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Definition
The diencephalon is a complex of
structures within the brain, whose
major divisions are the thalamus and
hypothalamus. It functions as a
relay system between sensory input
neurons and other parts of the
brain, as an interactive site for
the central nervous and endocrine
systems, and works in tandem with
the limbic system.
Description
The diencephalon is composed of
several structures, the whole about
the size of an apricot, situated
near the core center of the brain,
just above the brainstem. It is made
up of the medulla oblongata, pons,
and midbrain, below the
telencephalon, the most basal part
of the cerebrum. The two major
components of the diencephalon are
the thalamus and the hypothalamus.
Other important structures within
the diencephalon complex are the
epithalamus, subthalamus, third
ventricle, mammillary bodies,
posterior pituitary gland, and the
pineal body. The diencephalon
interconnects with a larger,
surrounding array of structures
called the limbic system, which is
the seat of emotions and memory.
The diencephalon functions in the
following ways:
As a junction and relay system that
receives and filters afferent
(incoming) sensory information, then
relays it on to other parts of the
brain, mainly the cerebral cortex,
but also to the cerebellum and
brainstem.
As an interactive site between the
central nervous system and the
endocrine system.
As an interactive complementary to
the limbic system.
The upper part of the diencephalon,
making up about 80% of its mass, is
the thalamus, a small pillow of
neural gray matter divided into two
egg-shaped lobes. The lobes' long
axes run toward the front and back
of the head, and are connected to
each other by a small stalk, the
intermediate mass. The two thalamic
lobes are filled with numerous pairs
of nuclei, which are concentrations
of synapsing afferent, or incoming,
and efferent, or outgoing, neurons.
Numerous such nuclei are situated
throughout the brain.
The thalamic nuclei are named and
classified according to their
positions within the thalamus
(medial, lateral, central, etc.), by
their neural connections, and by
their functions. In terms of
function, there are three types of
thalamic nuclei: sensory, motor, and
arousal.
Layered sheets of myelinated axons,
the internal thalamic medullary
laminae, run vertically through the
lobes of the thalamus. These laminae
are full of neurons that
interconnect various thalamic
nuclei. The edges of the internal
lamina reach the surfaces of the
lobes. They show as narrow, whitish,
cable-like bands, running across
either lobe from its posterior
underside, across the top, and
forward, bifurcating into two bands
(two vertical layers) toward the
front. The main lamina divide the
lobes of the thalamus into portions
containing the medial and lateral
geniculate nuclei, while the
anterior bifurcations enclose the
anterior nuclei.
The thalamus, the basal ganglia, and
the cerebellum, which is the main
movement coordination center of the
brain, are neurally linked to the
cerebral motor cortex in reciprocal,
or feedback, fashion. Together, they
regulate and fine-tune motor
functions. The basal ganglia, which
are part of the telencephalon, are
groupings of gray matter within the
white matter of the cerebral
hemispheres. The basal ganglia
function directly with the
cerebellum to modify and fine-tune
body movements.
A small part of the diencephalon,
the epithalamus, extends rearward
from, and slightly higher than, the
thalamus. It holds the habenular
nuclei, the stria medullaris thalami
nerve tracts, and the pineal body,
or epiphysis. The habenular nuclei
play a role in emotional responses
to odors. They receive afferent
nerves from the septum, a complex of
structures within the telencephalon
and limbic system, and from the
lateral preoptic nuclei of the basal
forebrain, which is the lowermost
region of the cerebrum; the stria
medullaris tracts and the basal
ganglia are the conduits. The
habenular nuclei send efferents to
the interpeduncular nucleus of the
midbrain via the
habenulo-interpeduncular nerve
tract.
The pea-sized, conically shaped
pineal body, on a short stalk,
projects rearward and downward from
the epithalamus. The pineal is a
gland-like organ whose functions are
still only poorly understood. It is
a functional, light-sensitive
remnant of an ancient and much more
complex system of visually oriented
organs, the pineal complex. The
pineal is neurally connected with
the suprachiasmatic nuclei of the
hypothalamus, which hold the
circadian internal clock. This is
located just above the optic chiasma,
the point at which the optic nerves
from both eyes cross. The human
pineal secretes melatonin, a hormone
that seems to have a calming effect
on the nervous system. The pineal,
in response to the level of
daylight, may induce sleepiness by
increasing the output of melatonin.
All sensory input, except the
olfactory (smell), passes through
the thalamus, where it is filtered,
integrated, and passed on to proper
sites in the brain, most of them
within the cerebral cortex. The
route is as follows:
Impulses from the auditory organs
synapse in the medial geniculate
thalamic nuclei, where they are sent
to the auditory centers of the
cerebral cortex.
