions.

The longitudinal cerebral fissure divides the brain into two hemispheres: the right and the left. Three surfaces are distinguished in each hemisphere: the superolateral surface, adjacent to the inner surface of the cranial vault (Atl. Figs. 126, 128); the inferior surface, the anterior and middle parts of which rest on the Skull base, while the posterior part lies on the tentorium cerebelli (Atl. Fig. 127); and the medial surface, facing the longitudinal fissure (Atl. Fig. 129). Where one surface transitions into another, the borders of the hemisphere are formed: superior, inferolateral, and inferomedial. Each hemisphere also has frontal, temporal, and occipital poles.

Fig. 3.36. Brain structures associated with the amygdala: afferent (A) and efferent (B) connections of the amygdala:

1 — thalamic nuclei; 2 — periaqueductal gray; 3 — parabrachial nucleus; 4 — locus coeruleus; 5 — raphe nuclei; 6 — nucleus of the solitary tract; 7 — dorsal nucleus of the Vagus nerve; 8 — temporal cortex; 9 — olfactory cortex; 10 — olfactory bulb; 11 — frontal cortex; 12 — cingulate gyrus; 13 — corpus callosum; 14 — olfactory nucleus; 15 — anteroventral and 16 — dorsomedial nuclei of the thalamus; 17 — central, 18 — cortical, and 19 — basolateral nuclei of the amygdala; 20 — hypothalamus; 21 — reticular formation; 22 — septum; 23 — substantia nigra; 24 — ventromedial nucleus of the hypothalamus; XXIII, XXIV, XXVIII — cortical areas

Throughout its extent, the cortex folds into numerous sulci (sulci cerebri), which divide the surface of the hemisphere into convex gyri and lobes. Sulci are classified as primary, secondary, and tertiary. Primary sulci are the deepest and most constant in their configuration; they are the first to form during prenatal ontogeny. Some of them are referred to as fissures (for example, the lateral sulcus or fissure, and the calcarine sulcus or fissure). Secondary sulci are shallower and have a more variable pattern. They also form prenatally, but later than the primary ones. Tertiary sulci are shorter, shallower, even more variable in shape, and emerge at the end of intrauterine development and during postnatal ontogeny. It is these sulci that give the surface of the cerebral hemisphere its unique, distinctive appearance, providing each individual with a specific, highly personalized sulcal pattern. Even the two hemispheres of the same individual are asymmetric.

Six lobes are distinguished in each hemisphere: frontal, parietal, temporal, occipital, limbic, and the insula. They are separated by the lateral, central, parieto-occipital, cingulate, and collateral sulci.

The lateral sulcus (sulcus lateralis) begins at the base of the hemisphere as a deep depression, the floor of which is formed by the insula, itself covered with sulci and gyri (Fig. 3.37). The lateral sulcus extends onto the lateral surface of the hemisphere, running gently backward and upward, separating the temporal lobe from the overlying lobes: the frontal lobe anteriorly and the parietal lobe posteriorly. Two branches arise from the Water/144.html">Origin of the lateral sulcus: the ascending ramus extending upward, and the anterior ramus extending forward (Atl. Figs. 127, 128).

The central sulcus (sulcus centralis) typically cuts through the medial border of the hemisphere at approximately its midpoint and extends downward and forward along the superolateral surface, usually not reaching the lateral sulcus. The central sulcus separates the frontal lobe from the parietal lobe (Atl. Figs. 126, 128).

Fig. 3.37. Superolateral surface of the hemisphere. Insula:

1 — precentral sulcus; 2 — superior frontal gyrus; 3 — superior frontal sulcus; 4 — middle and 5 — inferior frontal gyri; 6 — orbital part of the frontal lobe; 7 — sulci and gyri of the insula; 8 — temporal pole; 9 — superior temporal gyrus; 10 — superior temporal sulcus; 11 — middle temporal and 12 — angular gyri; 13 — superior parietal lobule; 14 — supramarginal gyrus; 15 — intraparietal and 16 — postcentral sulci; 17 — postcentral gyrus; 18 — central sulcus; 19 — precentral gyrus

The parieto-occipital sulcus (sulcus parietooccipitalis) runs vertically along the medial surface of the hemisphere, separating the parietal lobe (precuneus) from the occipital lobe. On the superolateral surface of the hemisphere, the boundary between these lobes is defined by an imaginary line connecting the superior end of the parieto-occipital sulcus with the preoccipital notch (an impression on the inferior margin of the hemisphere caused by the petrous part of the Temporal bone).

The cingulate sulcus (sulcus cinguli) runs along the medial surface of the hemisphere parallel to the corpus callosum, separating the frontal and parietal lobes from the cingulate gyrus (limbic lobe). The sulcus ends in a vertical branch (Atl. Fig. 129).

On the inferior surface of the hemisphere, the collateral sulcus (sulcus collateralis) separates the temporal lobe from the limbic and occipital lobes (Atl. Figs. 127, 129).

On the inferior surface of the hemisphere, in its anterior part, lies the olfactory sulcus (sulcus olfactorius), which extends to the anterior perforated substance (substantia perforata anterior). The olfactory bulb (bulbus olfactorius) lies within the olfactory sulcus and continues as the olfactory tract (tractus olfactorius). Posteriorly, it bifurcates into the lateral and medial olfactory striae, which form the olfactory triangle. The anterior perforated substance is located at its center.

