ESTIMATES OF PERSISTENT INWARD CURRENTS INCREASE WITH THE LEVEL OF VOLUNTARY DRIVE IN LOW-THRESHOLD MOTOR UNITS OF PLANTAR FLEXOR MUSCLES

2021 ◽  
Author(s):  
Lucas B R Orssatto ◽  
Karen Mackay ◽  
Anthony James Shield ◽  
Raphael Luiz Sakugawa ◽  
Anthony John Blazevich ◽  
...  
Keyword(s):  
Motor Unit ◽  
Repeated Measures ◽  
Discharge Rate ◽  
Motor Units ◽  
Plantar Flexors ◽  
Force Output ◽  
Persistent Inward Currents ◽  
Gastrocnemius Medialis ◽  
Inward Currents ◽  
Maximal Discharge

This study tested if estimates of persistent inward currents (PICs) in the plantar flexors would increase with the level of voluntary drive. High-density surface electromyograms were collected from soleus and gastrocnemius medialis of 21 participants during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise and decline of 2%/s and 30-s duration) of each participant's maximal torque. Motor units identified in all the contraction intensities were included in the paired-motor unit analysis to calculate delta frequency (ΔF) and estimate the PICs. ΔF is the difference in discharge rate of the control unit at the time of recruitment and de-recruitment of the test unit. Increases in PICs were observed from 10% to 20% (Δ=0.6 pulse-per-second, pps; p<0.001) and 20% to 30% (Δ=0.5pps; p<0.001) in soleus, and from 10% to 20% (Δ=1.2pps; p<0.001) but not 20% to 30% (Δ=0.09pps; p=0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% (respectively, Δ=1.75pps, p<0.001; and Δ=2.43pps, p<0.001) and 20% to 30% (respectively, Δ=0.80pps, p<0.017; and Δ=0.92pps, p=002). The repeated-measures correlation identified associations between ΔF and increases in maximal discharge rate for soleus (r=0.64; p<0.001) and gastrocnemius medialis (r=0.77; p<0.001). An increase in voluntary drive tends to increase PIC strength, which has key implications for the control of force but also for comparisons between muscles or studies when relative force levels are different. Increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit discharging, and thus force output modulation.

scholarly journals Persistent Inward Currents Increase With The Level Of Voluntary Drive In Plantar Flexor Low-Threshold Motor Units

2020 ◽  
Author(s):  
Lucas B. R Orssatto ◽  
Karen Mackay ◽  
Anthony J Shield ◽  
Raphael L. Sakugawa ◽  
Anthony J. Blazevich ◽  
...  
Keyword(s):  
Motor Unit ◽  
Repeated Measures ◽  
Discharge Rate ◽  
Motor Units ◽  
Plantar Flexors ◽  
Force Output ◽  
Persistent Inward Currents ◽  
Gastrocnemius Medialis ◽  
Inward Currents ◽  
Maximal Discharge

This study tested the hypothesis that estimates of persistent inward currents (PICs) in the human plantar flexors would increase with the level of voluntary drive. Twenty-one participants volunteered for this study (29.2±2.6 years). High-density surface electromyograms were collected from soleus and gastrocnemius medialis during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise of 2%/s and 30-s duration) of each participant's maximal torque. Motor units identified in all the contraction intensities were included in the paired-motor unit analysis to calculate delta frequency (ΔF) and estimate the PICs. Increases in PICs were observed from 10% to 20% (Δ=0.6 pps; p<0.001) and 20% to 30% (Δ=0.5 pps; p<0.001) in soleus, and from 10% to 20% (Δ=1.2 pps; p<0.001) but not 20% to 30% (Δ=0.09 pps; p=0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% (respectively, Δ=1.75 pps, p<0.001; and Δ=2.43 pps, p<0.001) and 20% to 30% (respectively, Δ=0.80 pps, p<0.017; and Δ=0.92 pps, p=002). The repeated-measures correlation identified associations between ΔF and increases in maximal discharge rate for both soleus (r=0.64; p<0.001) and gastrocnemius medialis (r=0.77; p<0.001). An increase in voluntary drive tends to increase PIC strength, which has key implications for the control of force but also for comparisons between muscles or between studies when relative force levels might be different. These data indicate that increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit firing, and thus force output modulation.

