Reversal of the late phase of spike frequency adaptation in cat spinal motoneurons during fictive locomotion

Abstract

Brownstone RM, Krawitz S, Jordan LM. Reversal of the late phase of spike frequency adaptation in cat spinal motoneurons during fictive locomotion. J Neurophysiol 105: 1045-1050, 2011. First published December 22, 2010 doi:10.1152/jn.00411.2010.-In spinal motoneurons, late spike frequency adaptation (SFA) is defined as the slowing of the firing rate over tens of seconds and can be seen during sustained or intermittent current injection. Although the function of late SFA is not known, it may result in a decrease in force production over time, or muscle fatigue. Because locomotion can persist for long periods of time without fatigue, late SFA was studied using intracellular recordings from adult cat motoneurons during fictive locomotion. Of eight lumbar motoneurons studied, all showed late adaptation during control conditions, but none demonstrated late adaptation during locomotor activity. The most consistent properties that correlated with the presence or absence of late SFA were those related to availability of fast, inactivating sodium channels, particularly action potential rate of rise. Evidence of the reversal of late SFA during locomotion was present for several minutes following locomotor trials, consistent with the suggestion that SFA is modulated through slow metabotropic pathways. The abolition of late adaptation in spinal motoneurons during fictive locomotion is an example of a statedependent change in the "intrinsic" properties of mammalian motoneurons. This change contributes to increased excitability of motoneurons during locomotion and results in robust firing during sustained locomotion.

Contribution of intrinsic motoneuron properties to discharge hysteresis and its estimation based on paired motor unit recordings: a simulation study

“…Our simulations indicate that raising spike threshold through increases in accommodation reduces ⌬F values; thus lowering spike threshold should enhance the effects of neuromodulatory drive on ⌬F. Similarly, reductions in late spike frequency adaptation during fictive locomotion that are thought to be mediated by neuromodulatory pathways (Brownstone et al 2011) would not be expected to affect ⌬F values if this late adaptation is mediated by either slow sodium channel inactivation or a slow outward current. Our simulations indicate that lower firing rates at de-recruitment mediated by AHP prolongation could enhance ⌬F values, but the differences in recruitment vs. de-recruitment firing rates appear to be similar in preparations with low and high levels of neuromodulatory drive (Wienecke et al 2009).…”

Neural Contributions to Muscle Fatigue

“…Such excitatory actions should aid in motor output rather than contribute to fatigue. Indeed, during rhythmic movements such as walking, the presence of neuromodulators may counteract the changes in intrinsic properties which lead to reduced excitability with repetitive firing of motoneurons (16). However, in addition to the 5-HT2 receptors, inhibitory 5-HT1A receptors have been identified on the axon initial segment of motoneurons.…”

Hind limb motoneurons activity during fictive locomotion or scratching induced by pinna stimulation, serotonin, or glutamic acid in brain cortex-ablated cats

“…An increase of synaptic input in these motoneurons, in the BCAC preparation, could be the cause of doublets or a suppression of slow after hyperpolarization, as observed in turtle spinal neurons during scratching-like activity (Alaburda et al 2005). The origin of the descending pathway implied in doublet generation may be localized in thalamic or subthalamic structures, as doublets were also seen in decerebrated cats during fictive locomotion, although spike frequency adaptation did not occur during this task (Brownstone et al 2011).…”

“…The origin of the descending pathway implied in doublet generation may be localized in thalamic or subthalamic structures, as doublets were also seen in decerebrated cats during fictive locomotion, although spike frequency adaptation did not occur during this task (Brownstone et al. ).…”