Histochemical identification and afferent connections of subdivisions in the lateralis posterior-pulvinar complex and related thalamic nuclei in the cat

Abstract Fiber pathways of the extrageniculate visual system have been shown in the cat to define an orderly sequence of adjoining, roughly parallel zones within the posterolateral thalamus. The present study provides evidence that these and related thalamic subdivisions can be identified histochemically by their differing contents of acetylthiocholinesterase. The brains of fifty cats were prepared for acetylthiocholinesterase histochemistry by the methods of Geneser-Jensen & Blackstad and Karnovsky & Roots. Serial sections were cut in each of the three Horsley-Clarke planes and a standard transverse cholinesterase series, containing occasional reference sections stained for myelin or Nissl substance, was used to assemble graphic reconstructions of the main cholinesterase subdivisions. Patterns of afferent connections were visualized by anterograde axon transport or fiber-degeneration techniques and recorded in thirteen sets of detailed chartings. In nine sets, serially adjoining sections processed by the cholinesterase and anterograde methods were directly compared and plotted together. The caudal part of the lateral thalamic region was divided into two main subdivisions: the lateralis posterior-pulvinar complex, which receives extrageniculate visual input, and the complex comprising the nuclei lateralis medialis and suprageniculatus (LM-Sg), which receives fiber projections from auditory association cortex and the deep layers of the superior colliculus. The latter complex occupies a ventromedial zone alongside the lateralis posterior-pulvinar complex. It is distinguished in the cholinesterase stain by prominent 0.5–1.5 mm wide patches of high enzyme activity that appear against a pale background. Within the lateralis posterior-pulvinar complex, three zones could be distinguished: (1) the medial part of the nucleus lateralis posterior (LPm) is rich in cholinesterase activity and receives input from the superficial collicular layers; (2) the lateral part of the nucleus lateralis posterior and the nucleus posterior of Rioch (LP1-NP) have much weaker cholinesterase activity and receive input from the striate cortex and other areas of visual cortex; (3) the pulvinar, the most lateral of the subdivisions, is a zone of high cholinesterase activity in which some pale patches appear. It receives input from the pretectum and is associated with a differentiated lateral marginal zone receiving direct projections from the retina and area 17. The intermediate nucleus of the lateral group was divided into two parts. The pars caudalis (LIc) has a mottled appearance in the cholinesterase stain; it receives an ascending input from the pretectotectal border zone and a descending projection from parietal cortex including area 7. The pars oralis of the intermediate nucleus (LIo) is rich in cholinesterase activity; it receives a descending projection from the anterior parietal cortex comprising in part area 5. These two intermediate subdivisions were flanked dorsally and rostrally by the nucleus lateralis dorsalis. The cholinesterase material suggested a division of the lateralis dorsalis into at least medial and lateral parts, but the connections of these regions were not analyzed in detail. The finding of clear chemoarchitectural subdivisions in the lateralis posterior-pulvinar complex and adjoining regions has practical significance as a guide to thalamic organization and raises the possibility that some extrageniculate and related transthalamic pathways may be differentiated from one another by the neurotransmitters they employ in the thalamus.