Basal Ganglia Pathways: Direct vs Indirect

Overview of Basal Ganglia

• Collection of Interconnected Structures
  • Subcortical structures below the cortex
  • Includes:
    • Caudate
    • Putamen (Caudate + Putamen = Striatum)
    • Globus Pallidus (Internis & Externus)
    • Subthalamic Nucleus
    • Substantia Nigra
• Function: Coordinates movement using excitatory and inhibitory interneurons
  • Glutamate = excitatory
  • GABA = inhibitory

Pathways

Direct Pathway

• Initiates movement
• Pathway Flow:
  1. Motor Cortex sends excitatory signals (Glutamate) to Striatum
  2. Striatum sends inhibitory signals (GABA) to Globus Pallidus Internis and Substantia Nigra
  3. Globus Pallidus Internis & Substantia Nigra send inhibitory signals to Thalamus
  4. Thalamus sends excitatory signals back to Motor Cortex
• Net Effect: Turn on Motor Cortex (disinhibition of Thalamic control of motor planning)

Indirect Pathway

• Terminates movement
• Pathway Flow:
  1. Motor Cortex sends excitatory signals to Striatum
  2. Striatum sends inhibitory signals to Globus Pallidus Externus
  3. Globus Pallidus Externus sends inhibitory signals to Subthalamic Nucleus
  4. Subthalamic Nucleus sends excitatory signals to Globus Pallidus Internis and Substantia Nigra
  5. Globus Pallidus Internis & Substantia Nigra send inhibitory signals to Thalamus
  6. Thalamus sends excitatory signals back to Motor Cortex
• Net Effect: Turn off Motor Cortex (activating inhibition)

Key Differences

• Direct Pathway: Disinhibits thalamic control, promotes motor activation
• Indirect Pathway: Enhances inhibition via the Substantia Nigra, suppresses motor activation
• Substantia Nigra:
  • Direct Pathway: Inhibition of Substantia Nigra leads to disinhibition
  • Indirect Pathway: Activation of Substantia Nigra promotes inhibition

Mnemonics for Remembering Pathways

• Direct Pathway: MSIT (Motor Cortex, Striatum, Internis, Thalamus)
• Indirect Pathway: MSIT + N (Add in Externus and Subthalamic Nucleus)
• Signal Nature:
  • Excitatory: First and last parts (M & T)
  • Inhibitory: Middle parts (S & I), except in indirect pathway where E is inhibitory, N is excitatory

Study Tips

• Memorize the flow using mnemonics
• Understand the role of glutamate (excitatory) and GABA (inhibitory)
• Practice writing out the pathways and their components

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These notes summarize the basal ganglia pathways, providing a clear distinction between the direct and indirect pathways, their components, functions, and how to remember them effectively.