Lobes of the hemisphere. The frontal lobe (lobus frontalis) is bounded posteriorly by the central sulcus (sulcus centralis) and the superior and inferior precentral sulci (sulcus precentralis) located anteriorly and parallel to it. The latter bound the precentral gyrus (gyrus precentralis), which extends onto the medial surface of the hemisphere as the anterior part of the paracentral lobule. Arising anteriorly from both precentral sulci at almost right angles are two parallel sulci—the superior frontal and inferior frontal sulci—which delineate three frontal gyri. The superior frontal gyrus (gyrus frontalis superior) extends onto the medial surface of the hemisphere, where it is bounded inferiorly by the cingulate sulcus (sulcus cinguli). The middle frontal gyrus (gyrus frontalis medius) is situated between the superior and inferior frontal sulci. The inferior frontal gyrus (gyrus frontalis inferior) is divided into three parts: the opercular part (pars opercularis)—between the inferior precentral sulcus posteriorly, the inferior frontal sulcus superiorly, and the ascending ramus of the lateral sulcus anteriorly; the triangular part (pars triangularis)—between the ascending and anterior rami of the lateral sulcus; and the orbital part (pars orbitalis)—below the anterior ramus of the lateral sulcus, extending onto the inferior surface of the frontal lobe. The most anteriorly projecting part of the frontal lobe is called the frontal pole.

The parietal lobe (lobus parietalis) contains the postcentral gyrus (gyrus postcentralis) in its anterior part, between the central sulcus and the parallel postcentral sulcus (sulcus postcentralis). On the medial surface of the hemisphere, it continues as the posterior part of the paracentral lobule (lobulus paracentralis). Running perpendicular to the postcentral sulcus, posteriorly and parallel to the medial margin of the hemisphere, is the intraparietal sulcus (sulcus intraparietalis), which divides the posterior part of the parietal lobe into the superior and inferior parietal lobules. The superior lobule extends onto the medial surface of the hemisphere as the precuneus. The posterior ends of the lateral sulcus and the underlying superior temporal sulcus project into the inferior parietal lobule. The portion of this lobule surrounding the end of the lateral sulcus is called the supramarginal gyrus (gyrus supramarginalis), while the portion surrounding the end of the superior temporal sulcus is the angular gyrus (gyrus angularis). The parietal lobe is separated from the occipital lobe by part of the parieto-occipital sulcus (sulcus parietooccipitalis).

The temporal lobe (lobus temporalis) is separated from the frontal and parietal lobes by the lateral sulcus. On its superolateral surface lie three parallel sulci. The superior temporal sulcus (sulcus temporalis superior) lies directly below the lateral sulcus and bounds the superior temporal gyrus (gyrus temporalis superior). The inferior temporal sulcus (sulcus temporalis inferior), which usually consists of separate segments, bounds the middle temporal gyrus (gyrus temporalis medius) from below. The inferior temporal gyrus (gyrus temporalis inferior) is bounded medially by the inferolateral margin of the hemisphere. On the inferior surface of the temporal lobe are the medial and lateral occipitotemporal gyri (gyrus occipitotemporalis medialis et lateralis), separated by the sulcus of the same name. The collateral sulcus (sulcus collateralis) separates the medial occipitotemporal gyrus from the limbic lobe. Anteriorly, the temporal lobe tapers to form the temporal pole.

The occipital lobe (lobus occipitalis) is separated from the parietal lobe by the parieto-occipital sulcus on the upper part of its superolateral surface and on its medial surface. It has no constant sulci on its superolateral surface. Its primary sulcus, the calcarine sulcus (sulcus calcarinus), runs horizontally on the medial surface from the occipital pole to the parieto-occipital sulcus, where they merge into a single trunk. Between these sulci lies a triangular gyrus—the cuneus. The calcarine sulcus and the posterior part of the collateral sulcus bound the medial occipitotemporal gyrus. The inferior surface of the occipital lobe rests on the tentorium cerebelli. At its posterior end, the lobe narrows to form the occipital pole.

The limbic lobe is located on the medial and inferior surfaces of the hemisphere. It includes the cingulate and parahippocampal gyri. The cingulate gyrus (gyrus cinguli) is bounded inferiorly by the sulcus of the corpus callosum and superiorly by the cingulate sulcus, which separates it from the frontal and parietal lobes. The parahippocampal gyrus (gyrus parahippocampalis) is bounded superiorly by the hippocampal sulcus (sulcus hippocampi), which serves as an antero-inferior continuation of the posterior end of the sulcus of the corpus callosum. Inferiorly,

the gyrus is separated from the temporal lobe by the collateral sulcus.

The anterior end of the parahippocampal gyrus curves around the anterior end of the hippocampal sulcus, forming the uncus (uncus).

The white matter lies beneath the cerebral cortex, forming a continuous mass above the corpus callosum. Below, the white matter is interrupted by collections of gray matter (basal ganglia) and is situated between them in the form of layers or capsules (Fig. 3.33).

The white matter consists of association, commissural, and projection fibers.

