The Diencephalon Functions of the Diencephalon

The Diencephalon Functions of the Diencephalon



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Kim Bengochea
Kim Bengochea, Regis University, Denver

Diencephalon

The human brain can be subdivided by many classification systems. One particular nomenclature that refers to the duality of the brain is the diencephalon. It is the caudal part of the forebrain (prosencephalon) that occupies the central region of the brain. The diencephalon is comprised of the:

  • Epithalamus
  • Thalamus
  • Subthalamus
  • Metathalamus
  • Hypothalamus
  • Hypophysis cerebri

In the following article we will explore these different parts of diencephalon as well as its function. 

Contents
  1. Function
  2. Embryology
  3. Gross Anatomy
    1. Borders
    2. Blood Supply
  4. Epithalamus
  5. Thalamus
    1. Nuclei
    2. Surfaces
  6. Subthalamus
    1. Characteristics & Components
    2. Connections
  7. The Metathalamus
    1. Medial Geniculate Body
    2. Lateral Geniculate Body
  8. Hypothalamus
    1. Relations
    2. Nuclei & Areas
  9. Hypophysis Cerebri
  10. Clinical Significance
    1. Diencephalic (Russell’s) Syndrome
    2. Panhypopituitarism Syndrome
  11. Related diagrams and images

    Function

    Diencephalon

    Each of the components of the diencephalon has specialized functions that are integral to life. The diencephalon acts as a primary relay and processing center for sensory information and autonomic control. The plethora of communicating pathways between these structures and other parts of the body makes the diencephalon a functionally diverse area. Some of these connections include pathways to the limbic system (seat of memory and emotion), basal ganglia (motor coordination), as well as primary sensory areas, such as auditory or visual.

    This article will explore the embryology of the diencephalon, as well as the gross anatomy and function of its various components. Clinically significant points relating to this region of the brain will also be discussed.

    Embryology

    The anterior most part of the  primary cerebral vesicles of the embryo gives rise to the prosencephalic portion of the foetal brain. At about the 5th week of development, the prosencephalon is further subdivided into a ventral telencephalon (that differentiates to form the cerebral hemispheres) and a caudal diencephalon (secondary cerebral vesicles). The latter originates from the median region of the prosencephalon. In utero, the region consists of two lateral alar plates (representing sensory areas), a roof plate (most dorsal part of the neural tube), and the third ventricle. The scientific community is unclear whether or not basal plates (representing motor areas) exist as it is not physically observed, but the corresponding biomarkers have been identified. 

    The alar plates give rise to the lateral walls of the third ventricle (and by convention, the medial walls of the thalamus). Rapid cellular proliferation results in medial projection of the thalamus into the diencephalic cavity (future space of the third ventricle), resulting in a midline fusion of a portion of the thalamus known as the massa intermedia or the interthalamic connexus (adhesions). A shallow groove known as the hypothalamic sulcus emerges and divides the walls of the diencephalon into the thalamus (dorsally) and the hypothalamus (ventrally). The roof plate is comprised of a monolayer of ependymal cells that is coated with vascular mesenchyme. This area subsequently differentiates into the choroid plexus of the third ventricle. Caudally, the roof plate differentiates into the epiphysis (pineal body).

    The hypothalamus further differentiates into a variety of nuclear regions that are responsible for an eclectic collection of body functions. These include, but are not limited to, digestion, thermoregulation, and circadian regulation. Caudal to the hypothalamus is the hypophysis cerebri (pituitary gland). It originates from both ectodermal and neuroglial tissue and as a result, it can be subdivided into the adenohypophysis and the neurohypophysis.

    Of note, the prosencephalon also gives rise to the optic cup and stalk, which subsequently develops into the retina of the eye .

    Gross Anatomy

    There are several structures between the brainstem and the cerebral cortex that make up the diencephalon. These include the epithalamus, thalamus, subthalamus, metathalamus, hypothalamus, hypophysis cerebri and the third ventricle as its cavity. The medial and lateral geniculate bodies, which are collectively called the metathalamus, along with the pulvinar, are frequently regarded as an integral part of the dorsal thalamus.

    Borders

    The third ventricle is a narrow vertical midline cleft between and below the two lateral ventricles and in between left and right thalami. The lateral ventricles communicate with the third ventricle via the interventricular foramen of Monro. It also communicates with the fourth ventricle posteroinferiorly via the cerebral aqueduct of Sylvius.

    Third ventricle – lateral-left view

    It possesses a roof, a floor and four walls: anterior, posterior and two lateral.

    • The roof is formed by the thin tela choroidea, which is a combination of two membranes, the ependyma and pia mater. Within the tela choroidea are two plexuses of blood vessels (one on either side of the middle line) that bulge downwards into the cavity of the third ventricle. These are the choroid plexuses of the third ventricle which functions as a point of production of the cerebrospinal fluid (CSF).  
    • The floor is made up of the optic chiasm, the tuber cinereum and infundibulum, the mammillary bodies, the posterior perforated substance and the uppermost part of the mesencephalic tegmentum.
    • The anterior wall is the delicate lamina terminalis, as well as the anterior commissure and anterior column of the fornix..
    • The short posterior wall is formed by the stalk of the pineal gland, posterior commissure and the Habenular commissures.
    • The lateral walls are of the cavity are formed by the medial walls of each thalami. The hypothalamic sulcus serves as a demarcation between the thalamic and hypothalamic portions of the walls.

    Blood Supply

    The diencephalon is richly supplied by several blood vessels, notably the thalamogeniculate branches of the posterior cerebral artery and thalamoperforating branches of the posterior cerebral artery and posterior communicating artery.

    Epithalamus

    The epithalamus lies in relation to the posterior part of the roof of the third ventricle and the little adjoining part of its lateral wall. It consists of the following parts:

    • Stria medullaris
    • Posterior commissure 
    • Habenular nuclei (medial and lateral)
    • Pineal body
    • Paraventricular nuclei (anterior and posterior)

    The stria medullaris thalami lie close to the taenia thalami as a bundle of fibres along the junction of the medial and superior surfaces of the thalamus. It begins near the anterior pole of the thalamus and runs posteriorly toward the Habenular trigone. From the stria medullaris thalami, some fibres cross in the superior or anterior lamina of the pineal stalk to reach the Habenular nuclei of the opposite side. These fibres constitute the Habenular commissure, some of which also connect the amygdaloid and hippocampal complexes of the right and left cerebral hemispheres. The posterior commissure lies in the inferior lamina of the stalk of the pineal body. A number of small nuclei are present in relation to the commissure, such as the interstitial and dorsal nuclei of the posterior commissure, the nucleus of Darkschewitsch and the interstitial nucleus of Cajal (both nuclei communicate with the vestibular nuclei via the medial longitudinal fasciculus).

    Stria medullaris of thalamus – cranial view

    The pineal gland is a small piriform structure located in relation to the posterior wall of the third ventricle. It is an endocrine gland of considerable significance and is made up of cells called pinealocytes, which secrete melatonin in response to darkness. The pineal body also secretes a number of hormones that have important regulatory influence on many endocrine organs including the hypophysis cerebri, thyroid , parathyroids, adrenals and gonads. Hormones of the pineal gland reach the hypophysis cerebri through the blood stream and the cerebrospinal fluid (CSF). As humans age, the pineal glands become calcified and form what are known as corpora arenacea or brain sand.

