Does Strict Validation Criteria for Individual Motor Units Alter Extrapolation Analyses of the Motor Unit Pool?

1. The tension produced by the combined stimulation of two to four single motor units of the cat tibialis posterior muscle was compared with the algebraic sum of the tensions produced by each individual motor unit. Comparisons were made under isometric conditions and during imposed changes in muscle length. 2. Under isometric conditions, the tension resulting from combined stimulation of units displayed marked nonlinear summation, as previously reported in other cat hindlimb muscles. On average, the measured tension was approximately 20% greater than the algebraic sum of the individual unit tensions. However, small trapezoidal movements imposed on the muscle during stimulation significantly reduced the degree of nonlinear summation both during and after the movement. This effect was seen with imposed movements as small as 50 microns. 3. The degree of nonlinear summation was not dependent on motor unit size or on stimulus frequency. The effect was also unrelated to tendon compliance because the degree of nonlinear summation of motor unit forces was unaffected by the inclusion of different amounts of the external tendon between the muscle and the force transducer. 4. Our results support previous suggestions that the force measured when individual motor units are stimulated under isometric conditions is reduced by friction between the active muscle fibers and adjacent passive fibers. These frictional effects are likely to originate in the connective tissue matrix connecting adjacent muscle fibers. However, because these effects are virtually eliminated by small movements, linear summation of motor unit tensions should occur at low force levels under nonisometric conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

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Recruitment of motor units in two fascicles of the semispinalis cervicis muscle

Journal of Neurophysiology ◽

10.1152/jn.00953.2011 ◽

2012 ◽

Vol 107 (11) ◽

pp. 3078-3085 ◽

Cited By ~ 11

Author(s):

Jochen Schomacher ◽

Jakob Lund Dideriksen ◽

Dario Farina ◽

Deborah Falla

Keyword(s):

Motor Unit ◽

Discharge Rate ◽

Maximum Voluntary Contraction ◽

Motor Units ◽

Voluntary Contraction ◽

Common Input ◽

Motor Unit Synchronization ◽

Motor Unit Action ◽

Input Strength ◽

Individual Motor

This study investigated the behavior of motor units in the semispinalis cervicis muscle. Intramuscular EMG recordings were obtained unilaterally at levels C2 and C5 in 15 healthy volunteers (8 men, 7 women) who performed isometric neck extensions at 5%, 10%, and 20% of the maximal force [maximum voluntary contraction (MVC)] for 2 min each and linearly increasing force contractions from 0 to 30% MVC over 3 s. Individual motor unit action potentials were identified. The discharge rate and interspike interval variability of the motor units in the two locations did not differ. However, the recruitment threshold of motor units detected at C2 ( n = 16, mean ± SD: 10.3 ± 6.0% MVC) was greater than that of motor units detected at C5 ( n = 92, 6.9 ± 4.3% MVC) ( P < 0.01). A significant level of short-term synchronization was identified in 246 of 307 motor unit pairs when computed within one spinal level but only in 28 of 110 pairs of motor units between the two levels. The common input strength, which quantifies motor unit synchronization, was greater for pairs within one level (0.47 ± 0.32) compared with pairs between levels (0.09 ± 0.07) ( P < 0.05). In a second experiment on eight healthy subjects, interference EMG was recorded from the same locations during a linearly increasing force contraction from 0 to 40% MVC and showed significantly greater EMG amplitude at C5 than at C2. In conclusion, synaptic input is distributed partly independently and nonuniformly to different fascicles of the semispinalis cervicis muscle.

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Numbers and contraction-values of individual motor-units examined in some muscles of the limb

Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character ◽

10.1098/rspb.1930.0032 ◽

1930 ◽

Vol 106 (745) ◽

pp. 326-357 ◽

Cited By ~ 224

Keyword(s):

Motor Unit ◽

Motor Nerve ◽

Motor Units ◽

Muscle Fibres ◽

Total Potential ◽

Quantitative Statement ◽

Effector Organ ◽

Quantum Reaction ◽

The Individual ◽

Individual Motor

“The muscle with its nerve may be thought of as an additive assemblage of motor-units, meaning by motor-unit an individual motor nerve-fibre with the bunch” [or “squad” (E. L. Porter, 1929 (1))]“ of muscle-fibres it activates.” (2) The components of such a unit can claim sufficiently close and sufficiently analysed interrelation to warrant acceptance for many purposes as a single functional entity. In application to reflexes, the unit thus resulting favours brevity and directness of quantitative statement. Its correspondence with a so-to-say quantum reaction, which forms the basis, by combinations temporal and numerical, of all grading of the muscle as effector-organ, fits it for measuring that grading. It is, moreover, applicable centrally as well as peripherally, since the motor-units active number the motoneurones discharging. Such mensuration, the total of the pool of motoneurones being known, evaluates per se the given reaction in terms of the total potential reaction. 1. Contraction-Tension of the Individual Motor-Unit. In the following experiments it was therefore sought to find the physiological size of the motor-unit, i. e. , to measure its contraction-tension. The muscles examined (cat) have been gastrocnemius (median head) soleus, semitendinosus, extensor longus digitorum , and, less fully, tibialis anticus and crureus.