Association fibers connect different areas of the cortex within the same hemisphere. Short fibers (arcuate fibers) run along the bottom of the sulci and connect the cortex of adjacent gyri, while long fibers connect the gyri of different lobes (Fig. 3.38). The long association fibers include:

— the superior longitudinal fasciculus connects the inferior frontal gyrus with the inferior parietal lobule, temporal, and occipital lobes; it is arc-shaped, curving around the insula, and extends along the entire hemisphere;

— the inferior longitudinal fasciculus connects the temporal lobe with the occipital lobe;

— the fronto-occipital fasciculus connects the frontal lobe with the occipital lobe and the insula;

— the cingulum connects the anterior perforated substance with the hippocampus and uncus; it is arched within the cingulate gyrus, curving over the corpus callosum;

— the uncinate fasciculus connects the inferior frontal lobe, the uncus, and the hippocampus.

Commissural fibers connect the cortex of symmetrical regions of both hemispheres. The corpus callosum is the largest commissural system, connecting corresponding areas of the neocortex of the right and left hemispheres (Fig. 3.39). It is located deep within the longitudinal fissure and is a flattened, elongated structure (Fig. 3.14). The anterior part of the corpus callosum curves forward and downward, ending in a tapered portion—the rostrum and terminal lamina. The middle part—the body (trunk)—is the longest and is convex. The posterior part—the splenium—overhangs the tectal plate and the pineal gland. The surface of the corpus callosum is covered by a thin layer of gray matter, which forms four longitudinal striae. Posteriorly, it continues as the dentate gyrus onto the parahippocampal gyrus, and anteriorly into the paraterminal gyrus. The fibers radiating from the corpus callosum form its radiation, which is divided into frontal, parietal, temporal, and occipital parts.

Fig. 3.38. Association fiber systems of the cerebral hemispheres:

A — dissection specimen prepared by the fiber-splitting method; B, C — diagrams (after S.P. Dzugaeva); 1 — short association (arcuate) fibers; 2 — long association fibers; 3 — projection fibers; 4 — parietal lobe; 5 — frontal lobe; 6 — superior longitudinal fasciculus; 7 — fronto-occipital fasciculus; 8 — temporal and 9 — occipital lobes; 10 — inferior longitudinal and 11 — cingulum fasciculi; 12 — fronto-temporal fasciculus

Fig. 3.39. Corpus callosum:

1 — radiation of the corpus callosum (frontal part); 2 — medial and lateral longitudinal striae of gray matter; 3 — corpus callosum; 4 — radiation of the corpus callosum (parietal part); 5 — radiation of the corpus callosum (occipital part)

For the phylogenetically ancient cortex, the anterior and posterior commissures serve as the commissural fiber systems. The anterior commissure (commissura anterior) connects the unci of the temporal lobes and the parahippocampal gyri, as well as the gray matter of the olfactory triangles (Fig. 3.14).

Projection fibers extend beyond the hemispheres as part of Projection Pathways. They provide bidirectional communication between the cortex and lower Divisions of the Central nervous system, down to the spinal cord.

All projection PATHWAYS OF THE hemispheres, both ascending and descending, pass through the internal capsule. It is a continuation of the crus cerebri (Fig. 3.30; see Atlas). Between the internal capsule and the cortex, the projection pathways fan out, forming the corona radiata.

The internal capsule is divided into the anterior limb, posterior limb, and genu. Descending projection pathways passing through the capsule connect various cortical areas with lower structures. The anterior limb contains the frontopontine tract (part of the corticopontine tract) and the anterior thalamic radiation. The genu contains the fibers of the corticonuclear (corticobulbar) tract, while the upper part of the posterior limb contains the corticospinal, corticorubral, and corticoreticular tracts, as well as the fibers of the thalamic radiation (thalamoparietal fibers). The most distal part of the posterior limb contains the corticotectal and temporopontine fibers, as well as fibers of the thalamic radiation heading to the occipital and temporal regions of the cortex (visual and auditory areas). The parieto-occipito-pontine bundle also runs here.

The descending projection pathways originating from the cortex are grouped into the pyramidal tract, which consists of the corticonuclear and corticospinal tracts.

The fornix occupies a special place in the fiber System of the cerebral hemispheres. It is a curved bundle of fibers consisting of a body, crura (legs), and columns (Figs. 3.14, 3.40). The body of the fornix is located beneath the corpus callosum and is fused with it. Anteriorly, the body of the fornix continues as the columns of the fornix, which curve downward, each ending in a mammillary body of the hypothalamus. The anterior part of the body of the fornix and, partially, the columns are fused with the septum pellucidum, which consists of two parallel laminae stretched between the body and columns of the fornix posteriorly and the corpus callosum superiorly, anteriorly, and inferiorly. The laminae of the septum pellucidum serve as the medial walls of the lateral ventricles of the forebrain. The columns of the fornix are located above the anterior PARTS OF THE thalami. Between each Column and the thalamus, there is an opening—the interventricular foramen. Anterior to the columns of the fornix, and fused with them, lies the anterior commissure. Posteriorly, the body of the fornix continues into the paired crura of the fornix, which curve laterally and downward, separating from the corpus callosum and fusing with the hippocampus to form its fimbria. The fimbria of the hippocampus ends in the uncus, with one of its sides facing the cavity of the lateral ventricle. The right and left hippocampi are connected to each other via the commissure of the fornix (psalterium), located between the crura. Thus, through the fornix, the temporal lobe of the hemisphere is connected to the mammillary bodies of the diencephalon. In addition, some fibers of the fornix project from the hippocampus to the thalamus, amygdala, and paleocortex.

