The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson’s disease, we have only limited information about its cellular composition and organizational principles. In this review, we describe recent advances in our understanding of the diversity in the molecular profile, anatomy, physiology, and corresponding behavior during movement of GPe neurons. Importantly, we attempt to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, we provide an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus. In conclusion, the GPe has emerged as a crucial node in the basal ganglia  macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

This article reviews the literature on neuron diversity in the GPe. The discovery of novel cellular markers revealed that the heterogeneity in GPe neurons’ anatomical and electrophysiological properties observed in early studies could be correlated to their molecular signatures. We have only begun to understand the cellular makeup of the rodent GPe. An important next step is to refine the classification schemes that have been developed and to identify the specific inputs and outputs of distinct GPe neuron classes. We can begin to accomplish this goal using currently available genetic tools, including the Npas1 mouse line developed by our group. The identification of novel cell-specific markers will undoubtedly continue to shape future research. Neuronal diversity is an emerging theme in the basal ganglia (Kreitzer, 2009; Tepper et al., 2010; Antal et al., 2014; Barrot, 2014; Poulin et al., 2014; Anderegg et al., 2015; Xiao et al., 2015), and it will be important to fully understand how distinct classes of GPe neurons are integrated into large-scale the full text here: