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Journal articles on the topic 'Motor Units (MUs)'

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Author: Grafiati

Published: 14 April 2023

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1

Knight, C. A., and G. Kamen. "Superficial motor units are larger than deeper motor units in human vastus lateralis muscle." Muscle & Nerve 31, no. 4 (2005): 475–80. http://dx.doi.org/10.1002/mus.20265.

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2

Fuglevand, Andrew J., Rosemary A. Lester, and Richard K. Johns. "Distinguishing intrinsic from extrinsic factors underlying firing rate saturation in human motor units." Journal of Neurophysiology 113, no. 5 (March 1, 2015): 1310–22. http://dx.doi.org/10.1152/jn.00777.2014.

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During voluntary contraction, firing rates of individual motor units (MUs) increase modestly over a narrow force range beyond which little additional increase in firing rate is seen. Such saturation of MU discharge may be a consequence of extrinsic factors that limit net synaptic excitation acting on motor neurons (MNs) or may be due to intrinsic properties of the MNs. Two sets of experiments involving recording of human biceps brachii MUs were carried out to evaluate saturation. In the first set, the extent of saturation was quantified for 136 low-threshold MUs during isometric ramp contractions. Firing rate-force data were best fit by a saturating function for 90% of MUs recorded with a maximum rate of 14.8 ± 2.0 impulses/s. In the second set of experiments, to distinguish extrinsic from intrinsic factors underlying saturation, we artificially augmented descending excitatory drive to biceps MNs by activation of muscle spindle afferents through tendon vibration. We examined the change in firing rate caused by tendon vibration in 96 MUs that were voluntarily activated at rates below and at saturation. Vibration had little effect on the discharge of MUs that were firing at saturation frequencies but strongly increased firing rates of the same units when active at lower frequencies. These results indicate that saturation is likely caused by intrinsic mechanisms that prevent further increases in firing rate in the presence of increasing synaptic excitation. Possible intrinsic cellular mechanisms that limit firing rates of motor units during voluntary effort are discussed.

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3

Brown, William F., Michael J. Strong, and Robert Snow. "Methods for estimating numbers of motor units in biceps-brachialis muscles and losses of motor units with aging." Muscle & Nerve 11, no. 5 (May 1988): 423–32. http://dx.doi.org/10.1002/mus.880110503.

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4

Defreitas, Jason M., Travis W. Beck, Xin Ye, and Matt S. Stock. "Synchronization of low- and high-threshold motor units." Muscle & Nerve 49, no. 4 (January 6, 2014): 575–83. http://dx.doi.org/10.1002/mus.23978.

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5

Johns, Richard K., and Andrew J. Fuglevand. "Number of motor units in human abductor hallucis." Muscle & Nerve 43, no. 6 (May 23, 2011): 895–96. http://dx.doi.org/10.1002/mus.22071.

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6

Bailey, E. Fiona, Keith W. Fridel, and Amber D. Rice. "Sleep/Wake Firing Patterns of Human Genioglossus Motor Units." Journal of Neurophysiology 98, no. 6 (December 2007): 3284–91. http://dx.doi.org/10.1152/jn.00865.2007.

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Although studies of the principal tongue protrudor muscle genioglossus (GG) suggest that whole muscle GG electromyographic (EMG) activities are preserved in nonrapid eye movement (NREM) sleep, it is unclear what influence sleep exerts on individual GG motor unit (MU) activities. We characterized the firing patterns of human GG MUs in wakefulness and NREM sleep with the aim of determining 1) whether the range of MU discharge patterns evident in wakefulness is preserved in sleep and 2) what effect the removal of the “wakefulness” input has on the magnitude of the respiratory modulation of MU activities. Microelectrodes inserted into the extrinsic tongue protrudor muscle, the genioglossus, were used to follow the discharge of single MUs. We categorized MU activities on the basis of the temporal relationship between the spike train and the respiration cycle and quantified the magnitude of the respiratory modulation of each MU using the eta (η2) index, in wakefulness and sleep. The majority of MUs exhibited subtle increases or decreases in respiratory modulation but were otherwise unaffected by NREM sleep. In contrast, 30% of MUs exhibited marked sleep-associated changes in discharge frequency and respiratory modulation. We suggest that GG MUs should not be considered exclusively tonic or phasic; rather, the discharge pattern appears to be a flexible feature of GG activities in healthy young adults. Whether such flexibility is important in the response to changes in the chemical and/or mechanical environment and whether it is preserved as a function of aging or in individuals with obstructive sleep apnea are critical questions for future research.

