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Department of Physiology and Neuroscience Program, Michigan State University, E. Lansing, Michigan 48824-3320
Submitted 6 November 2002; accepted in final form 6 March 2003
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 imp ·
s–1 · s–1) or
was bimodal, with an initial exponential phase (
450 ms) followed by
a linear decline (–0.02 to –6.3 imp ·
s–1 · s–1);
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 ms) and subsequent linear decline
(–1 to –78 imp · s–1 ·
s–1). Phasic cells were activated preferentially
by fast current ramps (slope, 70 pA/s–2.2 nA/s) 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 (0.5–4 s) and discharged during
current ramps with slopes less than 100 pA/s. 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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