THE BASAL GANGLIA
WHAT MAKES UP THE BASAL GANGLIA?
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Also known as the BASAL NUCLEI
Group of subcortical gray structures found deep within
the white matter of the brain
The grouping of the nuclei is related to function rather
than anatomy – its components are not part of a single
anatomical spread deep within the brain
They form part of the extrapyramidal motor system
It is part of a basic feedback circuit, receiving information
several sources including the cerebral cortex. The basal
ganglia feed this information back to the cortex, via the
thalamus.
Work in tandem with the pyramidal and limbic systems.
It consist of five pairs of nuclei:
o Caudate nucleus
o Putamen
o Globus pallidus
o Subthalamic nucleus
o Substantia nigra
BLOOD SUPPLY TO THE BASAL GANGLIA & INTERNAL
CAPSULE
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TERMINOLOGIES USED TO
DESCRIBE THE BASAL
GANGLIA
NEUROLOGICAL STRUCTURE
BASAL NUCLEI
Corpus striatum
Amygdala
Claustrum
Neostriatum
Paleostriatum
Caudate nucleus
Lentiform nucleus
Caudate nucleus + lentiform nucleus
Amygdaloid nucleus
Claustrum
Caudate nucleus + putamen
Globus pallidus
Caudate nucleus
Globus pallidus + putamen
PRIMARY FUNCTION OF THE BASAL GANGLIA
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The internal capsule and basal ganglia are supplied by the
perforating arteries of the MCA & ACA
The BG primarily supplied by the lenticulostriate arteries,
which are branches of the middle cerebral artery.
Note that the lenticulostriate arteries are prone to
hemorrhage in patients with uncontrolled hypertension.
IC is supplied either by the lateral lenticulostriate arteries
of the MCA or the recurrent artery of Heubner of the ACA.
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These nuclei are grouped into broader clusters:
o Striatum, consists of:
 Dorsal striatum (neostriatum)
 caudate nucleus
 Putamen
 Ventral striatum:
 Nucleus accumbens
 Olfactory tubercle
this part of striatum is considered
part of the limbic system
o Globus Pallidus
 Globus Pallidus interna
 Globus Pallidus externa
o Subthalamic Nucleus
o Substancia Nigra
Control conscious and proprioceptive movements. It
receives signals from the cortex, weighs those signals, and
determines what actions to “disinhibit
Serves to fine-tune the voluntary movements.
It does so by receiving the impulses for the upcoming
movement from the cerebral cortex, which they process
and adjust.
Then convey their instructions to the thalamus, which
then relays this information back to the cortex.
Ultimately, the fine-tuned movement instruction is sent to
the skeletal muscles through the tracts of the pyramidal
motor system.
Basal ganglia mediate some and other higher cortical
functions well: planning and modulation of movement,
memory, eye movements, reward processing, and
motivation.
OVERVIEW
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The basal ganglia are one of the components in the neural
chain that controls the voluntary motor activity
The supreme component of this chain is the CEREBRAL
CORTEX.
o Generates the commands that define the motor
activity of all skeletal muscles in the body.
These commands descend through the pathways of the
pyramidal system and synapse with the cranial nerve
nuclei and motor neurons of the spinal cord. From here,
the motor commands travel via the cranial and spinal
nerves in order to reach the target muscles.
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However, some extent of modulation and refinement of
these cortical signals is necessary so that their motor
execution at the muscular level happens as smoothly and
precisely as planned.
These adjustments are performed in the “accessory motor
centers”, with the most important one being the basal
ganglia
Despite being physically separated from each other, the
basal ganglia are interconnected with many pathways
making them a strong functional unity.
Functionally, the basal ganglia are referred to as the
extrapyramidal motor system although this term
nowadays is not used widely.
The BG receive and process the inputs from wide areas of
the cerebral cortex, after which they relay it back to the
thalamus.
The thalamus then forwards those refined inputs further
across the brain, mainly back to the cortex, and to the
brainstem
Phylogenetically, the oldest motor centers:
o Spinal cord
o Reticular formation of the brainstem
With the development of the vertebrates, the brain
gained new motor centers which grew together with the
cerebral cortex:
o Paleostriatum (globus pallidus)
o Neostriatum (caudate nucleus and putamen)
Over time, the cerebral cortex and pyramidal system grew
larger and developed a myriad of functional properties.
