The way I understand it, the substantia nigra pars compacta has a lot of
dopaminergic neurons that go to the striatum (caudate and putamen) that have
a lot of GABAergic neurons that go to the globus pallidus interna that has a
load of GABAergic neuron that go to the thalamus which goes to the motor
cortex, so exiting the substantia nigra pars compacta has an inhibitory
effect on the cerebral motor cortex. So, if the substantia nigra pars
compacta degenerates, as in Parkinson's, the striatum is less excited and
sends less inhibition to the motor cortex.
NOT TRUE! Parkinson's is too much inhibition of the cortex, rigidity and
slowness of movement, not too much movement (as is caused by giving them
levodopa). So somehow, killing the dopaminergic projections from the
substantia nigra pars compacta to the striatum must INCREASE striatal
inhibition of the motor cortex. Where is my logic failing me?