Basal ganglia are the scattered masses of gray matter submerged in subcortical substance of cerebral hemisphere. Basal ganglia form the part of extra pyramidal system, which is concerned with motor
activities.
COMPONENTS OF BASAL GANGLIA
Basal ganglia include
three primary components:
1. Corpus striatum
2. Substantia nigra
3. Subthalamic nucleus of
Luys.
CORPUS STRIATUM
Corpus striatum is a mass
of gray matter situated at the base of cerebral hemispheres in close relation
to
thalamus. Corpus striatum
is incompletely divided into two parts by internal capsule:
i. Caudate nucleus
ii. Lenticular nucleus.
Caudate Nucleus
Caudate nucleus is an
elongated arched gray mass, lying medial to internal capsule. Throughout its
length, the caudate nucleus is related to lateral ventricle. Caudate nucleus
has a head portion and a tail portion Head is bulged into lateral ventricle and
situated rostral to thalamus. The tail is long and arched. It extends along the
dorsolateral surface of thalamus and ends in amygdaloid nucleus.
Lenticular Nucleus
Lenticular nucleus is a
wedgeshaped gray mass, situated lateral to internal capsule. A vertical plate
of white matter called external medullary lamina, divides lenticular nucleus into
two portions:
a. Outer putamen
b. Inner globus pallidus.
Putamen and caudate nucleus
are the phylogenetically newer parts of corpus striatum and these two
parts are together called neostriatum
or striatum. Globus pallidus is phylogenetically older
part of corpus
striatum. And, it is
called pallidum or paleostriatum. Globus
pallidus has two parts, an outer part and an
inner part.
SUBSTANTIA NIGRA
Substantia nigra is situated
below red nucleus. It is made up of large pigmented and small nonpigmented
cells. The pigment
contains high quantity of iron.
SUBTHALAMIC NUCLEUS
OF LUYS
Subthalamic nucleus is situated
lateral to red nucleus and dorsal to substantia nigra.
CONNECTIONS OF BASAL GANGLIA
In addition to afferent
and efferent connections, different components of corpus striatum of the same
side are
interconnected by intrinsic fibers.
1. Putamen to globus pallidus
2. Caudate nucleus to
globus pallidus
3. Caudate nucleus to
putamen.
Different components of
corpus striatum in each side are connected to those of the opposite side by
commissural
fibers.
FUNCTIONS OF BASAL GANGLIA
Basal ganglia form the
part of extrapyramidal system, which is concerned with
integration and regulation
motor activities. Various
functions of basal ganglia are:
CONTROL OF MUSCLE
TONE
Basal ganglia control the
muscle tone. In fact, gamma motor neurons of spinal cord are responsible for
development of tone in the muscles. Basal ganglia decrease the muscle
tone by inhibiting gamma
motor neurons through descending
inhibitory reticular system in brainstem. During the lesion of basal
ganglia, muscle tone
increases leading to rigidity.
CONTROL OF MOTOR
ACTIVITY
Regulation of
Voluntary Movements
Movements during voluntary
motor activity are initiated by cerebral cortex. However, these movements are controlled
by basal ganglia, which are in close association with cerebral cortex. During
lesions of basal ganglia, the control mechanism is lost and so the movements become
inaccurate and awkward.
Basal ganglia control the
motor activities because of the nervous (neuronal) circuits between basal
ganglia and other parts of the brain involved in motor activity. Neuronal
circuits arise from three areas of the cerebral cortex:
a. Premotor area
b. Primary motor area
c. Supplementary motor
area
All these
nerve fibers from cerebral cortex reach the caudate nucleus. From here, the
fibers go to putamen. Some of the fibers from cerebral cortex go directly to putamen
also. Putamen sends fibers to globus pallidus. Fibers from here run
towards the thalamus, subthalamic nucleus
of Luys and substantia nigra. Subthalamic nucleus
and substantia nigra are in turn, projected into thalamus.
Now, the fibers from thalamus are projected back into primary motor area and
other two motor areas, i.e. premotor area and supplementary
motor area.
3. CONTROL OF REFLEX MUSCULAR
ACTIVITY
Some
reflex muscular activities, particularly visual and labyrinthine reflexes are important in maintaining
the posture. Basal ganglia
are responsible for the coordination and integration of
impulses for these reflex
activities. During lesion
of basal ganglia, the postural movements, especially the visual and labyrinthine reflexes become
abnormal. These abnormal movements are associated with rigidity. Rigidity is
because of the loss of inhibitory influence from the
cerebral cortex on spinal cord via basal ganglia.
4. CONTROL OF
AUTOMATIC ASSOCIATED MOVEMENTS
Automatic associated movements are the
movements in the body, which take place along with
some motor activities. Examples are the swing
of the arms while walking, appropriate facial
expressions while talking or doing any work.
Basal ganglia are responsible for the automatic
associated movements.
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