Short Essay on the Circuitry of the Basal Ganglia



The afferents to the corpus striatum come from the cerebral cortex (corticostriate and corticopallidal), the thalamus (thalamostriate and thalamopallidal) and the substantia nigra (nigrostriate, nigropallidal).

Most the afferents reach the neostriatum, which gives a robust projection to the globus pallidus. The efferents arise mostly from the globus pallidus (pallidofugal) and go the thalamus, the substantia nigra, the red nucleus and the midbrain reticular formation.

Physiologically, the connections of the corpus striatum are best understood in terms of functional circuits or loops. While considering the circuits, it is observed that (i) the corpus striatum does not have any direct connections with the spinal cord - either afferent or efferent, (ii) the globus pallidus internus (GPi) and the substantia nigra pars reticulata (SNpr) behave as the lateral and medial parts of a single functional entity (GPi/SNpr).

(1) The main connection of the corpus striatum is cortex -> striatum GPi/SNpr thalamus SMA. This circuit has two inhibitory neurons: the one running from striatum to the globus pallidus internus (GPi) and the other, from globus pallidus to thalamus. Thus, the striatum disinhibits the thalamus, and the effect of the entire circuit is excitatory. The part of the circuit that passes through the caudate nucleus is called the caudate loop while the part which passes through the putamen of the striatum is called the putamen loop.

(2) There is another indirect pathway via the sub-thalamic nucleus. This circuit contains three inhibitory neurons: one from the striatum to GPe, another from GPe to sub-thalamic nucleus, and the third from GPi to thalamus. Thus, the striatum disinhibits the subthalamic nucleus (STN). The STN in its turn stimulates GPi, which then increases its inhibitory output to the thalamus. The overall effect of the circuit is therefore inhibitory.

(3) The third circuit involving the substantia nigra pars compacta (SNpc). The SNpc is inhibited by the striatum through the striatonigral pathway. The SNpc in turn exerts both a facilitatory as well as an inhibitory effect on the striatum. The facilitatory effect is exerted on excitatory striatopallidal pathway that goes directly to the GPi, while the inhibitory effect is exerted on the inhibitory striatopallidal pathway that reaches the GPi through the GPe and STN.