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DIENCEPHALON 05. 05. 2014 Kaan Yücel M.D., Ph.D. https://yeditepeanatomyfhs122.wordpress.com Dr.Kaan Yücel yeditepeanatomyfhs122.wordpress.com Diencephalon The diencephalon is located at the dorsal end of the brain stem surrounded by the internal capsule laterally and the lateral ventricles and corpus callosum superiorly. It is divided into symmetrical halves separated by the narrow third ventricle but connected by the massa intermedia. As you will see the structures of the diencephalon are named according to their position to the thalamus. See yourself below: The diencephalon can be divided into four parts: (1) thalamus (2) subthalamus [-sub: 'inferior to”] (3) epithalamus [-epi: “superior to”] (4) hypothalamus [-hypo: “under” ] The diencephalon extends posteriorly to the point where the third ventricle becomes continuous with the cerebral aqueduct and anteriorly as far as the interventricular foramina. Thus, the diencephalon is a midline structure with symmetrical right and left halves. Obviously, these subdivisions of the brain are made for convenience, and from a functional point of view, nerve fibers freely cross the boundaries. Gross Features The inferior surface of the diencephalon is the only area exposed to the surface in the intact brain. It is formed by hypothalamic and other structures, which include, from anterior to posterior: 1. optic chiasma, with the optic tract on either side 2. infundibulum, with the tuber cinereum 3. mammillary bodies. The superior surface of the diencephalon is concealed by the fornix. The actual superior wall of the diencephalon is formed by the roof of the third ventricle. The roof contains a thin epithelial membrane called ependyma. It is continuous with the rest of the ependymal lining of the third ventricle. The ependymal is involved in CSF production. It is covered superiorly by a vascular fold of pia mater, called the tela choroidea of the third ventricle. From the roof of the third ventricle, a pair of vascular processes, the choroid plexuses of the third ventricle, project downward from the midline into the cavity of the third ventricle. The choroid plexus is the place where the CSF is produced. The lateral surface of the diencephalon is bounded by the internal capsule of white matter and consists of nerve fibers that connect the cerebral cortex with parts of the brainstem and spinal cord. Since the diencephalon is divided into symmetrical halves by the slitlike third ventricle, it also has a medial surface. The medial surface of the diencephalon (i.e., the lateral wall of the third ventricle) is formed in its superior part by the medial surface of the thalamus and in its inferior part by the hypothalamus. These two areas are separated from one another by a shallow sulcus, the hypothalamic sulcus. A bundle of nerve fibers, which are afferent fibers to the habenular nucleus, forms a ridge along the superior margin of the medial surface of the diencephalon and is called the stria medullaris thalami. 1. Thalamus L. thalamus "inner chamber," from Gk. thalamos "inner chamber, bedroom" The thalamus is a large ovoid mass of gray matter that forms the major part of the diencephalon. The thalamus is situated on each side of the third ventricle. The superior surface of the thalamus is covered medially by the tela choroidea and the fornix, and laterally, it is covered by ependyma and forms part of the floor of the lateral ventricle; the lateral part is partially hidden by the choroid plexus of the lateral ventricle. The inferior surface is continuous with the tegmentum of the midbrain. The medial surface of the thalamus forms the superior part of the lateral wall of the third ventricle and is usually connected to the opposite thalamus by a band of gray matter, the interthalamic connection (interthalamic adhesion; adhesio interthalamica; massa interrmedia). The interthalamic adhesion is found in 7080% of humans. The lateral surface of the thalamus is separated from the lentiform nucleus by the very important band of white matter called the internal capsule. The thalamus is a very important cell station that receives the main sensory tracts (except the olfactory pathway). It should be regarded as a station where much of the information is integrated and relayed to the http://www.youtube.com/yeditepeanatomy 2 Dr.Kaan Yücel yeditepeanatomyfhs122.wordpress.com Diencephalon cerebral cortex and many other subcortical regions. It also plays a key role in the integration of visceral and somatic functions. The activities of the thalamus are closely related to that of the cerebral cortex and damage