Based on their relationship to the internal medullary lamina, a Y-shaped band of myelinated axons coursing rostrocaudally through the thalamus, the thalamic nuclei are classified into four groups: (1) anterior, (2) medial, (3) lateral, and (4) intralaminar (eFig. 9.30). In addition to this structural categorization, the nuclei are divided into three major functional classes: (1) relay, (2) intralaminar, and (3) reticular. As mentioned earlier, most of the thalamus is composed of relay nuclei, which have reciprocal excitatory connections with the cortex. Relay nuclei are further subdivided into specific and nonspecific based on their projections to specific areas of the primary sensory and motor cortex or more diffuse cortical projections. The majority of specific relay nuclei reside in the lateral thalamus—in fact, all sensory modalities, with the exception of olfaction, have relays in the lateral thalamus before reaching their primary cortical target. Details of specific and nonspecific nuclei and their cortical connections can be reviewed in eTable 9.4.