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7

McNulty, Penelope A., and Vaughan G. Macefield. "Intraneural microstimulation of motor axons in the study of human single motor units." Muscle & Nerve 32, no. 2 (2005): 119–39. http://dx.doi.org/10.1002/mus.20324.

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8

Emeryk-Szajewska, Barbara, and Jerzy Kopeć. "PS-54-4 Reorganization of motor units (MUS) in motor neuron disease (MND)." Electroencephalography and Clinical Neurophysiology/Electromyography and Motor Control 97, no. 4 (September 1995): S227. http://dx.doi.org/10.1016/0924-980x(95)93285-2.

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9

Palmer, S. S., and E. E. Fetz. "Discharge properties of primate forearm motor units during isometric muscle activity." Journal of Neurophysiology 54, no. 5 (November 1, 1985): 1178–93. http://dx.doi.org/10.1152/jn.1985.54.5.1178.

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Activity of single motor units (MUs) was recorded in forelimb muscles of rhesus macaques while they generated isometric ramp-and-hold torques about the wrist. Multiunit electromyographic (EMG) activity was recorded from 10-12 identified flexor and extensor muscles of the wrist and digits with implanted EMG wire electrodes. Single MUs from these muscles were recorded with a remotely controlled tripolar microelectrode array. The parent muscle of each MU was determined by compiling MU-triggered averages of multiunit EMGs. The MU firing patterns during the isometric task were determined from response histograms aligned with change in torque. At moderate torque levels, MUs (n = 86) exhibited four types of discharge patterns during the ramp-and-hold trajectory: phasic-tonic (23%), tonic (33%), decrementing (39%), and phasic (5%). Phasic-tonic MUs exhibited a phasic burst of activity during the torque ramp which exceeded the firing rate during the static hold period. Both phasic-tonic and tonic MUs exhibited a constant mean firing rate during the hold period; the discharge of decrementing MUs gradually decreased during the static hold. Phasic MUs fired only during the change in force. The relation between MU firing rate and torque was investigated as the monkeys generated responses of different levels of static torque during the hold period. Mean firing rate during the hold was found to be proportional to static torque up to moderate torque levels, where it reached a maximum. In the linear range, the mean rate-torque slope was 3.4 +/- 1.9 imp/s per 10(5) dyn . cm (n = 9).

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10

Sokoloff, Alan J. "Localization and Contractile Properties of Intrinsic Longitudinal Motor Units of the Rat Tongue." Journal of Neurophysiology 84, no. 2 (August 1, 2000): 827–35. http://dx.doi.org/10.1152/jn.2000.84.2.827.

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Tongue dysfunction is a hallmark of many human clinical disorders, yet we lack even a rudimentary understanding of tongue neural control. Here, the location and contractile properties of intrinsic longitudinal motor units (MUs) of the rat tongue body are described to provide a foundation for developing and testing theories of tongue motor control. One hundred and sixty-five MUs were studied by microelectrode penetration and stimulation of individual motor axons coursing in the terminal portion of the lateral (retrusor) branch of the hypoglossal nerve in the rat. Uniaxial MU force was recorded by a transducer attached to the protruded tongue tip, and MU location was estimated by electromyographic (EMG) electrodes implanted into the anterior, middle, and posterior portions of the tongue body. All MUs produced retrusive force. MU twitch force ranged from 2–129 mg (mean = 35 mg) and tetanic force ranged from 9–394 mg (mean = 95 mg). MUs reached maximal twitch force in 8–33 ms (mean = 15 ms) and were resistant to fatigue; following 2 min of stimulation, MUs ( n= 11) produced 78–131% of initial force. EMG data were collected for 105 MUs. For 65 of these MUs, the EMG response was confined to a single electrode location: for 26 MUs to the anterior, 21 MUs to the middle, and 18 MUs to the posterior portion of the tongue. Of the remaining MUs, EMG responses were observed in two (38/40) or all three (2/40) tongue regions. These data provide the first contractile measures of identified intrinsic tongue body MUs and the first evidence that intrinsic longitudinal MUs are restricted to a portion of tongue length. Localization of MU territory suggests a role for intrinsic MU in the regional control of the mammalian tongue observed during feeding and speech.

