Connections of basal ganglia

Component	Afferent connections from	Efferent connections to
Corpus striatum	1. Thalamic nuclei to caudate nucleus and putamen 2. Cerebral cortex to caudate nucleus and putamen 3. Substantia nigra to putamen 4. Subthalamic nucleus to globus pallidus	1. Thalamic nuclei 2. Subthalamic nucleus 3. Red nucleus 4. Substantia nigra 5. Hypothalamus 6. Reticular formation (most of the fibers leave from globus pallidus)
Substantia nigra	1. Putamen 2. Frontal lobe of cerebral cortex 3. Superior colliculus 4. Mamillary body of hypothalamus 5. Medial and lateral lemnisci 6. Red nucleus	1. Putamen
Subthalamic nucleus of Luys	1.Globus pallidus	1. Globus pallidus 2. Red nucleus

FUNCTIONS OF BASAL GANGLIA

The basal ganglia form the part of extrapyramidal system, which is concerned with motor activities. The various functions of basal ganglia are:

Fig.31. Afferent connections of corpus striatum.

Fig.32. Efferent and intrinsic connections of corpus striatum.

1. Control of voluntary motor activity

The 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, this controlling mechanism is lost and so 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. The neuronal circuits arise from three areas of the cerebral cortex.

1. Premotor area,

2. Primary motor area and

3. Supplementary motor area

All these nerve fibers from cerebral cortex reach the caudate nucleus. From here, the fibers go to putamen. Some of fibers from cerebral cortex go directly to putamen also. Putamen sends fibers to globus pallidus. The fibers from here run towards the thalamus, subthalamic nucleus of Luys and substantia nigra. The subthalamic nucleus and substantia nigra are in turn, projected into thalamus. Now, the fibers from thalamus are projected back into the primary motor area and the other two motor areas, i.e. premotor area and supplementary motor area.

Fibers between cerebral cortex and caudate nucleus are concerned with regulation of conscious movements known as the cognitive control of activity. The cortical fibers reaching putamen are directly concerned with control of subconscious execution of some movements during performance of trained motor activities, i.e. skilled activities.

2. Control of muscle tone

Gamma motor neurons of spinal cord are responsible for maintaining the tone of muscles, which is important for posture. The tone of the muscle also depends on actions of muscle spindle. Gamma motor neurons, muscle spindle and muscle tone are all controlled by basal ganglia especially substantia nigra. In the lesion of basal ganglia, tone increases leading to rigidity.