to the thalamus causes great loss of cerebral function. The thalamus is actually a relay centre subserving both sensory and motor mechanisms. Thalamic nuclei (50–60 nuclei) project to one or a few well-defined cortical areas. Multiple cortical areas receive afferents from a single thalamic nucleus and send back information to different thalamic nuclei. The anterior part of the thalamus contains the anterior thalamic nuclei, which receive the mammilothalamic tract from the mammillary nuclei. The function of the anterior thalamic nuclei is closely associated with of that of the limbic system and is concerned with emotional tone and the mechanisms of recent memory. The medial part of the thalamus contains the large dorsomedial nucleus and several smaller nuclei. The dorsomedial nucleus has 2 connections with the whole prefrontal cortex of the frontal lobe of the cerebral hemisphere. It also has similar connections with the hypothalamic nuclei. The medial part of the thalamus is responsible for the integration of a large variety of sensory information, including somatic visceral and olfactory information and the relation of this information to one’s emotions. The lateral part is subdivided in dorsal and ventral components. 2. Subthalamus The subthalamus lies inferior to the thalamus and, therefore, is situated between the thalamus and the tegmentum of the midbrain; craniomedially, it is related to the hypothalamus. The nucleus has important connections with the corpus striatum; as a result, it is involved in the control of muscle activity. 3. Epithalamus (dorsal thalamus) The epithalamus consists of the habenular nuclei and their connections (stria medullaris thalami & habenulointerpeduncular tract ; fasciculus retroflexus).) and the pineal gland. Habenular Nucleus The habenular nucleus is a small group of neurons situated just medial to the posterior surface of the thalamus. The habenular nucleus is believed to be a center for integration of olfactory, visceral, and somatic afferent pathways. Pineal Gland (Body) The pineal gland is a small, conical structure that is attached by the pineal stalk to the diencephalon. The superior part of the base of the stalk contains the habenular commissure; the inferior part of the base of the stalk contains the posterior commissure. The pineal gland possesses no nerve cells, but adrenergic sympathetic fibers derived from the superior cervical sympathetic ganglia enter the gland and run in association with the blood vessels and the pinealocytes. The pineal gland, once thought to be of little significance, is now recognized as an important endocrine gland capable of influencing the activities of the pituitary gland, the islets of Langerhans of the pancreas, the parathyroids, the adrenal cortex and the adrenal medulla, and the gonads. The pineal secretions, produced by the pinealocytes, reach their target organs via the bloodstream or through the cerebrospinal fluid. Their actions are mainly inhibitory and either directly inhibit the production of hormones or indirectly inhibit the secretion of releasing factors by the hypothalamus. Animal experiments have shown that pineal activity exhibits a circadian rhythm that is influenced by light. The gland has been found to be most active during darkness. Melatonin and the enzymes needed for its production are present in high concentrations within the pineal gland. 4. Hypothalamus The hypothalamus is that part of the diencephalon that extends from the region of the optic chiasma to the caudal border of the mammillary bodies. It lies below the hypothalamic sulcus on the lateral wall of the third ventricle. It is thus seen that anatomically the hypothalamus is a relatively small area of the brain that is strategically well placed close to the limbic system, the thalamus, the ascending and descending tracts, and the hypophysis. Microscopically, the hypothalamus is composed of small nerve cells that are arranged in groups or nuclei. Physiologically, there is hardly any activity in the body that is not influenced by the hypothalamus. The hypothalamus controls and integrates the functions of the autonomic nervous system and the endocrine systems and plays a vital role in maintaining body homeostasis. It is involved in such activities as regulation of body temperature, body fluids, drives to eat and drink, sexual behavior, and emotion. 