Estimates of persistent inward currents in tibialis anterior motor units during standing ramped contraction tasks in humans

2021 ◽  
Author(s):  
Obaid U Khurram ◽  
Francesco Negro ◽  
CJ Heckman ◽  
Christopher K. Thompson
Keyword(s):  
Motor Unit ◽  
Visual Feedback ◽  
Discharge Rate ◽  
Motor Units ◽  
Firing Patterns ◽  
Persistent Inward Currents ◽  
Analysis Technique ◽  
Motor Unit Firing ◽  
Inward Currents ◽  
Unit Analysis

Persistent inward currents (PICs) play an essential role in setting motor neuron gain and shaping motor unit firing patterns. Estimates of PICs in humans can be made using the paired motor unit analysis technique, which quantifies the difference in discharge rate of a lower-threshold motor unit at the recruitment onset and offset of a higher-threshold motor unit (∆F). Because PICs are highly dependent on the level of neuromodulatory drive, ∆F represents an estimate of level of neuromodulation at the level of the spinal cord. Most of the estimates of ∆F are performed under constrained, isometric, seated conditions. In the present study, we used high-density surface EMG arrays to discriminate motor unit firing patterns during isometric seated conditions with torque or EMG visual feedback and during unconstrained standing anterior-to-posterior movements with RMS EMG visual feedback. We were able to apply the paired motor unit analysis technique to the decomposed motor units in each of the three conditions. We hypothesized that ∆F would be higher during unconstrained standing anterior-to-posterior movements compared to the seated conditions, reflecting an increase in the synaptic input to MNs drive while standing. In agreement with previous work, we found that there was no evidence of a difference in ∆F between the seated and standing postures, although slight differences in the initial and peak discharge rates were observed. Taken together, our results suggest that both the standing and seated postures are likely not sufficiently different--both being "upright" postures--to result in large changes in neuromodulatory drive.

Motor Unit Rotation in a Variety of Human Muscles

2009 ◽  
Vol 102 (4) ◽  
pp. 2265-2272 ◽  
Author(s):  
Parveen Bawa ◽  
Chantelle Murnaghan
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Motor Command ◽  
Emg Activity ◽  
Persistent Inward Currents ◽  
Small Hand ◽  
Abductor Digiti Minimi ◽  
Inward Currents ◽  
Low Threshold ◽  
The Subject

The phenomena of substitution and rotation among motor units of a muscle were examined in seven different muscles. Intramuscular motor unit activity and surface electromyographic (EMG) activity were recorded from one of the following muscles: abductor digiti minimi, first dorsal interosseous, extensor digitorum communis, flexor and extensor carpi radialis, tibialis anterior, and soleus. The subject was asked to discharge a discernible unit at a comfortable constant or rhythmically (pseudosinusoidally) modulated rate with audio and visual feedback. Results are reported from a total of 42 sets of motor units from all seven muscles. We observed that when a subject fired a motor unit for a long period, an additional motor unit frequently started to discharge after a few minutes. When the subject was asked to keep activity down to one unit, very often it was Unit 1 that dropped and Unit 2 continued to fire. Whereas Unit 2 had fired for a few minutes, Unit 1 resumed firing without any conscious effort by the subject. If the subject was then asked to retain just one unit, it was Unit 2 that dropped. Rhythmic modulation of firing rate of a tonically firing unit showed that whereas the threshold of this unit increased, the threshold of a phasically discharging unit decreased substantially. The increase in threshold of a tonically discharging unit is suggested to arise from inactivation of Na+ and Ca2+ channels and the decrease in threshold of higher-threshold units is suggested to arise from an increase in persistent inward currents that may occur during prolonged contractions. Whether a unit stops or starts to fire is suggested to depend on a balance between the strength of the central motor command, persistent inward currents, and inactivation of voltage-gated channels. Such rotations among low-threshold motoneurons would ensure low-level sustained contractions to be viable not only in small hand muscles but also in larger limb muscles.

