A comparative analysis of the morphology of corticothalamic projections inmammals

Recent anatomical tracing methods have revealed new principles underlying t he organization of corticothalamic connections in the mammalian nervous sys tem. These data demonstrated the distribution of two types of synaptic cont acts in the corticothalamic projection: small (<1 <mu>m) and giant (2-10 pm ) axon terminals. We compare the organization of corticothalamic projection s in the auditory, somatosensory, visual, and motor systems of a variety of mammalian species, including the monkey. In all these systems and species, both types of corticothalamic terminals have been observed. Small endings formed the major corticothalamic terminal field, whereas giant terminals we re less numerous and formed additional terminal fields together with small terminals. After comparing their spatial distribution, as well as the degre e of reciprocity between the corticothalamic and thalamocortical projection s, different roles are proposed for small and giant endings, Small terminal s are typically present in the projection serving the feed-back control of the cerebral cortex on the thalamic nucleus from which it receives its main projection. In contrast, giant terminals are involved in feed-forward proj ections by which activity from a cortical area is distributed, via the thal amus, to other parts of the cerebral cortex, The cross-species and cross-sy stems comparison reveals differences in the mode of feed-forward projection , which may be involved in the activation of other parts of the same cortic al area or form part of a projection that activates other cortical areas, ( C) 2001 Elsevier Science Inc.