Fig. 3.40. The fornix (after Sapin):

1 — anterior commissure; 2 — columns of the fornix; 3 — mammillothalamic tract; 4 — mammillary body; 5 — fimbria of the hippocampus; 6 — hippocampus; 7 — crus of the fornix; 8 — commissure of the fornix

The lateral ventricles (ventriculi laterales) of the hemispheres consist of a central part and three horns extending from it (Fig. 3.32).

The central part contains the choroid plexus and is located as a narrow horizontal slit at the level of the parietal lobe of the hemisphere, above the thalamus and the caudate nucleus. Superiorly, the central part of the lateral ventricle is bounded by the radiation of the corpus callosum, and its medial wall is formed by the body of the fornix. Along the boundary between the thalamus and the caudate nucleus runs the stria terminalis, which contains fibers connecting the amygdala with the septum pellucidum, hypothalamic nuclei, and the anterior perforated substance.

The anterior horn, which has a triangular cross-section, is located in the frontal lobe (Atlas Fig. 131). It is separated from the anterior horn of the opposite hemisphere by the septum pellucidum, situated between the corpus callosum and the column of the fornix. Its lateral wall and part of its inferior wall are formed by the head of the caudate nucleus. Posteriorly, the middle part of the lateral ventricle widens significantly and transitions into the central part of the lateral ventricle. The interventricular foramen, which connects the lateral and third ventricles, is also located here.

The posterior horn, which extends into the occipital lobe, is small; its medial wall features a prominence—the calcar avis—formed by the indentation of the deep calcarine sulcus; the floor is also slightly elevated by the collateral sulcus.

The inferior horn runs forward and downward within the temporal lobe. Its floor is elevated by the collateral sulcus; the medial wall is deeply invaginated by the hippocampal sulcus, forming the hippocampus. The latter consists of the archicortex (of lower vertebrates) that has shifted into the ventricle.

Receiving impulses from multiple sensory systems, it is involved in the regulation of generalized body movements and emotions.

The interventricular foramina open into the lateral ventricles between their central part and the anterior horn. Through these openings, the choroid plexuses of the third and both lateral ventricles communicate with each other.

Cortical architectonics. Cortical architectonics refers to the Specific features of the microscopic STRUCTURE OF THE cerebral cortex. A distinction is made between cytoarchitectonics (cellular structure) and myeloarchitectonics (fibrous structure of the cortex) (Fig. 3.41). The Study of cortical architectonics dates back to the late 18th century, when in 1782 Gennari first discovered the heterogeneous structure of the cortex in the occipital lobes of the hemispheres. In 1868, Meynert divided the cross-section of the cerebral cortex into layers. In Russia, the first researcher of the cortex was V.A. Betz (1874). In the second half of the 20th century, the study of cortical architectonics reached its peak, which was reflected in the works of neuromorphologists and physiologists worldwide.

Fig. 3.41. Cytoarchitectonics of the cerebral cortex (area 17 (1) and 19 (2))

The gray matter On the surface of the cerebral hemispheres is primarily represented by the neocortex, which accounts for 96% of the cortex in humans. Phylogenetically older cortical structures—the paleocortex, archicortex, and mesocortex—are poorly developed in humans.

The paleocortex is represented mainly by the septum pellucidum of the lateral ventricles, the anterior perforated substance, and the lateral olfactory stria.

The archicortex (archeocortex) includes the medial olfactory stria, the hippocampus, the uncus of the parahippocampal gyrus, and the band of gray matter in the depth of the sulcus of the corpus callosum.

With the exception of the uncus, the surface of the parahippocampal gyrus has a transitional structure between the archicortex and neocortex, and is classified as the mesocortex. A similar type of cortex is also preserved in humans in very small amounts between the paleocortex and neocortex.

The thickness of the neocortex varies across different regions, averaging 2–3 mm. In the upper parts of the precentral and postcentral gyri and the paracentral lobule, it reaches 5 mm, whereas in the depths of the sulci, it is significantly thinner. The Nerve Cells and fibers making up the cortex are arranged in layers (Fig. 3.42). These layers differ in thickness, Cell density, cell shape and size, as well as the orientation, density, and diameter of the fibers. Six such layers are distinguished in the motor areas of the cortex.

Fig. 3.42. Cellular (A, B) and fibrous (C) structure of the cerebral cortex in cross-section (after Ramón y Cajal, Brodmann, and Vogt). (I–VI — cortical layers)

Layer I — the molecular layer, contains a few very small horizontal cells with axons running parallel to the brain surface (tangentially) (Fig. 3.43). These cells provide local regulation of efferent neuron activity. This layer is common to the neocortex, archicortex, paleocortex, and mesocortex.

Layer II — the external granular layer, contains predominantly small, irregularly shaped neurons (round, stellate, pyramidal). The dendrites and axons of some neurons ascend into the molecular layer, where they synapse with horizontal neurons. Most of the axons enter the white matter. This layer is poor in myelinated fibers.

