Topic 16: Basal Ganglia

What are the 5 major nuclei of the basal ganglia?

1. Caudate nucleus

2. Putamen nucleus

3. Globus pallidus nucleus

4. Subthalamic nucleus (diencephalon)

5. Substantia nigra (midbrain)

Which three nuclei make up the "Corpus Striatum"?

Caudate, Putamen, and Globus Pallidus.

What is the "Striatum" or "Neostriatum"?

The caudate + putamen; it is the major input portion of the basal ganglia.

What is the "Lentiform Body" (Lenticular nucleus)?

Putamen + Globus Pallidus.

What are the two parts of the Globus Pallidus?

GP Externum (lateral) and GP Internum (medial).

Which part of the basal ganglia is the major output nucleus?

Globus Pallidus Internum (GPi).

What neurotransmitter does the Substantia Nigra use?

Dopamine (in the nigrostriatal tract).

Loss of dopaminergic neurons in the substantia nigra leads to what condition?

Hypokinesia in Parkinson's disease.

How is basal ganglia output sent to the motor cortex?

Indirectly via the ipsilateral thalamus (VA and VL nuclei).

What is the effect of basal ganglia output (from GPi) on the thalamus?

Inhibits the thalamus, decreasing excitation of the motor cortex.

What is the role of the Direct Pathway in the basal ganglia?

Striatum → GPi → Thalamus → Motor Cortex.

Reduces inhibition on the thalamus, allowing more movement ("Green Light Pathway").

What is the role of dopamine in the Direct Pathway?

Dopamine excites the direct pathway, facilitating movement.

What is the role of the Indirect Pathway in the basal ganglia?

Striatum → GPe → Subthalamic Nucleus → GPi → Thalamus.

Increases inhibition on the thalamus, suppressing movement ("Red Light Pathway").

What is the effect of dopamine on the Indirect Pathway?

Dopamine inhibits the indirect pathway, reducing braking and facilitating movement.

What effect does the Subthalamic Nucleus have on the Globus Pallidus Internum?

Excitatory effect, which increases inhibition of the thalamus and motor cortex.

What effect does the Globus Pallidus Externum have on the Subthalamic Nucleus?

Inhibitory effect.

How does the Substantia Nigra interact with the Striatum?

Via the nigrostriatal tract; dopamine can excite the direct pathway and inhibit the indirect pathway, overall facilitating movement.

In summary, how does dopamine affect the thalamus?

Dopamine disinhibits the thalamus by exciting the direct pathway and inhibiting the indirect pathway.

What is the Hyperdirect Pathway in the basal ganglia?

A direct connection from the motor cortex to the subthalamic nucleus, providing a rapid "braking" mechanism.

What is the main input nucleus of the basal ganglia?

The Striatum (caudate + putamen); it receives excitatory input from the ipsilateral motor cortex.

What is the major output nucleus of the basal ganglia?

The Globus Pallidus Internus (GPi); it sends inhibitory projections to the thalamus (VA/VL).

How does basal ganglia output affect the motor cortex?

GPi inhibits the thalamus, decreasing excitation of the ipsilateral motor cortex.

Through which thalamic nuclei does basal ganglia output reach the motor cortex?

VA and VL nuclei of the thalamus.

What is the function of the Direct Pathway?

Facilitates movement by disinhibiting the thalamus, allowing more excitation of the motor cortex ("Green Light Pathway").

What is the sequence of the Direct Pathway?

Striatum → GPi → Thalamus (VA/VL) → Cortex.

How does dopamine affect the Direct Pathway?

Dopamine excites the direct pathway via D1 receptors, promoting movement.

What is the function of the Indirect Pathway?

Suppresses movement by increasing inhibition of the thalamus ("Red Light Pathway").

What is the sequence of the Indirect Pathway?

Striatum → GPe → Subthalamic Nucleus → GPi → Thalamus → Cortex.

How does dopamine affect the Indirect Pathway?

Dopamine inhibits the indirect pathway via D2 receptors, reducing braking and promoting movement.

What is the role of the Subthalamic Nucleus in basal ganglia circuitry?

Excites the GPi, increasing inhibition of the thalamus and motor cortex.

What is the role of the Globus Pallidus Externus (GPe) in the indirect pathway?

GPe inhibits the Subthalamic Nucleus.

What is the Nigrostriatal Circuit?

Dopaminergic input from the substantia nigra pars compacta to the striatum; facilitates movement by exciting the direct pathway and inhibiting the indirect pathway.

What is the net effect of dopamine on basal ganglia function?

Dopamine disinhibits the thalamus, overall facilitating movement.

What is the Hyperdirect Pathway?

A fast pathway where the motor cortex projects directly to the Subthalamic Nucleus, providing rapid braking by increasing inhibition of the thalamus.

How does loss of dopamine in Parkinson's disease affect basal ganglia circuits?

Causes excessive inhibition of the thalamus, leading to bradykinesia, rigidity, resting tremor, and postural instability.

What is the sequence of the Direct Pathway?

Cortex (E) → Striatum (E) → GPi (I) → Thalamus (disinhibited, E) → Cortex (E).

What is the effect of cortical input on the striatum in the Direct Pathway?

Excitatory (glutamate).

What is the effect of the striatum on the GPi in the Direct Pathway?

Inhibitory (GABA).

What is the effect of the GPi on the thalamus in the Direct Pathway?

Inhibitory; but inhibition of GPi = disinhibition of thalamus.

What is the overall effect of the Direct Pathway on motor output?

Facilitates movement (less braking, excitatory net effect).

What is the sequence of the Indirect Pathway?

Cortex (E) → Striatum (E) → GPe (I) → Subthalamic Nucleus (E) → GPi (I) → Thalamus (inhibited, ↓E) → Cortex (↓E).

What is the effect of the striatum on the GPe in the Indirect Pathway?

Inhibitory (GABA).

What is the effect of the GPe on the Subthalamic Nucleus in the Indirect Pathway?

Inhibitory; inhibition of GPe disinhibits the Subthalamic Nucleus.

What is the effect of the Subthalamic Nucleus on the GPi in the Indirect Pathway?

Excitatory (glutamate).

What is the effect of the GPi on the thalamus in the Indirect Pathway?

Inhibitory; increased inhibition reduces thalamic excitation of motor cortex.

What is the overall effect of the Indirect Pathway on motor output?

Inhibits movement (more braking, net inhibitory).

How does dopamine affect the Direct Pathway?

Excites striatal D1 receptors → increases inhibition of GPi → disinhibits thalamus → facilitates movement.

How does dopamine affect the Indirect Pathway?

Inhibits striatal D2 receptors → decreases inhibition of GPe → more GPe inhibition on STN → less excitation of GPi → disinhibits thalamus → facilitates movement.

What is the overall effect of dopamine on basal ganglia output?

Disinhibits the thalamus → promotes movement.

What is the role of the Subthalamic Nucleus in basal ganglia circuitry?

Provides excitatory input to the GPi, which increases inhibition of the thalamus.

What is the effect of subthalamic nucleus activation on motor output?

Inhibits movement (via increased GPi inhibition of thalamus).

What is the sequence of the Hyperdirect Pathway?

Cortex (E) → Subthalamic Nucleus (E) → GPi (I) → Thalamus (↓E) → Cortex.

What is the purpose of the Hyperdirect Pathway?

Provides a fast "brake" on movement by strongly exciting the GPi, which inhibits the thalamus.

What is the overall effect of the Hyperdirect Pathway on motor output?

Rapid suppression of movement (strong inhibition).