Prolonged Firing in Motor Units -Evidence of Plateau Potentials in Human Motoneurons? Ole Kiehn and Torsten Eken. Division of Neurophysiology, Department of Medical Physiology, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen, Denmark and2Institute of Neurophysiology, University of Oslo, PO Box 1104, N-0317 Oslo, Norway.
APStracts 4:169N, 1997.
ABSTRACT
5-HT and noradrenaline-dependent plateau potentials are found in spinal motoneurons in reduced turtle and cat preparations. Triggering the plateau potential by short-lasting synaptic excitation causes a prolonged self- sustained firing, which can be terminated by short-lasting synaptic inhibition. The presence of plateau potentials can also allow neurons to fire in a bistable manner, i.e. shifting between stable low and high firing frequencies. Such a bistable firing behavior has been found in soleus motor units in unrestrained rats. In the present study single-motor-unit activity was recorded from low-threshold units in human soleus and tibialis anterior muscles in order to evaluate whether a bistable firing behavior and/or prolonged firing could be evoked. Vibration of the homonymous muscle tendon (30-100 Hz, 2-10 s) was used as excitatory input to the motoneuron pool. Brief excitation while the muscle was electrically silent induced firing during the vibration and sometimes recruited units into prolonged stable firing outlasting the vibratory stimulus. However, a bistable firing behavior, i.e. vibration-induced maintained shifts between two stable levels of firing, could not be convincingly demonstrated. The reason for this was twofold. Firstly, low-threshold human motor units tended to jump to a `preferred firing range' shortly after voluntary recruitment. This firing range was the same as when units were recruited from silence into prolonged firing by vibration. Below the preferred firing range maintained firing was unstable and usually only possible when subjects were listening to the spike potentials or had visual force-feedback. Secondly, vibration when units were firing in the preferred firing range caused a transient increase in firing frequency, but no maintained frequency shifts. Recordings from pairs of motor units showed that short-lasting vibration could recruit one unit into prolonged firing while a second unit, which already fired in its preferred firing range, only transiently increased its firing rate during the vibration. This suggests that the prolonged firing was not due to an increase in the common synaptic drive to the motoneuron pool. We conclude that a bistable firing behavior as seen in intact rats most probably is absent in human low-threshold motor units, but that prolonged firing could be seen in response to short-lasting excitation. This latter phenomenon is compatible with the existence of plateau potentials, which have to have a threshold close to the threshold for sodium spike generation. 

Received 23 May 1997; accepted in final form 24 July 1997.
APS Manuscript Number J435-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997