Indirect pathway The
basal ganglia functions to tonically inhibit movement, mainly in the absence of
motor cortex command, via
GABAergic inhibition of the
ventral lateral nucleus and
ventral anterior nucleus of the
thalamus, as well as the
superior colliculus and
mesopontine tegmentum of the brain stem. When movement is required, the
cerebral cortex sends commands to the
striatum, which directly inhibits the
medial globus pallidus and
substantia nigra pars reticulata, decreasing thalamus and brainstem inhibition. As the pathway from the
striatum to the
medial globus pallidus is monosynaptic (containing one
synapse), it is called the
direct pathway. The
indirect pathway, which contains the GPe and the
subthalamic nucleus, functions to modulate the effects of the direct pathway. The GPe acts as an inhibitory "control device", adjusting subthalamic nucleus neuronal activity via GABAergic output. When movement adjustment is required, striatal inhibitory
GABAergic axons are sent to the GPe, decreasing inhibition of the
subthalamic nucleus. The
subthalamic nucleus'
glutamatergic neurons then stimulate the GPi and
substantia nigra pars reticulata. This
multisynaptic indirect striatopallidal pathway allows for regulated excitatory input from the subthalamic nucleus to the GPi and
substantia nigra pars reticulata. This combines with direct pathway inhibition in the GPi, allowing for fine tuned basal ganglia output, and more controlled movement. == Related pathology ==