Convergence and branching patterns of round, type 2 corticopulvinar axons.
Corticopulvinar connections consist of at least two morphologically distinct subpopulations. In one subgroup (E, type 1), axons have an "elongated" terminal field and thin, spinous terminations; in the other (R, type 2), axons have a small, round arbor and large, beaded terminations. Previous work (Rockland, 1996) indicates that E-type axons from several occipitotemporal areas branch extensively within and sometimes between pulvinar subdivisions, but that R-type axons tend to have spatially delimited arbors. The present report is a further investigation of R-type axons from areas V1 and MT and was initiated to test the generality of the previous findings. There are four main results: 1) By serial section reconstruction of anterogradely labeled axons, 10 of 25 axons originating in area V1 had two or three spatially separate arbors (8 and 2 axons, respectively). Sixteen axons analyzed from area MT, however, all had single arbors, although the arbors were often formed by the convergence of widely separate branches. 2) Multiple (at least 2-5) R-type corticopulvinar axons, from V1 or from MT, can converge in a single focus. 3) R-type axons originating from both areas V1 and MT can branch to other structures; namely, the superior colliculus, the pretectal area, and/or the reticular nucleus of the thalamus. 4) Finally, corticopulvinar terminations from area V1 are predominantly R-type, whereas those from MT are more predominantly E-type. These results thus provide additional evidence of the special relationship of area V1 to the pulvinar. They also emphasize that the idea of corticopulvinocortical "feedback loops," although convenient as a shorthand nomenclature, does not adequately convey the full complexity of the system.
Rockland
K S
KS
Department of Neurology, University of Iowa, College of Medicine, Iowa City 52242-1053, USA. rockland@blue.weeg.uiowa.edu
D000818:Animals; D001369:Axons / Q000648:ultrastructure*; D001931:Brain Mapping; D002540:Cerebral Cortex / Q000502:physiology* / Q000648:ultrastructure; D009411:Nerve Endings / Q000502:physiology; D009434:Neural Pathways / Q000502:physiology; D012453:Saimiri / Q000502:physiology*; D013477:Superior Colliculi / Q000502:physiology; D013787:Thalamic Nuclei / Q000502:physiology* / Q000648:ultrastructure