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THE DIENCEPHALONThe diencephalon is the lower central part of
theforebrain surrounded by the cerebralhemisphere. It is derived
from the embryonicProsencephalon and consists of four
componentparts viz. Thalamus HypothalamusEpithalamus
andSubthalamus
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The components of the diencephalon arebilaterally represented
and surround the thirdventricle. They are bounded laterally by the
internal capsule and Medially by the third ventricle. The
diencephalon is continuous above with
the telencephalon and Below with the midbrain
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THE HYPOTHALAMUSGeneral Features:The Hypothalamus forms the
lateral wall of the 3rd ventricle inferior to the hypothalamic
sulcus and weighs about 4gms. The internal capsule and subthalamus
are located on its lateral aspect. Ventral surface structures (See
diagram 1)These include on each side:Anteriorly and in the midline
- The optic chiasma.Laterally - The optic tract and junction of the
internal capsule and the crus cerebri.Posteriorly - The mammillary
body.
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Functions: The principal function of the hypothalamus is the
integration of the Autonomic and the Endocrine Systems. Thus it is
implicated in:Initiation of responses to emotional
changes.Initiation of responses to changes in visceral
functions.Maintenance of a constant internal environment
(Homeostasis).Initiation of outward expressions of emotions
and moods.Reproductive functions
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Route of Manifestations of its Functions:Hypothalamic functions
are executed through:The Autonomic systemThe Reticular formationThe
endocrine System:
a. Adenohypophisisb. NeurohypophisisIn general, most
hypothalamic neurons functionas neurohaemal cells.
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Internal structure/Organization: See Diagram 3Internally, the
hypothalamus is subdivided into two zones. These are: Medial Zone
(Area).This is further subdivided into the
followingRegions:Suprachiasmatic Region which contains:Supraoptic
nucleusParaventricular nucleusSuprachiasmatic nucleus and Anterior
nucleus
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2.Tuberal (Infundibular) Region which contains:Ventromedial
nucleusDorsomedial nucleusInfundibular (Arcuate) nucleus
3.Mamillary Region which contains:Mamillary bodyPosterior
nucleus
The Lateral Zone (Area) is made up of two nucleiLateral
hypothalamic nucleusLateral Tuberal nucleusLongitudinally
traversing fibres
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Hypothalamic Connections: The principal connections of the
hypothalamus are to the following centres: Cerebral cortex.
Thalamus Amygdaloid body The brainstem Preganglionic autonomic
nuclei of the brainstem and
the spinal cord6. Epithalamus.7. Substantia innominata.
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Hypothalamic Tracts: The following tracts are utilized by fibres
projecting into and out of the hypothalamus Fornix. Medial
forebrain bundle. Dorsal longitudinal fasciculus. Stria terminalis.
Corticohypothalamic fibers. Mamillothalamic tract
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Control of some Visceral Functions:Sympathetic control Via
Posterior & Lateral
nucleiParasympathetic Control Via Anterior &
Preoptic nucleiControl of body temperature is effected via the
Sympathetic and Parasympathetic componentsFood Intake: This is
controlled via two centres: Hunger centre Lateral zone Satiety
centre Ventromedial nucleus
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Hunger and satiety centres are influenced by:Blood glucose
level.Autonomic inputs from visceral afferent centres.Emotional
responses initiated by:The Thalamus.The cerebral cortex.The
olfactory system.The limbic system
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Adiposogenital Syndrome: This condition results from concurrent
damage to the satiety centre and centres responsible for the
secretion of Gonadotrophin releasing factor. The two main features
of this condition are: Obesity. Deficiency of secondary sexual
characteristics.
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Water Intake: This is controlled by the same centre for food
intake with the participation of other centres outside the
Hypothalamus, Viz. The Zona Incerta of the subthalamus. The
subfornical organ in the neighborhood of
the fornix.
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The Hypothalamus and the Pituitary Gland:
The hypothalamus controls the activities of the pituitary gland
and by extension the entire endocrine system via connections to the
two structurally and functionally distinct parts of the pituitary
gland named (See Diagram 4): The Neurohypophysis and The
Adenohypophysis.
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The anterior pituitary is controlled by Hormone Releasing
Factors from the median eminence of the hypothalamus while the
posterior pituitary is under the control of two nuclei of the
medial zone of the hypothalamus. These are: Paraventricular nucleus
which secretes
Oxytocin and2. Supraoptic nucleus which secretes Vasopressin
(Also called Anti Diuretic Hormone {ADH})