The incidence of initial doublets in the discharges of motoneurones of two different inspiratory muscles in the cat.

Kirkwood, Peter; Munson, John B.

doi:10.1113/jphysiol.1996.sp021405

Cited by 24 publications

(23 citation statements)

References 37 publications

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“…The relative threshold for doublet generation (measured as a ratio of current necessary to evoke doublet to the rheobase) exceeded two times rheobase, and there were no statistical differences between groups. It should be mentioned that doublets were observed frequently with increased intensity of current injection, but not in all the cases, which confirms earlier observations (Bawa and Calancie 1983;Kirkwood and Munson 1996;Mrówczyński et al 2010). Doublets could be determined in 11 out of 14 slow and 29 out of 35 fast motoneurons in the control group, in 9 out of 12 slow and 27 out of 31 fast motoneurons in the 5wkOL group, and 11 out of 13 slow and 24 out of 29 fast motoneurons in the 12wkOL group.…”

Section: Resultssupporting

confidence: 75%

Adaptations of motoneuron properties to chronic compensatory muscle overload

Krutki

Haluszka

Mrówczyński

et al. 2015

Journal of Neurophysiology

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Krutki P, Hałuszka A, Mrówczyński W, Gardiner PF, Celichowski J. Adaptations of motoneuron properties to chronic compensatory muscle overload. J Neurophysiol 113: 2769 -2777, 2015. First published February 18, 2015 doi:10.1152/jn.00968.2014.-The aim of the study was to determine whether chronic muscle overload has measurable effect on electrophysiological properties of motoneurons (MNs), and whether duration of this overload influences intensity of adaptations. The compensatory overload was induced in the rat medial gastrocnemius (MG) by bilateral tenotomy of its synergists (lateral gastrocnemius, soleus, and plantaris); as a result, only the MG was able to evoke the foot plantar flexion. To assure regular activation of the MG muscle, rats were placed in wheel-equipped cages and subjected to a low-level treadmill exercise. The intracellular recordings from MG motoneurons were made after 5 or 12 wk of the overload, and in a control group of intact rats. Some of the passive and threshold membrane properties as well as rhythmic firing properties were considerably modified in fast-type MNs, while remaining unaltered in slow-type MNs. The significant changes included a shortening of the spike duration and the spike rise time, an increase of the afterhyperpolarization amplitude, an increase of the input resistance, a decrease of the rheobase, and a decrease of the minimum current necessary to evoke steady-state firing. The data suggest higher excitability of fast-type MNs innervating the overloaded muscle, and a shift towards electrophysiological properties of slow-type MNs. All of the adaptations could be observed after 5 wk of the compensatory overload with no further changes occurring after 12 wk. This indicates that the response to an increased level of chronic activation of MNs is relatively quick and stable. membrane properties; rat MUSCLE OVERLOAD CAN BE ACHIEVED by workload training (Fry 2004;McDonagh and Davies 1984), ablation or tenotomy of synergists (Gardiner et al. 1991;Noble and Pettigrew 1989;Olha et al. 1988), peripheral nerve damage, or denervation of synergistic muscles (Degens et al. 1995). Overload induces measurable adaptive effects in the whole muscle, its motor units (MUs), and individual muscle fibers. The most prominent response is muscle hypertrophy, reflected in the increase of muscle mass, cross-section area, and the increased strength (Allbrook 1981;Seynnes et al. 2006). Some biochemical adaptations related to this hypertrophy have been reported in muscle fibers (Baldwin et al. 1982;Dearth et al. 2013;Ianuzzo and Chen 1979). Several studies have reported changes in contractile properties of MUs in response to chronic muscle overload. They point on increased tetanic forces of all MU types, longer contraction times of fast resistant and slow MUs, decreased fatigue resistance of fast fatigable units, and altered proportions of MUs within an overloaded muscle (Noble and Pettigrew 1989;Olha et al. 1988).It is well documented that motoneurons are highly susceptible to altered levels of physical...

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Section: Resultssupporting

confidence: 75%

Adaptations of motoneuron properties to chronic compensatory muscle overload

Krutki

Haluszka

Mrówczyński

et al. 2015

Journal of Neurophysiology

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“…It has been suggested that doublets occur most often in higher-threshold motor units, but it has also been suggested that any motor unit may be capable of firing doublets (Bawa and Calancie 1983;Garland and Griffin 1999;Kirkwood and Munson 1996). Therefore from the current literature, it is not clear which motor units are more likely to fire doublets.…”

Section: Introductioncontrasting

confidence: 40%

Doublet Discharges in Motoneurons of Young and Older Adults

Christie

Kamen²

2006

Journal of Neurophysiology

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Christie, Anita and Gary Kamen. Doublet discharges in motoneurons of young and older adults. J Neurophysiol 95: 2787-2795, 2006. First published February 1, 2006 doi:10.1152/jn.00685.2005. The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 Ϯ 3.56 yr) and eight older (74.1 Ϯ 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a fourwire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges.

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“…Initial doublets, with instantaneous frequencies of 50-300 Hz, are seen in spinal motoneurons during endogenous motor behavior (488,639,673,1417), suggesting that they are not an experimental artifact. Initial doublets may permit motoneurons to generate extra force at the onset of a contraction (476,838), perhaps to overcome inertia, but the phenomenon is not necessarily correlated with a physiological need for strong contractions (639,673).…”

Section: Integration Of Synaptic Input and Firing Modes Of Motoneumentioning

confidence: 99%

“…Initial doublets may permit motoneurons to generate extra force at the onset of a contraction (476,838), perhaps to overcome inertia, but the phenomenon is not necessarily correlated with a physiological need for strong contractions (639,673). An increase in the magnitude and duration of spike AHP contributes to the initial adaptation (48, 62, 1312), dependent on Ca 2+ entry (activating I K Ca ) during the first few spikes (1088,1301).…”

Section: Integration Of Synaptic Input and Firing Modes Of Motoneumentioning

confidence: 99%

Synaptic Control of Motoneuronal Excitability

Rekling

Funk

Bayliss

et al. 2000

Physiological Reviews

532

482

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Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre-and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre-and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K + current, cationic inward current, hyperpolarization-activated inward current, Ca 2+ channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

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The incidence of initial doublets in the discharges of motoneurones of two different inspiratory muscles in the cat.

Cited by 24 publications

References 37 publications

Adaptations of motoneuron properties to chronic compensatory muscle overload

Adaptations of motoneuron properties to chronic compensatory muscle overload

Doublet Discharges in Motoneurons of Young and Older Adults

Synaptic Control of Motoneuronal Excitability

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