Layer III — the pyramidal layer, consists of pyramidal cells whose size increases from 10 to 40 µm in the deeper parts. They are typically arranged in columns separated by projection fibers. An apical dendrite arises from the apex of the pyramidal neuron and reaches the molecular layer. Other dendrites, originating from the lateral surfaces and the base of The Cell body, form synapses with neighboring cells of this layer. The axon always arises from the base of the cell body. Axons of smaller neurons remain within the cortex, while those of larger ones form association and commissural fibers of the white matter. Along with pyramidal cells, stellate cells are also found in this layer.

Fig. 3.43. Types of cortical neurons:

horizontal neurons of layer I (1); pyramidal neurons of layers II, III (2) and V (3) layers; stellate neuron of layers II and III (4); basket neuron of layer IV (5); fusiform neuron of layer VI (6)

Layer IV — the internal granular layer, is formed by densely packed stellate and basket cells and a dense concentration of horizontally oriented myelinated fibers. Most projection afferent fibers entering the cortex terminate on the neurons of this layer, and their axons penetrate into the adjacent upper and lower layers, thereby relaying afferent impulses to the efferent neurons of layers III and V. Its thickness varies across different cortical areas—it is almost absent in the precentral gyrus, but is well-developed in the visual cortex.

Layer V — the ganglionic layer, contains pyramidal cells, including very large ones known as Betz cells. Their height reaches up to 120 µm, and their width is 80 µm. The axons of these neurons form the Pyramidal Tracts. Before leaving the cortex, the axons forming the tract give off numerous collaterals that transmit inhibitory impulses to neighboring neurons. After exiting the cortex, the collaterals of these fibers reach the striatum, red nucleus, reticular formation, pontine nuclei, and inferior olives. The latter two relay signals to the cerebellum. In addition, there are neurons that send their axons directly to the caudate nucleus, red nucleus, and reticular nuclei of the brainstem. Pyramidal neurons also receive a large number of afferent inputs from various parts of The Nervous System, arriving via radial and horizontal fibers. Afferent synapses cover the cell body and dendrites. On the dendrites, synaptic contacts are formed mainly on spines—protrusions on the dendritic surface. The number of spines increases during cortical maturation and the formation of new connections.

Layer VI — the multiform layer, contains a large number of spindle cells and is characterized by Variability in the distribution and density of cells and fibers. In the outer part of the layer, the cells are larger, whereas in its deeper parts, the size of the neurons decreases and the distance between them increases. The axons of spindle neurons form efferent pathways, while their short apical dendrites ascend to the molecular layer or terminate in synapses on neurons of layers V and IV.

Moving away from the cortical surface, layer VI transitions into the white matter, where the number of fibers increases significantly and the proportion of cells decreases. Sometimes this transitional zone is classified as layer VII of the cortex.

All cortical cells are interneurons. Structurally, they are classified into long-axon and short-axon neurons, which perform different functional roles.

Cells with long axons are represented by pyramidal and fusiform cells (Fig. 3.43). These are the main cellular elements of layers V–VI. The long descending axon of these cells gives off numerous collaterals along its entire path and, leaving the cortex, continues into the white matter as a descending projection fiber. The latter terminates in the subcortical ganglia, motor nuclei of the brainstem, or on motor neurons of the spinal cord. The ascending dendrite of pyramidal cells rises to the first layer of the cortex, where it forms a dense terminal arborization. Along its path, like other dendrites of pyramidal neurons, it gives off branches to the neurons of all layers through which it passes. Thus, pyramidal cells collect impulses from all layers of the cortex.

In the upper layers, long axons are possessed by pyramidal cells of layer III. The axons of these cells enter the white matter primarily as association fibers, which establish connections between different areas of the cortex, and as commissural fibers, which connect the cortex of the two hemispheres.

Cells with short axons are distinguished by the fact that their axon does not extend beyond the cortex. These are mainly stellate and basket cells. In humans, they are more numerous than in animals, more diverse in shape, and found in all layers of the cortex. In layer IV, they are the primary elements. Their role is to receive afferent impulses and distribute them to the pyramidal cells of layers III and V. Stellate cells also facilitate the circular Circulation of impulses within the cortex. By transmitting an impulse from one stellate cell to another, these neurons combine into neural networks. Having received a Nerve Impulse, they can remain in a state of latent activity (not manifested in external reactions) for a long time, even after the stimulus has ceased. This represents one form of the material substrate of memory, providing the anatomical and functional Prerequisites for the dynamic fixation of excitation traces, and the retention and effective use of information accumulated by a person throughout their life.

According to modern concepts, the cerebral cortex is built of interacting functional blocks—modules or local networks. They can be represented by laminae or columns (Atl. Fig. 139). This organization is most clearly expressed in the sensory areas of the cortex (visual, auditory, somatosensory). Columns are vertical modules with a diameter of approximately 300 µm. The basis for the organization of this module is an afferent fiber entering the cortex. Such fibers can be processes of neurons of the thalamus, lateral geniculate body, etc. They (the fibers) terminate synaptically on stellate neurons of layer IV and on the basal dendrites of pyramidal neurons. From here, excitation spreads to higher and lower-lying neurons. Thus, information from a small group of subcortical neurons enters a localized area of the cortex. This achieves precision in processing sensory signals. Corticocortical fibers form contacts with neurons of all layers and can extend beyond a given module. This allows for more complex processing of information received from various receptors.

