The Thalamus

Abstract

The thalamus is the largest structure of the mammalian diencephalon. It comprises many nuclear groups, each concerned with transmitting characteristic afferent signals to specific areas of the cerebral cortex. The thalamus is often described as the gateway to the cerebral cortex. The term thalamus is a Greek word meaning "inner chamber." Its origin dates back to the 2nd century AD, when Galen traced the optic-nerve fibers to an oval mass closely associated with the ventricles. This part of the brain, known as the optic thalamus, was later defined as a large mass of gray matter involved with visual stimuli and in the processing of all sensory modalities except olfaction. The size of the thalamus is relatively small compared with that of the neocortex, but the functions of each major neocortical area largely depend on the interactions with well-defined thalamic cell groups. For this reason, an increase in size of any neocortical area is correlated with a corresponding increase in the related thalamic nuclei. The nomenclature of the different nuclear subdivisions of the primate thalamus is more complex than in rodents. Although the thalamic cytoarchitecture in monkeys and humans is relatively similar, the nomenclature used to define thalamic nuclei in simians and humans has diverged so much over the years that nonspecialist readers surely believe that the thalami of humans and monkeys are fundamentally different. The main difference between the human and non-human primate thalamus is the relative growth of specific thalamic nuclei relative to other nuclear subdivisions; the pulvinar is the most representative example of a thalamic nucleus that has overgrown in the human brain. This chapter provides an overview of the main features that characterize the anatomy, chemistry, and functional organization of the primate thalamus. For additional information, the reader is referred to other comprehensive reviews and compendiums (Steriade and Deschenes, 1984; Steriade and McCarley, 1990; Sherman and Koch, 1998; Kultas-Ilinsky and Ilinsky, 2001; Sherman and Guillery, 2001; Pinault, 2004; Jones, 2007).