Medium Spiny Neuron - Indirect Pathway

Indirect Pathway

In the indirect pathway, excitatory (e.g. cortical input to the basal ganglia) results in net inhibition of upper motor neurons. In this pathway the medium spiny neurons in the striatum project to the external segment of the globus pallidus. These neurons in turn project to the internal segment of the globus pallidus and to the subthalamic nuclei which form a closed loop by projecting back to the internal globus pallidus.

Cortical excitement of medium spiny neurons causes them to inhibit external globus pallidus neurons. These tonically inhibiting neurons thus decrease their inhibitory influence on the internal globus pallidus and the subthalamic nuclei.

Let's first look at the internal globus pallidus neurons which are also tonically inhibiting VTh and SNpr neurons. Since the inhibitory influence from the external globus pallidus is now reduced, these neurons show stronger activity thus increasing their inhibition of SNpr and VTh neurons.

The projections of the external globus pallidus to the subthalamic nuclei causes these neurons to increase their firing rate, since the globus pallidus neurons are inhibited by medium spiny neurons. The subthalamic nuclei have excitatory projections to the internal globus pallidus thus causing the internal globus pallidus neurons to increase their inhibititory influence on SNpr and VTh.

Eventually excitatory inputs from the cortex results in net inhibition of upper motor neurons thus preventing them from initiating a movement.