Diencephalon

• Diencephalon is a part of forebrain situated cranial to mesencephalon (midbrain) and caudal to telencephalon.

  • Diencephalon consists of two symetrical right and left parts with the cavity of third ventricle between them.
• Sagittal view of diencephalon

  
• Thalamus are large, paired masses of gray matter located between cerebral cortex above and midbrain bellow.
• There are two thalami, each situated on each side of a slit-like cavity - the 3 ventricle.

  1. Each half is narrow at the anterior end and widest at posterior end.
  2. Each talamis ia about 4cm in lenght. The 2 thalami are connected by interthalamic adhesion, this is not a commissure and posses no neurons.
• The thalami has 2 ends

  • Anterior end: narrow, also known as tubercle of thalami. It forms the poste boundary of inter-ventricular foramen
  • Posterior end: expanded; also known as pulvinar. It overhangs the lateral and medial geniculate bodies.
• Thalami medial and lateral surfaces:

  • Lateral surface: forms the medial boundary of posterior limb of internal capsule. It’s separated from lentiform nucleus by posterior limb of internal capsule.
  • Medial surface: forms lateral wall of third ventricle. Covered by ependyma that lines third ventricle.
• Thalami is dense mass of gray matter organised into three main groups of nuclei: these are separated by the internal medullary lamina which is Y shaped

  • The anterior group lies between the limbs on internal lamina
  • medial and lateral group parts lies on esch side of the main stem of the Y
• The anterior nucleus of thalamus is located in the anterior part. It receives afferents from mamillary body, limbic system and hippocampal formation and gives efferents to cingulate gyrus.

  It is involved in:

  • emotional regulation, memory consolidation
  • In the limbic system playing role in attention and cognitio.
  • Its a relay station within the papez circuit, facilitating communication between the hippocampus and other limbic structures.
• The median nuclear group contains:

  • Mediodorsal nuclei (main nuclei of importance) cognition, emotional response, decision-making. It is responsible for adjusting thoughts, behaviour and physical state of person in response to various sensory experiences. Receives input from all senses system, amygdala, papez circuit, hypothalamus and its main output is to the prefrontal cortex.
• Intralaminar nuclei of thalamus are located within the internal medullar lamina:

  • Centromedian nucleus: applies emotional component to pain sensation: painful stimulus is transmitted by C fibers to posterior grey horn of the spinal cord →decussation → spinal lemniscus → (reticular formations) → Centromedian nucleus → nonspecific cortical areas
  • Parafascicular nucleus: similar functions to centromedian. Involved in pain processing and arousal
• Lateral group of thalami are subdivided into dorsal and ventral tier
• The dorsal tier of lateral nuclei group of thalamus has:

  • Pulvinar: large nuclei that plays role in processing and attentio. Receives input from superior colliculus and auditory projecting inti parieto-occipital associated areas.
• The ventral tier of lateral group nuclei of thalamus has:

  • VentroANT nucleus: involed in motor contro, receiving inout from basal ganglia and cerebellum projecting into motor cortex
  • Ventro Lateral nucleus: motor control; inputs from basal ganglia + cerebellum (dentate → red nucleus ) and project to primary motor cortex
  • Ventro posteromedial: receives all somatosensory info from face (trigeminothalamic tract) + gustation info and protects into somatosensory cortex.
  • Ventri Posteriolateral: receives sensory info via DCMLP + spinothalamic and projects into primary somatosensory cortex
• Reticular nuclei Of thalamus froms a thin sell around thalamus and is involved in modulating activity of other thalamic nuclei. Receives input from cortex and thalamus and projects back to thalamic relay nuclei.
• White matter of thalamus is composed of myelinated fibers that facilitate communication between different parts of thalamus and between thalamus and other regions of brain.

  • Internal medullary lamina: Y-shaped layer of white matter that divides thalamus into anterior , lateral and medial group nuclei.
  • External medullary lamina: thin layer of white matter that covers lateral aspect of thalamus. It contains thalamocortical and corticothalamic fibers that connects thalamus to cerebral cortex.
• Thalamus


  A vital structure that acts as a relay station and integration center for various types of information
• Sensory relay


  1. Visual info (via lat geniculate body)

  2. Auditory info (via medial geniculate body)

  3. Somatosensory info (via ventral posterolateral: sensory info from body via spinal cord and ventral posteromedial nucleus: sensory info from face via trigeminal nerve)
• Motor relay


  1. Via ventral anterior and ventro-lateral nucleus

  2. Receives input from basal ganglia and cerebellum
• Emotional and memory functions

  Anterior nucleus
• Arousal and attention


  Intralaminar nuclei
• Modulation


  Reticular nuclei
• The metathalmus is part of diencephalon represented by a pair of geniculated bodies.

  They function as primary thalamic relay station for audi and visual information.
• The medial geniculate body is situated on the post aspect of thalamus just bellow pulvinar and lateral to the superior colliculi. Its primary involved in auditory processing.

  • Afferents from: lateral leminiscus, fibers from both inferior colliculi via inferior brachium
  • Efferents to: auditory radiation going to primary auditory cortex in temporal lobe.

  Consists of 3 subdivisions each receiving different types of auditory stimuli.

  Play role in processing and perception of sound.
• The lateral geniculate body is located advanced to MGD.

