DIENCEPHALON Dr Zeenat Zaidi Diencephalon Paired structure l

DIENCEPHALON Dr. Zeenat Zaidi

Diencephalon Paired structure l Located between the brain stem and the cerebral hemisphere l Continuous with the rostral part of the midbrain l Forms the lateral wall of the 3 rd ventricle l C mb p mo

Almost entirely surrounded by the cerebral hemispheres l A little part seen externally on the base of the brain caudal to optic chiasma, includes: • Infundibulum • Tuber cinerium • Mamillary bodies l Other parts seen on sagittal & coronal sections l tc I mb

l On the medial surface, the diencephalon is subdivided, by hypothalamic sulcus (indicated by black line) into: § Dorsal part § Ventral part Fornix CC Dorsal Ventral Optic chiasma Midbrain Cerebral aqueduct

Dorsal part Thalamus & Epithalamus H Subthalamus & Hypothalamus Ventral part

Relations Dorsal: Lateral ventricle Lateral: Internal capsule Medial: 3 rd ventricle Ventral: Exposed on the base of the brain

EPITHALAMUS

Epithalamus l Relatively small part, located in most caudal and dorsal region l Lies immediately rostral to superior colliculus l Consists of: § Pineal gland & § Habenular nuclei

Pineal Gland l An endocrine organ l Synthesizes melatonin l Controls: § Sleep/awake cycle § Regulation of onset of puberty

Habenular nuclei l Located in habenular triangle (area in the posterior part of the diencephalon, just anterior to pineal gland) l Have connections with limbic system l Serves autonomic function and emotional drives

THALAMUS

Thalamus Large mass of grey matter, in shape and size, resembles small hen’s egg l Forms the lateral wall of the 3 rd ventricle l Separated from hypothalamus by hypothalamic sulcus l May be connected to opposite thalamus by interthalamic adhesion (massa intermedia) l thalamus Interthalamic adhesion Hypothalamic sulcus

Thalamus: In horizontal sections of brain Lower level Higher level

Thalamus has a narrow anterior end called tubercle of thalamus, thalamus that lies in the posterior boundary of the interventricular foramen The expanded posterior end, called pulvinar, lies above the superior colliculi

Relations Dorsal: Dorsal lateral ventricle Lateral: Lateral Internal capsule Anterior: Anterior interventricular foramen Medial: Medial 3 rd ventricle Ventral: Subthalamus & Hypothalamus Caudal: Caudal midbrain

Surfaces l 4 • • Surfaces: Superior Inferior Medial Lateral S L M l

Superior Surface Bounded laterally by caudate nucleus, thalamostriate vein and a nerve fiber bundle called stria terminalis l Lateral part lies in the floor of the lateral ventricle & is covered by ependyma l Medial part is related to the choroid plexus of the 3 rd ventricle l stria terminalis caudate nucleus LV thalamostriate vein choroid plexus ependyma

Lateral Surface l Related to the internal capsule Inferior Surface l Rests on the subthalamus & hypothalamus

Medial Surface Stria medullaris thalami (a fascicle of nerve fibers) courses along its dorsomedial margin l Below is limited by hypothalamic sulcus l Forms the upper part of the lateral wall of the 3 rd ventricle l Covered by ependyma l Stria medullaris thalami Hypothalamic sulcus

Internal Organization l Thalamus is composed of grey matter, matter interrupted by two vertical sheaths of white matter called medullary laminae. l External medullary lamina: Located laterally, separates reticular nucleus from the rest of the thalamic mass § Contains thalamocortical & corticothalamic fibers §

l Internal medullary lamina Y- shaped band, divides thalamus into Anterior, Medial & Lateral nuclear groups § Contains: § Fibers connecting thalamic nuclei with one another § Neuronal collections called intralaminar nuclei §

Nuclear Groups l Anterior nuclear group Lateral nuclear group: Divided into: § dorsal & § ventral tiers Medial nuclear group Intralaminar nuclei Reticular nucleus l Midline nuclei l l