    Pineal gland – dorsal view

    Thalamus

    The thalamus is the largest mass of grey matter of the diencephalon that is laterally related to the third ventricle. The symmetrical halves of this midline structure are located between the cerebral cortices and the midbrain. It has an anterior and a posterior pole, as well as superior, inferior, medial and lateral surfaces. Its superior part is covered by a thin layer of white matter called the stratum zonale and its lateral surface is covered by a similar layer called the external medullary lamina.

    Nuclei

    The thalamus also comprises several nuclei, such as the:

    • Dorsomedial nucleus 
    • Midline nuclei 
    • Geniculate bodies (lateral and medial) 
    • Pulvinar 
    • Anterior nuclei 
    • Intralaminar nuclei 
    • Lateral dorsal nuclei 
    • Lateral posterior nuclei
    • Ventral anterior nuclei 
    • Ventral posterior (medial and lateral) 
    • Ventral lateral nuclei

    Thalamus

    Surfaces

    The anterior pole of the thalamus lies just behind the interventricular foramen of Monro.

    Its posterior pole makes up the pulvinar, and lies just superior and lateral to the superior colliculus. The medial and lateral geniculate bodies are connected to the inferior and superior colliculi by the inferior and superior brachium quadrigeminum, respectively.

    The medial surface forms the greater part of the lateral wall of the third ventricle, and is lined by ependyma. A mass of grey matter called the interthalamic adhesion (connexus) is found attached to the medial surfaces of the two thalami and this connexus interconnects the left and right thalami. Inferiorly, the medial surface is separated from the hypothalamus by the hypothalamic sulcus.

    The internal capsule is related to the lateral surface of the thalamus and separates this surface from the lentiform nucleus (globus pallidus and putamen) of the basal ganglia .

    The thalamostriate vein and a bundle of fibres called the stria terminalis are in close apposition to the dorsal or superior surface of the thalamus, and these structures separate the thalamus from the caudate nucleus of the basal ganglia.

    The inferior surface of the thalamus is related to the hypothalamus.

    Thalamus – cross-sectional view

    The thalamus and caudate nucleus form the floor of the central part of the lateral ventricles. However, the medial part of the superior surface of the thalamus is separated from these ventricles by the fornix, and by a fold of pia mater called the tela choroidea. At the junction of the medial and lateral surfaces of the thalamus the ependyma of the third ventricle is reflected from the lateral wall to the roof. The line of reflection is marked by a line called the taenia thalami, underneath which there is a narrow bundle of fibres called the stria medullaris thalami.

    Subthalamus

    Characteristics & Components

    The subthalamus refers to the part of the diencephalon that lies below the posterior part of the thalamus just behind and lateral to the hypothalamus. It includes nuclei and grey matter such as the zona incerta, reticular nucleus, and the perigeniculate nucleus. The zona incerta provides gamma-aminobutyric acid stimulation (GABAergic) to the thalamus; while the reticular nucleus provides GABAergic regulation to the thalamocortical pathway. Collectively, the pre-geniculate nucleus and the lateral geniculate nucleus form the lateral geniculate complex.

    The subthalamus is continuous with the upper ends of the red nucleus and substantia nigra of the tegmentum of the midbrain inferiorly. Laterally it reaches the lowest part of the internal capsule.

    Connections

    The pre-geniculate nucleus has connections similar to the lateral geniculate nucleus and includes fibres from the retina, pretectal region and superior colliculus, thus playing a role in vision and eye movements. The zona incerta has strong connections with the reticular nucleus of the thalamus which it connects as a thin lamina of grey matter. It is believed to be involved in the regulation of visceral activities such as sexuality, hydration and food intake as well as cardiovascular activity.

    Fibres emerging from the dorsal thalamus (thalamus proper) traverse the ventral thalamus through the reticular nucleus. These fibres give the nucleus a reticulated appearance, hence the name. The reticular nucleus is made of a thin layer of neurons covering the lateral aspect of the dorsal thalamus, and is also related to the internal capsule. Inferiorly it becomes partially continuous with the zona incerta. The main efferent fibers of the reticular nucleus pass to the dorsal thalamus; however afferents reach it from the nucleus cuneiformis (in the reticular formation of the midbrain).

    Other connections of the subthalamus include afferents from the cerebral cortex, putamen, globus pallidus, trigeminal nuclei, cerebellar nuclei and pretectal region. These afferents reach the reticular nucleus and zona incerta through which it sends efferent fibers to those structures and the spinal cord. The ventral thalamus also has connections with the dentate nucleus, cerebral peduncle, fasciculus lenticularis, ansa lenticularis, pre-rubral field and thalamic fasciculus.

    The Metathalamus

    The metathalamus consists of two oval eminences (the geniculate bodies) on the caudal surface of the diencephalon, just inferior to the caudal end of the dorsal thalamus.  The lateral and medial geniculate bodies function as primary thalamic relay stations for the auditory and optic system , respectively.

    Medial Geniculate Body

    The medial geniculate body receives tonotopically (arrangement of sound based on frequency on the brain) organized auditory information from the inferior colliculus of the quadrigemini, via the brachium of the inferior colliculus. Processed information from the medial geniculate body is then relayed via the auditory radiation to the primary auditory cortex on the transverse gyrus of Heschel (i.e. the superior temporal gyrus). The medial geniculate body consists of three major divisions:

    • the dorsal division, consisting of the posterior part of the parvocellular nucleus
    • the medial division, or magnocellular nucleus
    • the ventral division, consisting of the ventral part of the parvocellular nucleus.

    Medial geniculate nucleus – dorsal view

    Lateral Geniculate Body

    The lateral geniculate body receives retinotopic (mapping of visual input on the brain) input from the contralateral visual field via the optic tract.  Information is relayed from the lateral geniculate in a topographic manner via the optic radiation (Meyer’s loop) to the primary visual cortex surrounding the calcarine sulcus in the occipital lobe. The lateral geniculate body consists of layers of neurons, and the visual information received is divided among the various layers.

    Lateral geniculate nucleus – dorsal view

    Hypothalamus

    Relations

    As the name suggests, the hypothalamus is the part of the diencephalon that lies below the thalamus. Like the thalamus, it comprises several subdivisions and nuclei, including the periventricular zone, the medial zone and the lateral zone. On the medial side, it forms the wall of the third ventricle below the level of the hypothalamic sulcus. Posteriorly, the hypothalamus merges with the ventral thalamus, and through it with the tegmentum of the midbrain.

    Anteriorly, it extends up to the lamina terminalis, and merges with certain olfactory structures in the region of the anterior perforated substance. Inferiorly, the hypothalamus is related to structures in the floor of the third ventricle. These are the tuber cinereum, the infundibulum, and the mammillary bodies, which are considered part of the hypothalamus.

    Nuclei & Areas

    The hypothalamus also includes the:

    • Preoptic area – involved in thermoregulation
    • Mammillary nuclei – aid in memory and cognition as part of the circuit of Papez
    • Paraventricular nucleus – an important autonomic control centre
    • Suprachiasmatic nucleus – circadian rhythm control
    • Arcuate (or infundibular) nucleus – provides inhibition of prolactin secretion by the pituitary gland and control of hunger
    • Posterior nucleus – involved in thermoregulation
    • Anterior nucleus – forms a part of the hippocampal system for episodic memory
    • Dorsomedial nucleus – aids in circadian rhythm regulation
    • Ventromedial nucleus – serves as the satiety centre for regulation of feeding behaviour and body weight
    • Lateral hypothalamus – aids in coordinating swallowing. Lesions in this area lead to aphagia and adipsia
    • Supra-optic nucleus – facilitates secretion of vasopressin for osmoregulation
    • Tuberomammillary nucleus – single source of histamine neurons
    • Lateral tuberal nucleus – involved in regulation of feeding and metabolism
    • Intermediate nucleus – regulates sleep

    Functionally the hypothalamus is involved in the control of several cognitive behaviours, mainly owing to its connections with anatomical areas responsible for or involved in these activities. However the main functions attributed to the hypothalamus include the regulation of eating and drinking behaviour, regulation of sexual activity and reproduction, and the control of autonomic activity. Other activities of the hypothalamus include regulation of emotional behaviour, control of several  endocrine activities, response to stress, temperature regulation, as well as the control of circadian rhythms.

