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The Journal of Neurophysiology Vol. 86 No. 4 October 2001, pp. 1972-1982
Copyright ©2001 by the American Physiological Society
Division of Neuroscience, University of Alberta, Edmonton, Alberta T6G 2S2, Canada
Bennett, David J.,
Yunru Li,
Philip J. Harvey, and
Monica Gorassini.
Evidence for Plateau Potentials in Tail Motoneurons of Awake
Chronic Spinal Rats With Spasticity. J. Neurophysiol. 86: 1972-1982, 2001. Motor units of segmental tail
muscles were recorded in awake rats following acute (1-2 days) and
chronic (>30 days) sacral spinal cord transection to determine whether
plateau potentials contributed to sustained motor-unit discharges after
injury. This study was motivated by a companion in vitro study that
indicated that after chronic spinal cord injury, the tail motoneurons
of the sacrocaudal spinal cord exhibit persistent inward currents (IPIC) that cause intrinsically
sustained depolarizations (plateau potentials) and firing
(self-sustained firing). Importantly, in this companion
study, the plateaus were fully activated at recruitment and
subsequently helped sustain the firing without causing
abrupt nonlinearities in firing. That is, after recruitment and plateau activation, the firing rate was modulated relatively
linearly with injected current and therefore provided a good
approximation of the input to the motoneuron despite the plateau. Thus
in the present study, pairs of motor units were recorded
simultaneously from the same muscle, and the firing rate
(F) of the lowest-threshold unit (control unit) was used as
an estimate of the synaptic input to both units. We then examined
whether firing of the higher-threshold unit (test unit) was
intrinsically maintained by a plateau, by determining whether more
synaptic input was required to recruit the test unit than to maintain
its firing. The difference in the estimated synaptic input at
recruitment and de-recruitment of the test unit
(i.e., change in control
unit rate, 
F) was taken as an estimate of the plateau
current (IPIC) that intrinsically sustained the firing. Slowly graded manual skin stimulation was used to
recruit and then de-recruit the units. The test unit was recruited when
the control unit rate was on average 17.8 and 18.9 Hz in acute and
chronic spinal rats, respectively. In chronic spinal rats,
the test unit was de-recruited when the control unit rate (re:
estimated synaptic input) was significantly reduced, compared with at
recruitment (F = 
5.5 Hz), and thus a plateau participated in maintaining the firing. In the lowest-threshold motor
units, even a brief stimulation triggered very long-lasting firing
(seconds to hours; self-sustained firing). Higher-threshold units
required continuous stimulation (or a spontaneous spasm) to cause
firing, but again more synaptic input was needed to recruit the unit
than to maintain its firing (i.e., plateau present). In contrast, in
acute spinal rats, the stimulation did not usually trigger
sustained motor-unit firing that could be attributed to plateaus
because F was not significantly different from zero. These results indicate that plateaus play an important role in sustaining motor-unit firing in awake chronic spinal rats and thus
contribute to the hyperreflexia and hypertonus associated with chronic injury.
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