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The indirect pathway is a circuit in the basal ganglia hypothesized to play a role in the
inhibition of movement.
If you’re not already familiar with the direct pathway of the basal ganglia, you might
want to watch my video on the direct pathway before you watch this video.
The indirect pathway model involves GABA neurons that project from the external segment of
the globus pallidus to a region called the subthalamic nucleus.
These globus pallidus neurons typically exert an inhibitory effect on glutamate neurons
in the subthalamic nucleus, but when the indirect pathway is activated by signals from the cerebral
cortex, this causes the activation of GABA neurons in the striatum, which project to
the globus pallidus external and inhibit the activity of neurons there.
This keeps the globus pallidus external neurons from being able to inhibit neurons in the
subthalamic nucleus.
The subthalamic nucleus neurons are activated by projections from the cortex, and they stimulate
GABA neurons in the globus pallidus internal segment and substantia nigra pars reticulata.
These GABA neurons in turn project to the thalamus, inhibiting thalamic neurons that
travel to motor regions of the cerebral cortex to stimulate movement.
The inhibition of these thalamic neurons thus inhibits movement.
This activity in the indirect pathway is thought to antagonize the activity of the direct pathway
and act to keep unwanted movements from occurring.
Neurons from the substantia nigra pars compacta travel to the striatum via the nigrostriatal
pathway, and they can modulate the activity of the indirect pathway through dopamine release
in the striatum.
One effect of this seems to be the inhibition of activity in the indirect pathway, which
leads to the facilitation of movement.
This is thought to be one reason why dopamine depletion in disorders like Parkinson’s
disease may lead to difficulties initiating movement.