    Hypophysis Cerebri

    The small projection from the ventral aspect of the base of the brain is often called the pituitary gland, but is otherwise known as the hypophysis cerebri. This 12 mm by 8 mm ovoid structure is a continuation of the infundibulum of the hypothalamus. It is encased by the sella turcica of the sphenoid bone in the middle cranial fossa and crossed superiorly by the optic chiasm.

    Pituitary gland – cranial view

    The pituitary gland has dual embryonic origins and is consequently also divided based on the functionality of each component. The highly vascular adenohypophysis (which originated from ectoderm) is responsible for the synthesis of trophic hormones such as adrenocorticotropic, follicle-stimulating hormone, growth hormone, luteinizing hormone and others. The neurohypophysis is a derivative of neuroglial cells and is responsible for storing hormones produced by the hypothalamus.

    Clinical Significance

    Diencephalic (Russell’s) Syndrome

    It should be noted that a lesion (vascular or neoplastic) of any part of the diencephalon may result in a deficit in the function associated with that component. For example, an insult to the supraoptic nucleus of the hypothalamus can impair the production and subsequent release of vasopressin.

    Diencephalic (Russell’s) syndrome is a rare disorder characterized by:

    • severe emaciation with normal caloric intake
    • euphoria
    • locomotor hyperactivity
    • non-anaemic pallor
    • hypotension
    • hypoglycaemia

    Neoplastic insults of the optic and hypothalamic region are the primary cause of this disorder. The syndrome is one of the major causes of failure to thrive among paediatric patients. Fortunately, it seldom prohibits the patient from achieving their developmental milestones.

    Panhypopituitarism Syndrome

    Global deficiency of pituitary hormones is clinically classified as a panhypopituitarism syndrome. This may result from either local impairment of the gland or dysregulation of the hypothalamus. Iatrogenic insults to the pituitary gland (radiation therapy), postpartum pituitary necrosis and traumatic brain injury can also result in hypopituitarism.

    Patient presentation is dependent on which hormone(s) is (are) predominantly deficient. For example, a deficiency in growth hormone will result in failure to thrive in children; while low levels of thyroid stimulating hormone would cause hypothyroidism.

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    Kim Bengochea, Regis University, Denver


    Show references

    References:

    • Corenblum, Bernard and James R Mulinda: “Hypopituitarism (Panhypopituitarism): Background, Pathophysiology, Etiology”. Emedicine.medscape.com. N.p., 2016. Web. 14 Aug. 2016.
    • Fukushima, K.: “The Interstitial Nucleus Of Cajal And Its Role In The Control Of Movements Of Head And Eyes”. Progress in Neurobiology 29.2 (1987): 107-192.
    • Helena Cardoso, Silvia: “Brain Ventricles: Third Ventricle”. Cerebromente.org.br. N.p., 1997. Web. 14 Aug. 2016.
    • Herrero, Maria-Trinidad, Carlos Barcia, and Juana Navarro: “Functional Anatomy Of Thalamus And Basal Ganglia”. Child’s Nervous System 18.8 (2002): 386-404.
    • Kim, Ahlee et al.: “Diencephalic Syndrome: A Frequently Neglected Cause Of Failure To Thrive In Infants”. Korean J Pediatr 58.1 (2015): 28. Web. 14 Aug. 2016.
    • Kremer, HP.: “The Hypothalamic Lateral Tuberal Nucleus: Normal Anatomy And Changes In Neurological Diseases”. Prog Brain Res 93 (1992): 249-261. Print.
    • Sadler, T. W and Jan Langman: Langman’s Medical Embryology. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2012. Print.
    • Sinnatamby, Chummy S and R. J Last: Last’s Anatomy. Edinburgh: Churchill Livingstone/Elsevier, 2011. Print.
    • Standring, Susan and Henry Gray: Gray’s Anatomy. [Edinburgh]: Churchill Livingstone/Elsevier, 2008. Print.

    Article, Review and Layout:

    • Lorenzo Crumbie
    • Prof. Elizabeth Johnson
    • Adrian Rad

    Illustrators:

    • Diencephalon – Mid-sagittal view – Paul Kim 
    • Third ventricle – lateral-left view – Paul Kim
    • Stria medullaris of thalamus – cranial view – Paul Kim
    • Pineal gland – dorsal view – Paul Kim
    • Thalamus – Paul Kim
    • Thalamus – cross-sectional view – Paul Kim
    • Medial geniculate nucleus – dorsal view – Paul Kim
    • Lateral geniculate nucleus – dorsal view – Paul Kim
    • Pituitary gland – cranial view – Paul Kim

    © Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

    Related diagrams and images

    Thalamus

    Hypothalamus

    Horizontal section of the brain

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    • Atlas


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    Diencephalon – want to learn more about it?

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    Sign up for your free Kenhub account today and join over 923,303 successful anatomy students.

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    “I would honestly say that Kenhub cut my study time in half.”

    Read more.

    Kim Bengochea
    Kim Bengochea, Regis University, Denver

    Diencephalon

    The human brain can be subdivided by many classification systems. One particular nomenclature that refers to the duality of the brain is the diencephalon. It is the caudal part of the forebrain (prosencephalon) that occupies the central region of the brain. The diencephalon is comprised of the:

    • Epithalamus
    • Thalamus
    • Subthalamus
    • Metathalamus
    • Hypothalamus
    • Hypophysis cerebri

    In the following article we will explore these different parts of diencephalon as well as its function. 

    Contents
    1. Function
    2. Embryology
    3. Gross Anatomy
      1. Borders
      2. Blood Supply
    4. Epithalamus
    5. Thalamus
      1. Nuclei
      2. Surfaces
    6. Subthalamus
      1. Characteristics & Components
      2. Connections
    7. The Metathalamus
      1. Medial Geniculate Body
      2. Lateral Geniculate Body
    8. Hypothalamus
      1. Relations
      2. Nuclei & Areas
    9. Hypophysis Cerebri
    10. Clinical Significance
      1. Diencephalic (Russell’s) Syndrome
      2. Panhypopituitarism Syndrome
    11. Related diagrams and images

      Function

      Diencephalon

      Each of the components of the diencephalon has specialized functions that are integral to life. The diencephalon acts as a primary relay and processing center for sensory information and autonomic control. The plethora of communicating pathways between these structures and other parts of the body makes the diencephalon a functionally diverse area. Some of these connections include pathways to the limbic system (seat of memory and emotion), basal ganglia (motor coordination), as well as primary sensory areas, such as auditory or visual.

      This article will explore the embryology of the diencephalon, as well as the gross anatomy and function of its various components. Clinically significant points relating to this region of the brain will also be discussed.

      Embryology

      The anterior most part of the  primary cerebral vesicles of the embryo gives rise to the prosencephalic portion of the foetal brain. At about the 5th week of development, the prosencephalon is further subdivided into a ventral telencephalon (that differentiates to form the cerebral hemispheres) and a caudal diencephalon (secondary cerebral vesicles). The latter originates from the median region of the prosencephalon. In utero, the region consists of two lateral alar plates (representing sensory areas), a roof plate (most dorsal part of the neural tube), and the third ventricle. The scientific community is unclear whether or not basal plates (representing motor areas) exist as it is not physically observed, but the corresponding biomarkers have been identified. 

