Neuroanatomy: Diencephalon and Thalamus

Diencephalon

• A mass of gray matter (nuclei)
• Located where the cerebrum meets the brainstem
• Composed of thalamus, hypothalamus, epithalamus, subthalamus, and metathalamus

Thalamus

• Large mass of gray matter
• Acts as a synaptic relay for pathways to the cerebral cortex
• Divided into two ends: anterior and posterior
• Has four surfaces: superior, inferior, medial, and lateral

Relations of Thalamus

• Anterior end: Forms the posterior boundary of the interventricular foramen of Monro
• Posterior end: Broad, called the pulvinar; overlies medial and lateral geniculate bodies
• Inferior surface: Related to the subthalamus, hypothalamus, and tegmentum of the midbrain
• Superior surface: Related to the body of caudate nucleus, body of lateral ventricle, thalamostriate vein, and stria terminalis

Internal Structure of Thalamus

• External medullary lamina: Covers the lateral and superior surfaces
• Internal medullary lamina: Y-shaped bundle dividing the thalamus into anterior, medial, and lateral nuclear groups. Each group is subdivided into multiple nuclei

Thalamic Nuclei

• Anterior group: Located between the anterior limbs of the Y-shaped internal medullary lamina
• Medial group: Medial to the internal medullary lamina
• Lateral group: Lateral to the internal medullary lamina, subdivided into nucleus lateralis dorsalis (LD), nucleus lateralis posterior (LP), and pulvinar nucleus (P)
• Ventral group: Constitutes nucleus ventralis anterior (VA), nucleus ventralis lateralis (VL), nucleus ventralis intermedius (VI), and nucleus ventralis posterior (further subdivided into ventralis posterior medialis (VPM) and ventralis posterior lateralis (VPL))

Thalamic Radiations

• Anterior thalamic radiation: Connects the frontal lobe with medial and anterior nuclei
• Superior thalamic radiation: Connects ventral and lateral nuclei with the precentral and postcentral gyri
• Posterior thalamic radiation: Connects the pulvinar with the occipital lobe, including the connection between the lateral geniculate body and occipital lobe (optic radiation)
• Inferior thalamic radiation: Connects the pulvinar with the temporal lobe, including the connection between the medial geniculate body and temporal lobe (auditory radiation)

Blood Supply of the Thalamus

• Supplied by branches of the posterior communicating, posterior cerebral, posterior choroidal, and basilar arteries

Hypothalamus

• Structures in the floor and lateral wall of the third ventricle
• Includes dorsomedial nucleus, paraventricular nucleus, anterior hypothalamic nucleus, preoptic nucleus, suprachiasmatic nucleus, pituitary gland, supraoptic nucleus, arcuate nucleus, posterior nucleus, and ventromedial nucleus, mammillary body

Parts

• Optic chiasma region
• Tuber cinerium and stalk of pituitary gland
• Mammillary bodies

Epithalamus

• Consists of pineal body (gland), habenular nuclei, habenular commissure, and posterior commissure

Subthalamus

• Lies between the midbrain and thalamus
• Contains mainly the subthalamic nucleus, which lies above the substantia nigra
• Has connections with GRS

Metathalamus

• Composed of medial and lateral geniculate bodies

• Situated on the inferior surface of the pulvinar of the thalamus

• Medial geniculate body: Lateral to the superior colliculus, connected to the inferior colliculus by the inferior brachium; a relay nucleus in the auditory pathway

• Lateral geniculate body: Lateral to the medial geniculate body, connected to superior colliculus by superior brachium; receives fibers of optic tract carrying visual impulses and sends efferent fibers (optic radiation) to the visual area

Test your self (True or False)

• Anterior end of the thalamus is called pulvinar. (False)
• Body of caudate nucleus is related to superior surface of thalamus. (True)
• Inferior thalamic radiation includes optic radiation. (False)
• Pineal body is a part of the epithalamus. (True)

Test Yourself (Multiple Choice)

• Which structure is included in the metathalamus? (Lateral geniculate body)

Explore the key structures and relations of the diencephalon and thalamus in neuroanatomy. Learn about the composition, functions, and internal structures of these crucial brain areas that serve as relays to the cerebral cortex.