basal ganglia top
TRANSCRIPT
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Basal ganglia
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The term basal nuclei is applied to a collection of masses of
gray matter situated within each cerebral hemisphere.
They are the corpus striatum, the amygdaloid nucleus, and
the claustrum.
The basal nuclei play an important role in the control of
posture and voluntary movement.
The subthalamic nuclei, the substantia nigra,
and the red nucleus are functionally closelyrelated to the basal nuclei, but they should
not be included with them.
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Corpus Striatum
The corpus striatum is situated
lateral to the thalamus and is
almost completely divided by aband of nerve fibers, the
internal capsule, into the
caudate nucleus and the
lentiform nucleus. The term
striatum is used here because
of the striated appearance
produced by the strands of gray
matter passing through the
internal capsule and connectingthe caudate nucleus to the
putamen of the lentiform
nucleus.
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Caudate Nucleus
The caudate nucleus
is a large C-shaped
mass ofgray matterthat is closely related
to the lateral ventricle
and lies lateral to the
thalamus. The lateral
surface ofthe nucleusis related to the
internal capsule,
which separates it
from the lentiform
nucleus .It can be
divided into a head, a
body, and a tail.
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The headofthe caudate
nucleus is large androunded
and
form
s the lateral wall
ofthe anterior horn ofthe
lateral ventricle The head is
continuous inferiorly with the
putamen ofthe lentiform
nucleus (the caudate nucleusand the putamen are
sometimes referred to as the
neostriatumor striatum). Just
superior to this point of
union, strands ofgray matter
pass through the internal
capsule, giving the region a
striated appearance, hence
the termcorpus striatum.
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The body ofthe caudate nucleus
is long and narrow and is
continuous with the head in the
region ofthe interventricularforamen. The body ofthe caudate
nucleus forms part ofthe floorof
the body ofthe lateral ventricle.
The tail ofthe caudate nucleus is
long and slender and iscontinuous with the body in the
region ofthe posterior endofthe
thalamus. It follows the contour
ofthe lateral ventricle and
continues forward in the roofof
the inferior horn ofthe lateral
ventricle. It terminates anteriorly
in the amygdaloid nucleus.
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Lentiform Nucleus
The lentiform nucleus is a
wedge-shapedmass ofgray
matter whose broadconvexbase is directed laterally and
whose blade is directed
medially. It is burieddeep in the
white matterofthe cerebral
hemisphere and is relatedmedially to the internal capsule,
which separates it from the
caudate nucleus and the
thalamus.
The lentiform nucleus is related
laterally to a thin sheet ofwhite
matter, the external capsule ,
which separates it from a thin
sheet ofgray matter, called the
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The claustrum, in turn,
separates the external
capsule from the subcortical
white matterofthe insula. Avertical plate ofwhite matter
divides the nucleus into a
larger, darker lateral portion,
the putamen, and an inner
lighterportion, the globuspallidus.
The paleness ofthe globus
pallidus is due to the
presence ofa high
concentration ofmyelinated
nerve fibers. Inferiorly at its
anterior end, the putamen is
continuous with the headof
the caudate nucleus.
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Amygdaloid Nucleus
The amygdaloid nucleus is
situated in the temporal
lobe close to the uncus.The amygdaloid nucleus is
considered to be part of
the limbic system. Through
its connections, it can
influence the body'sresponse to environmental
changes. In the sense of
fear, for example, it can
change the heart rate,
bloodpressure, skin color,
andrate ofrespiration.
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Substantia Nigra and
Subthalamic Nuclei
The substantia nigra ofthemidbrain and the subthalamic
nuclei ofthe diencephalon are
functionally closely related to
the activities ofthe basal nucleiThe substantia nigra is a brain
structure located in the
mesencephalon (midbrain) that
plays an important role in reward,addiction, and movement.
Substantia nigra is Latin for "black
substance", as parts of the
substantia nigra appear darker
than neighboring areas
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Claustrum
The claustrum is a thin sheet ofgray
matter that is separatedfrom the lateral
surface ofthe lentiform nucleus by theexternal capsule. Lateral to the claustrum
is the subcortical white matterofthe
insula. The function ofthe claustrum is
unknown.
Connections ofthe basal ganglia
The caudate nucleus and the putamen form
the main sites forreceiving input to the
basal nuclei. The globus pallidus forms the
major site from which the output leaves the
basal nuclei.
They receive nodirect input fromoroutput
to the spinal cord.
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In order tounderstand the complex circuitry ofthe basal
ganglia, one has tofirst understand the important
participants in this circuit. The basal ganglia is in direct
communication with the thalamus and the cortex. Cortex,
thalamus, and the basal ganglia are therefore the three
main participants in the circuit created by the basal
ganglia.
