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1. Still searching for a valuable measure of spinal motoneuron excitability

   • Pietro Balbi, Neurologist, Research coordinator

   Salvatore Maugeri Foundation, IRCCS, Scientific Institute of Pavia via Boezio

   The interesting paper by Perez and Rothwell makes use of F-wave amplitude and persistence measurements to detect changes in spinal motoneuron excitability. However, as previously described (Balbi et al, 2014; Burke, 2014; Espiritu et al, 2003; Gogan et al, 1984; Hultborn and Nielsen, 1995) and partially mentioned in the paper, the F wave is a poor measure of motoneuron excitability. To explain this inadequacy, apart from those reported in the paper, two other mechanisms, at least, have to be considered.

   Firstly, when an antidromic wave is set in all the motor fibres of a peripheral nerve by means of a distal supramaximal stimulus, it is conducted without interruption to the initial segment of each motor axon. Depending on what happens at the junction between axonal initial segments and motoneuron somata, three distinct sets of neurons can be theoretically distinguished, in which: 1) the antidromic action potential (AP) fails to invade the soma, 2) the antidromic AP invades the soma after a junctional delay able to start a recurrent discharge, 3) the antidromic AP invades the soma but no recurrent discharge develops (too short junctional delay). Among these three distinct sets, the second one is responsible for the F wave and accounts for about 2-5% of the entire motoneuronal pool (Kimura, 2001). The relative proportions of these three sets of neurons, which are undetectable, depend on the biophysical features of single neurons and on the net sum of the excitatory and inhibitory inputs. In these conditions it is not always true that an increase in excitatory drive results in a wider size of the second set (i.e. increased amplitude and persistence of the F wave). Increased excitatory inputs, indeed, will shift some neurons from set 1 to set 2, as well as some others from set 2 to set 3; paradoxically, if the second transition (from set 2 to set 3) is wider than the first one, a decrease of the F-wave parameters will be detected in presence of an increase of excitatory inputs.

   Secondly, the motoneuron pool tested by the F wave (representing only a minor portion of the entire motoneuron pool) and the one activated by the voluntary contraction (or by the TMS) are very presumably two distinct and only partially, or even not, overlapping sets of neurons.

   These critical observations do not limit, in my opinion, the value of the present and past studies by the Authors' group, but raise a methodological issue for motor control investigations: is it more helpful to admit that a valid measure of motoneuron excitability is still lacking or to continue using an incorrect one?

   Balbi P, Martinoia S, Colombo R, Massobrio P. Modelling recurrent discharge in the spinal alpha-motoneuron: Reappraisal of the F wave. Clin Neurophysiol 2014 Feb; 125(2): 427-9.

   Burke D. Inability of F waves to control for changes in the excitability of the motoneuron pool in motor control studies. Clin Neurophysiol 2014 Feb; 125(2): 221-2.

   Espiritu MG, Lin CS-Y, Burke D. Motoneuron excitability and the F wave. Muscle Nerve 2003; 27: 720-727.

   Gogan P, Gustafsson B, Jankowska E, Tyc-Dumont S. On re-excitation of feline motoneurones: its mechanism and consequences. J Physiol 1984; 350: 81-91.

   Hultborn H, Nielsen JB. H-reflexes and F-responses are not equally sensitive to changes in motoneuronal excitability. Muscle Nerve. 1995 Dec;18(12):1471-4.

   Kimura J. The F wave and the A wave. In: Kimura J, editor. Electrodiagnosis in diseases of nerve and muscle: principles and practice. New York: Oxford University Press (Ed. 3rd), 2001; pp. 439-465.

   Conflict of Interest:

   None declared

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   Published April 13, 2015