ORGANIZATION OF THE PROJECTIONS FROM THE PERICRUCIATE CORTEX TO THE PONTOMEDULLARY BRAIN-STEM OF THE CAT - A STUDY USING THE ANTEROGRADE TRACER PHASEOLUS-VULGARIS-LEUKOAGGLUTININ

The anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was used to study the distribution and density of the projections that ori ginate from four identified subdivisions of the pericruciate cortex (n amely, the forelimb and hind limb representations of area 4, area 6a b eta, and area 6a gamma) and that terminate in the pontomedullary brain stem in the cat. Injections of PHA-L in all areas of the pericruciate cortex labelled numerous fibers and their terminal swellings in the br ainstem. The major target regions of all four cortical areas were the pontine nuclei and the pontomedullary reticular formation (PMRF). Inje ctions into both the forelimb and hind limb representations of area 4 and into area 6a beta resulted in a dense pattern of terminal labellin g in restricted regions of the medial and lateral parts of the ipsilat eral pontine nuclei. The labelling following the area 6a beta injectio n was spatially distinct from that seen following the area 4 injection s. Injections into the forelimb representation of area 4 as well as in to area 6a beta and 6a gamma resulted in the labelling of numerous ter minal swellings bilaterally in the PMRF; in contrast, there were few l abelled terminal swellings in the PMRF following injections into the h ind limb representation of area 4. Terminal swellings on individual co rticoreticular fibers were far less densely aggregated than those in t he pontine nuclei. The dense pattern of innervation to restricted regi ons of the pontine nuclei supports previous suggestions that the corti copontine projections retain a high degree of topographical specificit y that could be used in the control of discrete voluntary movements. I n contrast, the more diffuse pattern of the projections to the PMRF ma y facilitate the selection and activation of the complex postural patt erns that accompany voluntary movement. (C) 1997 Wiley-Liss, Inc.