Motoneuron double discharges: only one or two different entities?

INTRODUCTION The firing behavior of spinal motoneurons is controlled by a complex interplay between the synaptic inputs that they receive and the intrinsic properties of motoneurons. In particular, the longlasting afterhyperpolarization which follows each motoneuron spike was found to be an important mechanism in the regulation of repetitive firing behavior (Kernell, 1965, 2006). As a result, motoneurons typically fire single discharges at relatively low firing frequencies, especially in humans (about 5–20 Hz during gentle and slow voluntary muscle contractions or posture maintenance). However, under the same conditions, some motoneurons can produce a quite different type of firing termed double discharges (doublets) with uniquely short interspike intervals (ISIs), commonly ranging from 2.5 to 20 ms. Similar short ISIs were also reported under very distinct conditions, during initiation of strong and fast ballistic movements; they were referred to as doublet ISIs as well. Considering properties and underlying mechanisms of doubling, we will attempt to answer the question of whether or not this unique firing pattern can be regarded as a single entity. Note that the study of doubling in healthy humans is of interest not only in itself as a very specific motoneuron firing during natural motor control but also because it can potentially provide valuable insight into the pathophysiological mechanisms underlying doublet phenomenon that is dramatically enhanced in a number of neuromuscular diseases: proximal neuropathies (Partanen, 1978), Parkinson’s disease (Baker et al., 1992), chronic cervical spinal cord injury (Thomas and Ross, 1997), spinal muscular atrophy (Rowiñska-Marciñska et al., 1999), neuromyotonia (Kleine et al., 2008), amyotrophic lateral sclerosis and Kennedy’s disease (Piotrkiewicz et al., 2008; Weber et al., 2009). The importance of the doubling investigation was pointed out by the introduction of the term “double discharge” in the glossaries of terms in clinical electromyography and electrodiagnostic medicine (Simpson, 1969; AAEE, 1987; AAEM, 2001).