Blockade of 5-HT2 receptors suppresses rate of torque development and motor unit discharge rate during rapid contractions

2021 ◽  
Author(s):  
Benjamin Ian Goodlich ◽  
Sean A Horan ◽  
Justin J Kavanagh
Keyword(s):  
Motor Unit ◽  
Unit Activity ◽  
Repeated Measures ◽  
Discharge Rate ◽  
Voluntary Contraction ◽  
Unit Discharge ◽  
Motor Unit Activity ◽  
Rate Of Torque Development ◽  
Motor Unit Discharge ◽  
Torque Development

Serotonin (5-HT) is a neuromodulator that is critical for regulating the excitability of spinal motoneurons and the generation of muscle torque. However, the role of 5-HT in modulating human motor unit activity during rapid contractions has yet to be assessed. Nine healthy participants (23.7 ± 2.2 yr) ingested 8 mg of the competitive 5-HT2 antagonist cyproheptadine in a double-blinded, placebo-controlled, repeated-measures experiment. Rapid dorsiflexion contractions were performed at 30%, 50% and 70% of maximal voluntary contraction (MVC), where motor unit activity was assessed by high-density surface electromyographic decomposition. A second protocol was performed where a sustained, fatigue-inducing dorsiflexion contraction was completed prior to undertaking the same 30%, 50% and 70% MVC rapid contractions and motor unit analysis. Motor unit discharge rate (p < 0.001) and rate of torque development (RTD; p = 0.019) for the unfatigued muscle were both significantly lower for the cyproheptadine condition. Following the fatigue inducing contraction, cyproheptadine reduced motor unit discharge rate (p < 0.001) and RTD (p = 0.024), where the effects of cyproheptadine on motor unit discharge rate and RTD increased with increasing contraction intensity. Overall, these results support the viewpoint that serotonergic effects in the central nervous system occur fast enough to regulate motor unit discharge rate during rapid powerful contractions.

scholarly journals Evidence for Increased Activation of Persistent Inward Currents in Individuals With Chronic Hemiparetic Stroke

2008 ◽  
Vol 100 (6) ◽  
pp. 3236-3243 ◽  
Author(s):  
Jacob G. McPherson ◽  
Michael D. Ellis ◽  
C. J. Heckman ◽  
Julius P. A. Dewald
Keyword(s):  
Repeated Measures ◽  
Afferent Feedback ◽  
Joint Torque ◽  
Emg Activity ◽  
Tonic Vibration Reflex ◽  
Stretch Reflexes ◽  
Persistent Inward Currents ◽  
Inward Currents ◽  
Hemiparetic Stroke ◽  
Chronic Hemiparetic Stroke

Despite the prevalence of hyperactive stretch reflexes in the paretic limbs of individuals with chronic hemiparetic stroke, the fundamental pathophysiological mechanisms responsible for their expression remain poorly understood. This study tests whether the manifestation of hyperactive stretch reflexes following stroke is related to the development of persistent inward currents (PICs) leading to hyperexcitability of motoneurons innervating the paretic limbs. Because repetitive volleys of 1a afferent feedback can elicit PICs, this investigation assessed motoneuronal excitability by evoking the tonic vibration reflex (TVR) of the biceps muscle in 10 awake individuals with chronic hemiparetic stroke and measuring the joint torque and electromyographic (EMG) responses of the upper limbs. Elbow joint torque and the EMG activity of biceps, brachioradialis, and the long and lateral heads of triceps brachii were recorded during 8 s of 112-Hz biceps vibration (evoking the TVR) and for 5 s after cessation of stimulation. Repeated-measures ANOVA tests revealed significantly ( P ≤ 0.05) greater increases in elbow flexion torque and EMG activity in the paretic as compared with the nonparetic limbs, both during and up to 5 s following biceps vibration. The finding of these augmentations exclusively in the paretic limb suggests that contralesional motoneurons may become hyperexcitable and readily invoke PICs following stroke. An enhanced tendency to evoke PICs may be due to an increased subthreshold depolarization of motoneurons, an increased monoaminergic input from the brain stem, or both.

