Projections from the parabigeminal nucleus to the dorsal lateral geniculate nucleus in the tree shrew Tupaia glis.

The parabigeminal nucleus receives its major input from the superficial layers of the superior colliculus via the tectoparabigeminal projection. An extensive reciprocal parabigeminotectal pathway has also been observed. This close connectional association between the superficial gray and the parabigeminal nucleus is reflected in the collicularlike response characteristics of parabigeminal neurons (see Sherk: Brain Res. 145:375-379, '78, J. Neurophysiol. 42:1640-1655, 1656-1668, '79a,b, for review). Further documentation of the connectional relationship between the superior colliculus and the parabigeminal nucleus comes from the present data. Thus, our retrograde and anterograde transport findings reveal an extensive projection from the parabigeminal nucleus to layers 3 and 6 and several interlaminar zones of the contralateral dorsal lateral geniculate nucleus. These same layers and interlaminar zones receive tectogeniculate axons and have been shown to contain small cells that project to layers 1 and 3 of area 17. In addition to the distribution of parabigeminal axons to tectally innervated, small-celled zones, considerable parabigeminal input also reaches layers 1 and 5 of the tree shrew lateral geniculate nucleus. Each of these layers is the ipsilaterally (i.e., retinal) innervated component of a matched pair (layers 1 and 2 are considered magnocellular, while 4 and 5 are parvicellular), and it has been shown that layer 1 projects to lamina IVa of area 17, while layer 5 projects to lamina IVB. When the total distribution of parabigeminogeniculate axons is considered, it is apparent that the cells of origin of each of the major (small-celled, parvi- and magnocellular) geniculocortical channels receives parabigeminal input. Such an extensive distribution of parabigeminal axons within the lateral geniculate nucleus suggests that the information they convey might play an important role in geniculocortical function(s).