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Effect of compensatory hypertrophy studied in individual motor units in medial gastrocnemius muscle of the cat

Journal of Neurophysiology ◽

10.1152/jn.1978.41.2.496 ◽

1978 ◽

Vol 41 (2) ◽

pp. 496-508 ◽

Cited By ~ 30

Author(s):

J. V. Walsh ◽

R. E. Burke ◽

W. Z. Rymer ◽

P. Tsairis

Keyword(s):

Motor Unit ◽

Muscle Fibers ◽

Motor Units ◽

Compensatory Hypertrophy ◽

Medial Gastrocnemius ◽

Cross Sectional ◽

Unit Type ◽

The Mean ◽

Tetanic Tension ◽

Individual Motor

1. Compensatory hypertrophy of the medial gastrocnemius (MG) muscle was produced by denervating or removing its synergists (i.e., the lateral gastrocnemius, soleus, and plantaris muscles) in adult cats. Following survival times of 14-32 wk, intracellular recording and stimulation techniques were used to study the motor-unit population in MG. The data obtained were compared with results from MG motor units in normal unoperated cats of the same body size and weight. 2. Using criteria employed for normal motor units, the units in hypertrophic MG muscles were readily classified into the same groups (types FF, F(int), FR, and S) as in normal MG. There was no detectable difference in the distribution of motor-unit types after hypertrophy. 3. When compared with a normal motor-unit sample, there was a large increase in mean tetanic tension, but no significant change in twitch tension, for each motor-unit type in the hypertrophied muscles. The most marked increase was found among the fatigue-resistant type S and type FR motor units. There was no alteration of twitch contraction times or fatigue resistance in any unit type after hypertrophy. 4. For each motor-unit type, the mean homonymous (MG) group Ia EPSP amplitude was the same in normal and hypertrophic MG populations. There was, however, a significant increase in the average conduction velocity of MG motor axons in the animals with uncomplicated MG synergist removal and maximal MG hypertrophy. 5. On the basis of histochemical staining, muscle fibers from comparable sections of hypertrophic and contralateral (unoperated) MG muscles were presumptively identified as belonging to FF, FR, or S units. There was no significant difference between hypertrophic and contralateral MG muscles in the percentage of each fiber type, although there was some variability in muscle composition from one cat to another. One muscle pair was studied in detail for fiber cross-sectional area. In this cat, with marked hypertrophy by muscle weight, there was a modest increase in the mean fiber areas of histochemical S and FR muscle fibers, but no evident change in FF fibers, on the hypertrophic side. 6. MG motor units were examined in several cats in which synergist removal resulted in scarring and marked limitation of passive ankle mobility, and no evident weight gain in MG. Motor units of all types in these animals showed a decrease in twitch tension and in mean twitch/tetanus ratios, with little alteration in mean tetanic tensions. 7. The main effect of compensatory hypertrophy under the present conditions was a large increase in tetanic tension output from individual motor units due, at least in part, to an increase in fiber cross-sectional area. There was no evidence indicating any "conversion" of motor units or of their muscle fibers from one type to another.

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A non-invasive brain-machine interface via independent control of individual motor units

10.1101/2021.03.22.436518 ◽

2021 ◽

Author(s):

Emanuele Formento ◽

Paul Botros ◽

Jose Carmena

Keyword(s):

Motor Unit ◽

High Performance ◽

Biceps Brachii ◽

Motor Units ◽

Performance Limits ◽

Non Invasive ◽

Machine Interface ◽

And Performance ◽

Isometric Muscle ◽

Individual Motor

Brain-machine interfaces (BMIs) have the potential to restore independence in people with disabilities, yet a compromise between non-invasiveness and performance limits their translational relevance. Here, we demonstrate a high-performance BMI controlled by individual motor units non-invasively recorded from the biceps brachii. Through real-time auditory and visual neurofeedback of motor unit activity, 8 participants learned to skillfully and independently control three motor units in order to complete a two-dimensional center-out task, with marked improvements in control over 6 days of training. Concomitantly, dimensionality of the motor unit population increased significantly relative to naturalistic behaviors, largely violating recruitment orders displayed during stereotyped, isometric muscle contractions. Finally, participants' performance on a spelling task demonstrated translational potential of a motor unit BMI, exceeding performance across existing non-invasive BMIs. These results demonstrate a yet-unexplored level of flexibility of the peripheral sensorimotor system and show that this can be exploited to create novel non-invasive, high-performance BMIs.

