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

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

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.

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

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.

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

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

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.

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

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

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

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.

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.

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.

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})