MORPHOLOGY OF FUNCTIONAL LONG-RANGING PRIMARY AFFERENT-PROJECTIONS INTHE CAT SPINAL-CORD

1. A beta-cutaneous primary afferent fibers were impaled in the dorsal columns of alpha-chloralose-anesthetized cats. Penetrations were made with the use of electrodes filled with 2 or 5% N-(2-aminoethyl) bioti namide hydrochloride (Neurobiotin, NE) in 0.1 or 1 M KCl. After determ ining its adequate stimulus, each fiber was activated by current pulse s (18 Hz) injected via the microelectrode. The resulting cord dorsum p otential (CDP) was recorded at four locations. NB was then injected in to the fiber with the use of positive current pulses (11-22 nA) and a 75% duty cycle. 2. After allowing 2-8 h for diffusion, animals were pe rfused with saline (37 degrees C) followed by 4% paraformaldehyde (4 d egrees C). Frozen 50-mu m sections were cut in either the transverse o r sagittal plane, processed on slides with the use of standard avidin- biotin protocols, and visualized by the nickel-enhanced diaminobenzidi ne (DAB) reaction. 3. A total of 15 A beta-cutaneous afferents innerva ting both rapidly (RA) and slowly adapting (SA) receptors were adequat ely stained and their central projections recovered. For selected fibe rs the rostrocaudal and laminar bouton distributions were determined a nd compared with the distribution of monosynaptic CDP amplitudes recor ded at the four surface locations. 4. The rostrocaudal extent of a sin gle A beta-afferent fiber bouton distribution visualized with NE range d from 8 to 17.5 mm (14.4 +/- 2.4 mm, mean +/- SD), or two to three ti mes greater than that previously shown with the use of horseradish per oxidase (HRP). 5. The strong correlation between the rostrocaudal dist ribution of boutons and monosynaptic CDP amplitudes, and the improved agreement between modeled and observed CDP amplitudes over that seen p reviously with the use of HRP (mean percent error, HRP = 23 +/- 2.9%; NB = 9 +/- 2.3%), suggest that boutons along the entire length of the visualized distribution contribute to the recorded potentials. 6. Take n together, these findings suggest that inputs from a given point on t he skin can directly influence sensory information processing over a m uch greater rostrocaudal extent than predicted by dorsal horn somatoto pic maps. These findings are discussed in terms of their implications for spinal cord plasticity.