Functional Organization All the nuclei of the thalamus except reticular nucleus, project to ipsilateral cerebral cortex l The whole of the cerebral cortex receives input from the thalamus l All thalamic nuclei receive corticofugal fibers in a basically reciprocal fashion l

l Based on their connection with the cerebral cortex, the thalamic nuclei are divided into: v. Specific nuclei v. Nonspecific nuclei

l Specific nuclei: v Have well-defined sensory and motor functions v Have highly organized point-to-point connection with sensory & motor regions of cerebral cortex v Lie within the ventral group of the lateral nuclear group l Non-specific v Receive Nuclei: less functionally distinct afferent input v Connect with wider area of cortex, including associative and limbic regions v Include nuclei of the dorsal tier of lateral group, and whole of the anterior and medial group

Lateral Nuclear Group Ventral Tier Ventral anterior • Ventral lateral • Ventral posterior: • • (VPL) • (VPM) Lateral geniculate • Medial geniculate •

Ventral Anterior Nucleus Influences motor activity Ipsilateral globus pallidus & substantia nigra Motor cortex Premotor & supplementary motor cortex

Ventral Lateral Nucleus Influences motor activity Ipsilateral globus pallidus & substantia nigra Contralateral dentate nucleus Primary motor cortex

Ventral Posterior Nucleus Chief sensory relay station General sensory afferents from the contralateral half of the Head & neck (VPM) and Body (VPL) Primary somatosensory cortex

Lateral Geniculate Body Part of the Visual Pathway Ipsilateral temporal hemiretina Contralateral nasal hemiretina Optic radiation to the primary visual cortex

Medial Geniculate Body Part of the Auditory Pathway Inferior colliculus Auditory radiation to the primary auditory cortex

Lateral Nuclear Group Dorsal Tier Lateral Dorsal • Lateral Posterior • Pulvinar •

Lateral dorsal nucleus § Part of Limbic System Hippocamus Cingulate gyrus Lateral posterior nucleus sensory association cortex of parietal lobe Pulvinar: Sensory association cortices of parietal, temporal & occipital lobes

Medial Nuclear Group Integrates emotion, thought, and judgment Mediodorsal nucleus & Nucleus reuniens Hypothalamus, amygdala, other thalamic nuclei, prefrontal cortex Prefrontal cortex & limbic structures

Anterior Nuclear Groups Functionally part of the limbic system. Involved in control of instinctive drives, emotional aspect of behaviour and in memory l 3 parts: Anteroventral § Anteromedial § Anterodorsal §

Mammillary body of hypothalamus via mammillothalamic tract cingulate gyrus

Midline Nuclei Located between medial nuclear group and the ependyma of 3 rd ventricle l Important in visceral functions l Brainstem reticular formation Cingulate gyrus and hypothalamus

Intralaminar Nuclei Located within the internal medullary lamina l Main nuclei: Centromedian & Parafascicular l Function as activator of the cerebral cortical mantle l Lesions reduce the perception of pain and level of conciousness l

Reticular formation, spinothalamic & trigeminothalamic systems Widespread regions of cerebral cortex, caudate & putamen of the basal ganglia

Reticular Nucleus Regulates the activity of thalamus l Located between the external medullary lamina & the internal capsule Collaterals of both Thalamocortical & Corticothalamic fibers Other thalamic nuclei

Functions of the thalamus Receives and analyses all the sensory information (except olfactory) from the body l Having extensive connections with the basal ganglia and the motor cortices, it plays a pivot role in voluntary motor activity. Thalamotomy (VA, VL) was once used to treat basal ganglia disorders l Connections with the limbic system makes it important in the control of mood, emotional and sexual behavior, and memory l

Thalamic Lesions Cerebrovascular lesions or tumors of thalamus lead to: v Loss of sensation in the contralateral side of face and body followed by distressing discomfort, & burning and diffuse pain in the anaesthetic areas (thalamic pain) v Thalamic syndrome: Abnormal voluntary movements (chorea or hemiballismus) with hemisensory disturbance l

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