scholarly journals Time course of human motoneuron recovery after sustained low-level voluntary activity

2016 ◽  
Vol 115 (2) ◽  
pp. 803-812 ◽  
Author(s):  
Martin E. Héroux ◽  
Annie A. Butler ◽  
Simon C. Gandevia ◽  
Janet L. Taylor ◽  
Jane E. Butler
Keyword(s):  
Time Course ◽  
Discharge Rate ◽  
Rest Period ◽  
Motor Units ◽  
Triceps Brachii ◽  
Voluntary Activity ◽  
Constant Frequency ◽  
Sustained Activity ◽  
Single Motor Units ◽  
Voluntary Contractions

Motoneurons often fire repetitively and for long periods. In sustained voluntary contractions the excitability of motoneurons declines. We provide the first detailed description of the time course of human motoneuron recovery after sustained activity at a constant discharge rate. We recorded the discharge of single motor units (MUs, n = 30) with intramuscular wire electrodes inserted in triceps brachii during weak isometric contractions. Subjects ( n = 15) discharged single MUs at a constant frequency (∼10 Hz) with visual feedback for prolonged durations (3–7 min) until rectified surface electromyogram (sEMG) of triceps brachii increased by ∼100%. After a rest of 1–2, 15, 30, 60, 120, or 240 s, subjects briefly resumed the contraction with the target MU at the same discharge rate. Each MU was tested with three to four rest periods. The magnitude of sEMG was increased when contractions were resumed, and the target motoneuron discharged at the test frequency following rest intervals of 2–60 s ( P = 0.001–0.038). The increased sEMG indicates that greater excitatory drive was needed to discharge the motoneuron at the test rate. The increase in EMG recovered exponentially with a time constant of 28 s but did not return to baseline even after a rest period of ∼240 s. Thus the decline in motoneuron excitability from a weak contraction takes several minutes to recover fully.

Properties of motor units after immobilization of cat peroneus longus muscle

1993 ◽  
Vol 74 (3) ◽  
pp. 1131-1139 ◽  
Author(s):  
J. Petit ◽  
M. Gioux
Keyword(s):  
Time Course ◽  
Relaxation Times ◽  
Motor Units ◽  
Short Length ◽  
Low Frequencies ◽  
Tetanic Force ◽  
Peroneus Longus ◽  
Single Motor Units ◽  
Longus Muscle ◽  
Selective Immobilization

Changes in contractile properties of cat peroneus longus motor units were studied 2, 5, and 8 wk after selective immobilization of this muscle, which was achieved by fixing the distal tendon of the peroneus longus to the fibula either at the muscle minimal physiological length ("short" length) or at the length for a 90 degree ankle joint ("neutral" length). In each muscle, 75–90% of the units [slow (S), fast resistant to fatigue (FR), fast intermediate (FI), and fast fatigable (FF)] were studied. Immobilization elicited a permanent decrease in tetanic force developed by single motor units, which was larger for resistant-to-fatigue units (S, FR). In most instances this decrease was not related to the immobilization length. In all units, twitch contraction and half-relaxation times underwent a transient increase, the extent and time course of which were influenced by immobilization length. The relationship between the frequency of motor units activation and the ratio of unfused to maximal tetanic force was studied. For fast units, there was a transient shift of the relation toward low frequencies after 2 and 5 wk of immobilization at neutral and short length, respectively.

Discharge Properties of Motor Units of the Abductor Hallucis Muscle During Cramp Contractions

2009 ◽  
Vol 102 (3) ◽  
pp. 1890-1901 ◽  
Author(s):  
Marco A. Minetto ◽  
Aleš Holobar ◽  
Alberto Botter ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Abductor Hallucis ◽  
Significant Peak ◽  
Discharge Rates ◽  
Abductor Hallucis Muscle ◽  
Active Motor ◽  
Number Of Discharges ◽  
Voluntary Contractions

We analyzed individual motor units during electrically elicited cramp contractions with the aim of characterizing the variability and degree of common oscillations in their discharges. Intramuscular and surface electromyographic (EMG) signals were detected from the abductor hallucis muscle of 11 healthy subjects (age 27.0 ± 3.7 yr) during electrically elicited cramps. In all, 48 motor units were identified from the intramuscular EMG. These motor units were active for 23.6 ± 16.2 s, during which their average discharge rate was 14.5 ± 5.1 pulses/s (pps) and their minimum and maximum rates were, respectively, 6.0 ± 0.8 and 25.0 ± 8.0 pps ( P < 0.001). The coefficient of variation for the interspike interval (ISI) was 44.6 ± 9.7% and doublet discharges constituted 4.1 ± 4.7% of the total number of discharges. In 38 motor units, the SD of the ISI was positively correlated to the mean ISI ( R2 = 0.37, P < 0.05). The coherence spectrum between smoothed discharge rates of pairs of motor units showed one significant peak at 1.4 ± 0.4 Hz for 29 of the 96 motor unit pairs and two significant peaks at 1.3 ± 0.5 and 1.5 ± 0.5 Hz for 8 motor unit pairs. The cross-correlation function between pairs of discharge rates showed a significant peak (0.52 ± 0.11) in 26 motor unit pairs. In conclusion, motor units active during cramps showed a range of discharge rates similar to that observed during voluntary contractions but larger ISI variability, probably due to large synaptic noise. Moreover, the discharge rates of the active motor units showed common oscillations.