With this, the extrapyramidal system fell under the
control of the new, pyramidal, motor system, being left
with the autonomy to control the nuances of cortical
activity, i.e. to modulate the movements
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SPINY NEURONS
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A complex nucleus located deep in subcortical structures
of the forebrain, inside the insular lobe.
Composition:
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component of the basal ganglia and usually,
it is this part that is called “striatum” in the
literature, when we describe the basal
ganglia
The dorsal striatum (or simply the striatum)
consists of two parts
 Caudate nucleus
 Putamen
VENTRAL STRIATUM
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DORSAL STRIATUM
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The ventral striatum is considered part of the
limbic system
The parts of striatum are separated by THE INTERNAL
CAPSULE, whose myelinated fibers radiate through
striatum, giving it a characteristic striped appearance.
Together with the GLOBUS PALLIDUS, the striatum forms
a structure called CORPUS STRIATUM
The striatum is the main input unit of the basal ganglia.
It receives excitatory glutamatergic inputs from the
cerebral cortex, whose synapsing pattern reflects the
topography of the cortex.
This means that the caudal parts of the cortex project to
the caudal part of the striatum, while the rostral parts of
the cortex project to the rostral part of the striatum.
The projection neurons are covered by numerous spines,
hence their name.
Functionally, they are inhibitory neurons that use GABA as
a neurotransmitter.
The axons of these neurons form the direct and indirect
pathways of basal ganglia, which project into the globus
pallidus and substantia nigra.
INTERNEURONS
STRIATUM
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The substance of the striatum is mainly (80-95%)
composed:
o Projection neurons (medium-sized spiny neurons)
o Minor interneurons
The interneurons of the striatum lack spines and are
classified into four groups:
o Cholinergic large aspiny neurons
o Parvalbumin-containing GABAergic neurons
o Somatostatin/nitric oxide synthase-containing
GABAergic neurons
o Calretinin-containing GABAergic
These neurons project to the thalamus, SNc, cerebral
cortex, and control the activity of those regions
CAUDATE NUCLEUS
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An elongated C-shaped nucleus.
Position:
o Anterior to the thalamus
o Lateral to the lateral ventricles
o Medial to the internal capsule
Parts of caudate nucleus:
o Head
o Body
o Tail
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THE FUNCTIONS:
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Integrates sensory information about the spatial
position of the body and according to that, it sends
the information about the necessary fine tunes of
the motor response to that stimuli to the
thalamus.
o Contributes to body and limb posture and the
speed and accuracy of directed movements.
Besides motor control, the caudate nucleus is involved in
many tasks, such as memory, goal-pursuit, learning,
language processing, emotions, etc.
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Both structures are not involved in the movements
regulation.
They play an important role in the “reward circuit”.
Referred to as “limbic-motor interface”.
When we do anything rewarding (e.g. food, drugs, sex),
dopamine neurons in an area of the brain called the
VENTRAL TEGMENTAL AREA (VTA) are activated.
These neurons project to the nucleus accumbens and the
olfactory tubercle, and when they are activated it results
in an increase in dopamine levels
PUTAMEN
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The putamen and globus pallidus are separated by a thin
layer of white matter called the medial medullary lamina.
The main function of the putamen is to regulate motor
functions and influence various types of learning and it
employs dopamine to perform its functions
GLOBUS PALLIDUS
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Paired subcortical structure, situated medially to the
putamen.
Composed of inhibitory GABAergic projection neurons,
which fire spontaneously and irregularly at high
frequency.
It is divided by a vertically placed sheet of white matter,
the medial (internal) medullary lamina, into external
(GPe) and internal (GPi) segments.
NUCLEUS ACCUMBENS ANF OLFACTORY TUBERCLE
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Paired structures at the base of the forebrain.
They are components of the ventral striatum and
component of input nuclei for the ventral tegmental area
(VTA).
The NUCLEUS ACCUMBENS is found in the rostral
forebrain, where the head of the caudate nucleus and
putamen meet.
The OLFACTORY TUBERCLE, however, is situated ventral
to the nucleus accumbens, between the optic chiasm and
olfactory tract.
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The SUPERIOR and MEDIAL aspects of the globus pallidus
are in contact with the INTERNAL CAPSULE.
The capsule separates the caudate nucleus from the
globus pallidus.
