Expansion
The direct pathway facilitates movement; the indirect pathway inhibits it
Expansion
- Direct pathway: cortex to striatum to internal globus pallidus, which inhibits the thalamus. Activating it disinhibits the thalamus, so it facilitates movement
- Indirect pathway: cortex to striatum to external globus pallidus to subthalamic nucleus to internal globus pallidus. Activating it inhibits movement
Dopamine from the substantia nigra pars compacta acts on D1 receptors to excite the direct pathway and on D2 receptors to inhibit the indirect pathway. Both effects therefore promote movement.
Disorders follow:
- Parkinson disease: loss of nigral dopamine, so the direct pathway is underactive and the indirect overactive, giving a hypokinetic picture of bradykinesia, rigidity and tremor
- Huntington disease: loss of striatal neurones of the indirect pathway, giving a hyperkinetic picture of chorea
- Hemiballismus: a lesion of the subthalamic nucleus, part of the indirect pathway, causing violent flinging movements of the opposite limbs
Deep brain stimulation of the subthalamic nucleus works by modulating this circuit.