Basal Ganglia

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.