Diencephalon

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Gross features of the Diencephalon

  • Inferior surface

  • Superior surface

  • Lateral surface

  • Medial surface

Thalamus

Subdivisions of the Thalamus

  • Anterior part

  • Medial part

  • Lateral part

Subthalamus

Epithalamus

  • Habenular nucleus

  • Pineal gland

Hypothalamus

  • Hypothalamic relations

Optic chiasma

Tuber cinereum

Mamillary bodies

Hypothalamic nuclei

Third ventricle

  • Anterior wall

  • Posterior wall

  • Lateral wall

  • Superior wall

  • Inferior wall

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Gross features of the Diencephalon

Inferior surface

  • Formed by hypothalamic and other structures

  • Includes optic chiasma, infundibulum, tuber cinereum, and mammillary bodies

Superior surface

  • Concealed by the fornix

  • Formed by the roof of the third ventricle

  • Covered by the tela choroidea of the third ventricle

  • Choroid plexuses of the third ventricle project downward from the roof

Lateral surface

  • Bounded by the internal capsule of white matter

  • Consists of nerve fibers connecting the cerebral cortex with other parts of the brainstem and spinal cord

Medial surface

  • Divided into symmetrical halves by the third ventricle

  • Formed by the medial surface of the thalamus and hypothalamus

  • Separated by the hypothalamic sulcus

  • Contains the stria medullaris thalami

Thalamus

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  • Large ovoid mass of gray matter

  • Serves as a cell station for sensory systems and integration of information

  • Situated on each side of the third ventricle

  • Anterior end forms the posterior boundary of the interventricular foramen

  • Posterior end expands to form the pulvinar

  • Superior surface covered by tela choroidea and fornix

  • Lateral geniculate body forms a small elevation on the under aspect of the lateral portion of the pulvinar

  • Lateral surface separated from the lentiform nucleus by the internal capsule

  • Inferior surface continuous with the tegmentum of the midbrain

  • Medial surface forms the superior part of the lateral wall of the third ventricle and connected to the opposite thalamus by the interthalamic connection

Subdivisions of the Thalamus

  • Divided into three parts by the internal medullary lamina

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A. ANTERIOR PART

  • Contains the anterior thalamic nuclei

  • Receives the mammillothalamic tract from the mammillary nuclei

  • Receives reciprocal connection with the cingulate gyrus and hypothalamus

  • Function is closely associated with that of the limbic system and is concerned with emotional tone and the mechanisms of recent memory

B. MEDIAL PART

  • Contains the large dorsomedial nucleus and several smaller nuclei

  • Dorsomedial nucleus has two-way connections with the whole prefrontal cortex of the frontal lobe and the hypothalamic nuclei

  • Responsible for the integration of a large variety of sensory information, including somatic, visceral, and olfactory information, and the relation of this information to one's emotional feelings and subjective states

C. LATERAL PART

  • Nuclei are subdivided into a dorsal tier and a ventral tier

  • Medial and lateral parts lie on the sides of the Y

  • Each part contains thalamic nuclei

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IV. SUBTHALAMUS

  • Lies inferior to the thalamus and between the hypothalamus and thalamus

  • Situated between the thalamus and the tegmentum of the midbrain

  • Related to the hypothalamus craniomedially

  • Contains the cranial ends of the red nuclei and the substantia nigra

  • Subthalamic nucleus has the shape of a biconvex lens and is involved in the control of muscle activity

  • Contains important tracts that pass up from the tegmentum to the thalamic nuclei

V. EPITHALAMUS

  • Consists of the habenular nuclei and their connections and the pineal gland

  • Pineal gland is a small, conical structure attached to the diencephalon and projects backward posterior to the midbrain

  • Pineal gland possesses no nerve cells and no cell bodies, contains adrenergic sympathetic fibers, and is capable of influencing the activities of various endocrine glands

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A. HABENULAR NUCLEUS

  • Small group of neurons situated just medial to the posterior surface of the thalamus

  • Receives afferent fibers from the amygdaloid nucleus and the hippocampal formation

  • Believed to be a center for integration of olfactory, visceral, and somatic afferent pathways

B. PINEAL GLAND (BODY)

  • Small, conical structure attached by the pineal stalk to the diencephalon

  • Base of the pineal stalk possesses a recess that is continuous with the cavity of the third ventricle

  • Contains lobules divided by connective tissue septa, two types of cells (pinealocytes and glial cells), and accumulates calcified material called brain sand with age

  • Influences the activities of various endocrine glands and has inhibitory actions

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VI. HYPOTHALAMUS

  • Part of the diencephalon that extends from the region of the optic chiasma to the caudal border of the mammillary bodies

  • Composed of small nerve cells arranged in groups or nuclei

  • Controls and integrates the functions of the autonomic nervous system and the endocrine systems and plays a vital role in maintaining body homeostasis

  • Involved in activities such as regulation of body temperature, body fluids, drives to eat and drink, sexual behavior, and emotion

A. HYPOTHALAMIC RELATIONS

  • Anterior to the hypothalamus is the preoptic area, which extends forward from the optic chiasma to the lamina terminalis and the anterior commissure

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  • Caudally, the hypothalamus merges into the tegmentum of the midbrain.