FUNCTION OF THE GLOBUS PALLIDUS
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The INFERIOR SURFACE of the globus pallidus is in
contact with:
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SUBTHALAMIC NUCLEUS
ZONA INCERTA
Which separate it from the thalamus.
(ZI) is a horizontally elongated region of gray
matter in the subthalamus below the
thalamus. Its connections project extensively
over the brain from the cerebral cortex down
into the spinal cord
ANTERIORLY, it is closely related to the SUBSTANTIA
INNOMINATA and the HYPOTHALAMUS.
More CAUDALLY, it is in close proximity to the OPTIC
TRACT.
Because the putamen and globus pallidus are in close
connection, with their combined shapes resembling a
bean, they are referred to as the LENTICULAR NUCLEUS
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Both the GPe and GPi play an essential role in the
modulation of the motor program:
o Direct pathway
o Indirect pathway
Both receive inhibitory GABA-ergic input from the
striatum, through striatopallidal fibers, also known as
Wilson’s pencils
Fibers that project from the striatum to the internal part
of the globus pallidus are part of the indirect pathway of
the motor loop.
Fibers that connect the striatum with the external part of
the globus pallidus are part of the direct pathway of the
motor loop.
The output fibers of the globus pallidus are the
PALLIDOTHALAMIC TRACTS.
They divide into:
o Ansa lenticularis
o Lenticular fascicles
o Thalamic fasciculus.
Together they are part of the Forel’s field
These structures are responsible for connecting the
globus pallidus and thalamic nuclei.
The globus pallidus is involved in the constant subtle
regulation of movement to create smooth and precise
motor actions and has a primarily inhibitory action that
balances the excitatory action of the cerebellum
SUBTHALAMIC NUCLEUS
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Also known as Luys’ bodies.
Small biconvex paired structures located within the
subthalamus.
Not an anatomical part of the basal ganglia.
However, given their functional connection, the
subthalamus is listed as a functional part of the basal
ganglia.
Lies at the junction of the diencephalon and midbrain,
ventral to the thalamus and ventro-lateral to the red
nucleus.
Anteriorly it’s bordered by the substantia nigra and
medially by the internal capsule.
STN is closely related to the Forel’s fields and the
pallidothalamic fibers, which entwine around its ventral
and medial borders before arching back over its
dorsomedial surface as the thalamic fasciculus.
These fibers thus tend to separate the zone incerta from
the subthalamic nucleus below and the thalamus above.
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PARS RETICULATA
o Lie ventral to pars compacta.
o larger than the pars compacta, but it contains fewer
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The subthalamic nuclei are composed of excitatory
glutamatergic projection neurons
It receives excitatory inputs from the frontal cortex in a
somatotopically organized manner.
Its three parts are:
o DORSAL (motor) PART
 receives inputs from the primary motor
cortex
o VENTROLATERAL (associative) PART
 receives inputs from the prefrontal cortex
and frontal eye fields
o VENTROMEDIAL (limbic) PART
 Receives inputs from the anterior cingulate
cortex.
The function of the subthalamic nucleus is unknown, but
some theories suggest its crucial role in the hyperdirect
pathway in order to modulate the planned motor
program.
Additionally, considering the nucleus firing pattern, the
subthalamic nucleus is considered the “pace-maker” of
the basal ganglia
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cells than it
Medially to the substantia nigra is a zone called the
VENTRAL TEGMENTAL AREA. It is a small group of
scattered cells that have similar functions to the
pars compacta and may really be considered as an
extension of this part.
Serves mainly as an input, conveying signals from
the basal ganglia to the thalamus.
CONNECTIONS
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The MAJOR EFFERENTS (outputs) OF THE BASAL
GANGLIA consist of the neurons that project towards
the thalamus and brainstem from the internal part of
globus pallidus and the reticular part of the substantia
nigra. These are ANSA LENTICULARIS and LENTICULAR
FASCICULUS.
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Afferents (inputs) to the basal ganglia include the
following:
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FROM THE ENTIRE CEREBRAL CORTEX
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FROM THE SUBSTANTIA NIGRA
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SUBSTANCIA NIGRA
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A small motor nucleus, within the
anterior part of the midbrain.
Located between the cerebral
peduncle and tegmentum of the
midbrain.
Despite its location in the midbrain,
function-wise it is considered part
of basal ganglia.