Impulses from the eyes, via the
optic nerves, synapse in the lateral
geniculate thalamic nuclei, and are
sent on to the calcarine cerebral
cortex.
Other sensory input synapses in the
ventral posterome-dial thalamic
nuclei, which receive, process, and
pass on somatosensory input from the
head, while the ventral
posterolateral thalamic nuclei do
likewise with input from the rest of
the body.
The thalamic nuclei also receive
input from subcortical sources and
feedback from the cortical areas.
These operate in tandem to filter
and control input to the cortex.
The ventral anterior and ventral
lateral thalamic nuclei, involved
with motor function, receive sensory
input relayed through the basal
ganglia and through the superior
cerebellar peduncle, the main neural
tract connecting the cerebellum and
the red nuclei. The ventral anterior
and ventral lateral thalamic nuclei
project to the premotor and motor
cerebral cortex. In addition, the
ventral anterior thalamic nuclei are
the main relay nuclei between the
thalamus and the limbic system,
receiving the mammillothalamic nerve
tract from the mammillary bodies in
the hypothalamus and projecting to
the cingulate gyrus.
The cingulate gyrus, which is not a
part of the
diencephalon, is the
part of the cerebrum closest to the
limbic system, and serves to
neurally connect the thalamus and
hippocampus. The cingulate gyrus
associates memories and emotional
responses with smells, sights, and
pain, and allows movement of
attention among objects or ideas.
The medial dorsal thalamic nuclei
receive nerve tracts from the
amygdala of the limbic system and
send efferents to the prefrontal
cerebral cortex (not part of the
diencephalon), which has numerous
feedback connections with the
thalamus, amygdala, and other
subcortical structures.
The anterior thalamic nuclei connect
with the mammillary bodies of the
hypothalamus, and through them, via
a nerve tract, the fornix, with the
hippocampus and the cingulate gyrus.
The centromedian thalamic nuclei
regulate excitability levels within
the cerebral cortex and thus play a
major role in arousal and alertness.
The centromedian thalamic nuclei
receive motor-related input from the
basal ganglia, cerebellum, and the
reticular formation of the brainstem
and midbrain, and send efferent
nerves to the cerebral cortex. The
reticular formation is a network of
nerves running through the brainstem
and hindbrain, and containing the
reticular activating system, which
plays a key role in inducing arousal
and alertness in tune with the
circadian rhythm (sleeping and
waking cycles). The reticular
thalamic nuclei, which receive
neural input from the reticular
formation, regulate general thalamic
output in accordance with the
circadian rhythm.
The dorsomedial thalamic nuclei are
involved with emotional arousal and
the expression of emotionally based
behavior, as well as memory,
foresight, and feelings of pleasure.
These nuclei receive input from many
sites and interconnect with the
prefrontal cerebral cortex.
That part of the diencephalon
immediately below the two lobes of
the thalamus is the subthalamus. It
contains several nerve tracts and
the subthalamic nuclei. Small
portions of the red nuclei and the
substantia nigra of the mid-brain
reach into the subthalamus. The
subthalamic nuclei are
interconnected with the basal
ganglia and are involved in
controlling motor functions.
The hypothalamus is the lowermost
structure of the diencephalon. The
thalamus, epithalamus, and
hypothalamus surround and define
most of the third ventricle of the
brain, which, like all the
ventricles, is filled with
cerebrospinal fluid. The third
ventricle communicates with the
lateral ventricles and, via the
cerebral aqueduct, with the fourth
ventricle.
The hypothalamus contains several
nuclei, nerve tracts, and the
pituitary gland. It is the
regulatory seat of the autonomic
nervous system, while the
hypothalamus and the pituitary are
the major sites in which the two
regulatory systems of the body, the
central nervous system and the
endocrine system, interact. The
hypothalamus regulates the
production of pituitary hormones,
influencing and being influenced by
emotional states, physical
appetites, autonomic functions,
temperature control, and diurnal
rhythms. It is thus the main control
center for homeostasis, or keeping
physiological maintenance systems
functioning at optimal states.
Efferent nerves from the
hypothalamus extend into the
brainstem and the spinal cord, where
they synapse with neurons of the
autonomic nervous system, which
regulates a number of involuntary
functions, among them the rate of
heartbeat, urine release, and
peristalsis. The hypothalamus
responds to sensations of
temperature extremes, the posterior
hypothalamus stimulating muscle
shivering to deal with cold, via
efferent neurons to motor neurons
within the spinal cord, and the
anterior hypothalamus producing
sweating as a reaction to
overheating.
