Introduction to the Striatum

The striatum is a brain region within the basal ganglia comprised of three major components: the caudate, the putamen and the ventral striatum. While the caudate and putamen are located dorsal of the internal capsule, the ventral striatum rests on the ventral side of the internal capsule and itself contains the nucleus accumbens as well as the olfactory tubercle.[1]

Comprised primarily of GABAergic (and therefore inhibitory) medium spiny neurons (MSNs), the striatum receives the majority of its input from the neocortex in addition to components of the limbic system including the amygdala, hippocampus, and thalamus. As such, synaptic activity in the striatum modulates these inputs before sending the resultant signals to other brain regions. Aside from MSNs, which comprise ~95% of the neurons found in the striatum, this structure also contains cholinergic, fast-spiking interneurons which may play a significant role in the learning behaviors governed by the balance of reward and punishment.[2]

Signals leaving the striatum are divided into two categories: direct pathway and indirect pathway efferents. The direct pathway efferents are carried by MSNs which express D1-type dopamine receptors and primarily target the substantia nigra pars reticulata. These neurons carry the signals which evoke voluntary movements based on motivational information received from afferent inputs. Conversely, the indirect pathway efferents are carried by MSNs which express D2-type dopamine receptors and project to the globus pallidus where they function to inhibit involuntary movements and thereby permit the prioritization of actions to carry out motivated behaviors.