Neuroanatomy Draw It To Know It

18. Basal Ganglia 305

Direct & Indirect Pathways: Circuitry

■ Th e direct pathway:

• Th e cortex excites the striatum


• Th e striatum inhibits the combined globus pallidus
  internal segment and the substantia nigra reticulata
  (GPi/STNr)


• Th e GPi/STNr inhibits the thalamus (the thalamus
  excites the cerebral cortex)


• Th e direct pathway is overall excitatory
  ■ Th e indirect pathway:


• Th e cortex excites the striatum


• Th e striatum inhibits the globus pallidus external
  segment


• Th e globus pallidus external segment inhibits the
  GPi/STNr


• Th e GPi/STNr inhibits the thalamus (the thalamus
  excites the cerebral cortex)


• Th e indirect pathway is overall inhibitory


• A parallel circuit also exists within the indirect
  pathway that passes through the subthalamic
  nucleus in which the globus pallidus external
  segment inhibits the subthalamic nucleus and the
  subthalamic nucleus excites the GPi/STNr


• Th is parallel indirect pathway through the
  subthalamic nucleus is also overall inhibitory
  ■ Dopamine from the substantia nigra compacta
  excites the direct pathway (via dopamine 1 receptors)
  and inhibits the indirect pathway (via dopamine 2
  receptors); both of these eff ects promote movement.

Cingulate sulcus

Cingulate gyrus

Corpus callosum

Lateral ventricle

Caudate nucleus

Insula

Lateral sulcus

Superior temporal:

Gyrus

Sulcus

Middle temporal gyrus

Inferior temporal:

Sulcus

Gyrus

Accumbens nucleus

Optic nerves

Entorhinal cortex

Parahippocampal gyrus

10 mm

Collateral sulcus

Diagonal band

Globus pallidus

Extreme capsule

Claustrum

External capsule

Internal capsule

Septal nuclei

Septum pellucidum

Putamen

FIGURE 18-1 Coronal anatomic section through the anterior region of the basal ganglia. Used with permission from the Michigan State University:
Brain Biodiversity Bank. https://www.msu.edu/∼brains/