Anatomy of the Diencephalon, NEUROANATOMY Lecture : 5 Anatomy of the Diencephalon, Limbic System Pituitary Gland & Prepared and presented by: Dr. Iyad Mousa Hussein, MD, Ph.D in Neurology Head of Neurology Department Nasser Hospital

LECTURE OBJECTIVES: 1. Definition, Site, Surfaces, and Subdivisions of the Diencephalon. 2. Definition, site, Parts, nuclei, and functions of the Thalamus. 3. Definition, Site, Parts, and Nuclei and functions of the Hypothalamus. 4. Definition, Site, Structures, and functions of the Epithalamus. 5. Definition, Site, and Structures of the Metathalamus. 6. Definition, Site, and Structures of the Subthalamus. 7. Anatomy and Functions of the Pituitary Gland. 8.Definition, Site structures, Connections and functions of the Limbic System. 9. Pathway of upper and lower motor neuron. 10. Pathway of th superfacial and deep sensation.

The Diencephalon Definition and Site: It is the part of the brain located above the midbrain and between the lower parts of the two cerebral hemispheres. Embryological Origin: from forebrain → prosencephalon. Cavity of the diencephalon: is the third ventricle.

Surfaces of the Diencephalon Two lateral surfaces: each lateral surface related to the internal capsule. The Lower surface: a. The middle (largest) part: is the subthalamus, which located upper the midbrain. b. The anterior part: is formed by the hypothalamus and is connected with pituitary gland. C. The posterior part: is formed by the metathalamus. 3. Upper surface: it is formed by the two thalami and the third ventricle in the middle.

Subdivisions of the Diencephalon It is subdivided into five parts: 1. Thalamus (bilateral): the largest part. 2. Subthalamus: it lies directly above midbrain. 3. Hypothalamus: it lies infront of the subthalamus. 4. Metathalamus (bilateral). 5. Epithalamus.

The Thalamus Definition: it is a large, egg-shaped mass of gray matter lying in the middle of the cerebrum. Site: it is located on each side of the third ventricle, immediately above the subthalamus and medial to the internal capsule. Parts of the Thalamus: 1. Anterior part. 2. Medial part. 3. Lateral part.

Functions of the Thalamus Sensory Function: the thalamus is the great sensory relay station on the pathway of all sensations to cerebral cortex (except smell). 2. Consciousness level. 3. Emotional function.

Nuclei of the Thalamus 1. Medial group. 2. Lateral group. 3. Anterior group.

Nuclei of the Thalamus Anterior nucleus. Dorsomedial nucleus. Lateral posterior nucleus. Lateral dorsal nucleus. Lateral pulvinar nucleus. Ventral anterior nucleus. Ventral posterior nucleus (Ventral Posterolateral & Posteromedial nuclei). Ventral lateral nucleus. Inttralaminar nucleus. Midline reticular nucleus.

The Hypothalamus Definition and Site: It is the part of the diencephalon located infront of subthalamus and anterioinferior to the thalamus. The hypothalamus, although small (0,3 % of the total brain), is very important part of the central nervous System called Neuroendocrine organ.

Parts and Nuclei of the Hypothalamus Optic part: immediately related to optic chiasma. It consists of two nuclei: a. Supraoptic nucleus. b. Paraventricular nucleus. 2. Tuberal part: it consists of three nuclei: a. Ventromedial nucleus. b. Dorsomedial nucleus. c. Tuberal nucleus. 3. Mamillary part: it consists of two nuclei: a. Posterior nucleus. b. Lateral nucleus. 4. Posterior perforated substance.

Connections of the Hypothalamus A. Afferent fibers: 1. Somato and Visceral afferents. 2. Visual afferents. 3. Olfaction. 4. Auditory afferents. 5. Thalamohypothalamic fibers. 6. Amygdaloidhypothalamic fibers. 7. Tegmental fibers: from midbrain. B. Efferent fibers: 1. Descending fibers to the brain stem and spinal cord. 2. To the limbic system. 3. To the pituitary gland.

Functions of the Hypothalamus Hypothalamus has important regulatory functions: 1. Temperature. 2. Emotional regulation. 3. Growth (via thyroid stimulating hormone). 4. Hunger and thirst. 5. Sexual behaviour. 6. Control of various endocrine and activity rhythms (via hormones). 7. Memory (visual and verbal memory).

Connections of the Hypothalamus with the Hypophysis (Pituitary Gland) 1. The hormones vasopressin (antidiuretic hormone) and oxytocin are synthesized in the hypothalamus → the posterior lobe of the pituitary gland. 2. The hypothalamus is play important role in the production of the releasing hormones and release-inhibitory hormones.

The Hypothalamic Releasing and Inhibitory Hormones Anterior Pituitary Hormones Hypothalamic Regulatory Hormones Growth hormone (GH) Growth hormone-releasing hormone (GHRH) Growth hormone (reduced production) Growth hormone-inhibiting hormone (GHIH) or Somatostatin Prolactin hormone Prolactin-releasing hormone (PRH) Prolactin hormone (reduced production) Prolactin-inhibiting hormone (PIH) Adrenocorticotropic hormone (ACTH) Corticotropin-relasing hormone (CRH) Thyroid-stimulating hormone (TSH) Thyrotropin-relasing hormone (TRH) Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) Lutenizing hormone-relasing hormone (LHRH)

The Epithalamus Definition and Site: it is a part of the diencephalon which is attached to posterior end of the upper surface of the diencephalon. Parts of the Epithalamus: 1. Right and left Habenular nucleus. 2. Habenular commissure. 3. Posterior commissure. 4. Pineal body.

