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Nervous System
proximal limbs. The basal ganglia are thought to be par-
ticularly important in starting, stopping, and monitor-
ing movements executed by the cortex, especially those
that are relatively slow and sustained, or stereotyped.
They also help to regulate the intensity of these move-
ments, and they act to inhibit antagonistic or unneces-
sary movements. The function of the basal ganglia is
not limited to motor functions; they also are involved in
cognitive and perceptual functions.
Functional Properties of The Basal Ganglia
The structural components of the basal ganglia include
the caudate nucleus, putamen, and globus pallidus.
3,32
They are located lateral and caudal to the thalamus,
occupying a large portion of the interior of both cerebral
hemispheres. The caudate and putamen are collectively
referred to as the
striatum,
and the putamen and globus
pallidus form a wedge-shaped region called the
lentiform
nucleus.
Two other structures, the
substantia nigra
of
the midbrain and the
subthalamic nucleus
of the dien-
cephalon, are considered part of the basal ganglia (Fig.
36-11). The dorsal part of the substantia nigra contains
cells that synthesize dopamine and are rich in a black
pigment called
melanin.
The high concentration of mela-
nin gives the structure a black color, hence the name
sub-
stantia nigra.
The axons of the substantia nigra form the
nigrostriatal pathway,
which supplies dopamine to the
striatum. The subthalamic nucleus lies just below
the thalamus and above the anterior portion of the sub-
stantia nigra. The glutaminergic cells of this nucleus are
the only excitatory projections to the basal ganglia.
The basal ganglia have input structures that receive
afferent information from the cerebral cortex and
thalamus, internal circuits that connect the various struc-
tures of the basal ganglia, and output structures that
deliver information to other brain centers. The neostria-
tum represents the major input structure for the basal
ganglia. Virtually all areas of the cortex and afferents
from the thalamus project to the neostriatum. The output
areas of the basal ganglia, including the lateral globus pal-
lidus, have both ascending and descending components.
The major ascending output is transmitted to thalamic
nuclei, which process all incoming information that is
transmitted to the cerebral cortex. Descending output is
directed to the midbrain, brain stem, and spinal cord.
The output functions of the basal ganglia are mainly
inhibitory. Looping circuits from specific cortical areas
pass through the basal ganglia to modulate the excit-
ability of specific thalamic nuclei and control the corti-
cal activity involved in highly learned, automatic, and
stereotyped motor functions. The most is known about
the inhibitory basal ganglia loop that is involved in
modulating cortical motor control. This loop regulates
the release of stereotyped movement patterns that add
efficiency and gracefulness to cortically controlled move-
ments. These movements include inherited patterns that
add precision, efficiency, and balance to motion, such as
the swinging of the arms during walking and running
and the highly learned automatic postural and follow-
through movements of throwing a ball or swinging a bat.
The basal ganglia also have a cognitive function in
that they monitor sensory information coming into the
brain and apply it to information stored in memory as a
means of planning and sequencing motor movements.
3
The cognitive control of motor activities determines,
subconsciously and within seconds, which patterns of
movement will be needed to achieve a goal. The caudate
nucleus, which receives large amounts of input from the
association areas of the brain, plays a major role in the
cognitive control of motor activity.
Basal Ganglia–Associated Movement
Disorders
Disorders of the basal ganglia comprise a complex
group of motor disturbances characterized by tremor
and other involuntary movements, changes in posture
and muscle tone, and poverty and slowness of move-
ment. They include tremors and tics, hypokinetic disor-
ders, and hyperkinetic disorders
1
(Table 36-1).
Unlike disorders of the motor cortex and corticospi-
nal (pyramidal) tract, lesions of the basal ganglia disrupt
movement but do not cause paralysis. The various types
of involuntary movements often occur in combination
and appear to have a common underlying cause. Recent
studies indicate that hypokinetic and hyperkinetic dis-
orders can be explained as specific disturbances in the
indirect and direct pathways that link the basal ganglia
with the thalamocortical motor circuit.
32
Accordingly,
overactivity of the indirect pathway relative to the direct
pathway would result in hypokinetic disorders such as
Parkinson disease, and underactivity of the indirect
pathway would result in hyperkinetic disorders such as
chorea and ballismus.
Caudate
nucleus
Putamen
Basal
ganglia
Caudate nucleus
Lateral ventricle
Putamen
Cortex
Globus pallidus
Subthalamic nucleus
Substantia nigra
Thalamus
A
B
FIGURE 36-11.
(A)
Lateral view of the basal ganglia.
(B)
Coronal section showing the basal ganglia in relation to
surrounding structures.
