Single fiber studies of ascending input to the cuneate nucleus of cats: II. Postsynaptic afferents.

The morphology of single postsynaptic afferent fibers terminating in the feline cuneate nucleus was investigated by using transport of Phasolus vulgaris leucoagglutinin from the cervical spinal cord and intraaxonal injections of horseradish peroxidase into identified postsynaptic fibers in the cuneate fasciculus. Injections of Phaseolus in C5 and C6 of both rhizotomized and non-rhizotomized cats gave similar results and confirmed previous observations with other techniques. In one animal with the smallest injection and the fewest labeled fibers in the cuneate nucleus, ten individual collaterals were reconstructed from serial sections. Most of these collaterals were at middle levels of the cuneate (from obex to about 4 mm caudal to it); they were largely confined to the rim and ventral regions of the nucleus, and their terminal fields were restricted rostrocaudally. Electrophysiologically identified fibers stained with horseradish peroxidase had large receptive fields on the ipsilateral forepaw, and latencies suggesting an oligosynaptic link to the periphery. Most of the collaterals from these fibers were also at middle cuneate levels and terminated mainly at the periphery of the nucleus but gave rise to larger terminal arbors, including sparse terminal branches to the core of the nucleus. Individual postsynaptic fibers differed in several respects from primary afferent fibers. While the spacing of collaterals of postsynaptic fibers was intermediate between that of G hair and Ia fibers, their arbors were larger than either, and could extend through the dorsoventral extent of the cuneate nucleus. The pattern of bifurcation of postsynaptic fibers resulted in stringier arbors which encompassed larger and less dense terminal fields than those of primary afferents. The number of boutons per collateral was intermediate between G hair and Ia fibers, but boutons of postsynaptic fibers were substantially smaller. These morphological differences are consistent with distinct functional roles for the two main ascending afferent systems, as suggested by electrophysiological data.