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The effect of activity during early postnatal development on motor unit size

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y04-082 ◽

2004 ◽

Vol 82 (8-9) ◽

pp. 777-783 ◽

Cited By ~ 1

Author(s):

G Vrbová ◽

M B Lowrie

Keyword(s):

Postnatal Development ◽

Motor Unit ◽

Motor Units ◽

Final Outcome ◽

Adult Size ◽

Unit Size ◽

Early Postnatal Development ◽

Longus Muscle ◽

Old Rats ◽

Individual Motor

At early stages of neuromuscular development, motor unit territory is expanded, with each muscle fibre being supplied by several axons. During postnatal development, some synapses are eliminated, motor unit size decreases, and the adult distribution of motor unit sizes emerges. This process depends on activity, since it proceeds more rapidly when the nerve is activated and is slower when activity is reduced. Here we studied whether, in addition to influencing the rate of retraction of motor unit territory, activity during the critical period of development affects the final outcome of the distribution of motor unit sizes. The sciatic nerve of 8- to 12-day-old rats was stimulated daily. One week later the tension of the extensor digitorum longus muscle and that of its individual motor units was recorded. The sizes of individual motor units were calculated and compared with those from animals that received no stimulation. The distribution of motor unit sizes from stimulated muscles was not significantly different from those from control muscles. Therefore, we conclude that although activity increases the rate at which motor units attain their adult size, it does not influence the final outcome of motor unit size distribution.Key words: motor unit, electrical stimulation, postnatal development, polyneuronal elimination.

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Frequency-response analysis of vestibular-induced neck reflex in cat. II. Functional significance of cervical afferents and polysynaptic descending pathways

Journal of Neurophysiology ◽

10.1152/jn.1978.41.2.459 ◽

1978 ◽

Vol 41 (2) ◽

pp. 459-471 ◽

Cited By ~ 35

Author(s):

K. Ezure ◽

S. Sasaki ◽

Y. Uchino ◽

V. J. Wilson

Keyword(s):

Frequency Response ◽

Motor Unit ◽

Motor Units ◽

Response Analysis ◽

Phase Lag ◽

Descending Pathways ◽

Vestibulocollic Reflex ◽

Afferent System ◽

Vestibulospinal Tract ◽

Individual Motor

1. Three major inputs to cervical motoneurons were analyzed with frequency-response methods in decerebrate and unanesthetized cats: the afferent system including the gamma-loop, the vestibulospinal tract, and some polysynaptic tract that transmits vestibular influences in the vestibulocollic reflex. 2. The dorsal roots C1-C4 were cut in order to open the feedback loop through the gamma-fiber spindle-afferent system. No effects were observed on the gain and the phase lag of motor-unit response. However, the DC components of the response were consistently decreased by deafferentation both in motor-unit and compound EMG responses. Firing of muscle spindle afferents was modulated during oscillation of the turntable. The majority of responses of spindle afferents was in phase with the simultaneously recorded extrafusal motor activity. These results indicate the existence of alpha-gamma coactivation in the vestibulocollic reflex. 3. Intracellular recording from cervical motoneurons showed that monosynaptic EPSP's were induced by dorsal root stimulation. By far the larger EPSP's, however, were observed with latencies of 10-15 ms, showing the existence of more powerful polysynaptic routes. It is postulated that the cervical afferent system controls the gain of the reflex by changing the number of motor units participating in the vestibulocollic reflex through some polysynaptic pathways, and that the afferent system scarcely changes, the gain or phase lag of individual motor units through a short stretch reflex loop. 4. The medial vestibulospinal tract in the MLF was interrupted, but no effects were detected on the dynamic characteristics of the frequency response. This shows the existence of other effective descending pathways which function as an integrator. 5. Remarkable phase advances were induced by intravenous infusion of Nembutal. This suggests that the Nembutal decreased the activity of the polysynaptic pathways and that the neural integrator, composed of polysynaptic networks, was prevented from fulfilling its ordinary function.

