Basal Ganglia Top

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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

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