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11

Gossen, E. Roderich, Tanya D. Ivanova, and S. Jayne Garland. "Ischemia sensitivity and motoneuron afterhyperpolarization in human motor units." Muscle & Nerve 30, no. 2 (2004): 195–201. http://dx.doi.org/10.1002/mus.20083.

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12

Callister, Robert J., Sanjay Sesodia, Roger M. Enoka, Patti M. Nemeth, Robert M. Reinking, and Douglas G. Stuart. "Fatigue of rat hindlimb motor units: Biochemical- physiological associations." Muscle & Nerve 30, no. 6 (2004): 714–26. http://dx.doi.org/10.1002/mus.20158.

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13

Del Valle, Alejandro, and Christine K. Thomas. "Firing rates of motor units during strong dynamic contractions." Muscle & Nerve 32, no. 3 (2005): 316–25. http://dx.doi.org/10.1002/mus.20371.

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14

Roy, Roland R., Alan Garfinkel, Melinda Ounjian, Jeremy Payne, Alan Hirahara, Emmie Hsu, and V. Reggie Edgerton. "Three-dimensional structure of cat tibialis anterior motor units." Muscle & Nerve 18, no. 10 (October 1995): 1187–95. http://dx.doi.org/10.1002/mus.880181015.

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15

Palmer, S. S., and E. E. Fetz. "Effects of single intracortical microstimuli in motor cortex on activity of identified forearm motor units in behaving monkeys." Journal of Neurophysiology 54, no. 5 (November 1, 1985): 1194–212. http://dx.doi.org/10.1152/jn.1985.54.5.1194.

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We examined the magnitude and extent of output effects elicited from focal cortical sites on the activity of individual motor units (MUs) by delivering single-pulse intracortical microstimuli (S-ICMS) (5-15 microA) during isometric wrist activity. Stimulation sites in the precentral gyrus (area 4) were chosen for study if stimulus-triggered averages (stimulus-TAs) of multiunit electromyograms (EMGs) revealed poststimulus facilitation (PStimF) of EMG activity in any of the coactivated wrist muscles. Single MUs were then isolated in the facilitated muscles with a remotely controlled tripolar microelectrode. MUs were identified by their signatures in their parent muscles (from MU-triggered averages of EMGs) and by their firing pattern during ramp-and-hold wrist responses. One objective was to quantify the magnitude and time course of the effects on single MUs by compiling peristimulus histograms of MU firing. The cross-correlation histograms between S-ICMS and MU action potentials showed peaks with onset latencies of 8.8 +/- 1.7 ms (mean +/- SD, n = 64) and durations of 1.8 +/- 1.2 ms (n = 104). The cumulative sums of the correlogram peaks resembled the rising phase of corticomotoneuronal excitatory postsynaptic potentials previously recorded in forelimb motoneurons. Comparison of correlogram peaks with stimulus-TAs of MU potentials suggests that the duration of PStimF of multiunit EMG can be accounted for, in approximately equal proportions, by l) the variation in firing time of single MUs (i.e., the width of the MU correlogram peaks), 2) the width of single MU potentials, and 3) the contribution of different MUs at different latencies. The sizes of the correlogram peaks relative to base line were larger than the PStimF of multiunit EMGs, and increased more rapidly with stimulus intensity, indicating appreciable cancellation in the multiunit records. A second objective was to determine whether S-ICMS affected all the MUs of a facilitated muscle, or only a particular subset. Of 104 MUs sampled in facilitated muscles, 99 (95%) were found to be individually facilitated (P less than 0.05). MU firing patterns during isometric ramp-and-hold torque responses were characterized as phasic, phasic-tonic, tonic, or decrementing; stimulation at a given cortical site was found to facilitate all four types of MUs. When more than one muscle showed PStimF from a site, MUs belonging to each of the facilitated muscles were facilitated individually by S-ICMS at that site.(ABSTRACT TRUNCATED AT 400 WORDS)

