Morphological features of lamina V neurons receiving nociceptive input in cat sacrocaudal spinal cord

Thirty-six neurons from laminae III-VII in cat sacrocaudal spinal cord were labeled by intracellular injection of horseradish peroxidase, following physiological characterization. Of these 36 neurons, 24 had cell bodies within lamina V. Twelve lamina V neurons were multireceptive; i.e., they responded differentially to innocuous and noxious mechanical stimuli. Most multireceptive neurons had the following morphological features: (1) large cell bodies; (2) extensive dendritic spreads in all directions; and (3) axons which ascended in the contralateral ventral white matter. Three labeled lamina V neurons were activated only by noxious stimuli. Compared to the multireceptive neurons, these nociceptive-specific (NS) units had smaller cell bodies but a similar dendritic spread. Seven lamina V neurons were excited by innocuous mechanical stimuli with no evidence of nociceptive input. These seven neurons had less extensive dendritic trees than the multireceptive and the NS neurons. Six neurons labeled in lamina VII (three multireceptive and three NS) contrasted to most lamina V cells by having smaller cell bodies and short, sparsely branched dendrites. Among the lamina VII neurons, there was no obvious morphological feature that distinguished the multireceptive group from the NS group. Fifteen fully stained neurons from laminae III-VII had late discharges which were correlated with C fiber input. The dendrites of three of these neurons extended into laminae II and I; the dendrites of two neurons reached into the inner portion of lamina II; and the dorsal dendrites of the remaining ten neurons extended no further than the nucleus proprius (laminae III and IV). Thus, deeper dorsal horn neurons evincing reliable, excitatory influences from C fibers do not necessarily have superficially situated dendrites. Tests for correlations between size of cutaneous, excitatory receptive field (RF) and dendritic spread revealed a significant positive correlation between the mediolateral extent of dendritic spread and the size of the low-threshold component of the RF for lamina V neurons.