Basal Ganglia Flashcards
What is hyperkinesia?
Excessive movement, abnormal uncontrolled muscular activity
Hypokinesiw?
Diminished or abnormally slow movement
What is disinhibition?
Loss of inhibition
What are the components of the basal ganglia?
Striatum (STR):
- Caudate
- Putamen
• Globus pallidus:
-External (lateral) part (Gpe/GPL) -Internal (medial) part (Gpi/GPM). The GPi is the chief output nucleus.
- Subthalamic nucleus (Sth)
- Substantia Nigra (SN):
- Pars reticulata (SNr)
- Pars compacta (SNc)
Whats the function of the basal ganglia?
The basal ganglia participates in the initiation and control of voluntary movement
- Basal ganglia receives excitatory(+) input from the cerebral cortex
- Outputs of the thalamus to the cortex are generally excitatory (+)
- Inhibitory inputs into the thalamus from the basal ganglia (−) serve to dampen thalamic excitation of cortical neurons.
Whats the role of the substantia Nigra?
• Substantia Nigra (SN):
- Pars reticulata (SNr)
- Pars compacta (SNc)
• Dopamine released from the SNc regulates the function of the basal ganglia
Where is dopamine released from?
- Dopamine released from the SNc acts on the D1 and D2 receptors in striatum.
- Dopamine released from the SNc indirectly regulates movements through its action on the basal ganglia.
What are the significance of the basal ganglia pathways?
- The thalamus and, indirectly, the frontal cortex are under tonic inhibitory control from GPi.
- This inhibition is modulated by two parallel basal gangliar pathways that originate in and diverge from the Striatum:
- one direct pathway
- one indirect pathway
What is the direct pathway?
The direct pathway disinhibits the thalamus resulting in the facilitation of movement
What is the indirect pathway?
The indirect pathway inhibits the thalamus which diminishes movement
Whats the purpose of the direct pathway?
The direct pathway causes excitation of the motor regions of the cortex and thus facilitates movement
Communication is via two neurotransmitters
- Glutamate (+)
- GABA(-)
How does the direct pathway perform?
- Cortex excites the striatum
- Striatal output inhibits activity of the GPi
- Inhibition the GPi increases thalamic activity (disinhibition of thalamus)
- Thalamic activity excites the cortex to facilitate movement
What is the function of the indirect pathway?
The indirect pathway inhibits the thalamus which diminishes movement.
• Communication is via two neurotransmitters
- Glutamate (+)
- GABA (-)
What is the mechanism of action of the indirect pathway?
- Cortex excites the striatum
- Striatum inhibits the activity of the GPe
- Inhibition of GPe releases the Sth to be active
- Increased output from the Sth excites the GPi
- Excitation of the GPi inhibits the thalamus
- Decreased thalamic output decreases cortical excitation, thereby diminishing movement
What is the modulatory role of dopamine?
- SNc via dopamine further modulates movement
- SNc facilitates movement via excitation of striatal cells (D1R)- direct pathway
- SNc facilitates movement via inhibition of striatal cells (D2R) – indirect pathway
What does research suggest of modulatory dopamine?
- Current research suggests that the dopamine excites the direct pathway but inhibits the indirect pathway.
- Therefore, the action of dopamine is to facilitate movement because it excites the facilitatory (direct) pathway via its excitatory effects on dopamine D1 receptors in regions of the neostriatum that project to the medial pallidal segment, while inhibiting the inhibitory (indirect) pathway that projects initially to the lateral pallidal segment because of the inhibitory effects on dopamine D2 receptors in the neostriatum.
- The result is diminution of excitation from the glutamate-mediated pathway arising from the subthalamic nucleus to the medial pallidal segment.
• In this manner, the overall output of the medial pallidal segment to the
thalamus is diminished, resulting in greater excitation of the motor
regions of cortex, the consequence of which is an increase in movement
What happens when either direct or indirect pathways?
Disturbed activity of direct (D) and indirect (I) circuits yield either hypo- or hyperkinesis