First and Higher Order Thalamic Relays

Abstract

A major aspect of the function of a thalamic relay is the nature of information being relayed. Thus, the function of the lateral geniculate nucleus can largely be described a relaying retinal information to cortex. That is, identification of the driver (i.e., information bearing) input to a thalamic relay largely defines that relay’s function. Identification of driving inputs to many thalamic nuclei reveal that there are two types: one that emanates from a subcortical source (e.g., retinal input to the lateral geniculate nucleus) and another that emanates from layer 5 of cortex (e.g., much or most of the pulvinar). The lateral geniculate nucleus is an exemplar of a first order thalamic relay, because it represents the first relay of a type of information (e.g., visual) to cortex, whereas the pulvinar is a higher order thalamic relay because it relays information already in cortex between cortical areas. We refer to the latter circuit as transthalamic. Examples of first order relays are the lateral geniculate nucleus (for vision), the ventral posterior nucleus (for somatosensation), and the ventral division of the medial geniculate nucleus (for hearing); the respective higher order relays are the pulvinar, posterior medial nucleus, and dorsal division of the medial geniculate nucleus. Other first and higher order thalamic relays are described, and the significance of the newly appreciated transthalamic pathways is discussed.