In this article we will discuss about:- 1. Meaning of Basal Ganglia 2. Connections of Basal Ganglia 3. Function.
Meaning of Basal Ganglia:
i. It is composed of nuclear groups present in subcortical region.
ii. Like cerebellum, this ganglia is involved in muscular activity.
iii. Differences between cerebellum and basal ganglia are:
a. Cerebellum receives proprioceptive impulses from peripheral part of body but basal ganglia have no proprioceptive inputs.
b. Cerebellar impulses coming to cerebral cortex come mainly to motor and premotor areas but influences from basal ganglia go to almost all areas of cerebral cortex.
c. Many nuclei in brainstem are influenced by cerebellum whereas influence of basal ganglia on brainstem is limited.
Basal ganglia is composed of the following nuclei:
1. Caudate nucleus
2. Putamen
3. Globus pallidus
4. Red nucleus
5. Substantia nigra
6. Subthalamic nucleus (body of Luys)
Caudate nucleus and putamen together is called striatum.
Putamen and globus pallidus together is known as lentiform nuclei.
Gobus pallidus is composed of two different parts namely globus pallidus externa and globus pallidus interna.
Substantia nigra is also made up two parts—pars reticulata and pars compacta.
Connections of Basal Ganglia:
There is a closed circuit connection: There is a lot of mutual influence between various nuclei of basal ganglia.
Striatum receives afferent inputs from motor and premotor areas of cerebral cortex. From striatum impulses, efferent impulses reach pars reticulata of substantia nigra. Efferent impulses from striatum also reach globus pallidus.
i. Striatum to substantia nigra—striatonigral pathway.
ii. Striatum to globus pallidus—striatopallidal pathway.
From pars compacta region, efferent connection going to striatum is known as nigrostriatal pathway. The neurotransmitter released by this pathway is dopamine. From globus pallidus, impulses go to subthalamic nucleus. Impulses will also come back to globus pallidus (efferent) from subthalamic nucleus. Also, efferent impulses from subthalamic nucleus will reach pars reticulata of substantia nigra.
From globus pallidus, impulses (efferent) come to centromedian, ventroanterior and ventrolateral nuclei of thalamus. From centromedian nucleus, efferent impulses reach striatum. From ventrolateral, ventroanterior and centromedian nuclei impulses are relayed back to motor and pre-motor areas of cerebral cortex. So whole of basal ganglia connection is like a closed loop (Fig. 9.42).
The neurotransmitters involved in basal ganglia activity are (Fig. 9. 43):
1. Dopamine
2. GABA
3. Glutamate
4. Substance P
5. ACh
One of the most important neurotransmitters is dopamine.
Functions of Basal Ganglia:
Planning and programming of movements:
Cortico- striato-pallido-thalamo-cortical tract (which is a closed loop circuit) brings impulses from motor and premotor cortex to nuclei of basal ganglia and after processing are relayed back to motor and premotor cortex.
Because of this connection, there is proper coordination of muscle activity at every step of movement. But unlike cerebellum, basal ganglia cannot function as servo comparator; because there are no afferent inputs to nuclei of ganglia from different parts of body.
Controls subconscious-associated movements, like swinging of arm while walking; nodding of head during conversations; various facial expressions; gestures; and modulation of voice, etc. Caudate nucleus is especially important for this function; it facilitates subconscious-associated movement.
Provides postural background for any movements that is during movements it is not only particular joint that has to take part directly in intended movement, but also other joints must be held in proper position to facilitate smooth movement. Globus pallidus has important role to play for this activity.
Regulation of muscle tone:
Basal ganglia influences can reach certain area in brainstem and alter activity of neurons present in red nucleus, reticular formation, etc. From these areas, impulses are sent to lower motor neurons through rubrospinal and reticulospinal tracts to regulate the muscle tone.
In lower animals in which motor cortex is not developed, basal ganglia performs role of motor cortex. And hence it has important role to play in all voluntary movements.
Conditions associated with problems in different nuclei or connections of basal ganglia dysfunction:
i. Parkinsonism
ii. Ballism
iii. Hemiballism
iv. Huntington’s chorea
v. Athetosis
Parkinsonism:
Nigrostriatal pathway is important in this aspect. The neurotransmitter released by this pathway is dopamine. If this tract is not functioning properly, it leads to Parkinsonism or paralysis agitans or Parkinson’s disease.
Features of Parkinsonism:
1. Hypertonia:
Increase in muscle tone in both flexor and extensor group of muscles. There will be an abnormal increase of muscle tone, which gives rises to rigidity.
2. Rigidity:
Rigidity can be of two types, namely cogwheel rigidity and lead-pipe rigidity. In lead-pipe rigidity, during the course of passive movements rigidity is experienced throughout the course of movements without any release phenomenon, for e.g. flexion of elbow is not possible.
The whole arm will move as one piece. In cog-wheel rigidity, the resistance and release phenomenon alternate in installments and hence passive movements would be possible in steps. The abnormal increase of muscle tone both in flexor and extensor group of muscle makes it difficult to elicit deep reflexes.
3. Tremors:
Tremors are also seen, unlike in cerebellar region, there will be static/resting tremors. As long as person is at rest, fine tremors are more prominent in distal parts of body. But as soon as person starts some voluntary movement, tremors disappear. One of the ways tremors get manifested will be in the form of pill rolling movements (rubbing thumb against index finger).
4. Akinesia (bradykinesia) (Kinesia means movement):
These patients can hardly initiate any voluntary movement. Because of this, they continue to be in a particular posture for long hours. Upper and lower limb joints are usually kept in flexion— universal flexion.
5. Loss of automatic associated movements:
They will also lose automatic associated movements, like facial expressions, modulation of voice, bodily expressions, etc. (in the form of gestures, facial expressions). Hence, they shall have mask like face (expression less face). These patients try to walk in short steps as if they are trying to catch their center of gravity after every step.
Treatment:
Giving L-dopa, which is precursor of dopamine. Unlike dopamine, which cannot cross blood-brain barrier, L-dopa can cross blood-brain barrier and gets converted to dopamine in basal ganglia to exert its influence.
Nowadays, transplantation of a part of fetal adrenal medullary tissue is also being tried since from adrenal medulla there is secretion of dopamine. This would make treatment easier for prolonged durations. Future research will no doubt focus on this potential for human embryonic stem cells to play such a therapeutic role.