Heat-evoked responses of dorsal horn nociceptive neurons in the monkey

Locations and response characteristics of nociceptive neurons in the dorsal horn of the spinal cord were studied in urethane-chloralose anesthetized Japanese macaques. There were two general categories of nociceptive neurons. Neurons of the first type, nociceptive specific (NS) neurons, responded only to intense mechanical stimuli. Neurons of the second type, wide dynamic range (WDR) neurons, were activated by touch and pressure but responded maximally to noxious mechanical stimuli. NS neurons were located in laminae I and IIo of the superficial dorsal horn. WDR neurons were located not only in laminae I and IIo but also in laminae IV-VI. NS and WDR neurons in laminae I and IIo were spatially not segregated from each other. Projecting neurons were included in both categories of nociceptive neurons, but lamina IIo neurons were exclusively non-projecting neurons. Noxious heat stimulation of the peripheral receptive field as defined by mechanical stimuli, activated a significant fraction of NS and WDR neurons. Nearly all heat-sensitive WDR neurons in both the superficial layers and nucleus proprius received combined A delta- and C-fiber input. Heat-sensitive NS neurons received either exclusively A delta-fiber input or combined A delta- and C-fiber input. All the heat-sensitive neurons tested showed a monotonous increase of heat-evoked discharges between the threshold temperature and the peak temperature. In both categories of heat-sensitive neurons, noxious heat neurons predominated over warming-noxious neurons. Hence, the thermal threshold did not necessarily parallel the mechanical threshold.