Diencephalon (Thalamus & Hypothalamus)

Part of Forebrain
  • Cerebrum
  • Diencephalon
 
Diencephalon:
 
Cavity within the diencephalon - 3rd ventricle.
*  This 3rd ventricle makes division of diencephalon into right and left symmetrical parts.
 
The hypothalamic sulcus that extends from the interventricular foramen to the cerebral aqueduct makes division of each sides of the diencephalon into
  • Dorsal and Ventral parts
 
Dorsal part of Diencephalon
  • Thalamus
  • Metathalamus
    (Medial & Lateral
    geniculate bodies)
  • Epithalamus, including the pineal body & habenula
 
Ventral part of the diencephalon
  • Hypothalamus
  • Subthalamus
 
 
Thalamus:

Thalamus - a huge mass of grey matter.
Consists of multiple nuclei.
*  It forms the lateral wall of the third ventricle & also the floor of the central part of the lateral ventricle.
*      On either side of the 3rd ventricle (a slit-like cavity), two thalami are present.
 
*  On the posterior part, it overhangs the lateral and medial geniculate bodies, and the superior colliculus with its brachium.
 
Surface:
Medial Surface:
Form Maximum part of the lateral wall of the 3rd ventricle.
 
Lateral Surface:
Forms the medial boundary of the internal capsule.
 
Note:
Interthalamic adhesion - Interconnect the medial surface of both thalami.
Thamalus forms the lateral wall of the 3rd ventricle in the posterosuperior part.
At the same time, anteroinferiorly, the hypothalamus forms the lateral wall of the 3rd ventricle.
 
 
Nuclei of thalamus:
Anterior part Anterior nucleus
Medial Part Medial Ventral nucleus
Medial dorsal nucleus
Lateral part    -------------
*Dorsal part Lateral dorsal
Lateral posterior
Pulvinar
*Ventral part Ventral anterior
Ventral lateral
Ventral posterolateral
Ventral posteromedial
 
Ventral posteromedial nucleus receives:
  • General sensory impulses from the face and head through trigeminal lemniscus
  • Tastes sensations from taste buds through solitariothalamic tract
 
Ventral posterolateral nucleus receives:
  • Exteroceptive sensations (pain, touch, temperature) through spinal lemniscus
  • Proprioceptive sensations through medial lemniscus. (ie. muscle and joint sense, vibration, two-point discrimination)
    (from the rest of the body except face and head)
 
Important Connection of Thalamus:
Ventral Posterolateral nucleus Trigeminal lemniscus
Solitariothalamic lemniscus
To postcentral gyrus (area no. 3,1,2) Relay station for impulses from face & taste impulse
Ventral Posteromedial Nucleus  Spinal lemniscus
Medial lemniscus
To postcentral gyrus (area no. 3,1,2)
Relay station for exteroceptive & proprioceptive impulse from body, except face

 Thus, the thalamus is considered as the chief integrating center where sensory impulses from all these sources is brought together.
 
* All sensory impulses reaching the ventral posterior nucleus complex are carried to the primary sensory area of the cerebral cortex by fibers passing through the posterior limb of the internal capsule.
 
 
Function of Thalamus as whole:
It acts as a sensory relay station of all the sensory pathways except for the olfactory pathway.
 
Thalamus is considered as the great sensory gateway to the cerebral cortex.
It receives sensory impulses from the opposite half of the body and transmits most of them to the Somatosensory area of the cerebral cortex (Brodmann area no.3, 2,1).
 
 
 
Note:
4 lemniscus system in brain stem:
Medial Lemniscus
Spinal Lemniscus
Lateral Lemniscus
Trigeminal lemniscus
Medial lemniscus comprises of Tract of Gall & Burdach.
Spinal Lemniscus comprises of Anterior & Lateral spinothalamic tract.
The major sensory supply in the face is by the three divisions of the trigeminal nucleus.
Exteroceptive Sensation - (Pain, Temperature, Pressure, Touch)
Proprioceptive Sensation - (Muscle & joint sense, Vibration, Two-point discrimination)
 
 
Metathalamus:

Consists of medial & lateral geniculate bodies present on either side of the midbrain, below the thalamus.
 
Medial geniculate body:
Present medial to midbrain
*  Medial geniculate body is connected to the inferior colliculus by Inferior Brachium.
 
Connection:
Afferents
Lateral Lemniscus
Fibers from Inferior colliculus
Efferents
Gives rise to auditory radiation
       I
Sublentiform part of internal capsule
      I
Auditory area of temporal lobe
 
 
Lateral Geniculate Body:
  • Present anterolateral to the medial geniculate body, below the thalamus.
  • Superior colliculus is connected to it by superior brachium.
  • Forms part of the visual pathway, site of termination of the optic tract.
 
Connection:
Afferent:
Optic tract
Efferent:
Give rise to optic radiation
    I
Retrolentiform part of internal capsule
    I
Visual area of cortex


Structure of Lateral geniculate:
It is a six-layer.
*  Layers 1, 4 & 6 receive optic fibers from the opposite eye through the ipsilateral optic tract.
*  Layer 2, 3 & 5 receive optic fibers from of same side-eye through ipsilateral optic tract
 
Note:
Nasal fibers cross at optic chiasma and goes to opposite sides.
Thus, the optic tract contains temporal fibers of the same sides and nasal fibers of the opposite side.
 
Function:
The lateral geniculate body is the last relay station of the visual pathway to the occipital cortex.
 