Based on the types of neurons making up its layers, the cortex can be divided into upper and lower tiers. The lower tier, represented by layers V–VI, performs a projection function, sending descending fibers to the motor nuclei of the BRAIN AND SPINAL cord. The upper tier, consisting of layers II–IV, distributes impulses arriving via ascending fibers from subcortical structures throughout the cortex, and sends association and commissural fibers to all cortical areas, meaning it is involved in more complex functions. In ontogeny and phylogeny, the upper tier of the cortex develops later than the lower tier; in humans, it is more prominent than in animals.

It is believed that the complexity and sophistication of the human cerebral cortex structure compared to its organization in animals depend mainly on an increase in the number of cells with short neurites, while the significant increase in the surface area of the human cortex is associated with the growth of association fibers.

Based on layer width, and the shape, size, and density of cells, the cortex is divided into regions and areas (Atl. Fig. 132). Regions appear earlier in ontogeny and are characterized by more general features than areas, which emerge As a result of later structural differentiation.

The occipital, temporal, and insular regions coincide with the corresponding lobes of the hemisphere. The parietal (superior and inferior) and postcentral regions are part of the parietal lobe. The former cover the lobules of the same name and are separated from the postcentral region by the postcentral sulcus. The precentral and frontal regions occupy the frontal lobe. The boundary between them more or less corresponds to the superior and inferior precentral sulci. The limbic region, which is part of the limbic system, coincides with the cingulate gyrus.

The Emergence of cytoarchitectonic regions precedes the formation of sulci and gyri on the surface of the hemispheres. The reason for the formation of sulci and gyri lies in the uneven growth of individual parts of the cortex, which leads to the displacement of some of its areas and the appearance of depressions and protrusions on the surface of the hemispheres.

The arrangement of sulci and gyri on the surface of the hemisphere, the distribution of architectonic structures, their relative size, shape, and qualitative features vary among individuals. On this basis, some scientists drew Conclusions about the superiority of one race over another. Subsequent studies have shown that individual differences in The ratio of sulci and gyri, and in the distribution, structure, and size of the architectonic formations of the cortex, are so great within each race that none of these features can be considered typical of any particular race.

Localization of functions. The cerebral cortex is the most highly organized matter, associated with Higher Nervous Activity and the Regulation of the functions of all Organs. Pavlov believed that even the smallest details of cortical structure would sooner or later find their explanation in the light of reflex theory.

When studying the activity of the cortex as the site of the most complex analysis and synthesis of various stimuli, it is necessary to consider the localization of functions within it. Already Galen knew that organ functions are connected to the brain. More than a hundred years ago, Gall pointed out the dependence of mental activity on the cerebral cortex. He spoke of the connections between The Development of certain brain regions and the external shape of the skull. His naive speculative ideas were the first attempts to correlate body functions with brain structure.

The views of these scientists did not prevent the cortex from being considered structurally and functionally homogeneous for a long time afterward. The study of cortical architectonics only expanded significantly at the beginning of the 20th century. In the early works of Western European scientists (Brodmann, Economo, Vogt, etc.), in contrast to previous views, attempts were made to attribute the localization of functions even to individual cytoarchitectonic areas.

And only through the work of Pavlov and his school, based on experiments, was The Doctrine of the dynamic localization of functions established. According to this doctrine, the cerebral cortex is a collection of the cortical ends of analyzers. Each peripheral receptor apparatus corresponds to a cortical region, which Pavlov termed the nuclear zone of the analyzer, or, in modern terminology, the cortical zone of the sensory system, the projection zone (Atl. Fig. 133).

The cortical zone of somatosensory sensitivity, which perceives stimuli from proprioceptors of joints, skeletal muscles, and tendons, is located in the precentral and postcentral regions, mainly in areas 3 and 4, where the ascending projection fibers of the ventral nuclei of the thalamus terminate. In area 4, the majority of fibers of the most powerful descending pathways of the cortex—the Corticospinal and Corticobulbar tracts—originate from the giant pyramidal cells of layer V. The fibers of these pathways terminate on the motor neurons of the anterior horns of the spinal cord and the neurons of the motor nuclei of the Cranial Nerves.

The zone of cutaneous sensitivity, associated with Temperature, pain, and tactile reception, mainly occupies the postcentral region (areas 3, 1, 2). The bulk of the fibers arriving here from the ventral nucleus of the thalamus terminate in area 3.

Within the cortical Zones of the Somatosensory system, there is a systematic projection of different parts of the body. Thus, the areas of both central gyri and the paracentral lobule located near the medial margin of the hemisphere receive impulses from the lower limb; the lower-lying areas of the gyri receive impulses from the trunk; even lower areas receive impulses from the upper limb; and finally, the Tongue, Larynx, Pharynx, and face project to the lowest parts of the central gyri.

The cortical zone of the Visual sensory system is located in the occipital region (areas 17, 18, 19). The bulk of the fibers of the optic radiation terminate in area 17, on the walls and floor of the calcarine sulcus.

The cortical zone of the auditory sensory system is located in the temporal region (areas 41, 42, 20, 21, 22). In areas 41 and 42 of the superior temporal gyrus, the majority of the fibers of the auditory radiation terminate.

The cortical zone of the Olfactory sensory system is associated with the paleocortex and archicortex of the olfactory triangle, septum pellucidum, uncus of the parahippocampal gyrus, hippocampus, etc.

In addition to the projection areas of the cortex, which receive impulses primarily from a single sensory system, inter-analyzer, so-called association areas can be distinguished in the cerebral cortex, which receive impulses from multiple systems. In these areas, such as the parietal and frontal regions, the projections of different sensory systems overlap, and higher integrative functions are carried out.

The regions of the cortical projection areas where the main bulk of the ascending fibers of the analyzers terminate (Fig. 3.44) are distinguished as central, or primary, areas.

They are characterized by a specific structure. For instance, the cortex of areas 3, 17, and 41 is distinguished by exceptional cellularity and an Abundance of small granule cells, with a well-developed layer IV. Area 4 is an exception. Here, in the adult human, granule cells are scattered throughout the entire thickness of the cortex. This is because this area simultaneously serves as THE ORIGIN OF a powerful system of descending fibers of the pyramidal corticospinal and corticobulbar tracts.

While the main bulk of the fibers ascending to the projection area terminate in its central area, fewer afferent fibers reach its peripheral or secondary areas (1 and 2, 18 and 19, 22, etc.) than the central one; however, in addition to afferent fibers, fibers from the central area also arrive there (Fig. 3.45). In the peripheral areas, there are significantly fewer granule cells, and pyramidal cells of various sizes predominate.

Fig. 3.44. Ascending projection fiber systems to the primary sensory areas:

1 — postcentral gyrus (somatosensory system) elevated to show incoming nerve fibers; 2 — calcarine sulcus (visual system); 3 — superior temporal gyrus (auditory system); 4 — ventral thalamic nucleus; 5—6 — lateral and medial geniculate bodies

I.P. Pavlov attributed these Structural and functional Features of the cortex to the first signal system of reality, which is shared by humans and animals.

In addition to the areas well-developed in the animal cortex, humans exhibit a highly developed set of other, phylogenetically younger areas.

Fig. 3.45. System of connections between the areas of the human cerebral cortex (after Polyakov):

I — primary (central) areas; II — secondary (peripheral) areas; III — tertiary (associative) areas (analyzer overlap zones). Bold lines indicate: the projection (cortical-subcortical) connection system of the cortex; the projection-associative connection system of the cortex; the associative connection system of the cortex. 1 — receptor; 2 — effector; 3 — sensory ganglion neuron; 4 — motor neuron; 5—6 — relay neurons of the spinal cord and brainstem; 7—10 — relay neurons of subcortical structures; 11, 14 — afferent fiber from the subcortex; 13 — layer V pyramidal cell; 16 and 18 — layer III pyramidal cells; 12, 15, 17 — cortical stellate cells; 2 — axon; 3 — neurons at various stages of migration; 4 — radial glial fibers

An example is area 40 of the inferior parietal region, which occupies the supramarginal gyrus. It is involved in regulating goal-directed movements acquired throughout life. Damage to this area leads to a loss of The ability to perform complex, coordinated motor acts.

Localization of speech functions.

During the historical development of human society, structural and functional features evolved in the cerebral cortex that depend on labor activity and the speech associated with it.

Cortical structures in humans that perceive excitation coming from the speech organs belong to the second signal system of reality. These are areas 44 and 45, which occupy the opercular and triangular parts of the inferior frontal gyrus (Broca's area) (Fig. 3.46). In their activity, they are closely linked to the lower part of the precentral gyrus, which receives proprioceptive impulses from the Muscles of the tongue, Lips, Cheeks, and larynx.

The cortical area associated with spoken speech is located anterior to the cortical region where pathways carrying proprioceptive impulses from the head terminate (area 4). The part of area 6 located in the posterior middle frontal gyrus is associated with written speech and lies anterior to the section of area 4 that receives proprioceptive impulses from the hand. The joint activity of these core zones is involved in the complex motor acts required for writing. Damage to this part of area 6 leads to Impairment of the fine movements performed by the hand when writing letters.

Fig. 3.46. Cortical areas associated with speech (after Geschwind):

A — word perception; B — word reading; 1 — motor cortex; 2 — arcuate fasciculus; 3 — Wernicke's area; 4 — primary auditory cortex; 5 — Broca's area; 6 — angular gyrus; 7 — primary visual cortex

Other cortical areas particularly closely associated with speech developed near the visual and auditory analyzers. Area 39, which occupies the angular gyrus of the inferior parietal region (Wernicke's area) and closely adjoins the core zone of the visual analyzer, is involved in the Visual Perception of written characters. Damage to area 39 leads to a loss of the ability to assemble letters into words and phrases. In area 22, located in the posterior part of the superior temporal gyrus, auditory perception of speech occurs with the involvement of areas 41 and 42 (the core zone of the Auditory Analyzer). If this part of area 22 is damaged, the ability to understand words is lost.

