Amadi O. Ihunwo, PhD

School of Anatomical Sciences

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Diencephalon Has a “nuclear” structure,

as opposed to the layer-like structure of cerebral cortex

Located between cerebral hemispheres & above midbrain

Forms central core of cerebrum + Telencephalon

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Surfaces of Diencephalon

Surfaces:

Medial – 3rd ventricle with interthalamic adhesion

Dorsal - Fornix

Lateral – Internal capsule

Ventral – front: optic chiasma; sides: optic tract

Subdivisions

Thalamus, Epithalamus, Subthalamus, Hypothalamus

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Thalamus Largest nuclear mass

in CNS Ovoid in shape Constitutes four-fifth

of diencephalon Functional

Classification Specific or Relay Nuclei

Association Nuclei

Non-specific nuclei

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Functions of thalamic Nuclei

Specific Nuclei have a reciprocal connection to specific motor or sensory areas of cerebral cortex. E.g. Medial & lateral geniculate

nuclei (metathalamus) for primary auditory & visual cortical areas respectively

Association Nuclei are reciprocally connected to association areas of cerebral cortex Anterior nucleus associated with

limbic system (cingulate gyrus for memory and behavioural functions (reproductive & defense)

Non-specific nuclei: Involved in different levels of consciousness & degree of alertness (arousal)

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Lateral

Geniculate body

Pineal gland

Pulvinar of

Thalamus

Posterior

Commissure 3rd ventricle

Fornix

Habenular trigone

Striae medullaris

EPITHALAMUS

Components: Habenular nuclei Habenular commissure Striae medullaris Epiphysis (pineal gland) Posterior commissure

Pineal gland Endocrine gland attached to

diencephalon by pineal stalk

2 cell types: principal pinealocytes - synthesize melatonin implicated in sleep-wake cycle (secreted mostly at night & inhibited in daytime) & Neuroglia

Calcification results in “brain sands” (corpora arenacea @ 30 yrs +)

Useful radiologic landmark for determining whether or not pineal gland is displaced by a space-occupying lesion as it lies in midline.

However, a normal pineal gland may lie slightly to left because right cerebral hemisphere is usually a little wider than left at this level

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Functional Roles of Epithalamus

Regulation of cyclic behaviour, such as circadian rhythm and reproductive cycles, in response to the day-night cycle (epiphysis mainly)

Modulation of other systems in the brain such as the limbic system, hypothalamus, raphe, and some motor-related pathways.

Subthalamus (Ventral thalamus)

Wedged between thalamus (T) & hypothalamus

Consist of: subthalamic nucleus & zona incerta (zi)

Subthalamic nucleus (sb) –lies against internal capsule (ic) & substantia nigra (SN)

Function: control of skeletal muscle activity

zi - Associated with regulation of thirst.

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Hypothalamus

Occupies a small part of brain. Weight – 4g

Boundaries Superior: hypothalamic sulcus

Inferior: Optic chiasma, tuber cinereum, mammillary bodies

Anterior: Lamina terminalis

Posterior: Tegmentum of midbrain

Medial: 3rd ventricle

Lateral: Internal capsule

Most hypothalamic nuclei have ill-defined boundaries

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T

Pituitary

gland

T-Thalamus

Functions of Hypothalamus Control of hypophysis (pituitary

gland) Control of autonomic centres Temperature regulation Regulation of food & water

intake Emotions & Behaviour: Rage &

fear; Sexual arousal Control of circadian rhythms:

sleeping & waking in association with pineal gland.

Memory: mammillary bodies in association with limbic system

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