Visceral and somatic afferent convergence onto neurons near the central canal in the sacral spinal cord of the cat.

One hundred and sixty extracellularly and intracellularly recorded unitary discharges from the sacral or caudal spinal segments of 30 anemically decerebrated cats were studied to examine the effects of somatic and visceral afferent stimulation on neurons near the central canal (CC). The recorded unitary activity was histologically verified (by dye marks or horseradish peroxidase, HRP) as having come from the gray matter surrounding the CC that approximates Rexed's lamina X. In the absence of intentional stimulation or apparent injury by the recording electrode, 62% of the units exhibited ongoing discharges. Each unit was tested for responses to the stimulation of somatic (cutaneous and subcutaneous) and visceral (bladder and colon) structures. Seventy-six (48%) of the units responded exclusively to the stimulation of somatic receptive fields, and 10 (6%) of the units were selectively responsive to stimulation of the pelvic viscera. The activity of the remaining 74 (46%) was influenced by activity in both somatic and visceral afferent fibers. Eighteen of the 160 neurons were intracellularly marked with HRP. Based on perikaryal size and dendritic extent, it was possible to divide these cells into two partially overlapping groups. One group consisted of seven neurons with small to medium-sized perikarya, dendritic arbors largely restricted to the gray matter surrounding the CC, and small, singular somatic receptive fields. The second group comprised 11 cells with medium to large-sized soma and dendrites extending out of lamina X. These larger neurons usually possessed multiple, widely distributed somatic receptive fields. The principal finding of the present study is that in the sacral spinal cord many cells near the CC receive primary afferent inputs converging from a wide range of receptor types in somatic and visceral structures. Such neurons are capable of integrating afferent information from somatic structures on both sides of the body with information originating in pelvic viscera and midline regions such as the genitals.