      The alar plates give rise to the lateral walls of the third ventricle (and by convention, the medial walls of the thalamus). Rapid cellular proliferation results in medial projection of the thalamus into the diencephalic cavity (future space of the third ventricle), resulting in a midline fusion of a portion of the thalamus known as the massa intermedia or the interthalamic connexus (adhesions). A shallow groove known as the hypothalamic sulcus emerges and divides the walls of the diencephalon into the thalamus (dorsally) and the hypothalamus (ventrally). The roof plate is comprised of a monolayer of ependymal cells that is coated with vascular mesenchyme. This area subsequently differentiates into the choroid plexus of the third ventricle. Caudally, the roof plate differentiates into the epiphysis (pineal body).

      The hypothalamus further differentiates into a variety of nuclear regions that are responsible for an eclectic collection of body functions. These include, but are not limited to, digestion, thermoregulation, and circadian regulation. Caudal to the hypothalamus is the hypophysis cerebri (pituitary gland). It originates from both ectodermal and neuroglial tissue and as a result, it can be subdivided into the adenohypophysis and the neurohypophysis.

      Of note, the prosencephalon also gives rise to the optic cup and stalk, which subsequently develops into the retina of the eye .

      Gross Anatomy

      There are several structures between the brainstem and the cerebral cortex that make up the diencephalon. These include the epithalamus, thalamus, subthalamus, metathalamus, hypothalamus, hypophysis cerebri and the third ventricle as its cavity. The medial and lateral geniculate bodies, which are collectively called the metathalamus, along with the pulvinar, are frequently regarded as an integral part of the dorsal thalamus.

      Borders

      The third ventricle is a narrow vertical midline cleft between and below the two lateral ventricles and in between left and right thalami. The lateral ventricles communicate with the third ventricle via the interventricular foramen of Monro. It also communicates with the fourth ventricle posteroinferiorly via the cerebral aqueduct of Sylvius.

      Third ventricle – lateral-left view

      It possesses a roof, a floor and four walls: anterior, posterior and two lateral.

      • The roof is formed by the thin tela choroidea, which is a combination of two membranes, the ependyma and pia mater. Within the tela choroidea are two plexuses of blood vessels (one on either side of the middle line) that bulge downwards into the cavity of the third ventricle. These are the choroid plexuses of the third ventricle which functions as a point of production of the cerebrospinal fluid (CSF).  
      • The floor is made up of the optic chiasm, the tuber cinereum and infundibulum, the mammillary bodies, the posterior perforated substance and the uppermost part of the mesencephalic tegmentum.
      • The anterior wall is the delicate lamina terminalis, as well as the anterior commissure and anterior column of the fornix..
      • The short posterior wall is formed by the stalk of the pineal gland, posterior commissure and the Habenular commissures.
      • The lateral walls are of the cavity are formed by the medial walls of each thalami. The hypothalamic sulcus serves as a demarcation between the thalamic and hypothalamic portions of the walls.

      Blood Supply

      The diencephalon is richly supplied by several blood vessels, notably the thalamogeniculate branches of the posterior cerebral artery and thalamoperforating branches of the posterior cerebral artery and posterior communicating artery.

      Epithalamus

      The epithalamus lies in relation to the posterior part of the roof of the third ventricle and the little adjoining part of its lateral wall. It consists of the following parts:

      • Stria medullaris
      • Posterior commissure 
      • Habenular nuclei (medial and lateral)
      • Pineal body
      • Paraventricular nuclei (anterior and posterior)

      The stria medullaris thalami lie close to the taenia thalami as a bundle of fibres along the junction of the medial and superior surfaces of the thalamus. It begins near the anterior pole of the thalamus and runs posteriorly toward the Habenular trigone. From the stria medullaris thalami, some fibres cross in the superior or anterior lamina of the pineal stalk to reach the Habenular nuclei of the opposite side. These fibres constitute the Habenular commissure, some of which also connect the amygdaloid and hippocampal complexes of the right and left cerebral hemispheres. The posterior commissure lies in the inferior lamina of the stalk of the pineal body. A number of small nuclei are present in relation to the commissure, such as the interstitial and dorsal nuclei of the posterior commissure, the nucleus of Darkschewitsch and the interstitial nucleus of Cajal (both nuclei communicate with the vestibular nuclei via the medial longitudinal fasciculus).

      Stria medullaris of thalamus – cranial view

      The pineal gland is a small piriform structure located in relation to the posterior wall of the third ventricle. It is an endocrine gland of considerable significance and is made up of cells called pinealocytes, which secrete melatonin in response to darkness. The pineal body also secretes a number of hormones that have important regulatory influence on many endocrine organs including the hypophysis cerebri, thyroid , parathyroids, adrenals and gonads. Hormones of the pineal gland reach the hypophysis cerebri through the blood stream and the cerebrospinal fluid (CSF). As humans age, the pineal glands become calcified and form what are known as corpora arenacea or brain sand.

      Pineal gland – dorsal view

      Thalamus

      The thalamus is the largest mass of grey matter of the diencephalon that is laterally related to the third ventricle. The symmetrical halves of this midline structure are located between the cerebral cortices and the midbrain. It has an anterior and a posterior pole, as well as superior, inferior, medial and lateral surfaces. Its superior part is covered by a thin layer of white matter called the stratum zonale and its lateral surface is covered by a similar layer called the external medullary lamina.

      Nuclei

      The thalamus also comprises several nuclei, such as the:

      • Dorsomedial nucleus 
      • Midline nuclei 
      • Geniculate bodies (lateral and medial) 
      • Pulvinar 
      • Anterior nuclei 
      • Intralaminar nuclei 
      • Lateral dorsal nuclei 
      • Lateral posterior nuclei
      • Ventral anterior nuclei 
      • Ventral posterior (medial and lateral) 
      • Ventral lateral nuclei

      Thalamus

      Surfaces

      The anterior pole of the thalamus lies just behind the interventricular foramen of Monro.

      Its posterior pole makes up the pulvinar, and lies just superior and lateral to the superior colliculus. The medial and lateral geniculate bodies are connected to the inferior and superior colliculi by the inferior and superior brachium quadrigeminum, respectively.

      The medial surface forms the greater part of the lateral wall of the third ventricle, and is lined by ependyma. A mass of grey matter called the interthalamic adhesion (connexus) is found attached to the medial surfaces of the two thalami and this connexus interconnects the left and right thalami. Inferiorly, the medial surface is separated from the hypothalamus by the hypothalamic sulcus.

      The internal capsule is related to the lateral surface of the thalamus and separates this surface from the lentiform nucleus (globus pallidus and putamen) of the basal ganglia .

      The thalamostriate vein and a bundle of fibres called the stria terminalis are in close apposition to the dorsal or superior surface of the thalamus, and these structures separate the thalamus from the caudate nucleus of the basal ganglia.

      The inferior surface of the thalamus is related to the hypothalamus.

      Thalamus – cross-sectional view

      The thalamus and caudate nucleus form the floor of the central part of the lateral ventricles. However, the medial part of the superior surface of the thalamus is separated from these ventricles by the fornix, and by a fold of pia mater called the tela choroidea. At the junction of the medial and lateral surfaces of the thalamus the ependyma of the third ventricle is reflected from the lateral wall to the roof. The line of reflection is marked by a line called the taenia thalami, underneath which there is a narrow bundle of fibres called the stria medullaris thalami.

