The striatum subthalamic nucleus substantia nigra globus pallidus( striatum, subthalamic nucleus, substantia nigra, globus pallidus ) is the main input region for the basal ganglia.

Dorsal striatum: Caudate nucleus subthalamic nucleus substantia nigra pars reticulata( Caudate nucleus, subthalamic nucleus, substantia nigra pars reticulata ) + putamen globus pallidus internal substantia nigra pars compacta( putamen, globus pallidus internal, substantia nigra pars compacta )
Ventral striatum: Nucleus accumbens Caudate nucleus Subthalamic nucleus( Nucleus accumbens, Caudate nucleus, Subthalamic nucleus ) + olfactory tubercle putamen globus pallidus external( olfactory tubercle, putamen, globus pallidus external )

Medium spiny neurons with D1 and D2 receptors can be found in the striatum globus pallidus subthalamic nucleus substantia nigra( striatum, globus pallidus, subthalamic nucleus, substantia nigra ).

D1 neurons project to the direct indirect hyperdirect( direct, indirect, hyperdirect ) pathway.
D2 neurons project to the indirect direct hyperdirect( indirect, direct, hyperdirect ) pathway

Medium spiny neurons in the striatum show a preference for the contralateral ipsilateral( contralateral, ipsilateral ) side.

In the classical model the globus pallidus is believed to be the relay amplifier supplier of feedback inhibition( relay, amplifier, supplier of feedback inhibition ) of the striatum.

The substantia nigra pars reticulata substantia nigra pars compacta subthalamic nucleus( substantia nigra pars reticulata, substantia nigra pars compacta, subthalamic nucleus ) and the globus pallidus internal globus pallidus external striatum( globus pallidus internal, globus pallidus external, striatum ) are the main output structures of the basal ganglia.

Output typically consists of GABAergic dopaminergic( GABAergic, dopaminergic ) projections to the thalamus cerebellum brain stem( thalamus, cerebellum, brain stem ).

The term "convergence of information" means that there are less more( less, more ) neurons in regions further down the processing pathway.

Movement initiation:

There is a sharp increase decrease( increase, decrease ) in the firing rate of medium spiny neurons in the striatum globus pallidus subthalamic nucleus( striatum, globus pallidus, subthalamic nucleus ).
This leads to a sharp decrease increase( decrease, increase ) in the firing of output neurons in the substantia nigra pars reticulata substantia nigra pars compacta subthalamic nucleus( substantia nigra pars reticulata, substantia nigra pars compacta, subthalamic nucleus ).
This in turn releases inhibition in target neurons, e.g. the superior colliculus thalamus globus pallidus( superior colliculus, thalamus, globus pallidus ) for eye movement.

Parkinson's disease is characterized by the loss of dopaminergic GABAergic( dopaminergic, GABAergic ) neurons in the substantia nigra pars compacta substantia nigra pars reticulata striatum( substantia nigra pars compacta, substantia nigra pars reticulata, striatum ).

This leads to a lack of excitation inhibition( excitation, inhibition ) of the direct pathway and a lack of inhibition excitation( inhibition, excitation ) on the indirect pathway, which results in slow fast( slow, fast ) movements.
Another result is hyperactivity hypoactivity( hyperactivity, hypoactivity ) in the subthalamic nucleus.