  It is a key relay center in visual information. 6 layer structure with layers 1,,4,6 receiving contralateral optic fibers and layers 2,3,5 receiving ipisilateral optic fibers.

  • Afferents from: retina of both eyes, fibers from primary visual cortex and superior colliculi via superior brachium.
  • Efferents: gives optic radiations (meyers loop) going to visual cortex area in occipital lobe.

  It plays crucial role in visual processing, perception and integration of visual informatio.
• Epithalamus is a small area located posteriosuperior to thalamus.

  • Consists of Habenular nuclei
  • Habenular commisure
  • pineal galnd
  • Posterior commisure
• The pineal gland is a small endocrine gland located between the 2 hemispheres, attached to the roof of 3 ventricle.

  • It consists of a small conical body and a peduncle which dived anteriorly into. 2 laminae separated by the pineal recess of third ventricle. The superior lamina contan the habenular commissure and inferior lamina contains the posterior commissure.
• The pineal gland has 2 types of cells: pineolocytes and neuroglial cells and it is rich in various sized concentrations of Calcium and magnesium salts called corpora arenacea aka brain sand.

  1. Pineolocytes produces melatonin which release is promoted by dark and inhibited by light.
  2. melatonin plays crucial role in regulating sleep-wake cycle (circadian rhythm) and other physiological functions such as mood, immune functions.
  3. Secretion is influenced by suprachiasmatic nucleus of hypothalamus which receives input from retina.
• The habenular nuclei are paired structure located on each side of pineal gland connected with the habenular commissure.

  • involved in processing negative reward signals, mood regulation, stress response.
  • connects limbic system and brain stem.

  Habenular commissure are bundle of nerve fibers that connecting habenular nuclei of left and right sides

  • facilitates communication btw the two habenular nuclei ensuring coordinated activity across both sides.
• Diencephalon diagram

  
• Hypothalamus extends from lamina terminalis to the mamillary bodies and from hypothalamic sulcus to base of brain beneath third ventricle

  
• Hypothalamus is bounded:

  • SUP: thalamus + floor of 3ventricle
  • Anterior: anterior commissure + lamina terminalis
  • Post: posterior commissure + cerebral aqueduct
  • INF: optic chiasm + pituitary stalk + brainstem (floor of 3ventricle)
  • Lateral: internal capsule
  • Medial: medial to 3 ventricle
• Sherington described hypothalamus as head ganglion of autonomic nervous system.

  Via autonomic and endocrine system it controls fluid and electrolytes balance, food ingestion, energy balance, reproduction, thermoregulation, immune and emotional response.
• Supraoptic region of hypothalamus contains:

  • Suprachiasmatic nuclei: gets info from retina via retinohypothalamic tract. Output is to pineal gland →regulates circadian rhythm.
  • Paraventricular nucleus: synthesises oxytocin→ released by post pituitary →Induces uterine contraction and lactation.
  • Anterior hypothalamic nucleus: has parasympathetic outflow + and is involved in decrease body T by vasodilation of cutaneous body vessel + sweatin.
  • Supraoptic nucleus: produces vasopressin which is released by post (pituitary (increases water reabsorption and causes vasoconstriction)
• Tuberal region os hypothalamus contains:

  • Arcuate nucleus: regulates appetite and food intake by modulating activity of ventromedial + lateral hypothalamic nuclei.
  • Ventromedial nucleus: satiety center; arcuate nucleus relase alpha- melanocyte stimulating hormone + cocaine-amphetamine regulating transcript → VMN released corticotropin releasing hormone → induces satiety.
  • Dorsomedial nucleus: involved in physiological circadian rhythm, eating, drinking and energy consumption.
• Mammilary region of hypothalamus contain:

  • Posterior hypothalamic nucleus: has sympathetic activity + is involved in thermoregulation by increasing body T through vasoconstriction of cutaneous vessels + shrivering.
  • Mammillary bodies: component of limbic system, involved in regulating emotions, memory processing. It receives input frim hyppocampus via fornix, amygdala and anterior thalamic nuclei. These allows for integration of emotional and memo processing.
• Arcuate nucleus of hypothalamus has two types of neurons

  • orexigenic neurons: stimulates feeding
  • Anorexigenic neurons: suppress feeding

  Increasing levels of insulin and leptin stimulates release of anorexigenic neuropeptides that target neurons in ventromedial nucleus (satiety center) which promotes satiety and inhibiton of feeding behaviour.

  Decreasing insulin and leptin levels leads to activation of orexigenic neurons that target neurons in lateral hypothalamic nuclei (feeding center) stimulation feeding behaviour and increase food intake.
• The arcuate nucleus in involved in primary regulation of anteior pituitary gland via hypothalamohypophyseal portal system.

  • produces hormones which regulates secretion of other hormones in pituitary gland: GHRH, CRH, TRH and so influences sexual activity, reproductio, gametogenesis, uterine and ovarian cycled and secondary sexual characteristics.
• The paraventricular nucleus of the hypothalamus is involved in regulation of posterior pituitary gland via hypothalamohypophyseal portal system. Produces OXYTOCIN which are stored in posterior lobe of pituitary gland and released into bloodstream when needed.
• Connections of hypothalamus

  
• Subthalamus is a part of diencephalon located just beneath thalamus and lateral to hypothalamus.

  • Consists primary of two structures: subthalamic nucleus, zona incerta and fields of forel.