Correlation of monosynaptic field potentials evoked by single action potentials in single primary afferent axons and their bouton distributions in the dorsal horn.

The relationship between structure and function of the projections of single identified primary cutaneous axons was investigated by recording cord dorsum potentials at 4 sites in response to electrical stimulation of the single axon and visualizing the boutons of the axon stained by intracellular injection of horseradish peroxidase. The rostrocaudal extent of boutons differed from fiber to fiber ranging from 4.14-11.50 mm; their location in the dorsal horn also varied in agreement with the known somatotopy of the presynaptic neuropil and dorsal horn neurons. Rostrocaudal distributions of cord dorsum potentials and boutons of individual fibers revealed good agreement. Cord dorsum potential amplitude and length of the spinal projection were positively correlated with number of boutons, but no correlation with bouton density was found. The spinal projection of afferents innervating slowly adapting type 1 mechanoreceptors exhibited a greater rostrocaudal extent (mean: 8.48 mm) than those innervating rapidly adapting mechanoreceptors (i.e., hair follicle and field receptors: mean: 5.87 mm). Although the mean total number of boutons was greater for axons with slowly adapting receptors (7,250/fiber) than for axons of rapidly adapting receptors (4,677/fiber), no differences in the longitudinal density of boutons (boutons/mm) were observed. Likewise, summed amplitudes of cord dorsum potentials at the 4 recording electrodes were larger for SA1 afferents than for those of field and hair follicle afferents. A major role for the number of boutons in determining these differences is supported by the finding that the calculated average contribution per bouton to cord dorsum potentials (expressed as an amplitude coefficient a) was similar for slowly and rapidly adapting afferents. No evidence was found for regions in which boutons did not contribute to the cord dorsum potential.