The striatum is the largest structure of the basal ganglia, receiving synaptic input from multiple regions including the neocortex, thalamus, external globus pallidus, and midbrain. Earlier schemes of striatal connectivity presented a relatively simple architecture which included primarily excitatory input from the neocortex, dopaminergic input from the midbrain, and intrastriatal connectivity between projection neurons and a small number of interneuron types. In recent years this picture has changed, largely due to the introduction of new experimental methods to reveal cell types and their connectivity. The striatal microcircuit is now considered to consist of several newly defined neuron types which are intricately and selectively interconnected. New afferent pathways have been discovered, as well as novel properties of previously known afferents such as the midbrain dopaminergic inputs. In this review we aim to provide a summary of these recent discoveries.
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