Discharge Variability of Motor Units in an Intrinsic Muscle of Transplanted Hand

2008 ◽  
Vol 99 (5) ◽  
pp. 2232-2240 ◽  
Author(s):  
Dario Farina ◽  
Marco Pozzo ◽  
Marco Lanzetta ◽  
Roger M. Enoka
Keyword(s):  
Motor Neurons ◽  
Action Potentials ◽  
Discharge Rate ◽  
Motor Units ◽  
Surface Emg ◽  
Intrinsic Muscle ◽  
Single Motor Units ◽  
Abductor Digiti Minimi ◽  
The Individual ◽  
Ramp Contractions

The study analyzed the discharge characteristics of the motor units in an intrinsic muscle of a transplanted hand. Multichannel electromyographic (EMG) recordings were obtained in 11 experimental sessions over 16 mo starting from day 205 after a hand was transplanted in a 35-yr-old man who had lost his right hand 22 yr earlier. The action potentials discharged by single motor units were identified from the surface EMG signals of the abductor digiti minimi muscle in the transplanted hand as the individual performed 60-s maximal and linearly increasing (ramp) contractions. Discharge rate decreased from 27.1 ± 8.4 pulses per second (pps) at the start of the maximal contractions to 17.2 ± 2.9 pps at the end ( P < 0.001) and increased from 17.4 ± 4.3 to 22.1 ± 5.0 pps during the ramp contractions ( P < 0.05). The SD of the interspike interval (ISI) nearly related to the mean ISI with a similar regression slope for the maximal (0.49 ± 0.09) and ramp contractions (0.43 ± 0.10). The coefficient of variation for ISI was higher than values in able-bodied persons and did not change during either the maximal (36.8 ± 10.8%) or the ramp contractions (35.9 ± 7.4%). High-frequency bursts of activity with <20 ms between two and six action potentials occurred during both maximal and ramp contractions. In conclusion, motor neurons that reinnervated a muscle in a transplanted hand discharged action potentials with a high degree of variability that suggested greater synaptic noise during the voluntary contractions.

Comparison of the contractile responses to irregular and regular trains of stimuli during microstimulation of single human motor axons

2014 ◽  
Vol 111 (7) ◽  
pp. 1499-1506 ◽  
Author(s):  
Michael Leitch ◽  
Vaughan G. Macefield
Keyword(s):  
Angular Displacement ◽  
Motor Units ◽  
Common Peroneal Nerve ◽  
Displacement Transducer ◽  
Motor Axons ◽  
Interspike Intervals ◽  
Contractile Responses ◽  
Single Motor ◽  
Single Motor Units ◽  
Voluntary Contractions

During voluntary contractions, human motoneurons discharge with a physiological variability of ∼20%. However, studies that have measured the contractile responses to microstimulation of single motor axons have used regular trains of stimuli with no variability. We tested the hypothesis that irregular (physiological) trains of stimuli produce greater contractile responses than regular (nonphysiological) trains of identical mean frequency but zero variability. High-impedance tungsten microelectrodes were inserted into the common peroneal nerve and guided into fascicles supplying a toe extensor muscle. Selective microstimulation was achieved for 14 single motor axons. Contractile responses were measured via an angular displacement transducer over the relevant toe. After the responses to regular trains of 10 stimuli extending from 2 to 100 Hz were recorded, irregular trains of 10 stimuli, based on the interspike intervals recorded from single motor units during voluntary contractions, were delivered. Finally, the stimulation sequences were repeated following a 2-min period of continuous stimulation at 10 Hz to induce muscle fatigue. Regular trains of stimuli generated a sigmoidal increase in displacement with frequency, whereas irregular trains, emulating the firing of volitionally driven motoneurons, displayed significantly greater responses over the same frequency range (8–24 Hz). This was maintained even in the presence of fatigue. We conclude that physiological discharge variability, which incorporates short and long interspike intervals, offers an advantage to the neuromuscular system by allowing motor units to operate on a higher level of the contraction-frequency curve and taking advantage of catch-like properties in skeletal muscle.

scholarly journals Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor

2000 ◽  
Vol 528 (1) ◽  
pp. 227-234 ◽  
Author(s):  
Vaughan G. Macefield ◽  
Andrew J. Fuglevand ◽  
John N. Howell ◽  
Brenda Bigland‐Ritchie
Keyword(s):  
Motor Units ◽  
Single Motor ◽  
Single Motor Units ◽  
Voluntary Contractions ◽  
Discharge Behaviour

Synchronization of single motor units during voluntary contractions in the upper and lower extremities

2001 ◽  
Vol 112 (7) ◽  
pp. 1243-1249 ◽  
Author(s):  
Myung-Shin Kim ◽  
Yoshihisa Masakado ◽  
Yutaka Tomita ◽  
Naoichi Chino ◽  
Young Sook Pae ◽  
...  
Keyword(s):  
Lower Extremities ◽  
Motor Units ◽  
Single Motor ◽  
Single Motor Units ◽  
Upper And Lower Extremities ◽  
Voluntary Contractions