      Subthalamus

      Characteristics & Components

      The subthalamus refers to the part of the diencephalon that lies below the posterior part of the thalamus just behind and lateral to the hypothalamus. It includes nuclei and grey matter such as the zona incerta, reticular nucleus, and the perigeniculate nucleus. The zona incerta provides gamma-aminobutyric acid stimulation (GABAergic) to the thalamus; while the reticular nucleus provides GABAergic regulation to the thalamocortical pathway. Collectively, the pre-geniculate nucleus and the lateral geniculate nucleus form the lateral geniculate complex.

      The subthalamus is continuous with the upper ends of the red nucleus and substantia nigra of the tegmentum of the midbrain inferiorly. Laterally it reaches the lowest part of the internal capsule.

      Connections

      The pre-geniculate nucleus has connections similar to the lateral geniculate nucleus and includes fibres from the retina, pretectal region and superior colliculus, thus playing a role in vision and eye movements. The zona incerta has strong connections with the reticular nucleus of the thalamus which it connects as a thin lamina of grey matter. It is believed to be involved in the regulation of visceral activities such as sexuality, hydration and food intake as well as cardiovascular activity.

      Fibres emerging from the dorsal thalamus (thalamus proper) traverse the ventral thalamus through the reticular nucleus. These fibres give the nucleus a reticulated appearance, hence the name. The reticular nucleus is made of a thin layer of neurons covering the lateral aspect of the dorsal thalamus, and is also related to the internal capsule. Inferiorly it becomes partially continuous with the zona incerta. The main efferent fibers of the reticular nucleus pass to the dorsal thalamus; however afferents reach it from the nucleus cuneiformis (in the reticular formation of the midbrain).

      Other connections of the subthalamus include afferents from the cerebral cortex, putamen, globus pallidus, trigeminal nuclei, cerebellar nuclei and pretectal region. These afferents reach the reticular nucleus and zona incerta through which it sends efferent fibers to those structures and the spinal cord. The ventral thalamus also has connections with the dentate nucleus, cerebral peduncle, fasciculus lenticularis, ansa lenticularis, pre-rubral field and thalamic fasciculus.

      The Metathalamus

      The metathalamus consists of two oval eminences (the geniculate bodies) on the caudal surface of the diencephalon, just inferior to the caudal end of the dorsal thalamus.  The lateral and medial geniculate bodies function as primary thalamic relay stations for the auditory and optic system , respectively.

      Medial Geniculate Body

      The medial geniculate body receives tonotopically (arrangement of sound based on frequency on the brain) organized auditory information from the inferior colliculus of the quadrigemini, via the brachium of the inferior colliculus. Processed information from the medial geniculate body is then relayed via the auditory radiation to the primary auditory cortex on the transverse gyrus of Heschel (i.e. the superior temporal gyrus). The medial geniculate body consists of three major divisions:

      • the dorsal division, consisting of the posterior part of the parvocellular nucleus
      • the medial division, or magnocellular nucleus
      • the ventral division, consisting of the ventral part of the parvocellular nucleus.

      Medial geniculate nucleus – dorsal view

      Lateral Geniculate Body

      The lateral geniculate body receives retinotopic (mapping of visual input on the brain) input from the contralateral visual field via the optic tract.  Information is relayed from the lateral geniculate in a topographic manner via the optic radiation (Meyer’s loop) to the primary visual cortex surrounding the calcarine sulcus in the occipital lobe. The lateral geniculate body consists of layers of neurons, and the visual information received is divided among the various layers.

      Lateral geniculate nucleus – dorsal view

      Hypothalamus

      Relations

      As the name suggests, the hypothalamus is the part of the diencephalon that lies below the thalamus. Like the thalamus, it comprises several subdivisions and nuclei, including the periventricular zone, the medial zone and the lateral zone. On the medial side, it forms the wall of the third ventricle below the level of the hypothalamic sulcus. Posteriorly, the hypothalamus merges with the ventral thalamus, and through it with the tegmentum of the midbrain.

      Anteriorly, it extends up to the lamina terminalis, and merges with certain olfactory structures in the region of the anterior perforated substance. Inferiorly, the hypothalamus is related to structures in the floor of the third ventricle. These are the tuber cinereum, the infundibulum, and the mammillary bodies, which are considered part of the hypothalamus.

      Nuclei & Areas

      The hypothalamus also includes the:

      • Preoptic area – involved in thermoregulation
      • Mammillary nuclei – aid in memory and cognition as part of the circuit of Papez
      • Paraventricular nucleus – an important autonomic control centre
      • Suprachiasmatic nucleus – circadian rhythm control
      • Arcuate (or infundibular) nucleus – provides inhibition of prolactin secretion by the pituitary gland and control of hunger
      • Posterior nucleus – involved in thermoregulation
      • Anterior nucleus – forms a part of the hippocampal system for episodic memory
      • Dorsomedial nucleus – aids in circadian rhythm regulation
      • Ventromedial nucleus – serves as the satiety centre for regulation of feeding behaviour and body weight
      • Lateral hypothalamus – aids in coordinating swallowing. Lesions in this area lead to aphagia and adipsia
      • Supra-optic nucleus – facilitates secretion of vasopressin for osmoregulation
      • Tuberomammillary nucleus – single source of histamine neurons
      • Lateral tuberal nucleus – involved in regulation of feeding and metabolism
      • Intermediate nucleus – regulates sleep

      Functionally the hypothalamus is involved in the control of several cognitive behaviours, mainly owing to its connections with anatomical areas responsible for or involved in these activities. However the main functions attributed to the hypothalamus include the regulation of eating and drinking behaviour, regulation of sexual activity and reproduction, and the control of autonomic activity. Other activities of the hypothalamus include regulation of emotional behaviour, control of several  endocrine activities, response to stress, temperature regulation, as well as the control of circadian rhythms.

      Hypophysis Cerebri

      The small projection from the ventral aspect of the base of the brain is often called the pituitary gland, but is otherwise known as the hypophysis cerebri. This 12 mm by 8 mm ovoid structure is a continuation of the infundibulum of the hypothalamus. It is encased by the sella turcica of the sphenoid bone in the middle cranial fossa and crossed superiorly by the optic chiasm.

      Pituitary gland – cranial view

      The pituitary gland has dual embryonic origins and is consequently also divided based on the functionality of each component. The highly vascular adenohypophysis (which originated from ectoderm) is responsible for the synthesis of trophic hormones such as adrenocorticotropic, follicle-stimulating hormone, growth hormone, luteinizing hormone and others. The neurohypophysis is a derivative of neuroglial cells and is responsible for storing hormones produced by the hypothalamus.

      Clinical Significance

      Diencephalic (Russell’s) Syndrome

      It should be noted that a lesion (vascular or neoplastic) of any part of the diencephalon may result in a deficit in the function associated with that component. For example, an insult to the supraoptic nucleus of the hypothalamus can impair the production and subsequent release of vasopressin.

      Diencephalic (Russell’s) syndrome is a rare disorder characterized by:

      • severe emaciation with normal caloric intake
      • euphoria
      • locomotor hyperactivity
      • non-anaemic pallor
      • hypotension
      • hypoglycaemia

      Neoplastic insults of the optic and hypothalamic region are the primary cause of this disorder. The syndrome is one of the major causes of failure to thrive among paediatric patients. Fortunately, it seldom prohibits the patient from achieving their developmental milestones.

