Fine structure of the basal ganglia

Question 1

I. Basics
1. What is basal ganglia?

Answer 1

Basal ganglia is a group of nuclei located below the cerebral cortex hidden inside the white matter

Question 2

I. Basics
2. What are The main nuclei belonging to the basal ganglia are the?

Answer 2

1. Corpus striatum = caudate nucleus + putamen
2. Globus pallidus (“pallidum”) consists of the Globus pallidus externus (GPe) and Globus pallidus internus (GPi)
3. Nucleus accumbens – important in addiction and reward mechanism, so the positive feedback of our action
4. Substantia innominata – Here we have the basal nucleus (of Meynert) and the nucleus of the diagonal tract (of Broca). These give the cholinergic innervation to the brain

Question 3

I. Basics
3. What is the function of basal ganglia?

Answer 3

Clinically, the most important function of the basal ganglia is the coordination of involuntary movements

Question 4

II. Pathways
1. What are the 2 types of pathways

Answer 4

1. Direct pathway (circuit) – enhancement of movement
2. Indirect pathway (circuit) – Inhibitory effect on movement

Question 5

II. Pathways
2. Describe Direct pathway (circuit) – enhancement of movement

Answer 5

• Excitatory neurons goes from the VA/VL (thalamus) to the premotor cortex (cerebral cortex) (Glutamate)
• The Corticostriatal pathway is excitatory (glutamate) going from the premotor cortex to the striatum (putamen and caudate nucleus)
• The striatum has inhibitory neurons that goes to the GPi with GABA as its neurotransmitter
• In the GPi we also have inhibitory neurons that goes to the VA/VL, finishing the circuit
• If we want to make a movement, (+) input from the cortex goes to the striatum. Here (-) input goes to the GPi, inhibiting the inhibitory neurons found here. Therefore, the inhibitory neurons of the GPi will not inhibit the VA/VL, so we get an excitation of the VA/VL which will enhance the movement

Question 6

II. Pathways
3. Describe Indirect pathway (circuit) – Inhibitory effect on movement

Answer 6

1. Excitation in the cortex goes to the striatum
2. Inhibitory neurons will inhibit the inhibitory neurons in the GPe, exciting the
   excitatory neurons in the subthalamic nuclei
3. This excitation goes to the GPi, which will inhibit the VA/VL through the
   inhibitory neurons found here
4. So, it will inhibit the signal from VA/VL to the cortex, so it cannot excite the
   premotor cortexreduce the movement

Question 7

II. Pathways
1. Explain the nigrostriatal pathway

Answer 7

Substantia nigra is located in the midbrain (tegmentum) containing neuromelanin because it produces dopamine
- Dopaminergic neurons in the substantia nigra will go to the striatum, terminating on:
+) The GABA (+ substance P) (direct pathway)
+) The GABA (+ Enkephalin) (indirect pathway)
=> This is called the nigrostriatal pathway
- GABA (+ substance P) containing neurons will express D1-R on their surface.
GABA (+ Enkephalin) containing neurons will express D2-
R on their surface. These are dopaminergic receptors
- Dopamine will always have a positive effect on the
movement
- On the D1-R it will have a excitatory effect, and on the
D2-R it will have a inhibitory effect
+) By the mentioned pathways, D1-Receptors will enhance the movement
+) D2-Receptors will inhibit the inhibitory indirect pathway and excite the excitatory direct pathway