Retrograde labeling of lumbosacral interneurons following injections of red and green fluorescent microspheres into hindlimb motor nuclei of the cat.

In order to map the laminar and segmental positions of lumbosacral interneurons that project to L7 motor nuclei, red and green fluorescent latex microspheres ("beads") were pressure-injected through micropipettes into the deep peroneal or posterior biceps-semitendinosus motoneuron pools of cats. Micropipette tips were positioned by recording the antidromic field potentials evoked by electrical stimulation of the muscle nerves. Projecting interneurons were identified by the presence of retrogradely transported beads. Small bead injections labeled large numbers of neurons. These were observed in all the spinal segments examined, L1-S2, and were most densely concentrated within laminae VI and VII ipsilateral to the injections and lamina VIII contralaterally. In addition, significant numbers of labeled cells were observed in lateral lamina V ipsilaterally and in lamina X. A few cells with bilateral projections were double-labeled following injections of red and green beads on opposite sides of the cord. These were most often observed in midlumbar segments (L3-L5) in medial regions of the gray matter. The results suggest that the intermediate zone (laminae V-VIII and X) of the lumbosacral spinal cord is a major source of interneuronal projections to the L7 ventral horn. This is true for both lateral and medial areas of the intermediate zone, as the fluorescent microspheres labeled neurons in medial regions of the cord largely undetected in previous studies in which other methodologies were employed.