      Panhypopituitarism Syndrome

      Global deficiency of pituitary hormones is clinically classified as a panhypopituitarism syndrome. This may result from either local impairment of the gland or dysregulation of the hypothalamus. Iatrogenic insults to the pituitary gland (radiation therapy), postpartum pituitary necrosis and traumatic brain injury can also result in hypopituitarism.

      Patient presentation is dependent on which hormone(s) is (are) predominantly deficient. For example, a deficiency in growth hormone will result in failure to thrive in children; while low levels of thyroid stimulating hormone would cause hypothyroidism.

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      References:

      • Corenblum, Bernard and James R Mulinda: “Hypopituitarism (Panhypopituitarism): Background, Pathophysiology, Etiology”. Emedicine.medscape.com. N.p., 2016. Web. 14 Aug. 2016.
      • Fukushima, K.: “The Interstitial Nucleus Of Cajal And Its Role In The Control Of Movements Of Head And Eyes”. Progress in Neurobiology 29.2 (1987): 107-192.
      • Helena Cardoso, Silvia: “Brain Ventricles: Third Ventricle”. Cerebromente.org.br. N.p., 1997. Web. 14 Aug. 2016.
      • Herrero, Maria-Trinidad, Carlos Barcia, and Juana Navarro: “Functional Anatomy Of Thalamus And Basal Ganglia”. Child’s Nervous System 18.8 (2002): 386-404.
      • Kim, Ahlee et al.: “Diencephalic Syndrome: A Frequently Neglected Cause Of Failure To Thrive In Infants”. Korean J Pediatr 58.1 (2015): 28. Web. 14 Aug. 2016.
      • Kremer, HP.: “The Hypothalamic Lateral Tuberal Nucleus: Normal Anatomy And Changes In Neurological Diseases”. Prog Brain Res 93 (1992): 249-261. Print.
      • Sadler, T. W and Jan Langman: Langman’s Medical Embryology. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2012. Print.
      • Sinnatamby, Chummy S and R. J Last: Last’s Anatomy. Edinburgh: Churchill Livingstone/Elsevier, 2011. Print.
      • Standring, Susan and Henry Gray: Gray’s Anatomy. [Edinburgh]: Churchill Livingstone/Elsevier, 2008. Print.

      Article, Review and Layout:

      • Lorenzo Crumbie
      • Prof. Elizabeth Johnson
      • Adrian Rad

      Illustrators:

      • Diencephalon – Mid-sagittal view – Paul Kim 
      • Third ventricle – lateral-left view – Paul Kim
      • Stria medullaris of thalamus – cranial view – Paul Kim
      • Pineal gland – dorsal view – Paul Kim
      • Thalamus – Paul Kim
      • Thalamus – cross-sectional view – Paul Kim
      • Medial geniculate nucleus – dorsal view – Paul Kim
      • Lateral geniculate nucleus – dorsal view – Paul Kim
      • Pituitary gland – cranial view – Paul Kim

      © Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

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      Hypothalamus

      Horizontal section of the brain

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      Central Nervous System

      The Diencephalon

      Functions of the Diencephalon

      Distinct parts of diencephalon perform numerous vital functions, from regulating wakefulness to controlling the autonomic nervous system.

      Learning Objectives

      Describe the functions of the diencephalon region of the brain

      Key Takeaways

      Key Points

      • The diencephalon is made up of four main components: the thalamus, the subthalamus, the hypothalamus, and the epithalamus.
      • The hypothalamus is an integral part of the endocrine system, with the key function of linking the nervous system to the endocrine system via the pituitary gland.
      • The thalamus is critically involved in a number of functions including relaying sensory and motor signals to the cerebral cortex and regulating consciousness, sleep, and alertness.
      • The epithalamus functions as a connection between the limbic system to other parts of the brain. Some functions of its components include the secretion of melatonin by the pineal gland (involved in circadian rhythms) and regulation of motor pathways and emotions.

      Key Terms

      • subthalamus: Receives afferent connections from the substantia nigra and striatum and regulates skeletal muscle movements.
      • thalamus: Either of two large, ovoid structures of gray matter within the forebrain that relay sensory impulses to the cerebral cortex.
      • hypothalamus: A region of the forebrain located below the thalamus, forming the basal portion of the diencephalon, and functioning to regulate body temperature, some metabolic processes, and the autonomic nervous system.
      • epithalamus: The dorsal posterior segment of the diencephalon, involved in the maintenance of circadian rhythms and regulation of motor pathways and emotions.
      • limbic system: A set of brain structures located on both sides of the thalamus, right under the cerebrum. Supports a variety of functions including emotion, behavior, motivation, long-term memory, and olfaction.

      The diencephalon (“interbrain”) is the region of the vertebrate neural tube that gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube that later forms both the diencephalon and the telencephalon. In adults, the diencephalon appears at the upper end of the brain stem, situated between the cerebrum and the brain stem. It is made up of four distinct components: the thalamus, the subthalamus, the hypothalamus, and the epithalamus.

      image

      Diencephalon: Three-dimensional view of the diencephalon

      Other structures that are part of the diencephalon are:

      • Anterior and posterior paraventricular nuclei
      • Medial and lateral habenular nuclei
      • Stria medullaris thalami
      • Posterior commissure
      • Pineal gland

      This diagram of the embryonic vertebrate brain indicates the telencephalon, diencephalon, prosencephalon (forebrain), mesencephalon (midbrain), rhombencephalon (hindbrain), metencephalon, and myelencephalon.

      Embryonic Brain: Subdivisions of the embryonic vertebrate brain that later differentiate into forebrain, midbrain, and hindbrain structures.

      Functions of Primary Diencephalon Structures

      The thalamus is a kind of switchboard of information, believed to act as a relay between a variety of subcortical areas and the cerebral cortex. In particular, every sensory system (with the exception of the olfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area. The thalamus also plays an important role in regulating states of sleep and wakefulness. Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with consciousness. The thalamus plays a major role in regulating arousal, awareness level, and activity. Damage to the thalamus can lead to permanent coma.

      The subthalamus connects to the globus pallidus, a basal nucleus of the telencephalon. It receives afferent connections from the substantia nigra and striatum and regulates skeletal muscle movements.

      The hypothalamus performs numerous vital functions (e.g., regulation of certain metabolic processes), most of which relate directly or indirectly to the regulation of visceral activities by way of other brain regions and the autonomic nervous system. It synthesizes and secretes certain neurohormones, often called hypothalamic-releasing hormones, and these in turn stimulate or inhibit the secretion of pituitary hormones. The hypothalamus controls body temperature, hunger, thirst, fatigue, sleep, and circadian cycles.

      The epithalamus functions as a connection between the limbic system and other parts of the brain. Some functions of its components include the secretion of melatonin by the pineal gland (involved in circadian rhythms) and regulation of motor pathways and emotions.

      Thalamus

      The thalamus is a small structure in the center of the brain that acts as a relay center for sensory and motor information.

      Learning Objectives

      Describe the thalamus and its functions

      Key Takeaways

      Key Points

      • The thalamus is the largest structure derived from the embryonic diencephalon.
      • Together, the two halves of the thalamus are a prominent bulb-shaped mass, about 5.7 cm in length, located obliquely and symmetrically on each side of the third ventricle.
      • The thalamus has a system of myelinated fibers that separate the different thalamic subparts. These areas are defined by distinct clusters of neurons.
      • Every sensory system (with the exception of the olfactory system) has a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area.
      • The thalamus’s functions include relaying sensory and motor signals to the cerebral cortex and the regulation of consciousness, sleep, and alertness.

