Thalamus and Prethalamus Cell Types and Their Interconnections

Abstract The interconnected neurons of the thalamus and prethalamus can be divided into 3 basic categories: (1) glutamatergic thalamic projection neurons that innervate the cortex and prethalamic reticular nucleus (TRN), and often other forebrain regions such as the striatum, and amygdala; (2) GABAergic intrinsic interneurons that form synaptic connections only within their nucleus of origin; and (3) GABAergic TRN neurons that are reciprocally connected with each thalamic nucleus. As briefly reviewed in this chapter, these 3 basic categories of neurons can be further subdivided based on gene and protein expression, developmental origin, soma size, dendritic morphology, physiological properties, axonal projection patterns, and synaptic connections. Nonetheless, the fundamental circuit patterns formed between these 3 categories of neurons are shared across thalamic nuclei. For example, intrinsic interneurons form dendrodendritic synaptic connections with projection neurons and interneurons in tightly entwined synaptic arrangements termed glomeruli, while TRN neurons are connected to projection neurons via reciprocal axonal projections to and from each thalamic nucleus. A description of these 3 essential thalamic elements sets the stage for the subsequent chapters of this book that describe wide-ranging features that underlie the dynamic interplay between the thalamus and the cortex.