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

Special Part
Nervous system, systema nervosum - Special Part
Brain - Forebrain - Diencephalon

The Forebrain, prosencephalon, consists of two parts: the Diencephalon and the Telencephalon.

The diencephalon is derived from the prosencephalic vesicle and is located between the Midbrain and the telencephalon (Fig. 143 B, 146).

It is divided into a phylogenetically older region—the Hypothalamus, which serves as the highest subcortical center of the Autonomic Nervous system—and a newer region, comprising the thalamus, epithalamus, and metathalamus, which act as centers for afferent pathways.

The diencephalon is divided into four regions**:

Thalamus, Thalamus;

Hypothalamus, Hypothalamus;

Metathalamus, Metathalamus;

Epithalamus, Epithalamus.

The thalamus, Thalamus, or optic thalamus, thalamus opticus, is a large, paired, egg-shaped Structure (weighing ~ 20 g) located on either side of the Third ventricle (Fig. 146). It is most highly developed in humans. The thalamus serves as the subcortical center for all sensory pathways, transmitting information from exteroceptors and interoceptors to the Cerebral Cortex.

* Theodor Meynert (1833—1892) was an Austrian neurologist. The dorsal tegmental decussation (Meynert's decussation) is named after him.

** Foreign neuroanatomists (J. P. Schadé and D. H. Ford, 1976) propose dividing the diencephalon into five regions, which is also emphasized in the latest version of Terminologia Anatomica (1998).

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Fig. 146. Diencephalon (thalamus, epithalamus, and ventriculus III) (superior view)

Based on topographical principles, the Gray matter of the thalamus is divided by layers of White matter into distinct nuclear groups*: the anterior nuclei of the thalamus, nuclei anteriores thalami; the ventrolateral nuclei of the thalamus, nuclei ventrolaterales thalami; the medial nuclei of the thalamus, nuclei mediales thalami; and the posterior nuclei of the thalamus, nuclei posteriores thalami. According to the Structural and functional differentiation of thalamic nuclei, they are classified into: 1) specific sensory (Relay) nuclei, each of which receives primarily one main type of sensory afferentation from the periphery; 2) non-specific nuclei, which receive and process impulses arriving mainly from the reticular Formation of the Brainstem and the hypothalamus; 3) associative nuclei, which are involved in the further Processing and integration of various sensory impulses.

The anterior nuclear group, located in the anterior tubercle of the thalamus, includes the anterodorsal and anteroventral nuclei, which are specific relay structures, and the anteromedial Nucleus, which belongs to non-specific structures. The relay nuclei receive impulses via the mammillothalamic tract and project them primarily to the limbic areas of the neocortex.

The posterior nuclear group is located in the posterior part of the thalamus (pulvinar). Structurally and functionally, the posterior group is involved not only in visual (primarily) but also in somatosensory and auditory projections (to some extent) directed to the cerebral cortex.

The ventral nuclei of the thalamus include the lateral anterior (ventral) nucleus and the ventral nuclear group, which is further divided into the anterior and posterior ventral nuclei. The lateral anterior nucleus is a phylogenetically older structure. Located posterior to the anterior nuclear group, it has direct connections with the fornix, allowing it to be classified as a relay STRUCTURE OF THE limbic system. The lateral posterior nucleus belongs to the associative nuclei of the thalamus.

The primary relay Nucleus of the thalamus that transmits somatic afferent signals to the cerebral cortex is the ventral posteromedial (VPM) nucleus, which receives the medial lemniscus, the spinothalamic tract, the cerebellorubrothalamic tract, and axons from the Cells of the spinal trigeminal nucleus.

The medial nuclei of the thalamus are located lateral to the central gray matter that lines the walls of the third ventricle. The most highly developed is the mediodorsal nucleus, which is an associative nucleus and has direct connections with the structures of the frontal lobe of the neocortex. The medioventral nucleus is located beneath the mediodorsal nucleus and is phylogenetically the oldest.

The hypothalamus, Hypothalamus (Fig. 147), is the part of the diencephalon located beneath the thalamus and the third ventricle. In the floor of the third ventricle, the gray matter of the hypothalamus forms between 15 and 48 paired nuclei (Fig. 148). Most authors distinguish three main nuclear groups in the hypothalamus: 1) the anterior group, which includes the medial preoptic, suprachiasmatic, supraoptic, paraventricular, and anterior hypothalamic nuclei; 2) the middle (or tuberal) group, which includes the dorsomedial, ventromedial, arcuate (infundibular), and lateral hypothalamic nuclei; 3) the posterior group, which includes the supramammillary, premammillary, and mammillary nuclei, the posterior hypothalamic and perifornical nuclei, and the subthalamic nucleus of Luys.

* Up to 60 pairs of nuclei are distinguished in the thalamus.

Fig. 147. Diencephalon. Metathalamus and hypothalamus.