      Key Terms

      • nuclear: In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons.
      • medial geniculate nucleus: A part of the auditory thalamus that is the relay between the inferior colliculus and the auditory cortex.
      • lateral geniculate nucleus: A relay center in the thalamus for the visual pathway that receives major sensory input from the retina.

      The thalamus (derived from the Greek meaning “inner chamber”) is a midline symmetrical structure within the brain, situated between the cerebral cortex and midbrain. Its functions include relaying sensory and motor signals to the cerebral cortex and regulating consciousness, sleep, and alertness. The thalamus surrounds the third ventricle and is the main product of the embryonic diencephalon.

      Overall Structure of the Thalamus

      Each half of the thalamus is about the size and shape of a walnut: approximately 3 centimeters long, 2.5 centimeters across at the widest point, and about 2 centimeters high. Together, the two halves of the thalamus are a prominent bulb-shaped mass, about 5.7 cm in length, located obliquely (about 30°) and symmetrically on each side of the third ventricle.

      The thalamus is part of a nuclear complex of composed of four parts: the hypothalamus, epithalamus, the ventral thalamus, and the dorsal thalamus. The thalamus has a system of myelinated fibers that separate the different thalamic subparts. These areas are defined by distinct clusters of neurons.

      The thalamus derives its blood supply from four arteries including the polar artery ( posterior communicating artery), paramedian thalamic-subthalamic arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral ) choroidal arteries. These are all derived from the vertebrobasilar arterial system except the polar artery. The thalamus is connected to the hippocampus via the mammillothalamic tract.

      This diagram indicates the nuclei of the thalamus and other structures, including intrathalmic adhesion, median, medial, internal medullary, lamina, anterior, pulvinar, medial and lateral geniculate body, lateral dorsal and posterior nuclei, ventral anterior and lateral nucleus, ventral intermediate nucleus, ventral posteromedial, and ventral posterolateral.

      The Nuclei of the Thalamus: The graphic details the various nuclei of the thalamus.

      Primary Functions of the Thalamus

      The thalamus has multiple functions, serving as a sort of switchboard of information. It is generally believed to act as a relay between a variety of subcortical areas and the cerebral cortex. In particular, every sensory system (with the exception of the olfactory system) has a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area. For the visual system, for example, inputs from the retina are sent to the lateral geniculate nucleus of the thalamus, which in turn projects to the primary visual cortex in the occipital lobe. The thalamus is believed to both process and relay sensory information. Each of the primary sensory relay areas receives strong back projections from the cerebral cortex. Similarly, the medial geniculate nucleus acts as a key auditory relay between the inferior colliculus of the midbrain and the primary auditory cortex. The ventral posterior nucleus is a key somatosensory relay, which sends touch and proprioceptive information to the primary somatosensory cortex.

      The thalamus also plays an important role in regulating states of sleep and wakefulness. Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with consciousness. The thalamus plays a major role in regulating arousal, awareness level, and activity. Damage to the thalamus can lead to permanent coma. Fatal familial insomnia is a hereditary prion disease characterized by degeneration of the thalamus and leading to a gradual progression to a state of total insomnia that is eventually fatal.

      Hypothalamus

      The hypothalamus serves as a gateway between the nervous system and endocrine system.

      Learning Objectives

      Describe the functions of the hypothalamus of the brain.

      Key Takeaways

      Key Points

      • The hypothalamus produces and secretes a wide variety of neurohormones that lead to the release or inhibition of pituitary gland
        hormone.
      • The posterior pituitary is composed of tissue derived from the hypothalamus, whereas the anterior pituitary is derived from epithelial tissue.
      • The hypothalamus can pass signals back and forth between the anterior and posterior pituitary by the neurohypophysis and median eminence.
      • The hypothalamus can sample the blood composition at two sites, the subfornical organ and the organum vasculosum of the lamina terminalis. This is important for the uptake of circulating hormones and to determine concentration of substances in the blood.
      • The outputs of the hypothalamus can be divided into neural projections and endocrine hormones. The neural projections tend to run bidirectionally.

      Key Terms

      • neurohypophysis: The posterior lobe of the pituitary gland, responsible for the release of oxytocin and antidiuretic hormone (ADH), also called vasopressin.
      • median eminence: Part of the inferior boundary for the hypothalamus and one of the seven areas of the brain devoid of a blood-brain barrier.
      • anterior pituitary: Also called the adenohypophysis, the glandular anterior lobe of the pituitary gland. The anterior pituitary regulates several physiological processes including stress, growth, reproduction, and lactation.

      The hypothalamus (derived from the Greek for “under chamber”) is a portion of the brain that contains a number of small, distinct nuclei with various functions and less anatomically distinct areas. The hypothalamus is located below the thalamus just above the brain stem. In the terminology of neuroanatomy, it forms the ventral part of the diencephalon. All vertebrate brains contain a hypothalamus. In humans, it is roughly the size of an almond.

      image

      The location of hypothalamus and pituitary: At center, hypothalamus is located just superior to the pituitary, with which it closely interacts.

      General Functions

      image

      Location of the hypothalamus: This is the location of the human hypothalamus in relation to the thalamus, pituitary gland, sella turcica, and optic chiasm.

      One of the most important functions of the hypothalamus is linking the nervous system to the endocrine system via the pituitary gland (hypophysis).

      The hypothalamus contains thyrotropin-releasing hormone, gonadotropin-releasing hormone, growth hormone-releasing hormone, corticotropin-releasing hormone, somatostatin, and dopamine, as well as vasopressin and oxytocin. These hormones are released into the bloodstream and target other organ systems, most notably the pituitary. The hypothalamus affects the endocrine system and governs emotional behavior such as anger and sexual activity. Most of the hypothalamic hormones generated are distributed to the pituitary via the hypophyseal portal system. The hypothalamus maintains homeostasis, including the regulation of blood pressure, heart rate, and temperature.

      The hypothalamus coordinates hormonal and behavioral circadian rhythms, complex patterns of neuroendocrine outputs, complex homeostatic mechanisms, and important behaviors. It must therefore respond to many different signals, some of which are generated externally and some internally. The hypothalamus is thus richly connected with many parts of the central nervous system, including the brainstem, reticular formation and autonomic zones, and the limbic forebrain (particularly the amygdala, septum, diagonal band of Broca, olfactory bulbs, and cerebral cortex).

      The hypothalamus can sample the blood composition at the subfornical organ and the organum vasculosum of the lamina terminalis. This is important for the uptake of circulating hormones and to determine concentration of substances in the blood.

      Temperature Regulation

      The hypothalamus functions as a type of thermostat for the body. It sets a desired body temperature and stimulates either heat production and retention to raise the blood temperature to a higher level, or sweating and vasodilation to cool the blood to a lower temperature. All fevers result from a raised setting in the hypothalamus; elevated body temperatures due to any other cause are classified as hyperthermia. Rarely, direct damage to the hypothalamus such as from a stroke causes a fever; this is sometimes called a hypothalamic fever. However, such damage more commonly causes abnormally low body temperatures.

      Appetite

      The extreme lateral part of the ventromedial nucleus of the hypothalamus is responsible for control of food intake. Stimulation of this area causes increased food intake. Bilateral lesion in this area causes complete cessation of food intake. Medial parts of the nucleus have a controlling effect on the lateral part. Bilateral lesion of the medial part of the ventromedial nucleus causes hyperphagia and obesity. Further lesion of the lateral part of the ventromedial nucleus in the same animal produces complete cessation of food intake.