Afterhyperpolarization time-course and minimal discharge rate in low threshold motor units in humans

2008 ◽  
Vol 189 (1) ◽  
pp. 23-33 ◽  
Author(s):  
Christopher W. MacDonell ◽  
Tanya D. Ivanova ◽  
S. Jayne Garland
Keyword(s):  
Time Course ◽  
Discharge Rate ◽  
Motor Units ◽  
Low Threshold

Changes in the Estimated Time Course of the Motoneuron Afterhyperpolarization Induced by Tendon Vibration

2010 ◽  
Vol 104 (6) ◽  
pp. 3240-3249 ◽  
Author(s):  
Christopher W. MacDonell ◽  
Tanya D. Ivanova ◽  
S. Jayne Garland
Keyword(s):  
Achilles Tendon ◽  
Time Constant ◽  
Time Course ◽  
Motor Units ◽  
Reciprocal Inhibition ◽  
Excitatory Input ◽  
Motor Unit Action Potential ◽  
Inhibitory Input ◽  
Tendon Vibration ◽  
Single Motor Units

Group Ia afferents are activated vigorously with high-frequency tendon vibration and provide excitatory input to the agonist muscle and inhibitory input to the antagonist muscle group via inhibitory interneurons. The purpose of this experiment was to determine whether the afterhyperpolarization (AHP) time course in humans is altered in response to tendon vibration. The AHP time course is estimated using the interval death rate (IDR) analysis, a transform of the motor unit action potential train. Single motor units from tibialis anterior (TA) were recorded as subjects held low force dorsiflexor contractions for 600 s with and without vibration. The vibratory stimulus was superimposed on the low force contraction either to the tendon of the TA or the antagonist Achilles tendon. During TA tendon vibration, the time course of the AHP, as expressed by its time constant (τ), decreased from 35.5 ms in the previbration control condition to 31.3 ms during the vibration ( P = 0.003) and returned to 36.3 ms after the vibration was removed ( P = 0.002). The AHP τ during vibration of the antagonist Achilles tendon (38.6 ms) was greater than the previbration control condition (33.6 ms; P = 0.001). It is speculated that the reduction in AHP time constant with TA vibration may have resulted alone or in combination with a modulation of motoneuron gain, an alteration of persistent inward currents and/or the restructuring of synaptic noise. A decrease in firing probability, possibly reflecting Ia reciprocal inhibition, may have been responsible for the larger AHP time constant.

Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction

1994 ◽  
Vol 76 (6) ◽  
pp. 2411-2419 ◽  
Author(s):  
S. J. Garland ◽  
R. M. Enoka ◽  
L. P. Serrano ◽  
G. A. Robinson
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Human Subjects ◽  
Absolute Threshold ◽  
Motor Units ◽  
Single Motor ◽  
Threshold Force ◽  
Single Motor Units ◽  
Discharge Rates

The activity of 50 single motor units was recorded in the biceps brachii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on motor unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval on the discharge rates of single motor units during a fatiguing contraction. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contraction, whereas the discharge rates of most newly recruited units were either constant or increased slightly. The absolute threshold forces of recruitment and derecruitment decreased, and the variability of interimpulse intervals increased after the fatigue task. The change in motor unit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be predicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units declined despite an increase in the excitatory drive to the motoneuron pool during the fatigue task.

Recruitment and Discharge Patterns of Single Motor Units During Speech Production

1977 ◽  
Vol 20 (4) ◽  
pp. 613-630 ◽  
Author(s):  
Harvey M. Sussman ◽  
Peter F. MacNeilage ◽  
Randall K. Powers
Keyword(s):  
Dynamic Range ◽  
Isometric Force ◽  
Discharge Rate ◽  
Motor Units ◽  
Size Principle ◽  
Single Motor ◽  
Single Motor Units ◽  
Movement Dynamics ◽  
Discharge Patterns ◽  
Anterior Belly

Recruitment and discharge patterns of single motor units (MUs) in the anterior belly of digastric were studied during speech in three subjects, using electrodes facilitating selective recording at high force levels. Fixed recruitment order was observed in over 99% of all comparisons. Later recruited units invariably possessed muscle action potentials of higher amplitude, suggesting that units were activated in accordance with the “size principle.” Additional evidence for this was that later recruited units, of a set of three studied during speech, motor unit training, and isometric force ramps, showed greater sensitivity to input, and greater dynamic range than earlier recruited units. Units in this set were much more sensitive to rapid changes in input associated with speech gestures than to static activation even at high force levels. Several significant relations between discharge characteristics and aspects of movement dynamics were observed, including relations between (1) recruitment interval (MU1 to MU3) and latency of mandibular lowering, (2) onset of initial discharge of MU1 and relative mechanical advantage of the mandible, (3) number of MUs active and velocity and displacement of the mandible, and (4) discharge rate of MU3 and velocity and displacement of the mandible.

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