Abstract
The double discharges are observed at the onset of contractions of mammalian motor units (MUs), especially during their recruitment to strong or fast movements. Doublets lead to MU force increase and improve ability of muscles to maintain high force during prolonged contractions. In this review we discuss an ability to produce doublets by fast and slow motoneurons (MNs), their influence on the course of action potential afterhyperpolarization (AHP) as well as its role in modulation of the initial stage of the firing pattern of MNs. In conclusion, a generation of doublets is an important strategy of motor control, responsible for fitting the motoneuronal firing rate to the optimal for MUs at the start of their contraction, necessary for increment of muscle force.
Highlights
A pair of action potentials at short interspike intervals (ISIs, below 10 ms) called ‘‘doublet’’ (Simpson, 1969) has been frequently observed at the beginning of discharge pattern of motoneurons (MNs)
Many experiments on human muscles (Bawa and Calancie, 1983; Kirkwood and Munson, 1996; Van Cutsem et al, 1998; Garland and Griffin, 1999; Christie and Kamen, 2006) and motor units (MUs) of various animal species (Zajac and Young, 1980a; Hennig and Lømo, 1987; Sandercock and Heckman, 1997) have suggested that doublets are responsible for considerable enhancement of muscle output force
A substantial increase of a number of doublets in muscles of trained athletes during dynamic voluntary contractions have suggested their contribution to an increase in the speed of Consequences of doublet discharges contraction after the dynamic training (Griffin et al, 1998; Van Cutsem et al, 1998)
Summary
A pair of action potentials at short interspike intervals (ISIs, below 10 ms) called ‘‘doublet’’ (Simpson, 1969) has been frequently observed at the beginning of discharge pattern of motoneurons (MNs) Such initial doublets in trains of action potentials of motor units (MUs) have been recorded from numerous human muscles during different types of voluntary activity (Person and Kudina, 1972; Kudina, 1974; Bawa and Calancie, 1983; Kudina and Alexeeva, 1992; Garland and Griffin, 1999) or from animal muscles during locomotion (Zajac and Young, 1980b; Hennig and Lømo, 1985; Hoffer et al, 1987; Gorassini et al, 2000). The consequences of the doublet discharges are discussed in relation to the MU force development
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