Basal Ganglia Pathways

• Cortex increases the inhibition power of the striatum on the GPi and substantia nigra

• Blocking inhibition on the thalamus originates from the GPi and substantia nigra

• The thalamus stimulates the cortex

Indirect Pathway

• Cortex → Striatum → GPe → STN → GPi → Thalamus

  • Glutamate (+) is involved in the Cortex to Striatum and STN to GPi steps
  • GABA (-) is involved in the Striatum to GPe, and GPe to STN steps

• Glutamate (+) is an important component of this pathway

Considerations

• Direct pathway utilizes GABA and Substance P (SP) to enhance movement

• Indirect pathway uses GABA and Enkephalin (ENK) to diminish movement

• D1 receptors are excitatory and stimulate dopamine production

• D2 receptors inhibit dopamine production

• Dopamine stimulates the direct pathway while inhibiting the indirect pathway

• Hypokinetic disorders are related to insufficient direct pathway activity, coupled with excessive indirect pathway activity

• Hyperkinetic disorders are related to excessive direct pathway activity with insufficient indirect pathway activity

• Parkinson's Disease involves the loss of the substantia nigra and dopamine projection

• Damage to the corticospinal tract is contralateral

  • Damage to the right side of the motor brain results in tremors on the left side

• Reduction in dopamine from the substantia nigra and D1/D2 receptors leads to increased inhibition strength of the thalamus

Symptoms

• Motoric symptoms include tremor, bradykinesia, rigidity, and loss of postural reflex
• Depression is linked to all motoric symptoms
• Dementia is correlated with decreased dopamine levels
• Resting tremor manifests as "pills rolling"
• Bradykinesia causes gait issues
• Loss of postural reflex is associated with mild tremor and bradykinesia
• Rigidity involves cogwheel rigidity and increased muscle tone