Subthalamus & HypothalamusDr Zeenat ZaidiDIENCEPHALON

SUBTHALAMUS

SUBTHALAMUSRegion of diencephalon located below the thalamus & dorsolateral to hypothalamusContinues caudally with the midbrainThHypothalamus

ContentsRostral extension of:Red nucleusSubstantia nigraBrainstem reticular formation as Zona incertaLong tracts passing through brain stem and heading toward thalamusSpinothalamic & Trigeminothalamic tractsMedial lemniscusDentatothalamic fibersPallidothalamic fibers (fasciculus lenticularis, Ansa lenticularis & thalamic fascicle) Subthalamic nucleus

Subthalamic Nucleus Resembles a biconvex lens in shapeLocated in the ventrolateral part of the subthalamusLies against the medial surface of the internal capsuleIC

ConnectionsHas reciprocal connections with ipsilateral:Globus pallidus via subthalamic fasciculus, which passes through the internal capsuleSubstantia nigra

Plays an important role in normal functioning of basal ganglia

RareUsually of cerebrovascular originResults in Hemiballism (sudden, forceful involuntary, violent or jerky, movements of the limbs) on the contralateral side

FunctionsLesions

Zona IncertaRostral extension of the brainstem reticular formationEnveloped by pallidothalamic fibers (lies between the lenticular fascicle and the thalamic fascicle)

HYPOTHALAMUS

HypothalamusMost ventral part of diencephalonLies beneath the thalamus and ventromedial to the subthalamusForms the floor and lower part of the lateral wall of the 3rd ventricleHTHFCC

Most of the hypothalamus is hidden except the inferior surface, that can be seen on the inferior surface of the brain, cranial to the cerebral pedunclesParts of hypothalamus seen on the base of the brain include: InfundibulumTuber cineriumMammillary bodiesmbtcocPI

FmbThe anterior column of the fornix passes vertically through the substance of hypothalamus (to terminate in the mamillary body) and divides it into medial and lateral zones

Medial NucleiLateral Nuclei

Lateral partLies medial and ventral to the subthalamusTraversed by many fibers including medial forebrain bundleControls food and water intake (feeding centre)Lesions cause aphagia and adipsia

Medial partForms lateral wall of the 3rd ventricleContains:Anterior nucleusSupraoptic nucleusPreoptic nucleusParaventricular nucleusDorsomedial nucleusVentromedial nucleusPosterior nucleusMammillary nucleiInfundibular nucleus

Supraoptic nucleus produces vasopressin which increases water reabsorption by the kidneyParaventricular nucleus produces oxytocinThe axons of cells in supraoptic and paraventricular nuclei pass to the neurohypophysis in the hypothalamo-hypophyseal tractThe hormones are transported in this tract and released into the capillary bed

Suprachiasmatic nucleus: concerned with the control of diurnal rhythm and sleep/awake cycleReceives some afferent fibers directly from the retinaVentromedial nucleus: acts as satiety centreLesions cause polyphagia, polydipsia

Mammillary nuclei:Part of the limbic systemReceive afferents from the hippocampus through fornixProject to the:Anterior nucleus of thalamus through mammillo-thalamic tractBrainstem through the mamillotegmental tract

Optic tract Mamillary body Column of fornix Thalamus Superior & inferior colliculi Caudate nucleus Anterior commissure Mamillothalamic tract

Hypothalamus also synthesizes releasing factors & release-inhibiting factors, that control the release of hormones by the adenohypophysisThese factors are released from the terminals of hypothalamic neurones into the capillary bed of the pituitary portal system, which conveys the release agents to the anterior pituitary

FunctionsCo-ordination of homeostatic mechanismControls the release of hormones from the pituitary gland.Center for regulation of autonomic activity --- controls medulla oblongata nuclei for cardiovascular, respirationActivation of posterior region associated with sympathetic responsesActivation of anterior region associated with parasympathetic responsesThe mammillary nuclei are associated with the emotional behaviour and memoryThe suprachiasmatic nucleus is concerned with diurnal rhythm & sleep/waking cycleThe lateral hypothalamus & the ventromedial nucleus regulate feeding and drinkingCenter for Feeding reflexeslicking, swallowing, etc.Controls subconscious skeletal muscle movementsfacial expressions, sexual movementsCoordinates autonomic response to conscious inputthought of fear produces accelerated heart rate, etc.

3rd Ventricle: The cavity of the Diencephalon

BoundariesAnterior: Lamina terminalis, a membrane stretching between anterior commissure (ac) & optic chiasma (oc)Posterior: Pineal gland Lateral walls:medial surface of thalamus above & hypothalamus below the hypothalamic sulcusTH pHacoc

Roof: Ependyma stretching between the two stria medullaris thalami

Floor: Anteroposteriorly:optic chiasmaInfundibulumtuber cineriummammillary bodiestegmentum of midbrainCavity is crossed by interthalamic adhesion (black arrow)

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RecessesAre extensions of the cavity:SupraopticInfundibularPinealSuprapineal

CommunicationsRostrally: communicates on each side with the lateral ventricle through interventricular foramen of MonroCaudally: with 4th ventricle through cerebral aqueduct

Interventricular Foramen of MonroPaired foraminaEach bounded:Anteriorly by column of fornix Posteriorly by anterior pole of the thalamusCommunicate 3rd ventricle with the two lateral ventricles

Choroid Plexus of 3rd ventricleTuft of capillaries enclosed within ependymaInvaginates from its roofExtends rostrally till the interventricular foramenContinues laterally with the choroid plexus of lateral ventricleProduces cerebrospinal fluid

Clinical AnatomyObstruction of cerebral aqueduct leads to dilatation of 3rd ventricle and both lateral ventriclesUnilateral/bilateral obstruction of interventricular foramen leads to dilatation of one or both lateral ventricles respectivelyIn both cases symptoms and signs of Hydrocephalus develop

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