Ankle Angle but Not Knee Angle Influences Force Fluctuations During Plantar Flexion

2021 ◽  
Author(s):  
Georgios Trypidakis ◽  
Ioannis G. Amiridis ◽  
Roger Enoka ◽  
Irini Tsatsaki ◽  
Eleftherios Kellis ◽  
...  
Keyword(s):  
Knee Joint ◽  
Coefficient Of Variation ◽  
Discharge Rate ◽  
Dominant Role ◽  
Plantar Flexion ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Knee Angle ◽  
Plantar Flexors ◽  
Ankle Angle

AbstractThe purpose of the study was to evaluate the influence of changes in ankle- and knee-joint angles on force steadiness and the discharge characteristics of motor units (MU) in soleus when the plantar flexors performed steady isometric contractions. Submaximal contractions (5, 10, 20, and 40% of maximum) were performed at two ankle angles (75° and 105°) and two knee angles (120° and 180°) by 14 young adults. The coefficient of variation of force decreased as the target force increased from 5 to 20% of maximal force, then remained unaltered at 40%. Independently of knee angle, the coefficient of variation for force at the ankle angle of 75° (long length) was always less (p<0.05) than that at 105° (shorter length). Mean discharge rate, discharge variability, and variability in neural activation of soleus motor units were less (p<0.05) at the 75° angle than at 105°. It was not possible to record MUs from medial gastrocnemius at the knee angle of 120° due to its minimal activation. The changes in knee-joint angle did not influence any of the outcome measures. The findings underscore the dominant role of the soleus muscle in the control of submaximal forces produced by the plantar flexor muscles.

Physiological responses of rat plantaris motor units to overload induced by surgical removal of its synergists

1988 ◽  
Vol 60 (6) ◽  
pp. 2138-2151 ◽  
Author(s):  
A. E. Olha ◽  
B. J. Jasmin ◽  
R. N. Michel ◽  
P. F. Gardiner
Keyword(s):  
Motor Unit ◽  
Fatigue Resistance ◽  
Motor Units ◽  
High Threshold ◽  
Surgical Removal ◽  
Type I ◽  
Myofibrillar Atpase ◽  
Force Output ◽  
Cross Sectional ◽  
Single Motor Units

1. Rat plantaris muscles were subjected to chronic overload by the surgical removal of the soleus and most of the gastrocnemius muscles. Twelve to 16 wk later whole muscle and motor unit (ventral root dissection technique) contractile properties as well as histochemistry were determined. 2. Motor units were categorized as fast, fatigable (FF), fast, intermediate fatigue-resistant (FI), fast, fatigue-resistant (FR), and slow (S) based on contractile characteristics. Muscle fibers were identified as type I and type II according to myofibrillar ATPase staining. 3. Whole muscles demonstrated increases in wet weight, tetanic force, proportion of type I fibers, and mean cross-sectional areas of both type I and II fibers, as a result of chronic overload. 4. Tetanic tension increased by the same relative magnitude in all motor units whereas twitch tension remained unchanged. A significant change in the proportions of the motor unit types occurred in overloaded muscles, such that the latter contained higher proportions of FF and S units, and lower proportions of FI and FR units, than normal muscles. 5. The fatigue profile of a composite constructed from a summation of motor unit responses revealed that the overloaded plantaris displayed fatigue resistance similar to that of the normal plantaris for a given absolute force output. 6. Glycogen-depleted fibers of hypertrophied single motor units demonstrated uniform myofibrillar ATPase and SDH staining characteristics suggesting that metabolic adaptations among fibers of the same unit were similar after 12-16 wk of overload. 7. The finding that overload caused a uniform increase in the tetanic strength of all motor units, whereas alterations in fatigue resistance varied in degree and direction among unit types, demonstrate that these two properties are not controlled in parallel in this model. The smallest units maintain or even increase their fatigue resistance during the hypertrophic process, whereas high threshold units actually decrease in fatigue resistance.

