Our current understanding of thalamocortical (TC) circuits is largely based on studies investigating so-called 'specific' thalamic nuclei, which receive and transmit sensory-triggered input to specific cortical target areas. TC neurons in these nuclei have a striking point-to-point topography and a stereotyped laminar pattern of termination in the cortex, which has made them ideal models to study the organization, plasticity, and development of TC circuits. However, despite their experimental importance, neurons within these nuclei only represent a fraction of all thalamic neurons and do not reflect the diversity of the TC neuron population. Here we review the distinct subtypes of projection neurons that populate the thalamus, both within and across anatomically-defined nuclei, with regard to differences in their morphology, input/output connectivity and target specificity, as well as more recent findings on their neuron type-specific gene expression and development. We argue that a detailed understanding of the biology of TC neurons is critical to understand the role of the thalamus in normal and pathological perception, voluntary movement, cognition and attention
