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The indirect pathway through the basal ganglia

Indirect pathway from the cortex through the basal ganglia towards the thalamus.

In short, it can be said that activity along the "Indirect pathway" involves a slowing of motor skills.

The "indirect pathway" from the cortex through the basal ganglia towards the thalamus can originate anywhere in the cerebral cortex but usually starts from the frontal and/or parietal lobes, runs via switching first in the striatum, then on over the outer part of the pale nucleus (GPe), then to the subthalamic nucleus (STN) to end in GPi-SNr .

The signals from the cerebral cortex to the striatum are excitatory (propulsive) and affect striatal inhibitory neurons (MSNs) equipped with dopamine receptors of type D2; here called D2 neuron. The D2 neuron, whose slowing activity is thus regulated from the cerebral cortex, contacts the GPe. The GPe neuron, which are inhibitory, in turn contacts the excitatory STN. The STN neuron, whose excitatory activity is driven by the cerebral cortex, contacts and stimulates the inhibitory GPi-SNr neurons. Finally, the GPi-SNr neuron slows down the activity of the thalamus, thereby reducing the signal flow from the thalamus to the cerebral cortex.

Three different activity patterns are conceivable along "The indirect path":

1/ The D2 neuron is in a state of rest. STN combines excitatory signals from the cerebral cortex and inhibitory signals from GPe, resulting in a moderate flow of excitatory signals to GPi-SNr. The result is a moderate brake on the thalamus and on the feedback to the cerebral cortex.

2/ The D2 neuron is excited by the cerebral cortex and slows down GPe's braking effect on STN. STN is disinhibited and increases the excitation on GPi-SNr, which in turn increases the inhibition in the thalamus. The result is reduced signal backflow to the cerebral cortex and suppressed motor skills.

3/ The situation is the same as in 2/, but!! here the D2 receptors are activated due to dopamine influence from the compact part of the black nucleus (SNc). The inhibitory action of the D2 neuron is attenuated. GPe's ability to slow down STN increases. The excitation of GPi-SNr decreases. The brake on the thalamus decreases. The signal flow to the cerebral cortex and motor skills recovers.

Normal activity along the "indirect pathway" thus means reduced signal flow from the thalamus to the cerebral cortex with a dampening effect on motor skills.

The nerve impulse flows along the indirect, direct and hyperdirect pathways are intertwined and coordinated from SNc.
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