3 http://twitter.com/hippocampusamyg Dr.Kaan Yücel yeditepeanatomyfhs122.wordpress.com Diencephalon Relations of the Hypothalamus Anterior to the hypothalamus is an area that extends forward from the optic chiasma to the lamina terminalis and the anterior commissure; it is referred to as the preoptic area. Caudally, the hypothalamus merges into the tegmentum of the midbrain. The thalamus lies superior to the hypothalamus, and the subthalamic region lies inferolaterally to the hypothalamus. The hypothalamus can be loosely divided into four distinct groups in the rostral-caudal plane of the third ventricle: preoptic (above and in front of the optic chiasm - actually telencephalic extension of the basal forebrain, but functionally considered with the diencephalon), chiasmatic (above and around the optic chiasm), tuberal (above and around the "tuber cinereum", i.e. pituitary stalk) and the posterior region which includes the mammillary bodies. When observed from below, the hypothalamus is seen to be related to the following structures, from anterior to posterior: (1) the optic chiasma, (2) the tuber cinereum and the infundibulum, and (3) the mammillary bodies. Optic Chiasma The optic chiasma is a flattened bundle of nerve fibers situated at the junction of the anterior wall and floor of the third ventricle. The superior surface is attached to the lamina terminalis, and inferiorly, it is related to the hypophysis cerebri, from which it is separated by the diaphragma sellae. A small recess, the optic recess of the third ventricle, lies on its superior surface. Pituitary gland Let me do an anology here. Pituitary gland is the “switch” of the body. Look at the functions of the gland for God’s sake. Metabolism in the body, reproduction, water balance, growing…. Pituitary gland releases hormones under the influence of the hormones released by the hypothalamus. The hypothalamus is considered as a part of the diencephalon but they do not count the pituitary gland in the diencephalon but still we talk about it when we talk diencephalon. It lies under the hypothalamus and sits on the sella turcicae part called “fossa hypophysis”. It has a stalk called infundibulum and has two parts; the anterior pituitary and posterior pituitary. The posterior pituitary is specific as it is formed by the axons coming from the distinct nuclei in the hypothalamus. The anterior pituitary is regulated by the hypothalamus by the help of a vascular network. Hormone Stimulated by the hypothalamic Does hormone Anterior pituitary gland (Adenohypoysis) Growth Hormone (GH) Growth Hormone-Releasing Hormone Growing (GHRH) Thyroid-stimulating Thyrotropin-Releasing Hormone (TRH) Metabolism of the body hormone (TSH) Adrenocorticotropic Corticotropin-Releasing Hormone (CRH) Production and release of corticosteroids from the hormone (ACTH) Prolactin (PRL) adrenal glands Long list of chemical substances, Stimulation of milk production in breasts inhibited by dopamine Luteinizing hormone (LH) Gonadotropin-Releasing Hormone (GnRH) Triggers ovulation http://www.youtube.com/yeditepeanatomy 4 Dr.Kaan Yücel Follicle-stimulating hormone (FSH) yeditepeanatomyfhs122.wordpress.com Diencephalon ICHS production of testosterone Gonadotropin-Releasing Hormone (GnRH) Regulates the development, growth, pubertal maturation, and reproductive processes of the body Posterior pituitary gland (Neurohypoysis) Oxytocin Secreted from the hypothalamus and Distension of the cervix and uterus during labor, carried to the pituitary gland facilitating birth, and after stimulation of the nipples, facilitating breastfeeding. Antidiuretic hormone Secreted from the hypothalamus and (ADH) carried to the pituitary gland Increases water absorption in the the kidney Tuber Cinereum The tuber cinereum is a convex mass of gray matter, as seen from the inferior surface. It is continuous inferiorly with the infundibulum. The infundibulum is hollow and becomes continuous with the posterior lobe of the pituitary gland. The median eminence is a raised part of the tuber cinereum to which is attached the infundibulum. Mammillary Bodies The mammillary bodies are two small hemispherical bodies situated side by side posterior to the tuber cinereum. They possess a central core of gray matter invested by a capsule of myelinated nerve fibers. They are parts of the limbic system. 5. Third Ventricle Anterior commissure (AC) The anterior commissure (AC) of the primate brain is a tract of axons that primarily connects the right and left neocortex of the middle and inferior temporal lobes. Posterior commissure (PC) The posterior commissure bridges the upper part of the midbrain and lies adjacent to the posterior end of the third ventricle. The posterior commissure interconnects the pretectal nuclei, mediating the consensual pupillary light reflex. It is also related to superior colluculi related to light reflex. 5 http://twitter.com/hippocampusamyg