Basal Ganglia

BASAL GANGLIA

• Scattered masses of gray matter 
• Submerged in sub-cortical substance of cerebral hemisphere.
• Part of extrapyramidal system, associated with motor activities.
• Constitute another accessory motor system
• Functions in close relationship with cerebral cortex & corticospinal motor control system

COMPONENTS OF BASAL GANGLIA
1. Corpus striatum – 

• Receives afferent inputs coming to basal ganglia.

Divided into 2 by internal capsule.

• Caudate nucleus
• Lenticular nucleus – 
• Divided into 2 by, 
• External medullary lamina – Vertical plate of white matter

Sub-divisions:

• Putamen
• Globus pallidus – Principle output nuclei & efferents arise here.

2. Subthalamic nucleus

3. Substania Nigra – 

• Principle output nuclei & efferents arise here.
• Divided into 2 – Pars compacta & pars reticulata

CONNECTIONS OF BASAL GANGLIA

1. Afferent connections:

• Stiratum(caudate & putamen)
• Receives afferents form cerebral cortex – Sensory, motor, association & limbic cortex.
• Also recieved from thalamus

2. Efferent connections:

• Globus pallidus & Substania Nigra – Major efferents arise
• Pathway – Efferent (GABAergic)→ thalamus→ prefrontal cortex→ premotor cortex→ supplementary motor area→ Primar motor cortex
• Discharges motor activities from 1º motor cortex region
• No direct connections between brainstem/spinal cord & basal ganglia.

3. Internuclear connections:

• Basal ganglia are richly connected with one another.

Connection 1:

• Dopaminergic nigrostriatal projection from “Substantia Nigra Pars Compacta” (SNPC)→ striatum (caudate nucleus & putamen).
• Corresponding GABAergic projection from stratium→ Substantia Nigra Pars Reticulata (SNPR).
• Dopamine – Major neurotransmitter in Substantia nigra

Connection 2:

• Inhibitory GABAergic projections from Striatum  → to both Globus Pallidus Inner Segment (IS) & External Segment (ES).

Connection 3:

• GABAergic projections from Globus pallidus ES→ Subthalamic nucleus & to Globus pallidus lS

Connection 4:

• Excitatory Glutamate-ergic projections from Subthalamic nucleus → Globus pallidus IS & ES

Connection 5:

NEUROTRANSMITTERS IN BASAL GANGLIA:

1. Dopaine –

• Neuromodulatory neurotransmitter.
• Used by SNPC→ selectively modulate areas of stratium(caudate/putamen).
• Excite via D1 receptors – Direct pathway.
• Inhibit via D2 receptors – Indirect pathway.

2. Glutamate

• Excitatory neurotransmitter
• From Subthalamic nucleus → Stimulate ventrolateral thalamus→ 1° localized motor cortex.

FUNCTIONS OF BASAL GANGLIA

1. In planning & programming of movement
2. In process of conversion of an abstract thought into voluntary action.
3. Putamen circuit – Complex motor activity & skilled movement.
4. For motor activity, final common pathway – Corticospinal tract
5. Part of extra-pyramidal system – Regulation of tone & posture.
6. Caudate nucleus – Major role in cognitive control of motor activity 

DISORDERS OF BASAL GANGLIA:

There are 3 distinct biochemical pathways of basal ganglia normally operating in balanced fashion- 

• Nigrostriatal dopaminergic system
• Intrastriatal cholinergic system
• GABAergic system
• Project from stratum to globus pallidus & substantia nigra
• Characteirsitics motor abnormalities occur with dysfucntion of these pathways.

1. Hyperkinetic:

• Excessive and abnormal movement
• Chorea, athetosis & ballism.

2. Hypokinetic:

• Paucity of movement
• Akinesia & Bradykinesia.