scholarly journals Motor unit contributions to activation reduction and torque steadiness following active lengthening: a study of residual torque enhancement

2020 ◽  
Vol 123 (6) ◽  
pp. 2209-2216 ◽  
Author(s):  
Jennifer M. Jakobi ◽  
Samantha L. Kuzyk ◽  
Chris J. McNeil ◽  
Brian H. Dalton ◽  
Geoffrey A. Power
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Electromyographic Activity ◽  
Torque Capacity ◽  
Active Motor ◽  
Motor Neuron Pool ◽  
Acute Condition ◽  
The Impact ◽  
Neuron Pool

Our findings indicate that lower electromyographic activity during the torque-enhanced condition following active lengthening compared with a purely isometric contraction arises from fewer active motor units and a lower discharge rate of those that are active. We used an acute condition of increased torque capacity to induce a decrease in net output of the motor neuron pool during a submaximal task to demonstrate, in humans, the impact of motor unit activity on torque steadiness.

Change in Muscle Fascicle Length Influences the Recruitment and Discharge Rate of Motor Units During Isometric Contractions

2005 ◽  
Vol 94 (5) ◽  
pp. 3126-3133 ◽  
Author(s):  
Benjamin Pasquet ◽  
Alain Carpentier ◽  
Jacques Duchateau
Keyword(s):  
Motor Unit ◽  
Ankle Joint ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Isometric Contractions ◽  
Neutral Position ◽  
Proprioceptive Information ◽  
Fascicle Length ◽  
Muscle Fascicle

This study examines the effect of fascicle length change on motor-unit recruitment and discharge rate in the human tibialis anterior (TA) during isometric contractions of various intensities. The torque produced during dorsiflexion and the surface and intramuscular electromyograms (EMGs) from the TA were recorded in eight subjects. The behavior of the same motor unit ( n = 59) was compared at two ankle joint angles (+10 and −10° around the ankle neutral position). Muscle fascicle length of the TA was measured noninvasively using ultrasonography recordings. When the ankle angle was moved from 10° plantarflexion to 10° dorsiflexion, the torque produced during maximal voluntary contraction (MVC) was significantly reduced [35.2 ± 3.3 vs. 44.3 ± 4.2 (SD) Nm; P < 0.001] and the average surface EMG increased (0.47 ± 0.08 vs. 0.43 ± 0.06 mV; P < 0.05). At reduced ankle joint angle, muscle fascicle length declined by 12.7% ( P < 0.01) at rest and by 18.9% ( P < 0.001) during MVC. Motor units were activated at a lower recruitment threshold for short compared with long muscle fascicle length, either when expressed in absolute values (2.1 ± 2.5 vs. 3.6 ± 3.7 Nm; P < 0.001) or relative to their respective MVC (5.2 ± 6.1 vs. 8.8 ± 9.0%). Higher discharge rate and additional motor-unit recruitment were observed at a given absolute or relative torque when muscle fascicles were shortened. However, the data indicate that increased rate coding was mainly present at low torque level (<10% MVC), when the muscle-tendon complex was compliant, whereas recruitment of additional motor units played a dominant role at higher torque level and decreased compliance (10–35% MVC). Taken together, the results suggest that the central command is modulated by the afferent proprioceptive information during submaximal contractions performed at different muscle fascicle lengths.

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