 
Summary of Pathway:
 
Auditory sense - Lateral lemniscus - Inferior colliculus - Medial geniculate body - Auditory radiation - Sublentiform part of internal capsule - auditory cortex
 
Visual sense - optic chiasma - optic tract - Lateral geniculate body - optic radiation - retrolentiform part of internal capsule - visual cortex
 

Part Afferent Fiber Efferent fiber Function
Medial geniculate body Auditory fibers from inferior colliculus Primary auditory area (41,42) Relay station for auditory impulses
Lateral geniculate body Optic tract Primary Visual Area (17) Relay station for visual impulses
 
Corpora quadrigemina:
In the brain, four colliculi (two superior & two inferior) are located on tectum at the dorsal aspect of the midbrain called Corpora Quadrigemina.
  • Superior colliculus related to visual reflexes
  • Inferior colliculus form auditory pathway
* Superior colliculus
         I
superior brachium - medial geniculate body

* Inferior colliculus -
        I
inferior brachium - lateral geniculate body
 
 
Epithalamus:

Part:
  1. Right & left habenular nuclei
  2. Pineal body
  3. Habenular commissure
  4. Posterior commissure
 
Pineal body /Pineal Gland:
Pineal gland is the small body projecting backward and downwards between the two superior colliculus.

*  Pineal gland is the neuroendocrine gland which contains parenchymal cells called pinealocytes and neuroglia cells.
*  It is the only part of the brain without neural tissue in it.
 
Function of pineal gland:
  • Pineal gland secretion including melatonin have inhibitory influences on endocrine glands and gonads.
  • It secretes hormones called melatonin which causes changes in skin color in some species.
 
 
Hypothalamus:

The hypothalamus is a part of the diencephalon.
It lies in the floor and lateral wall of the third ventricle.
 
Anatomically, it includes:
a. The floor of the third ventricle
b. The lateral wall of the third ventricle below the hypothalamic sulcus.
 
It means:
The floor and lateral wall of the 3rd ventricle is also the part of the hypothalamus.
 
Boundaries
* Superiorly:
Thalamus

* Inferiorly:
Floor of 3rd ventricle

* Posteriosuperiorly:
Hypothalamic sulcus

* Anteriorly:
Lamina terminalis (extends from otic chiasma to anterior commissure)
 
 
Parts & Nuclei of Hypothalamus:
 
Optic part
1. Preoptic nucleus
2. Supraoptic nucleus
3. Paraventricular nucleus
4. Anterior nucleus
Mammillary part
1. Arcuate nucleus
2. Ventromedial nucleus
3. Dorsomedial nucleus
Tuberal part
1. Posterior nucleus
2. Mammillary nuclei
3. Lateral nucleus
 
 
 
Connection of hypothalamus:
 
Axons of para-ventricular nuclei & Supraoptic nuclei go through the pituitary stalk forming the hypothalami-hypophyseal tract & go downwards to the posterior pituitary gland (neurohypophysis).
  *  Vasopressin & oxytocin (synthesized in supraoptic & paraventricular nucleus) are transported through this tract to the posterior pituitary gland.
 
Axons of other nuclei transfer their secretion to the hypothalamohypophyseal portal system of blood vessels.
*  And controls hormones secretion from the anterior pituitary gland (adenohypophysis).
 
Note:
Vasopressin & oxytocin are the hormones secreted from the hypothalamus, but they remain stored in the nerve fibers ending of the hypothalamo-hypophyseal tract & these tract descends up to the posterior pituitary gland.
  *  Thus, in a simple manner, it can be said that the posterior pituitary gland acts as a storehouse for vasopressin(ADH) & oxytocin.
When nerve fibers get stimulated, then these tract drain hormones to systemic circulation.
Tracts are the bundle of nerve fibers (axon or dendrites) within CNS.
 
 
Function of hypothalamus:

1) Autonomic control
  • Anterior part controls the parasympathetic nervous system.
  • The posterior part of the hypothalamus controls the sympathetic nervous system.
Thus, the hypothalamus controls the cardiovascular, respiratory and alimentary function.
 
2) Regulation of body temperature
  • Anterior part helps in decreasing body temperature by causing cutaneous vasodilation & increases sweating
The posterior part of the hypothalamus is stimulated in cold climate increasing sympathetic activity.
       |
Causes vasoconstriction of skin blood vessels
       |
Decreases heat loss & sweating
 
3) Regulation of sensation of thirst & water intake
  • Lateral part of hypothalamus acts a thirst center
4) Regulation of Hunger/Food intake
  • The feeding center is placed laterally. Stimulation of the feeding center causes hyperphagia (overeating)
  • The satiety center is placed medially, stimulation of this causes hypophagia.
5) Regulation of endocrine control
  • By secreting releasing & inhibiting hormones, it controls the secretion from the anterior pituitary gland.
  • Formation of posterior pituitary gland hormone & their regulation of secretion (neurosecretion).
6) Reproduction & sexual behavior:
  • Regulates the secretion of gonadotrophin hormones by pituitary gland influencing the sexual behavior & reproduction
7) Biological clock:
  • Cyclic activity regulation like sleeping & waking cycle.
 
Simply,
1) Regulation of Endocrine Function by secreting releasing & inhibiting hormones.
2) Regulation of body temperature
3) Regulation of Cardiovascular system
4) Regulation of water intake & sensation of thirst
5) Regulation of hunger, feeding & GI activity
 
Lesion in hypothalamus could causes
1) Obesity
2) Hyperglycemia & glycosuria
3) Diabetes insipidus
4) Sexual disturbance
5) Sleep disturbances