Single fiber studies of ascending input to the cuneate nucleus of cats: I. Morphometry of primary afferent fibers.

The morphology of afferent fibers ascending to the cuneate nucleus has been examined in this and the subsequent paper in order to quantify the pattern of arborization and bouton arrangement of selected classes of primary afferents and to compare these data with data from postsynaptic fibers ascending to the cuneate nucleus. Electrophysiologically identified G hair and Ia muscle afferent fibers in the cuneate fasciculus were intraaxonally injected with horseradish peroxidase. Cutaneous afferents terminated dorsal to proprioceptive afferents, especially at middle levels of the cuneate nucleus. The spacing of collaterals along G hair fibers was variable, but averaged 1.46 collaterals per mm; collateral density was higher at middle cuneate levels than in the rest of the nucleus. Collateral density of Ia fibers was lower than for G hair fibers and was lowest at caudal levels of the nucleus. Branches of G hair collaterals, though often initially diverging, usually converged to terminate in a single focus in the dorsal part of the nucleus. The probability of bifurcation of Ia collaterals decreased steadily at successive branch points. These collaterals branched less symmetrically than G hair collaterals, and terminated in the ventral cuneate with less dense arbors, stretched mediolaterally, but of comparable cross-sectional area. Individual G hair collaterals gave rise to more boutons than Ia collaterals; in both cases they were mostly of the en passant type. Boutons were restricted to distal branches of G hair collaterals, whereas boutons of Ia collaterals were also located on proximal branches. Bouton size was similar for the two classes of collaterals. The data reported here, in combination with the published literature, suggest that the collaterals of roughly 300 G hair fibers overlap at any given point at middle levels of the cuneate nucleus. This high degree of anatomical convergence is not predicted by the functional segregation described with electrophysiological mapping, implying the presence of intrinsic nuclear mechanisms enhancing response specificity.