The substantia nigra consists of two parts with very
different connections and functions:
o pars compacta (SNc)
o pars reticulata (SNr).
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Comprises the dorsal portion of the substantia
nigra.
Consists of numerous closely packed melanin-filled
neurons that give the substantia nigra its distinctive
dark color.
Serve mainly as an output to the basal ganglia
circuit, supplying the striatum with dopamine,
through specific D1 and D2 neurons within the
nigrostriatal pathways.
The loss of dopamine neurons in SNc is believed to
be the reason for the development of Parkinson's
disease and some other parkinsonic syndromes
Fibers arising in the pars compacta of the
substantia nigra reach the striatum, forming
the nigrostriatal connections.
This very important connection of the basal
ganglia ensures a continuous supply of
dopamine to the striatum, which promotes
the regulation of direct, indirect and
hyperdirect pathways.
FROM THE THALAMUS
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PARS COMPACTA
Through the corticostriatal pathway
 Largest afferent connection of the
basal ganglia.
The fibers are glutamatergic
 Releasing the neurotransmitter
glutamate to excite the striatal
neurons.
Fibers from the thalamus to the basal ganglia
form the thalamostriatal connections or the
thalamostriatal afferents.
Those connections or pathways are
glutamatergic and responsible for excitatory
effects on the cerebral cortex and brainstem.
FROM THE RETICULAR FORMATION OF THE
BRAINSTEM (specifically from the midbrain)
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Afferents from the reticular formation are
noradrenergic and responsible, besides vital
functions, for modulation and regulation of
flexor and extensor muscles tonus in
voluntary movements
SUMMARY
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In summary, the basal nuclei can be grouped functionally
into four categories:
o INPUT NUCLEI: striatum and subthalamic nucleus,
which receive cortical inputs
o OUTPUT NUCLEI: internal part of globus pallidus
and reticular part of substantia nigra, which project
outside the basal ganglia to the thalamus and
brainstem
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CONNECTING NUCLEUS: external part of globus
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pallidus, which connects the input nuclei to the
output nuclei.
MODULATORY NUCLEUS: compact part of
substantia nigra, which modulates the activity of the
basal ganglia
INDIRECT PATHWAY
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MOTOR & NON-MOTOR LOOPS OF THE BASAL
GANGLIA
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Inhibits motor activity.
The dorsal striatal neurons expressing the D2-family of
dopamine receptors are inhibited by dopamine from the
SN.
These D2R neurons send inhibitory GABAergic
connections to the GPe.
The GPe inhibits the subthalamic nucleus, which excites
the GPi/SNpr, completing the “Indirect pathway.”
The GPi sends inhibitory GABAergic signals to the VA and
VL of the thalamus.
Activation of the “Indirect pathway” results in stimulation
of the GPi which then inhibits the VA/VL of the thalamus,
ultimately inhibiting movement.
NET RESULT: reduction of movement - hyperactive in
hypokinetic disorders.
DIRECT PATHWAY:
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Comprised of inhibitory projections from the caudate or
putamen (STRIATUM).
Activity in the “Direct pathway” releases or “disinhibits”
motor movement by inhibiting the GPI/SNpr.
DOPAMINE, via the nigrostriatal pathway, synapses on
striatal neurons that express the D1-family of receptors.
The excite D1R neurons send inhibitory projections to
inhibit the GPi/SNpr.
The GPi/SNpr has GABAergic inhibitory neurons that
project to the VA and VL of the thalamus both of which
send excitatory projections to the motor cortex.
In this way, the “Direct pathway” inhibits the GPi which in
turn is no longer inhibiting the VA and VL of the thalamus.
As a result, the VA/VL are said to be “disinhibited” and can
excite the motor cortex to promote movement through its
corticospinal projections.
END RESULT: Movement
MODULATION OF THE BG
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The neuronal circuits that modulate the function of the
basal ganglia are:
o Nigrostriatal pathway
o Thalamostriatal pathway
NIGROSTRIATAL PATHWAY serves as a basal ganglia
input and modulates the direct and indirect pathways
o The substantia nigra pars compacta (SNpc) sends
dopamine to the striatum via this pathway.
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The striatum has a population of neurons that are
excited by dopamine because they express the D1family of dopamine receptors as part of the “Direct
pathway.”
o Therefore, dopamine will depolarize this population
of striatal neurons and increase activity in the
“Direct pathway.”
