‫عالعافية‬....‫السالم عليكم‬
21/3/2011
The Diencephalon
Definition:
It is the third ventricle and the structures that form its boundaries.
It includes:
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Thalamus
Hypothalamus
Subthalamus
Epithalamus
Thalamus
 It is the largest part of the diencephalon.
 It is a large ovoid mass of gray matter that forms a major part of the
diencephalon.
 There are two thalami, one in each hemisphere.
 It is a station for all sensory systems except olfactory sensation system.
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Boundaries of the thalamus
 The anterior border of the thalamus is surrounded by part of the lateral
ventricle and interventricular foramen, (the anterior end of the thalamus
forms the posterior boundary of interventricular foramen).
 The posterior border is formed by the pulvinar which is the largest
nucleus inside the thalamus.
 The superior border is formed by the fornix and choroid plexus.
 The inferior border is formed by the midbrain.
 The thalamus is surrounded medially by the third ventricle from both
sides and the interventricular adhesion (which is a commissural tract
that connects the two thalami together).
 The lateral boundary of the thalamus is formed by the internal capsule
and lateral to the internal capsule lies a nucleus in the white matter
known as lentiform nucleus which is a part of the basal nuclei (basal
ganglion).
Notes:
 The internal capsule is an example of a projection fiber.
 The name “basal ganglion” is a wrong nomenclature
because it is located inside the central nervous system so
it should be called “basal nuclei”.
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The thalamus is subdivided into anterior, medial and lateral parts by the
internal medullary lamina, and in each part we have a group of thalamic nuclei.
Each nucleus is named according to its location and each has its own function.
Note:
 The internal medullary lamina is a Y shaped
part of white matter.
Functions of thalamus:
 Relaying sensations, spatial sense and motor signals to the cerebral
cortex, along with the regulation of consciousness, sleep and alertness.
Sensory function:
It relays all sensory information from the spinal cord to the S1 area inside the
cortex.
Motor function:
The basal ganglia, cerebral cortex and the cerebellum are responsible for
motor functions.
The thalamus also has a role in motor functions through its nuclei which are
connected to the basal ganglia.
Subthalamus
 The subthalamus is located inferior to the thalamus, and its structure is
extremely complex because many sensory tracts pass through it.
 It has subthalamic nuclei that have connections with corpus striatum.
 Generally, the subthalamus influences motor functions.
Epithalamus
 The epithalamus consists of habenular nucleus and pineal gland.
 The right and left habenular nuclei are connected together by the
habenular commissure.
 The function of the habenular nucleus is the integration between all
sensations including olfactory and visceral sensations.
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 The pineal gland is a conical shaped endocrine gland, connected to the
diencephalon by a stalk.
 This stalk is bounded superiorly by the habenular commissure and
inferiorly by the posterior commissure.
 The pineal gland inhibits all endocrine glands either directly through CSF
or indirectly through the blood stream.
Note:
 We have two thalami, one hypothalamus, two habenular
nuclei and one pineal gland.
Hypothalamus
 The hypothalamus extends from optic chiasm and ends posteriorly at
mammilary bodies.
 It lies below the hypothalamic sulcus on the lateral wall of the third
ventricle.
 The hypothalamic sulcus separates the thalamus from the
hypothalamus.
Functions of Hypothalamus
There is hardly any activity in the body that is not influenced by the
hypothalamus.
 It has an effect on temperature e.g. it affects mechanisms that dissipate
or collect heat through sweating and dilatation of blood vessels
underneath the skin.
 Emotional regulation.
 Sexual behaviour.
It affects the emotional and sexual behaviours through its connection to the
prefrontal cortex.
 Hunger and thirst.
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Neurosecretion.
Endocrine control
The hypothalamus is subdivided into medial and lateral parts.
The lateral part consists of nuclei responsible for hunger (hunger centre)
and thirst (thirst centre).
A bilateral destruction to the hunger centre causes anorexia which is a
loss of appetite.
Stimulation of the thirst centre causes the person to drink more water.
The medial part of the hypothalamus contains the satiety centre (‫)شبع‬.
A destruction to the satiety centre results in obesity.
The hypothalamus, in addition to its effects on other glands (either stimulating
or inhibiting them to release their hormones), it secretes its own hormones
(cytosine and vasopressin).
"Whoever follows patience, success will follow him”
Dedicated to: Farah Tsey
(pure Cherkess)
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Best wishes
Isra M. Abu Ghoush
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