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16

Dubose, Laniel, Teresa B. Schelhorn, and H. Peter Clamann. "Changes in contractile speed of cat motor units during activity." Muscle & Nerve 10, no. 8 (October 1987): 744–52. http://dx.doi.org/10.1002/mus.880100811.

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17

Dengler, Reinhard, Richard B. Stein, and Christine K. Thomas. "Axonal conduction velocity and force of single human motor units." Muscle & Nerve 11, no. 2 (February 1988): 136–45. http://dx.doi.org/10.1002/mus.880110209.

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18

Hamm, Thomas M., Patti M. Nemeth, Lata Solanki, Debra A. Gordon, Robert M. Reinking, and Douglas G. Stuart. "Association between biochemical and physiological properties in single motor units." Muscle & Nerve 11, no. 3 (March 1988): 245–54. http://dx.doi.org/10.1002/mus.880110309.

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19

Robinson, Andrew J., and H. Peter Clamann. "Effects of glucocorticoids on motor units in cat hindlimb muscles." Muscle & Nerve 11, no. 7 (July 1988): 703–13. http://dx.doi.org/10.1002/mus.880110706.

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20

Dorfman, Leslie J., Jane E. Howard, and Kevin C. McGill. "Triphasic behavioral response of motor units to submaximal fatiguing exercise." Muscle & Nerve 13, no. 7 (July 1990): 621–28. http://dx.doi.org/10.1002/mus.880130711.

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21

Thomas, Christine K., Roland S. Johansson, and Brenda Bigland-Ritchie. "Incidence of F waves in single human thenar motor units." Muscle & Nerve 25, no. 1 (2001): 77–82. http://dx.doi.org/10.1002/mus.10005.

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22

Tam, Siu Lin, Vey Archibald, Neil Tyreman, and T. Gordon. "Effect of exercise on stability of chronically enlarged motor units." Muscle & Nerve 25, no. 3 (February 19, 2002): 359–69. http://dx.doi.org/10.1002/mus.10057.

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23

Galea, Victoria, Darcy Fehlings, Susan Kirsch, and Alan McComas. "Depletion and sizes of motor units in spinal muscular atrophy." Muscle & Nerve 24, no. 9 (2001): 1168–72. http://dx.doi.org/10.1002/mus.1128.

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24

Konstanzer, A., R. Dengler, S. Hesse, J. Elek, and W. Wolf. "Weakness of motor units (MUs) in late amyotrophic lateral sclerosis (ALS)." Electroencephalography and Clinical Neurophysiology 75 (January 1990): S74. http://dx.doi.org/10.1016/0013-4694(90)91986-y.

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25

Spielmann, J. M., and E. K. Stauffer. "Morphological observations of motor units connected in-series to Golgi tendon organs." Journal of Neurophysiology 55, no. 1 (January 1, 1986): 147–62. http://dx.doi.org/10.1152/jn.1986.55.1.147.

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The glycogen-depletion technique (17, 32) has been used to examine the functional and morphological relationships between single isolated motor units (MUs) and single isolated Golgi tendon organs (GTOs) that were excited by the MUs in the soleus muscle of the cat. All MUs whose twitch contraction generated a brisk discharge from the GTOs during the rising and plateau phase of force development had a muscle fiber attached specifically to the proximal end of the GTOs. A significant (P less than 0.05) linear relationship was found between GTO discharge rate and the cross-sectional area of the muscle fibers that connected to a receptor. This was true when the correlation was calculated between firing rate and 1) the cross-sectional area of the entire collection of muscle fibers that connected in series to the GTOs; and 2) for the cross-sectional areas of the individually depleted muscle fibers that inserted on the GTO sample. These findings support the notion that the most physiologically relevant input for GTOs arises from the MUs that are attached directly in-series with the receptor.

