Journal of neurophysiology
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The physiological characteristics of antidromically identified lamina I spinothalamic (STT) neurons in the lumbosacral spinal cord were examined using quantitative thermal and mechanical stimuli in barbiturate-anesthetized cats. Cells belonging to the three main recognized classes were included based on categorization with natural cutaneous stimulation of the hindpaw: nociceptive-specific (NS), polymodal nociceptive (HPC), or thermoreceptive-specific (COOL) cells. The mean central conduction latencies of these classes differed significantly; NS = 130.8 +/- 55.5 (SD) ms (n = 100), HPC = 72.1 +/- 28.0 ms (n = 128), and COOL = 58.6 +/- 25.3 ms (n = 136), which correspond to conduction velocities of 2.5, 4.6, and 5.6 m/s. ⋯ The 20 HPC lamina I STT cells and 10 NS cells tested with quantitative pinch stimuli showed fairly linear responses above a threshold of approximately 130 g/mm(2) for HPC cells and a threshold of approximately 100 g/mm(2) for NS cells. All of these response functions compare well (across species) with the available data on the characteristics of thermoreceptive and nociceptive primary afferent fibers and the appropriate psychophysics in humans. Together these results support the concept that these classes of lamina I STT cells provide discrete sensory channels for the sensations of temperature and pain.
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Small dorsal root ganglion (DRG) neurons, which include nociceptors, express multiple voltage-gated sodium currents. In addition to a classical fast inactivating tetrodotoxin-sensitive (TTX-S) sodium current, many of these cells express a TTX-resistant (TTX-R) sodium current that activates near -70 mV and is persistent at negative potentials. To investigate the possible contributions of this TTX-R persistent (TTX-RP) current to neuronal excitability, we carried out computer simulations using the Neuron program with TTX-S and -RP currents, fit by the Hodgkin-Huxley model, that closely matched the currents recorded from small DRG neurons. ⋯ The persistent TTX-R current also enhanced the response to depolarizing inputs that were subthreshold for spike electrogenesis. In addition, the presence of persistent TTX-R current predisposed the cell to anode break excitation. These results suggest that, while the persistent TTX-R current is not a major contributor to the rapid depolarizing phase of the action potential, it contributes to setting the electrogenic properties of small DRG neurons by modulating their resting potentials and response to subthreshold stimuli.