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

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

The Diencephalon is located above the Midbrain and beneath the corpus callosum. It consists of the thalamus, epithalamus, metathalamus, and Hypothalamus.

The thalamus is a paired, egg-shaped Structure. Its medial surface faces the cavity of the Third ventricle and is therefore clearly visible in a Brain section. Its anterior (inferior) side is fused with the hypothalamus, while its lateral side lies against the internal capsule. The thalamus serves as the primary Relay center for all types of general sensation (pain, Temperature, tactile, and proprioceptive). The anterior end (anterior tubercle) of the thalamus is pointed, whereas the posterior end (pulvinar) is rounded. The medial surfaces of the right and left thalami face each other, forming the lateral walls of the diencephalic cavity—the third ventricle—and are connected by the interthalamic adhesion.

The part of the diencephalon located inferior to the thalamus and separated from it by the hypothalamic sulcus constitutes the subthalamus proper. The structures of the tegmentum of the cerebral peduncles continue here, and the red nuclei and substantia nigra of the midbrain terminate in this region.

The epithalamus comprises the Pineal Gland, habenulae, and habenular trigones. The pineal gland, or epiphysis, Functions as an endocrine gland. It is suspended, as it were, by two habenulae connected by a commissure and linked to the thalamus via the habenular trigones. The habenular trigones contain nuclei related to the olfactory system.

The metathalamus is formed by the paired medial and lateral geniculate bodies located posterior to each thalamus. The medial geniculate body lies posterior to the pulvinar of the thalamus and, together with the inferior colliculi of the tectum of the midbrain (corpora quadrigemina), serves as a subcortical auditory center. The lateral geniculate body is situated inferior to the pulvinar. Together with the superior colliculi of the tectum, it acts as a subcortical visual center. The nuclei of the geniculate bodies are connected via Neural Pathways to the cortical centers of the visual and auditory systems.

The hypothalamus represents the ventral part of the diencephalon, situated anterior to the cerebral peduncles. It includes the optic chiasm, optic tracts, tuber cinereum, infundibulum, mammillary bodies, and the subthalamic region proper.

The spherical mammillary bodies are formed by Gray matter covered by a thin layer of White matter. The nuclei of the mammillary bodies serve as subcortical olfactory centers. The tuber cinereum is a thin lamina of gray matter in the floor of the third ventricle that extends inferiorly and anteriorly, forming the infundibulum. The distal end of the infundibulum transitions into the hypophysis (Pituitary Gland), an endocrine gland located in the hypophyseal fossa of the sella turcica. The optic chiasm, positioned anterior to the tuber cinereum, continues anteriorly into the optic nerves and posteriorly and laterally into the optic tracts.

The cavity of the diencephalon—the third ventricle—is a narrow, slit-like space situated in the sagittal plane. The third ventricle is bounded laterally by the medial surfaces of the thalami, inferiorly by the hypothalamus, anteriorly by the columns of the fornix, the anterior commissure, and the terminal lamina, posteriorly by the epithalamic (posterior) commissure, and superiorly by the fornix, above which lies the corpus callosum. The superior wall proper is formed by the tela choroidea of the third ventricle, which contains its choroid plexus. Posteriorly, the cavity of the third ventricle opens into the cerebral aqueduct of the midbrain, while anteriorly and laterally it communicates with the Lateral ventricles of the Telencephalon via the interventricular foramina.

Structural and functional Organization OF THE thalamus and hypothalamus. All information about the surrounding world and the state of our body reaches the Cerebral Cortex through the thalami of the diencephalon. Therefore, the thalamus acts as a gateway through which all sensory signals, except for Olfaction, reach the CEREBRAL CORTEX AND enter conscious awareness. The nerve Cells of the thalamus are grouped into numerous nuclei (up to 40), which are topographically divided into anterior, median, medial, lateral, and posterior groups.

