Medium Spiny Neuron - Direct Pathway Within The Basal Ganglia

Direct Pathway Within The Basal Ganglia

The direct pathway within the basal ganglia makes excitatory inputs coming from e.g. the cortex cause a net excitation of upper motor neurons in the motor areas of the cortex. In the direct pathway, the medium spiny neurons project to the internal division of the globus pallidus which in turn sends axons to the substantia nigra pars reticulata (SNpr) and the ventroanterior and ventrolateral thalamus (VTh). The SNpr projects to the deep layer of the superior colliculus thus controlling fast eye movements (saccades). The VTh projects to upper motor neurons in the primary motor cortex (precentral gyrus).

Neurons in the globus pallidus are also inhibitory, thus inhibiting the excitatory neurons in the SNpr and VTh. But in contrast to the medium spiny neurons, globus pallidus neurons are tonically active when not activated. Thus in the absence of cortical stimulation, SNpr and VTh neurons are tonically inhibited thus preventing involuntary spontaneous movements.

Once the medium spiny neurons receive sufficient excitatory cortical input, they are excited and fire a burst of inhibitory action potentials to globus pallidus neurons. These tonically active neurons are then inhibited, causing their inhibitory influence on SNpr and VTh to decline. Thus SNpr and VTh neurons are disinhibited resulting in net excitement causing them to activate upper motor neurons commanding a movement. Cortical activation of the basal ganglia thus eventually results in excitement (disinhibition) of motor neurons causing movement to take place.