The striatum also has a population of neurons, the
“Indirect pathway” which are inhibited by dopamine as
they express the D2-family of dopamine receptors.
Dopamine will hyperpolarize this population and inhibit
the “Indirect pathway.”
The SNpc amplifies the response of the basal ganglia by
exciting the direct and inhibiting the indirect.
A tonic release of dopamine preferentially modulates the
D1 receptor, and this is a powerful modulator of the
nigrostriatal pathway.
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movements on only one side of the body, that affect
only the proximal muscles of a limb
BALLISMUS: the equivalent to the
hemiballismus, with the difference that it
affects the entire body. It is the most
extreme type of dyskinesia.
TICS:
 Brief, stereotyped semi-voluntary
movements, which mean that unlike other
movement disorders, they are partially
suppressible.
 Can be either motor (motor tics) or sounds
(vocal tics).
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CLINICAL SIGNIFICANCE
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Disorders of the basal ganglia are classified into two
categories:
o Hypertonic-hypokinetic
o Hypotonic-hyperkinetic
HYPERTONICITY is an abnormal increase of the muscle
tone in response to passive stretch.
o When the indirect pathway of the basal ganglia is
stimulated, it sends signals to the motor cortex and
brainstem, which ultimately inhibit muscle tone.
o Following a lesion of the basal ganglia, this
inhibitory influence is lost and hypertonicity is
manifested contralateral to the side of the lesion.
DYSKINESIA is a presence of the unintentional
purposeless movements & classified as:
o Hypokinesia
o Hyperkinesia
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HYPERTONIC-HYPOKINETIC DISORDERS
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These disorders result from the degeneration of the
neurons that form the direct pathway. Since this is the
pathway that serves for the planning of the movement,
the problems that patients will have been presented in
two form
o BRADYKINESIA that is presented with slow
movement
o AKINESIA is presented with the inability to move at
all because the individual is unable to plan or to
direct a movement toward a desired position or
target.
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PARKINSON’S DISEASE is the most
HYPOTONIC-HYPERKINETIC DISORDERS
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These disorders are caused by a disturbance of the
indirect loop that causes a loss of the inhibition of the
thalamic neurons, which ultimately results in
excess cortical activity and movement.
o TREMOR: rhythmic, low amplitude
movement that may be manifested as the
nodding of the head, or in the hands and
feet.
o CHOREA: sequence of rapid involuntary
movements involving mostly the hands and
feet, the tongue, and facial muscles.
o HEMIBALLISMUS: patient exhibits
involuntary ballistic (violent striking)
Common in children and can appear as the
result of direct brain injury (ex. head trauma
or encephalitis).
 Most of them are idiopathic and are part of
the spectrum of Gilles de la Tourette
syndrome or another idiopathic tic
disorder.
DYSTONIA is characterized by involuntary,
sustained muscle contraction that leads to
abnormal postures of the neck, toes, hands,
or other parts of the body.
MYOCLONUS:
 jerky, involuntary, and usually
arrhythmic movement.
 To imagine how myoclonus
looks like, think of body jerks
as one is falling asleep, this is physiological
myoclonus.Motor & Vocal Tic
prevalent disorder associated with basal
ganglia.
o Result of the degeneration of the
dopaminergic neurons of the
pars compacta of the substantia
nigra.
o This is actually the place of origin of the
nigrostriatal pathway that is essential for the
promotion of the direct pathway of the basal
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ganglia.
Because of its damage, the excitation of the
supplementary motor area which is of key
significance for the movement planning is lost.
ADDITIONAL NOTES
NIGROSTRIATAL PATHWAY
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Pars compacta of the substancia nigra produce
DOPAMINE that act on the dopaminergic receptors found
in the striatium - D1 and D2 receptors.
D1 receptors once stimulated -> activate the direct
pathway (DP)
D2 receptors once stimulated -> activate the indirect
pathway (IP)
DOPAMINE VS ACETYLCHOLINE EFFECTS
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DOPAMINE ==> activates the DP and inhibits IP
ACETYLCHOLINE ==> activates the IP and inhibits the DP
PARKINSON'S DISEASE:
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ACETYLCHOLINE > DOPAMINE
o since pars compacta cells undergo degeneration,
hence, cannot produce dopamine
o this leads to BRADYKINESIA
 slowness of movement and they are less
able to execute or perform the motor plans