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DIENCEPHALON-HYPOTHALAMUS. Diencephalon [“ between brain”]. Mostly hidden from view Between cerebral hemispheres 2% of CNS by weight Widespread and important sensory connections. Majority of sensory, motor and limbic pathways involve one or more stops in this region

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diencephalon between brain
Diencephalon [“ between brain”]
  • Mostly hidden from view
  • Between cerebral hemispheres
  • 2% of CNS by weight
  • Widespread and important sensory connections

Majority of sensory, motor and limbic pathways involve one or more stops in this region

  • 4 parts – each part includes the term ‘thalamus’ [ inner chamber]

Epithalamus –including pineal gland and few nearby neural structures

  • Dorsal thalamus=thalamus
  • Subthalamus
  • Hypothalamus

Visible part of diencephalon is inferior surface of hypothalamus

  • Includes mammillary bodies and infundibulum
  • Entire medial surface is wall of 3rd ventricle, visible in a hemisected brain

Superiorly, it borders body of lateral ventricle

  • Laterally- internal capsule
  • Caudal boundary-plane through posterior commissure and caudal edge of mammillary bodies
  • Rostral boundary-plane through back of anterior commissure and front of optic chiasm

Boundaries are approximate

  • Neural tissue is continuous across boundaries
  • Certain thalamic nuclei protrude through posterior boundary to a position alongside midbrain
  • Includes pineal gland and habenular nuclei
pineal gland
Pineal gland
  • Midline, unpaired
  • Resembles a pine cone
  • Rostral to superior colliculi
  • Once considered to be the seat of the soul

Pineal tumours compress midbrain leading to

  • Hydrocephalus
  • Deficits in eye movements and pupillary reactions
  • Altered sexual development

Receives light – regulated input by a circuitous pathway

  • Retina →hypothalamus→ intermediolateral cell column→ postganglionic fibres of superior cervical ganglion→pineal gland
  • No known neural output

Secretes a hormone- melatonin [derived from serotonin]

  • Secretion increases during darkness
  • Related in humans to sleep-wake cycles
  • Gland undergoes calcification after the age of 17

Calcified gland is a useful radiologic landmark

  • Slight shifts in pineal position can be indicative of expanding masses of different types
basic facts
Basic facts
  • Small part of diencephalon [ 4g in weight]
  • Important as a nodal point in pathways concerned with autonomic, endocrine, emotional and somatic functions designed to promote homeostasis

Widespread sets of connections

  • Various components of limbic system
  • Outputs influencing pituitary gland
  • Interconnections with various visceral and somatic nuclei[ motor and sensory,of brainstem and spinal cord]
boundaries of inferior surface
Boundaries of inferior surface
  • Optic tracts, optic chiasma, mammillary bodies
  • This area exclusive of mammillary bodies is called tuber cinerium [‘gray swelling’]
  • Medial eminence protrudes from surface of tuber cinerium , and is continuous with infundibular stalk, which in turn is continuous with posterior lobe of pituitary

Infundibular stalk +posterior lobe of pituitary=neurohypophysis

medial surface
Medial surface
  • Anterior extent-lamina terminalis
  • Superiorly- hypothalamic sulcus
  • Posteriorly- caudal edge of diencephalon

Neural tissue anterior to a plane passing through anterior edge of optic chiasma and posterior edge of anterior commissure is functionally continuous with hypothalamus=preoptic area

  • Considered a part of anterior hypothalamus
divisions anteroposterior
Divisions [anteroposterior]
  • Anterior
  • Tuberal
  • Posterior

Anterior region- above optic chiasma

  • Tuberal – above and including tuber cinerium
  • Posterior – above and including mammillary bodies
parasagittal section regions a anterior p posterior t tuberal po preoptic
Parasagittalsection.Regions ;a-anterior, p-posterior,t-tuberal, po-preoptic
longitudinal zones
Longitudinal zones
  • Periventricular- in the wall of 3rd ventricle [rostral continuation of PAG]
  • Lateral –lateral to fornix
  • Medial zone [in between the two] –populated by series of hypothalamic nuclei
  • The 1st 2 zones contain neurons and are avenues via which ascending and descending axons enter, leave or traverse hypothalamus
periventricular zone
Periventricular zone
  • Traversed by dorsal longitudinal fasciculus[bundle of hypothalamic afferents and efferents]
  • Contains suprachiasmatic and arcuate nuclei
  • Suprachiasmatic – tiny – less than 1 mm square and fewer than 10,000 neurons
  • ‘master clock’ for our circadian rhythms

Receives direct retinal projections which entrain it to the actual day length

  • Its neurons also contain melatonin receptors
  • Night-time rise in pineal melatonin secretion probably helps ‘set’ the circadian rhythm
lateral zone
Lateral zone
  • Mainly scattered cells interspersed among longitudinally running fibers of Medial forebrain bundle
  • Anteriorly- continuous with lateral preoptic nucleus- an important sleep-promoting area
  • Caudally- continuous with midbrain reticular formation

Also has

  • Parts of supraoptic nucleus
  • Lateral tuberal nuclei
  • Tuberomammillary nucleus [source of histaminergic fibers that project widely to cerebral cortexand thalamus-participate in sleep-wake cycles]
medial zone
Medial zone
  • Anteriorly has 2 nuclei containing large neurosecretory cells-paraventricular , supraoptic
supraoptic nucleus
Supraoptic nucleus
  • Sits astride optic tract
  • Extends to lateral hypothalamic zone
paraventricular nucleus
Paraventricular nucleus
  • Located higher up in the wall of 3rd ventricle
  • Most cells of supraoptic nucleus and many cells of paraventricular nucleus secrete hormones that travel down axons of these cells and are released in neurohypophysis
medial tuberal zone
Medial tuberal zone
  • Divided into dorsomedial and ventromedial nuclei
  • Also has clusters of orexin-containing neurons near fornix extending into lateral and medial hypothalamus
  • Source of second set of wakefulness promoting neurons
medial mammillary region
Medial mammillary region


  • Mammillary body [complex of many nuclei]
  • Posterior hypothalamic nuclei continuous with PAG [periaqueductal gray matter]of midbrain