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26

Doherty, Timothy J., and William F. Brown. "Age-related changes in the twitch contractile properties of human thenar motor units." Journal of Applied Physiology 82, no. 1 (January 1, 1997): 93–101. http://dx.doi.org/10.1152/jappl.1997.82.1.93.

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Doherty, Timothy J., and William F. Brown. Age-related changes in the twitch contractile properties of human thenar motor units. J. Appl. Physiol. 82(1): 93–101, 1997.—The purpose of this study was to examine the effects of aging on the contractile and electrophysiological properties of human thenar motor units (MUs). Percutaneous electrical stimulation of single motor axons within the median nerve was used to isolate and examine the twitch tensions, contractile speeds, and surface-detected MU action potential (S-MUAP) sizes of 48 thenar MUs in 17 younger subjects (25–53 yr) and 44 thenar MUs in 9 older subjects (64–77 yr). A wide range of twitch tensions, contractile speeds, and S-MUAP sizes was observed in both age groups. However, older subjects had significantly larger MU twitch tensions and slower MU twitch contraction and half-relaxation times. These changes were accompanied by increased S-MUAP sizes. These findings suggest that the human thenar MU pool undergoes significant age-related increase in MU size and slowing of contractile speed. Such adaptation may help to overcome previously reported age-related losses of thenar MUs.

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27

Galea, Victoria, Hubert De Bruin, Richard Cavasin, and Alan J. McComas. "The numbers and relative sizes of motor units estimated by Computer." Muscle & Nerve 14, no. 11 (November 1991): 1123–30. http://dx.doi.org/10.1002/mus.880141114.

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28

Kryściak, Katarzyna, Jan Celichowski, Hanna Drzymała-Celichowska, Phillip F. Gardiner, and Piotr Krutki. "Force regulation and electrical properties of motor units in overloaded muscle." Muscle & Nerve 53, no. 1 (November 26, 2015): 96–106. http://dx.doi.org/10.1002/mus.24690.

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29

Chan, K. Ming, Timothy J. Doherty, and William F. Brown. "Contractile properties of human motor units in health, aging, and disease." Muscle & Nerve 24, no. 9 (2001): 1113–33. http://dx.doi.org/10.1002/mus.1123.

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30

Turkawski, Stan J. J., and Theo M. G. J. van Eijden. "Action potential shape of rabbit masseter motor units and jaw angle." Muscle & Nerve 24, no. 11 (2001): 1551–53. http://dx.doi.org/10.1002/mus.1182.

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31

Doherty, Timothy J., and William F. Brown. "A method for the longitudinal study of human thenar motor units." Muscle & Nerve 17, no. 9 (September 1994): 1029–36. http://dx.doi.org/10.1002/mus.880170910.

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32

Rafuse, V. F., T. Gordon, and R. Orozco. "Proportional enlargement of motor units after partial denervation of cat triceps surae muscles." Journal of Neurophysiology 68, no. 4 (October 1, 1992): 1261–76. http://dx.doi.org/10.1152/jn.1992.68.4.1261.

