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1 Physiology, Michigan State University, E. Lansing, MI, USA
* To whom correspondence should be addressed. E-mail: schnei98{at}msu.edu.
Using whole-cell recordings, I analyzed the intrinsic discharge properties for 285 neurons in Rexed's laminae III-V of isolated hamster spinal cord preparations. Neurons were characterized by their responses to step-wise and ramp-hold depolarizing current applied through the recording pipettes. Tonic cells (133/285; 47%) fired repetitively during step-wise current application. Firing decayed linearly (-0.14 to -4.3 impulses/sec/sec) or was bimodal, with an initial exponential phase (
450 msec) followed by a linear decline (-0.02 to -6.3 impulses/sec/sec); discharge frequency was unrelated to current trajectory. Phasic-firing cells (108/285; 38%) responded with a burst discharge having an initial rapid, exponential decrease (30 msec) and subsequent linear decline (-1 to -78 impulses/sec/sec). Phasic cells were activated preferentially by fast current ramps (slope 70 pA/sec - 2.2 nA/sec) with the number and frequency of impulses increasing with current slope. Delayed-firing cells (44/285; 15%), responded to current steps with an accelerating firing following a substantial latent period (0.5 to 4 sec) and discharged during current ramps with slopes less than 100 pA/sec. Intracellular staining revealed a significant association between electrophysiological profile and neuronal morphology. A majority of presumed projection cells (22/30; 73%) exhibited tonic firing to step-wise activation. The preponderance of phasic and delayed firing cells, 93% (42/45) and 71% (12/17), respectively, were interneurons with local or intersegmental terminations. Differential sensitivity to static and time-varying components of membrane current suggest differences in neuronal signaling properties that may have important implications for integration of mechanosensory information in the deep spinal dorsal horn.
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