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DIENCEPHALON
05. 05. 2014
Kaan Yücel
M.D., Ph.D.
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Dr.Kaan Yücel
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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
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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.
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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
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Follicle-stimulating
hormone (FSH)
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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.
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