The Metathalamus Definition and Site: It is the part of the diencephalon which is attached to posterior part of the inferior surface of the thalamus. Parts of the Metathalamus: 1. Lateral geniculate body (LGB): visual function. 2. Medial geniculate body (MGB): auditory function.

The Subthalamus Definition and Site: it is the part of the diencephalon which located between the thalamus and midbrain. Parts of the Subthalamus: A. Posterior smaller part: containing of five bundles: 1. Medial lemniscus. 2. Spinal lemniscus. 3. Trigeminal lemniscus. 4. Reticuo-thalamic tract. 5. Superior cerebellar peduncle. B. Anterior larger part: containing of five bundles. 1. The upper end of the red nucleus. 2. The upper end of the substantia nigra. 3. The subthalamic nucleus. 4. The ansa lenticularis bundle. 5. The fasciculus lenticularis.

Pituitary Gland (Hypophysis or Master Gland) Site: it lies below the hypophyseal fossa below the diaphragm sella. Shape: it is an avoid body, its transverse diameter is 12mm and its anteroposterior diameter is 8mm. Relation: 1. Above: diaphragm sella which separates the gland from optic chiasma. 2. Below: body of sphenoid and sphenoid sinus separating the gland from nasopharynx. 3. On each side: cavernous sinus and its contains. Connection: with hypothalamus. Components: 1. Anterior lobe (Adenohypophysis). 2. Posterior lobe (Neurohypophysis).

Functions of the Pituitary Gland Production of the following hormones: A. Anterior pituitary hormones (from anterior lobe): 1. Growth hormone (GH). 2. Prolactin hormone. 3. Adrenocorticotropic hormone (ACTH). 4. Thyroid-stimulating hormone (TSH). 5. Luteinizing hormone (LH). 6. Follicle-stimulating hormone. B. Posterior pituitary hormones (from posterior lobe): 1. Vasopressin (Antidiuretic hormone). 2. Oxytocin.

The Limbic System (Emotional Brain) Definition and Site: It is the number of cortical and subcortical structures lying between the cerebral cortex and the hypothalamus.

Structures of the Limbic System Cingulate gyrus. Parahippocampal gyrus. Uncus. Hipocampal formation. Mammilary bodies. Septum pollucidum. Amygdaloid nucleus. The fornix.

Connections of the Limbic System Cerebral cortex. Thalamus. Hypothalamus. Epithalamus.

Functions of the Limbic System 1. Control the endocrine system. 2. Control the emotional behavior. 3. Recent memory (hippocampus). 4. There is no evidence that the limbic system has an olfactory function.

Pathway of the Upper Motor Neuron (Pyramidal or Cortico-Spinal tract) From Betz cells in motor area 4 (precentral gyrus of frontal lobe) → descends in the corona radiata → pass in the internal capsule → descends in mid brain, pons and medulla oblongata, some fibers of the pyramidal tract (cortico-nuclear fibers) supply the motor nuclei of the cranial nerves of both sides except the lower 1/2 of facial nuclei & the hypoglossal nuclei which are supplied only from opposite pyramidal tract. In the lower medulla 80% of fibers decussate to descend → in the lateral column of the opposite side of the spinal cord, while the another fibers (20%) descend directly → in the anterior column of the same side. All fibers are terminate at different level of anterior horn cells (AHCs).

Pathway of the Lower Motor Neuron System (Spino-muscular fibers) From AHCs of the spinal cord → fibers exit from the spinal cord as the anterior roots of the same side → peripheral nerves → motor end plate → voluntary muscles.

Pathway of the Superficial Sensations (Pain, Temperature & Crude Touch) All somatic sensation (superficial and deep) pass through three order neurons from receptors in the skin & deep structure to reach cortical sensory area. 1. The first order neuron (muscular-spinal neuron): started from receptors in the skin → peripheral (spinal) nerves → posterior roots & ganglion cells. 2. The second neuron (spino-thalamic neuron): started from posterior ganglion cells → posterior horns → decussating (crossing) to the opposite side of the spinal cord: a. For pain and temperature sensations: ascends in the lateral column of the spinal cord (lateral spino-thalamic tract). b. For crude touch sensation: ascends in the ventral column of the spinal cord (ventral spino-thalamic tract). Both lateral & ventral spino-thalamic tracts ascends through spinal cord → medulla oblongata → pons → mid brain → ends in the thalamus. 3. The third order neuron (Thalamo-cortical neuron): from the thalamus, sensation ascends through the internal capsule to reach cortical sensory area (1, 2, 3) in the postcentral gyrus of the parietal lobe.

Pathway of the Deep & Fine Touch Sensation The first order neuron (muscular-spinal tract): started from receptors in the skin → peripheral (spinal) nerves → posterior roots & ganglion cells. 2. The second neuron (Gracile & Cuneate tract): from the posterior roots, fibers pass directly into the posterior column of the same side of the spinal cord reaching the Gracile and Cuneate nuclei in the medulla → decussating (crossing) to the opposite side of the medulla oblongata and ascends through pons → mid brain → ends in the thalamus. 3. The third order neuron (Thalamo-cortical tract): from the thalamus, sensation ascends through the internal capsule to reach cortical sensory area (1, 2, 3) in the postcentral gyrus of the parietal lobe.

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