The Anterior Group of nuclei occupies the region of the anterior tubercle. The nuclei of the medial and median groups lie along the wall of the third ventricle. The lateral group is located lateral to the medial group, separated from it by the internal medullary lamina. The posterior group is formed by the nuclei of the pulvinar. The axons of the geniculate body cells establish direct connections with the projection fields of the visual cortex in the occipital lobe of the cerebrum (area 17); those of the medial geniculate body connect with the cortical fields of the temporal lobe (area 41). The efferent fibers of the nuclei of the dorsolateral complex project to the cortex of the postcentral gyrus of the cerebrum (areas 1, 2, 3) and the superior parietal lobule (areas 5 and 7).

The hypothalamus integrates the Functions of the autonomic, somatic, and endocrine systems, and ensures the constancy of the body's internal environment. The Nerve Cells of the hypothalamus are organized into nuclei (over 30 nuclei) located in the floor and walls of the lower part of the third ventricle. Topographically, the preoptic, supraoptic, paraventricular, tuberal, and other nuclei are distinguished within the hypothalamus.

As a vital integrative center, the hypothalamus features highly developed systems of afferent and efferent connections. Its intrinsic hypothalamic pathways, which connect the hypothalamic nuclei to one another and to the nuclei of adjacent brain regions, include the paraventricular and supraoptic fibers, mammillotegmental, mammillothalamic, and superior longitudinal fasciculi, among others. The hypothalamohypophyseal tract connects the neurosecretory nuclei of the hypothalamus with the structures of the posterior pituitary.

The hypothalamohypophyseal system. Regulation of pituitary function is one of the most crucial roles of the hypothalamus. While part of the diencephalon, the hypothalamus simultaneously functions as an endocrine organ, transforming neural impulses into endocrine processes within its structures. The secretory Neurons of the supraoptic Nucleus secrete the bioactive substance vasopressin, while those of the paraventricular nucleus secrete oxytocin. These bioactive substances are transported to the capillary network of the neurohypophysis. Vasopressin, or antidiuretic hormone, exerts a vasoconstrictive effect (raising Blood pressure) and regulates Water reabsorption in the renal tubules (nephron tubules). Oxytocin regulates the tone of smooth Muscle in the digestive tract, milk secretion by the Mammary Glands, and uterine contractions.

However, whereas the Hormones of the posterior pituitary are produced by neurosecretory cells of the hypothalamic nuclei, the hormones of the anterior pituitary are synthesized by cells within the anterior lobe itself (the adenohypophysis). The secretion of all adenohypophyseal hormones is controlled by hypophysiotropic hormones of the hypothalamus. These hypophysiotropic hormones are classified into stimulatory factors, or releasing factors (liberins), and inhibitory factors (statins). Released from nerve terminals in the neurosecretory nuclei of the hypothalamus, hypophysiotropic hormones travel via the Vessels of the hypothalamohypophyseal system to the adenohypophysis, where they act on its endocrine cells.

The hypothalamic nuclei house the centers regulating autonomic functions, such as increasing or decreasing Heart rate, enhancing or inhibiting intestinal peristalsis, and constricting or dilating the pupils.

The structures of the hypothalamus concentrate the centers of thermoregulation. The paraventricular nucleus ensures the integration of heat dissipation processes. Stimulation of neurons in this nucleus induces cutaneous vasodilation, increased sweating, and heightened Respiration. The hypothalamus also contains structures responsible for regulating feeding behavior. The lateral hypothalamic nuclei control the drive for food intake or refusal (the hunger and satiety centers). Additionally, the centers regulating drinking behavior are located in the hypothalamus. The primary role in drinking behavior is played by the osmoreceptors of the preoptic nucleus, which exhibit high sensitivity to cellular dehydration.

Hypothalamic structures play a significant role in The regulation of Sexual Behavior. Hypothalamic cells synthesize gonadotropin-releasing hormone (gonadoliberin), which regulates the synthesis of gonadotropic hormones by adenohypophyseal cells.

Review and Self-Assessment Questions:

1. Name the major Divisions of the brain and describe the boundaries between them.

2. List the cranial nerve nuclei located within the Brainstem.

3. Name the PARTS OF THE diencephalon. Describe the structural features and functions of the hypothalamus.

4. Describe the functions of each part of the brainstem (Medulla Oblongata, Pons, midbrain, and diencephalon) and the Cerebellum.