In the highest position ofauthority, andresponsible for
conscious perception ofthe universe, lies the human
cerebral cortex. All actions undertaken by the nervous
systemdirectly or indirectly relate to the cortex. Cortex
has many different areas with different functions. One
such cortical area is called the pre-central gyrus, also
known as the "motorcortex."
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Specialized neurons from the
cortex in the motorcortex region
(precentral gyrus) extend their
axons all the way to the striatum
portion ofthe basal ganglia. These
cortical neurons release the
neurotransmitter which is
excitatory in nature. Once excitedby the cells in the striatumproject
in twodifferent directions giving
rise to twomajorpathways: The
"direct" and the "indirect"
pathway:
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In the direct pathway, once activated
by the cortex, the cells of striatum,
project inhibitory neurons ( an
inhibitory neurotransmitter) onto the
cells of the "SNr-GPi complex" (SNrand GPi are separate spatially but
due to similar function it is correct to
think of them as a complex). SNr and
GPi, are constantly in connection with
the thalamus through pathway, in an
attempt to inhibit the thalamus. Due
to the inhibition (via the striatal
inhibition) of "SNr-GPi" (inhibitors of
thalamus themselves), the end result
is lack of inhibition of the thalamus.
Thalamus interestingly projects to thecortex itself, constantly stimulating
the cortex.
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. The direct pathway therefore
results in the thalamus being
allowed to stimulate the cortex.
Once stimulated the cortex willthen send this message of
"stimulation" down its motor
pathway via the lateral
corticospinal tract to the muscles,
resulting in a hyper-kinetic
behavior (meaning increased
motion). The following diagram
depicts the "direct" pathway:
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The end result is an actual
inhibition of the thalamus and
therefore decreased stimulation of
the cortex by the thalamus. Thisresults in the cortex stimulating the
muscles less through the lateral
corticospinal tract and favoring a
hypo-kinetic state or decreased
motion. In essence, the direct andthe indirect pathway are antagonist
in function.
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Cortex -(stimulates)-> Striatum -(inhibits)-> GPe -(inhibits)-> STN
-(stimulates)-> "SNr-GPi" complex -(inhibits)-> thalamus -(is
stimulating less)-> Cortex -(is stimulating less)-> Muscles, etc. ->
(hypokinetic state)
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Cortex -(stimulates)-> Striatum -(inhibits)-> "SNr-GPi" complex -(inhibits)-> Thalamus -(stimulates)-> Cortex -(stimulates)-> Muscles,
etc. -> (hyperkinetic state)
Cortex -(stimulates)-> Striatum -(inhibits)-> GPe -(inhibits)-> STN -
(stimulates)-> "SNr-GPi" complex -(inhibits)-> thalamus -(is
stimulating less)-> Cortex -(is stimulating less)-> Muscles, etc. ->
(hypokinetic state)
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Functions ofthe Basal Nuclei
The basal nuclei are joined together andconnected with many
different regions ofthe nervous system by a very complexnumberofneurons.
Basically, the corpus striatumreceives afferent information
frommost ofthe cerebral cortex, the thalamus, the
subthalamus, and the brainstem, including the substantia
nigra. The information is integrated within the corpus striatum,and the outflow passes back to the areas listed above.
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the basal nuclei assist in the regulation ofvoluntary
movement and the learning ofmotor skills.
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Writing the letters ofthe alphabet, drawing a diagram, passing a
football, using the vocal cords in talking and singing, andusing the
eye muscles when looking at an object are a few examples where
the basal nuclei influence the skilledcortical motor activities.Destruction ofthe primary motorcerebral cortex prevents the
individual fromperforming fine discrete movements ofthe hands
andfeet on the opposite side ofthe body . However, the individual
is still capable ofperforming gross crude movements ofthe
opposite limbs. Ifdestruction ofthe corpus striatum then takes
place, paralysis ofthe remaining movements ofthe opposite side of
the body occurs.
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The basal nuclei not only influence the execution ofa
particularmovement of, say, the limbs but also help
prepare for the movements. This may be achieved bycontrolling the axial and girdle movements ofthe body
and the positioning ofthe proximal parts ofthe limbs. The
activity in certain neurons ofthe globus pallidus increases
before active movements take place in the distal limbmuscles. This important preparatory function enables the
trunk and limbs to be placed in appropriate positions
before the primary motorpart ofthe cerebral cortex
activates discrete movements in the hands andfeet.
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Disorders of the basal nuclei are of two general types.
Hyperkinetic disorders are those in which there are
excessive and abnormal movements, such as seen withchorea, athetosis(slow involuntary movements in hands),
and ballism(violent dyskinetic movements caused by the
contraction of the proximal limb muscles). Hypokinetic
disorders include those in which there is a lack or slowness
of movement. Parkinson disease includes both types ofmotor disturbances.
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Diseases of the Basal Ganglia
Parkinsons:
Akinesia
Bradykinesia
Resting tremor
Rigidity
Huntingtons disease
Chorea
Psychiatric disturbances
Dementia