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Amplitude cancellation reduces the size of motor unit potentials averaged from the surface EMG

Journal of Applied Physiology ◽

10.1152/japplphysiol.01282.2005 ◽

2006 ◽

Vol 100 (6) ◽

pp. 1928-1937 ◽

Cited By ~ 78

Author(s):

Kevin G. Keenan ◽

Dario Farina ◽

Roberto Merletti ◽

Roger M. Enoka

Keyword(s):

Motor Unit ◽

Conduction Velocity ◽

Motor Units ◽

Surface Emg ◽

Excitation Level ◽

Bipolar Electrode ◽

Emg Signal ◽

Motor Unit Synchronization ◽

Rate Coding ◽

Simulated Surface

The purpose of the study was to evaluate the influence of selected physiological parameters on amplitude cancellation in the simulated surface electromyogram (EMG) and the consequences for spike-triggered averages of motor unit potentials derived from the interference and rectified EMG signals. The surface EMG was simulated from prescribed recruitment and rate coding characteristics of a motor unit population. The potentials of the motor units were detected on the skin over a hand muscle with a bipolar electrode configuration. Averages derived from the EMG signal were generated using the discharge times for each of the 24 motor units with lowest recruitment thresholds from a population of 120 across three conditions: 1) excitation level; 2) motor unit conduction velocity; and 3) motor unit synchronization. The area of the surface-detected potential was compared with potentials averaged from the interference, rectified, and no-cancellation EMGs. The no-cancellation EMG comprised motor unit potentials that were rectified before they were summed, thereby preventing cancellation between the opposite phases of the potentials. The percent decrease in area of potentials extracted from the rectified EMG was linearly related to the amount of amplitude cancellation in the interference EMG signal, with the amount of cancellation influenced by variation in excitation level and motor unit conduction velocity. Motor unit synchronization increased potentials derived from both the rectified and interference EMG signals, although cancellation limited the increase in area for both potentials. These findings document the influence of amplitude cancellation on motor unit potentials averaged from the surface EMG and the consequences for using the procedure to characterize motor unit properties.

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Bioengineered model of the human motor unit with physiologically functional neuromuscular junctions

Scientific Reports ◽

10.1038/s41598-021-91203-5 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Rowan P. Rimington ◽

Jacob W. Fleming ◽

Andrew J. Capel ◽

Patrick C. Wheeler ◽

Mark P. Lewis

Keyword(s):

Neuromuscular Junction ◽

Motor Neuron ◽

Motor Unit ◽

Motor Nerve ◽

Motor Units ◽

Extracellular Matrices ◽

Synaptic Membrane ◽

Type I ◽

Dependent Manner ◽

Human Motor

AbstractInvestigations of the human neuromuscular junction (NMJ) have predominately utilised experimental animals, model organisms, or monolayer cell cultures that fail to represent the physiological complexity of the synapse. Consequently, there remains a paucity of data regarding the development of the human NMJ and a lack of systems that enable investigation of the motor unit. This work addresses this need, providing the methodologies to bioengineer 3D models of the human motor unit. Spheroid culture of iPSC derived motor neuron progenitors augmented the transcription of OLIG2, ISLET1 and SMI32 motor neuron mRNAs ~ 400, ~ 150 and ~ 200-fold respectively compared to monolayer equivalents. Axon projections of adhered spheroids exceeded 1000 μm in monolayer, with transcription of SMI32 and VACHT mRNAs further enhanced by addition to 3D extracellular matrices in a type I collagen concentration dependent manner. Bioengineered skeletal muscles produced functional tetanic and twitch profiles, demonstrated increased acetylcholine receptor (AChR) clustering and transcription of MUSK and LRP4 mRNAs, indicating enhanced organisation of the post-synaptic membrane. The number of motor neuron spheroids, or motor pool, required to functionally innervate 3D muscle tissues was then determined, generating functional human NMJs that evidence pre- and post-synaptic membrane and motor nerve axon co-localisation. Spontaneous firing was significantly elevated in 3D motor units, confirmed to be driven by the motor nerve via antagonistic inhibition of the AChR. Functional analysis outlined decreased time to peak twitch and half relaxation times, indicating enhanced physiology of excitation contraction coupling in innervated motor units. Our findings provide the methods to maximise the maturity of both iPSC motor neurons and primary human skeletal muscle, utilising cell type specific extracellular matrices and developmental timelines to bioengineer the human motor unit for the study of neuromuscular junction physiology.

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