Proceedings of The Physiological Society
University College London 2006 (2006) Proc Physiol Soc 3, PC61
Estimation of post-synaptic potentials in motoneurons evoked by cutaneous stimulation in man
Jonathan A Norton1, David J Bennett1, Monica A Gorassini1
1. Centre for Neuroscience & Department of Biomedical Engineering, University of Alberta, Edmonton, AB, Canada.
The mechanisms underlying the triggering of muscle spasms from non-noxious sensory stimulation after spinal cord injury (SCI) remain unknown. Following SCI in rats, long-lasting reflexes (spasms) can be triggered from weak, single shock cutaneous stimulation due to the activation of persistent inward calcium currents (PICs) in the motoneuron (Li & Bennett, 2003). Such cutaneous stimulation produces NMDA-mediated excitatory post-synaptic potentials (EPSPs) in the motoneuron that are very long-lasting, ranging from 0.5-2s. These long EPSPs are able to recruit the slowly activating PIC, which requires depolarizations of 200ms or more. The purpose of this on-going study is to examine if, in man, EPSPs from non-noxious, spasm-triggering sensory inputs are similarly prolonged after spinal cord injury compared to non-injured controls to determine if they more readily activate motoneuron PICs. To estimate PSPs in man, the technique of peristimulus frequencygrams (PSFs) was used where the mean instantaneous firing rate relative to a stimulus is plotted. Changes in firing rate from background are thought to reflect changes in the underlying membrane potential and circumvent count-related errors associated with other measures such as the peristimulus time histogram (Turker & Powers, 2005). In non-injured controls, single shock stimulation to the medial arch of the foot, a stimulation that readily triggers muscle spasms in patients with SCI, was tested in 3 non-injured control subjects. The duration of the estimated PSP (recorded from the tibialis anterior muscle) produced from a non-noxious stimulation was surprisingly long, ~281±30ms (n=9 units, 2 units from 2 subjects and 3 units from 1 subject), as determined by the corner points of the CUSUM of the PSF (Ellaway, 1978). To determine if these prolonged increases in firing rate above background were due to a long PSP and not a result of history-dependent effects of tonic cell discharge (see Turker & Powers, 2005), direct intracellular recordings of motoneurons were made from the sacral spinal cord of an adult rat. Here, current injections that produced PSPs similar to that predicted from the PSF in humans were directly compared. Intracellular recordings revealed that for long EPSPs (>200 ms), with a slowly falling membrane potential following the initial depolarisation, the PSF profile faithfully followed the membrane voltage trajectory. Synchronization and history dependent effects on motoneuron discharge, which give a false representation of the PSP, only became a problem in cases where motoneurons had weak sodium-mediated PICs, currents that have been shown to be critical in motoneuorn firing (Lee et al. 2003; Harvey et al. 2005). In summary, we have shown that even in non-injured controls, very long estimated PSPs (~281ms) can be produced from single shock cutaneous stimulation as reflected in PSF profiles from motor unit recordings. The next step is to investigate effects of chronic SCI on these PSPs and its role in triggering muscle spasms.
Where applicable, experiments conform with Society ethical requirements