The hypothalamic nuclei form numerous connections with each other, as well as with higher- and lower-lying structures of the CNS. They control and regulate the constancy of the body's internal environment (Homeostasis), fluid-electrolyte balance, Sleep, the autonomic nervous system, METABOLISM, body Temperature, and the Functions of the Digestive System (hunger and satiety), Cardiovascular system (Blood Circulation), and the respiratory and endocrine systems through control over the adenohypophysis. Hypothalamic nuclei also influence The formation of emotional responses such as anger and fear. The hypothalamus includes:

- preoptic area;

- optic chiasm, chiasma opticum;

- tuber cinereum, tuber cinereum;

- infundibulum, infundibulum;

- posterior pituitary, neurohypophysis:

- mammillary bodies, corpora mamillaria.

The optic chiasm, chiasma opticum, is formed by the fibers of the optic nerves, which, after a partial decussation, form a quadrangular plate whose posterior angles continue into the optic tracts, tractus opticus. The optic tracts wind around the cerebral peduncle and reach the subcortical visual centers (the lateral geniculate body, the pulvinar of the thalamus, and the superior colliculi of the tectum). Along with the Optic nerve fibers, commissural fibers pass through the optic chiasm, connecting the hypothalamus of the right and left hemispheres.

Beneath the thalamus lies a small area of Brain matter known as the subthalamic region, regio subthalamica (BNA), or the subthalamus, subthalamus. Functionally, the subthalamic region (subthalamus) is connected to the red nucleus and the substantia nigra, which are located in the midbrain. All of this indicates that the function of the subthalamic region is linked to the Basal Ganglia. The subthalamic region includes the following structures: the subthalamic nucleus, nucleus subthalamicus (corpus Luysi), the zona incerta, and the perizonar nuclei of the fields (H, H1, H2). The subthalamic nucleus is an accumulation of gray matter interspersed with fibers. It belongs to the centers of the extrapyramidal system. This nucleus lies adjacent to the Medial surface of the internal capsule, which separates it from the globus pallidus. The subthalamic nucleus has numerous connections with the globus pallidus via the subthalamic fasciculus, fasciculus subthalamicus.

Fig. 148. PROJECTION OF THE hypothalamic nuclei onto the lateral wall of the third ventricle (diagram).

The zona incerta, zona incerta, is located in the subthalamic region, adjacent to the nuclei, in the form of a thin plate of gray matter. It can be traced throughout the diencephalon and is separated from the thalamus by the fibers of the thalamic fasciculus, fasciculus thalamicus, and bounded inferiorly by the fibers of the lenticular fasciculus.

The tuber cinereum, tuber cinereum, is a gray-colored protrusion located between the optic chiasm and the mammillary bodies. Inferiorly, the tuber cinereum narrows and transitions first into the infundibulum, infundibulum, and then into the neurohypophysis, neurohypophysis. On the inner side of the third ventricle, a depression, the recessus infundibuli, corresponds to the infundibulum. The tuber cinereum and the infundibulum are composed of a thin plate of gray matter, which gradually thins out and transitions into the terminal lamina, lamina terminalis.

The mammillary bodies, corpora mamillaria, are two pea-sized, spherical white structures. They are located in the area between the posterior perforated substance and the tuber cinereum. Inside the mammillary bodies are two nuclei: lateral and medial. The fibers of the columns of the fornix and the Pons terminate in the mammillary bodies. Two tracts arise from the medial nuclei of the mammillary bodies: the mammillothalamic tract, fasciculus mamillothalamicus, and the mammillotegmental tract, fasciculus mamillotegmentalis.

The metathalamus, Metathalamus (Fig. 147), or (post-Thalamic region) is a subcortical center for Hearing and Vision. It consists of the lateral and medial geniculate bodies, which are oblong-oval white eminences. They contain the nuclei of the same name, nucl. corporis geniculati laterales et mediales. The lateral geniculate body is located beneath the pulvinar of the thalamus on the inferolateral surface. The lateral geniculate body is larger and flatter than the medial one. The fibers of the lateral ROOT of the optic tract terminate in the cells of the lateral geniculate body. The fibers of the lateral geniculate body and the fibers of the pulvinar of the thalamus form the optic radiation, corona optica, which reaches the region of the calcarine sulcus (occipital lobe).

The epithalamus, Epithalamus (Fig. 146), or epithalamic region, is located above the thalamus and consists of the following structures:

- pineal body, corpus pineale [gl. pinealis];

- striae medullares, stria medulares;

- habenulae, habenulae, and habenular nuclei;

- habenular trigone, trigonum habenulae;

- posterior epithalamic commissure, commissura epithalamica posterior.

The right and left habenulae are connected to each other by a commissure, commissura habenularum, and continue into the medullary lamina, which connects to the pineal body, corpus pineale (gl. pinealis), an endocrine gland. The epithalamic region is anatomically connected to the Limbic System and the reticular formation of the midbrain, and likely integrates their functions.