Phylogenetically new cortical areas, which are particularly closely associated with speech, are asymmetrical; they are represented in the left hemisphere in right-handed individuals, and in the right hemisphere in left-handed individuals.

It has now been shown that the other hemisphere is also involved in speech functions (it perceives vocal intonations and gives speech its emotional coloring). Hemispheric specialization is also manifested in the organization of memory and the regulation of emotional states.

The existence of areas in humans whose destruction leads to the loss of speech functions does not mean that these functions are linked exclusively to specific cortical regions. Here, as with localization in the first signal system, certain areas merely play a predominant role. Speech has a highly complex localization and is carried out with the involvement of the entire cortex. As new experience is acquired, speech functions can shift to other cortical areas (e.g., reading in the blind, writing with a FOOT in armless individuals, etc.).

The limbic system includes phylogenetically ancient structures: the hippocampus and olfactory bulbs; and younger formations: the limbic cortex (an area on the medial surface of the hemisphere, including the cingulate and parahippocampal gyri, and the gyrus of the corpus callosum). Additionally, the limbic system includes the amygdala, septum, mammillary bodies, anterior thalamic nuclei, fornix, and the periaqueductal gray of the midbrain (Fig. 3.47, A, B). These structures are integrated into a unified system via the hypothalamus. This system is of great importance in subserving a complex array of diverse motivational, emotional, and adaptive responses. Neuroanatomist Papez described the interconnected structures that mediate the generation and processing of emotions. This is the so-called 'Papez circuit': hippocampus — mammillary bodies — anterior thalamic nuclei — cingulate gyrus — hippocampus (Fig. 3.47, C). The limbic system is also connected to interoreceptors and the regulation of endocrine and autonomic functions, and participates in maintaining homeostasis, Learning and Memory, and regulating the sleep-wake cycle.

Fig. 3.47. The limbic system:

A — limbic cortical areas on the medial surface of the hemispheres; B — diagram of the limbic system structure: 1 — anterior thalamic nucleus; 2 — lateral olfactory stria; 3 — mammillary body; 4 — medial olfactory stria; 5 — olfactory bulb; 6 — olfactory triangle; 7 — septum pellucidum; 8 — corpus callosum; 9 — fornix; 10 — brainstem; 11 — dentate gyrus; 12— hippocampus; 13 — amygdala; 14 — frontal lobe; 15 — uncus; 16 — parahippocampal gyrus; 17 — cingulate gyrus; 18 — subcallosal area

Humans exhibit significant morphofunctional brain Asymmetry, which has been quite well studied. For instance, it has been shown that the motor speech center in right-handed individuals is located in the left hemisphere, specifically in areas 44 and 45 of the inferior frontal gyrus. Accordingly, it has been established that in right-handers, the area of the auditory-speech center in the superior temporal gyrus is larger in the left hemisphere than in the right (Fig. 3.48).

It is well known that left-hemisphere dominant individuals are characterized by rational, analytical thinking, well-developed speech, and an aptitude for the exact sciences; in musical perception, they grasp rhythm more easily than melody, etc. Conversely, right-hemisphere dominant individuals possess more holistic, imaginative thinking, an artistic mindset, are more musical, and tend to be more emotional.

Morphological brain asymmetry is expressed in The structure of sulci and gyri, as well as at the microscopic level in the degree of development of individual layers and cell sizes. At the same time, it is most pronounced in phylogenetically younger and functionally more complex [areas]. Very deep sulci, the so-called fissures (e.g., calcarine, lateral), are formed, which invaginate the hemispheric wall deep into the lateral ventricle. In a six-month-old fetus (Fig. 3.49), the hemispheres significantly overhang individual parts of the brain, the fissures deepen greatly, and the so-called insula becomes visible at the bottom of the lateral fissure. Later, less deep primary sulci (e.g., central) and secondary ones appear. During the first years of a child's life, tertiary sulci are also formed—these are mainly branches of Primary and secondary sulci (Fig. 3.54). On the medial surface of the hemisphere, the hippocampal and cingulate gyri appear first. After this, the formation of sulci and gyri proceeds very rapidly.

Although all the main gyri already exist at birth, the pattern of sulci has not yet reached a high degree of complexity. One year after birth, individual differences in the distribution of sulci and gyri appear, and their structure becomes more complex. Due to the uneven growth of individual cortical areas during ontogenesis, certain regions in some areas are seemingly pushed deep into the sulci by the overgrowth of neighboring, functionally more important ones. An example of this is the gradual sinking of the insula deep into the lateral sulcus due to the massive growth of adjacent cortical areas that develop with the child's articulate speech. These are the so-called frontal operculum and temporal operculum (motor speech areas of the cortex, for example, in the region of the motor speech, auditory speech, visual speech centers, and the written language center.

Fig. 3.48. Human brain asymmetry: Horizontal section of the human brain. Asymmetry of the superior surface of the temporal lobe; the size of the so-called temporal plane (planum temporale) is larger on the left (after Geschwind, 1981):

1 — right side; 2 — transverse gyrus 1; 3 — transverse gyrus 2; 4 — planum temporale; 5 — posterior margin; 6 — occipital lobe; 7 — lateral sulcus; 8 — transverse gyrus; 9 — intermediate sulcus; 10 — temporal pole; 11 — left side