The pair of globular mammillary
bodies are partially embedded in the
underside of the hypothalamus. They
are involved in olfactory reflexes
and emotional responses to odors.
Also on the underside of the
hypothalamus, and toward the front,
is the optic chiasma, where the two
optic nerve cables of the eyes
cross.
From the floor of the hypothalamus,
the posterior pituitary gland, or
neurohypophysis, extends forward and
downward at the end of a long
peduncle or stalk, the infundibulum.
Efferent hypothalamic nerves extend
through the infundibulum to the
posterior portion of the pituitary
gland, others extend to the
trigeminal and facial nerve nuclei,
to help control the head muscles
involved in swallowing.
The posterior pituitary is an
extension of the hypothalamus, but
the anterior part of the pituitary
is glandular tissue with an
embryonic origin separate from that
of the posterior pituitary. During
embryonic development, the anterior
and posterior lobes of the pituitary
eventually meet and fuse.
The hypothalamus plays a pivotal
role in regulating the endocrine
system via its control of the
pituitary gland's production of
several hormones, while the
hypothalamus is influenced in turn
by hormones in the bloodstream and
by nerve input. A partial list of
hormones secreted by the pituitary
includes cortisol, prolactin,
antidiuretic hormone (ADH), oxytocin,
growth hormone (GH), thyroid
stimulating hormone (TSH),
adrenocorticotropic hormone (ACTH),
lipotropins, beta-endorphins,
melanocyte stimulating hormone,
luteinizing hormone, and follicle
stimulating hormone.
Hormones influence functions as
diverse as metabolism, growth and
maturation, reproduction, dealing
with stress, urine production, ion
balance, sexual development, and
sexual function. The hypothalamus
regulates physical appetites for
food, water, and sex. Afferent
fibers synapsing in the hypothalamus
carry input from the internal
organs, the taste receptors of the
tongue, the limbic system, the
nipples, and the external genitalia.
The hypothalamus responds to and
accords with emotional states, and
thus plays a major role in affecting
emotions and moods, among them
sexual pleasure, tranquility, rage,
and fear.
The hypothalamus contributes to the
regulation of the circadian rhythm
via an internal clock within the
suprachiasmatic nuclei. This
internal clock communicates with the
reticular formation of the midbrain.
The reticular formation contains the
reticular activating system, which
plays a key role in inducing arousal
and alertness, in tandem with the
circadian rhythm.
The diencephalon is interconnected
with a surrounding complex of brain
structures, the limbic system, which
functions as the center of emotional
states and responses, and of memory.
Besides the various structures
within the diencephalon, the limbic
system includes the olfactory
cortex, hippocampus, amygdala,
cingulate gyrus, septal nuclei, the
dorsomedial nuclei of the thalamus,
and the anterior nuclear complex of
the thalamus.
Memories of vividly emotional
experiences are recorded and kept
within easy reach of consciousness
within the limbic system.
Connections between, and functions
of, the hypothalamus and limbic
system are intimately intertwined.
The ventral anterior thalamic nuclei
are the main relay nuclei connecting
the thalamus and the limbic system,
receiving the mammillothalamic tract
and projecting to the cingulate
gyrus.
The olfactory sense is the only one
whose neurons directly connect with
a processing center within the
limbic system and outside the
thalamus. Within the hypothalamus,
relayed olfactory impulses are used
to regulate appetite and sexual
behavior, and to regulate autonomic
reactions initiated by odors. Since
the limbic system processes memory
and stores important memories, the
direct connection of the olfactory
neurons to the limbic system helps
explain why odors serve as alarms
(e.g., the odor of smoke) and can
trigger strong emotional responses
and vivid, detailed memories of
events and emotional states.
The hippocampus, the main processor
of memory, is a paired structure
looping over the tops of the
thalamic lobes and rearwards,
curving downward and forward and
ending at the paired, globular,
cherry-sized amygdala, below and in
front of the hypothalamus. The
amygdala connect with the
hippocampus, the septal nuclei, the
prefrontal area of the cerebrum, and
the medial dorsal nucleus of the
thalamus. The amygdala also send
nerves to the hypothalamus via the
ventral amygdalofugal pathway.
The amygdala are centers for
associating strong emotions, good or
bad, with memories of the
experiences that triggered those
emotions. Fear responses and
fear-charged memories are centered
in the amygdala, which can retain
vivid memories of traumatic
experiences, and initiate the
survival fight-or-flight response.
The hippocampus sends efferents, via
a cable of nerves, the fornix, to
the mammillary bodies within the
hypothalamus. The mammillary bodies
send efferents to the anterior
nuclei of the thalamus via the
mammillothalamic tract.
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