  • The thalamus lies superior to the hypothalamus.

  • The subthalamic region lies inferolaterally to the hypothalamus.

  • The hypothalamus is related to the following structures from anterior to posterior:

    • Optic chiasma

    • Tuber cinereum and the infundibulum

    • Mammillary bodies

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  • The optic chiasma is a flattened bundle of nerve fibers situated at the junction of the anterior wall and floor of the third ventricle.

  • The superior surface of the optic chiasma is attached to the lamina terminalis.

  • The inferior surface of the optic chiasma is related to the hypophysis cerebri, separated by the diaphragma sellae.

  • The anterolateral corners of the chiasma are continuous with the optic nerves, and the posterolateral corners are continuous with the optic tracts.

  • The tuber cinereum is a convex mass of gray matter, continuous inferiorly with the infundibulum.

  • The infundibulum is hollow and becomes continuous with the posterior lobe of the hypophysis cerebri.

  • The mammillary bodies are two small hemispherical bodies situated posterior to the tuber cinereum.

  • The mammillary bodies possess a central core of gray matter invested by a capsule of myelinated nerve fibers.

  • Posterior to the mammillary bodies lies an area of the brain called the posterior perforated substance, which is pierced by small apertures transmitting the central branches of the posterior cerebral arteries.

  • The hypothalamus is divided into medial and lateral zones.

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  • The third ventricle is a slit-like cleft between the two thalami.

  • It communicates anteriorly with the lateral ventricles through the interventricular foramina and posteriorly with the fourth ventricle through the cerebral aqueduct.

  • The third ventricle has anterior, posterior, lateral, superior, and inferior walls lined with ependyma.

  • The anterior wall is formed by the lamina terminalis, across which runs the anterior commissure.

  • The posterior wall is formed by the opening into the cerebral aqueduct, with the small posterior commissure and pineal recess above it.

  • The lateral wall is formed by the medial surface of the thalamus superiorly and the hypothalamus inferiorly, separated by the hypothalamic sulcus.

  • The superior wall or roof is formed by a layer of ependyma, with the tela choroidea of the third ventricle projecting downward to form the choroid plexuses.

  • The inferior wall or floor is formed by the optic chiasma, tuber cinereum, infundibulum, and mammillary bodies, with the tegmentum of the cerebral peduncles posterior to them.

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Pineal Gland

  • True of the Pulvinar except:

    • Posterior end of the Thalamus

    • Has a small elevation under the aspect of its lateral portion called the lateral geniculate body

    • Separated from the lentiform nucleus by the very important band of white matter called the internal capsule

Inferior aspect of the Diencephalon

  • True of the Inferior aspect of the Diencephalon except:

    • Is the only area exposed to the surface in the intact brain.

    • Formed by hypothalamus and other structures

    • Includes, from anterior to posterior, the optic chiasma, with the optic tract on either side; the infundibulum, with the tuber cinereum; and the mammillary bodies.

    • Concealed by the fornix, which is a thick bundle of fibers that originates in the hippocampus of the temporal lobe

Subthalamic nucleus

  • The subthalamic nucleus has the shape of a:

    • Biconcave lens

    • Funnel

    • The letter Y

    • Biconvex lens

Epithalamus

  • Consists of the habenular nuclei and their connections and the pineal gland.

Habenular nucleus

  • Small group of neurons situated just medial to the posterior surface of the thalamus.

Pineal Gland

  • The pineal gland is a small, conical structure that is attached by the pineal stalk to the diencephalon.

Hypothalamus

  • Physiologically, nearly all the activities in the body are influenced by the hypothalamus.

Lamina terminalis

  • This structure is a flattened bundle of nerve fibers found at the junction of the anterior wall of the brainstem and floor of the third ventricle.

Tuber cinereum

  • The infundibulum is continuously inferiorly with this structure, which is a convex mass of gray matter seen from the inferior surface of the brainstem.

Optic recess of the 3rd ventricle

  • This is a small recess lying on the superior surface of the optic chiasm.

Mamillary bodies

  • It is located posterior to the tuber cinereum, and posterior to this structure is the posterior perforated substance.

Posterior perforated substance

  • There are two small spherical hemispherical bodies pierced with structures that transmit the central branches of the posterior cerebral arteries. The piercing structures are:

    • Mamillary bodies

    • Tuber cinereum

    • Posterior perforated substance

    • Apertures

Recess of the 4th ventricle

  • Formed by a layer of ependyma that is continuous with the lining of the ventricle.

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Lamina terminalis

  • Formed by a thin sheet of gray matter, the lamina terminalis, across which runs the anterior commissure.

Hypothalamic nuclei

  • Small groups of nerve cells in the hypothalamus.