ORGANIZATION OF THE MEDIAL PULVINAR NUCLEUS IN THE MACAQUE

Background: The medial pulvinar appears to subserve the integration of associative cortical information and projects to visuomotor-related c ortex, In contrast to the other pulvinar subdivisions, the medial pulv inar is a polymodal structure, Therefore, we studied the structural or ganization of the medial pulvinar to determine how it differs from the surrounding unimodal nuclei. Methods: Nissl-stained sections were exa mined to determine the boundaries of, and the distribution of neuronal sizes within, the medial pulvinar, In addition, Golgi-impregnated neu rons were examined and drawn for analysis, Only rhesus monkey specimen s were used, and the material had been prepared previously for other s tudies. Results: Projection neurons have round to oval somata and mode rate numbers of primary dendrites that extend for short distances befo re branching into many secondary branches, Two variations of projectio n neurons (P1 and P2) were distinguished on the basis of the diameters of their dendritic tree, Both varieties have short dendrites that rad iate in all directions, They differ in that P2 cells have longer secon d tier dendrites than P1 cells, Three types of local circuit neurons, tufted, radiating and varicose, were distinguished on the basis of the ir dendritic morphology, Four types of afferent fibers were identified . Type 1 afferents form cone-shape terminal arbors. Type 2 afferents a re similar to those reported for retinal or cortical terminals, Type 3 afferents are of medium thickness and of an unknown origin, Type 4 af ferents are thin and have small varicosities consistent with previousl y described cortical afferents, Afferent fibers are predominantly orie nted along the mediolateral axis of the nucleus. We observed putative contacts between some afferents and local circuit neurons and between local circuit neurons and projection neurons, Conclusions: Medial pulv inar neurons are generally smaller and rounder than those found in the adjacent pulvinar nuclei, These results provide additional evidence f or structural distinctions between thalamic nuclei having different fu nctions, However, the observed differences are subtle, In addition, th e data in this report provide morphological evidence that cortical sig nals are likely to be integrated by means of the circuitry located wit hin the nucleus. (C) 1998 Wiley-Liss, Inc.