      Fear Processing

      The medial zone of hypothalamus is part of a circuitry that controls motivated behaviors, such as defensive behaviors and social defeat. Analyses of Fos-labeling showed that a series of nuclei in the behavioral control column is important in regulating the expression of innate and conditioned defensive behaviors. Nuclei in the medial zone are also mobilized during an encounter with an aggressor. The defeated animal has an increase in Fos levels in sexually dimorphic structures.

      Sexual Dimorphism

      Several hypothalamic nuclei are sexually dimorphic, with clear differences in both structure and function between males and females. Some differences are apparent even in gross neuroanatomy, most notably is the sexually dimorphic nucleus within the preoptic area. However, most are subtle changes in the connectivity and chemical sensitivity of particular sets of neurons. In neonatal life, gonadal steroids influence the development of the neuroendocrine hypothalamus. For instance, they determine the ability of females to exhibit a normal reproductive cycle and of males and females to display appropriate reproductive behaviors in adult life.

      In 2004 and 2006, two studies by Berglund, Lindström, and Savic used Positron Emission Tomography (PET) to observe how the hypothalamus responds to smelling common odors, the scent of testosterone found in male sweat, and the scent of estrogen found in female urine. These studies showed that the hypothalamus of heterosexual men and homosexual women both respond to estrogen. Also, the hypothalamus of homosexual men and heterosexual women both respond to testosterone. In all four groups, common odors were processed similarly involving only the olfactory brain.

      Epithalamus and Pineal Gland

      The epithalamus connects the limbic system to other parts of the brain.

      Learning Objectives

      Describe the function of the epithalamus of the brain

      Key Takeaways

      Key Points

      • The epithalamus is a dorsal posterior segment of the diencephalon, which includes the habenula and their interconnecting fibers, the habenular commissure, the stria medullaris, and the pineal body.
      • A main function of the epithalamus is the secretion of melatonin by the pineal gland.
      • The epithalamus is connected with both the limbic system and the basal ganglia.

      Key Terms

      • circadian rhythm: The “internal body clock” that regulates the 24-hour cycle of biological processes in animals and plants.
      • pineal gland: A small, pinecone-shaped endocrine gland found near the centre of the brain that produces melatonin.
      • melatonin: A hormone related to serotonin that is secreted by the pineal gland and stimulates color change in the skin of reptiles. It is involved in the sleep/wake and reproductive cycles in mammals.

      The epithalamus is a dorsal posterior segment of the diencephalon (as shown in the figure below).

      This diagram depicts the hypothalamus and other structures in the brain, including foramen of Monro, middle commissure, choroid plexus of third ventricle, taenia thalami, habenular commissure, posterior commissure, pineal body, aqueduct, quadrigeminal lamina, superior medullary vellum, fourth ventricle, oblongata, pons, midbrain, thalamus, fornix, callosum, genu, splenium, septum lucidum, oculomotor nerve, corpus albicans, tuber cinereum, optic nerve, pituitary body, optic chiasm, lamina terminalis, anterior commissure, copula, and rostrum.

      The Epithalamus: A brain sectioned in the median sagittal plane. The epithalamus is labeled in red.

      It includes the habenula and their interconnecting fibers (the habenular commissure), the stria medullaris, and the pineal gland. The habenular commissure is a band of nerve fibers situated in front of the pineal gland that connects the habenular nuclei on both sides of the diencephalon. The stria medullaris, also known as stria medullaris thalami, is a fiber bundle containing afferent fibers from the septal nuclei, lateral preoptic hypothalamic region, and anterior thalamic nuclei to the habenula.

      Pineal Gland

      The pineal gland (also called the pineal body, epiphysis cerebri, epiphysis, conarium, or the “third eye”) is the only unpaired midline brain structure. It is about the size of a grain of rice (5–8 mm) in humans. The pineal gland lies between the laterally positioned thalamic bodies and behind the habenular commissure. It is located behind the third ventricle and is bathed in cerebrospinal fluid supplied through a small pineal recess of the third ventricle.

      Epithalamic Function

      The epithalamus acts as a connection between the limbic system and other parts of the brain. Some functions of its components include the secretion of melatonin by the pineal gland (involved in circadian rhythms) and regulation of motor pathways and emotions. It is wired with the limbic system and basal ganglia.

      Circumventricular Organs

      Circumventricular organs are situated adjacent to the brain ventricles and sense concentrations of various compounds in the blood.

      Learning Objectives

      Describe the circumventricular organs of the brain

      Key Takeaways

      Key Points

      • Circumventricular organs have incomplete blood-brain barriers.
      • Circumventricular organs secrete or are sites of action of a variety of different hormones, neurotransmitters, and cytokines. They are sometimes classified by whether they are secretory or sensory.
      • The sensory organs are able to sense plasma molecules and then pass that information into other regions of the brain.
      • The secretory organs are responsible for secreting hormones and glycoproteins into the peripheral vascular system using feedback from both the brain environment and external stimuli.

      Key Terms

      • circumventricular organs: So named because they are positioned at distinct sites around the margin of the ventricular system of the brain. They are among the few sites in the brain which have an incomplete blood-brain barrier and, as a result, can directly sense the concentrations of various compounds, particularly peptide hormones, in the bloodstream.
      • median eminence: Part of the inferior boundary for the hypothalamus and one of the seven areas of the brain devoid of a blood-brain barrier.
      • subcommissural organ: A gland in the brain and one of the circumventricular organs, consisting of ependymal cells which secrete SCO-spondin, a protein that contributes to neuron growth and maintenance.

      Circumventricular organs (CVOs) are positioned at distinct sites around the margin of the ventricular system of the brain. They are among the few sites in the brain that have an incomplete blood-brain barrier. As a result, neurons located in circumventricular organs can directly sense the concentrations of various compounds, particularly peptide hormones, in the bloodstream without the need for specialized transport systems that move those compounds across the blood-brain barrier. A useful mnemonic device for remembering this aspect of their function, though not the source of the name, is that they allow factors to circumvent’ the blood-brain barrier. These organs secrete or are sites of action of a variety of different hormones, neurotransmitters, and cytokines. They are sometimes classified by whether they are secretory or sensory.

      CVOs allow for linkage between the central nervous system (CNS) and peripheral blood flow, and are an integral part of neuroendocrine function.

      This diagram of the third and fourth ventricles delineates the superior sagittal sinus, choroid plexus, interventricular foramen, cerebral aqueduct, lateral aperture, median aperture, central canal, right lateral ventricle, meningeal dura mater, subarachnoid space, and arachnoid granulation.

      CVO: View of the third and fourth ventricles. The CVOs border these ventricles.

      Sensory Circumventricular Organs

      The sensory organs are able to sense plasma molecules and pass that information into other regions of the brain. Therefore, they provide direct information to the autonomic nervous system from the systemic circulation. These organs include:

      • Area postrema: Site of the chemoreceptor trigger zone for vomiting, sends major and minor efferents to sections of the brain involved in the autonomic control of cardiovascular and respiratory activities.
      • Subfornical organ: Active in osmoregulation, cardiovascular regulation, and energy homeostasis.
      • Vascular organ of lamina terminalis: Responsible for the homeostatic conservation of osmolarity.

      Secretory Circumventricular Organs

      The secretory organs are responsible for secreting hormones and glycoproteins into the peripheral vascular system using feedback from both the brain environment and external stimuli. These include:

      • Subcommissural organ (SCO): Secretion of the glycoprotein SCO-spondin.
      • Posterior pituitary: Stores and releases oxytocin and vasopressin, also known as antidiuretic hormone, produced in the hypothalamus.
      • Pineal gland: The main function is the secretion of melatonin.
      • Median eminence: Allows for the transport of neurohormones between the CSF and the peripheral blood supply.

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