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1. To determine the capacity of motoneurons to increase their motor unit (MU) size by collateral sprouting and to assess this capacity in relation to the size of the motor nerve, we partially denervated soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) muscles in adult and neonatal cats. Isometric force and extracellular nerve potentials were recorded from > or = 7% of the remaining MUs, 2.5-18 mo later. S1 or L7 roots were sectioned unilaterally and the number of remaining MUs was quantified by use of charge and force measurements. 2. The mean unit force increased inversely with MU number in partially denervated muscles, but the increase was less than predicted for extensive denervations (> or = 90%). The same enlargement of MU size occurred whether muscles were partially denervated in neonatal or adult animals. 3. The force distribution of MUs in partially denervated muscles was similar to normal but was shifted to larger force values in direct proportion to the extent of partial denervation (PD). All MUs increased in size by the same factor to preserve the normal force distribution. 4. Normal size relationships among force, contractile speed, and axon potential amplitude were observed for MUs in partially denervated muscles. Because changes in muscle fiber size could not account for the increase in unit force, these data show that increase in MU size, with respect to unit force and innervation ratio (muscle fibers per motoneuron), is proportional to the size of the motor nerve. 5. Enlargement of MU size in partially denervated muscles did not have a retrograde effect on nerve fiber caliber because axon potential amplitude and conduction velocity were not changed after PD. 6. Under conditions of extensive PD (> 85%), regenerated nerves from the cut spinal root reinnervated the gastrocnemius muscles. It is likely that nerves supplied muscle fibers that were not innervated by sprouts from nerves in the uncut root as well as displacing sprouts from terminals in extensively enlarged MUs. 7. We conclude that all motoneurons within a motor pool compensate for partial nerve injuries by collateral sprouting and that enlargement of MU size is a function of motor nerve size, consistent with Henneman's size principle.

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33

Doherty, Timothy J. "Effects of Short-Term Training on Physiologic Properties of Human Motor Units." Canadian Journal of Applied Physiology 25, no. 3 (June 1, 2000): 194–203. http://dx.doi.org/10.1139/h00-015.

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The human neuromuscular system possesses a remarkable ability to adapt to any change in the demands placed upon it. Adaptation to training or disuse is reflected in the activation patterns and physiologic properties of the motor unit (MU) pool in a given muscle group. This review summarizes the studies that have examined such adaptation at the level of the single MU. Disuse, as revealed by electrophysiologic studies, results in impaired MU recruitment and smaller twitch tensions in tow and high threshold MUs. Alternatively, short-term training improves MU recruitment and generally results in larger MU twitch tensions. A method has been developed to physiologically characterize and longitudinally follow single thenar MUs. Studies utilizing these methods have demonstrated that MUs differing in their baseline physiologic characteristics respond very differently to identical short-term training programs. These observations at the single MU level provide insight into training adaptations in whole muscles and muscle groups. Key words: motor unit, training, disuse, electromyography

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34

Posa, Andreas, Andrej Temneanu, Alexander Emmer, Thomas Langer, and Malte Kornhuber. "Jitter patterns in conventional concentric needle electromyography recordings of regenerating motor units." Muscle & Nerve 62, no. 5 (August 13, 2020): 593–96. http://dx.doi.org/10.1002/mus.27033.

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35

Clamann, H. Peter, and Teresa B. Schelhorn. "Nonlinear force addition of newly recruited motor units in the cat hindlimb." Muscle & Nerve 11, no. 10 (September 1988): 1079–89. http://dx.doi.org/10.1002/mus.880111012.

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36

Dengler, Reinhard, Annette Konstanzer, Gerald Küther, Stefan Hesse, Werner Wolf, and Albrecht Strupplerdr. "Amyotrophic lateral sclerosis: Macro-EMG and twitch forces of single motor units." Muscle & Nerve 13, no. 6 (June 1990): 545–50. http://dx.doi.org/10.1002/mus.880130612.

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37

Wiegner, Allen W., M. Margaret Wierzbicka, Llewelyn Davies, and Robert R. Young. "Discharge properties of single motor units in patients with spinal cord injuries." Muscle & Nerve 16, no. 6 (June 1993): 661–71. http://dx.doi.org/10.1002/mus.880160613.

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38

Gobbo, Massimiliano, Jan Celichowski, Piotr Krutki, Hanna Drzymała-Celichowska, Moshe Solomonow, and Claudio Orizio. "A novel method to attain sinusoidal mechanical responses from single motor units." Muscle & Nerve 51, no. 1 (November 22, 2014): 134–36. http://dx.doi.org/10.1002/mus.24464.

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39

Troiani, D., G. M. Filippi, and F. Andreasi Bassi. "Nonlinear Tension Summation of Different Combinations of Motor Units in the Anesthetized Cat Peroneus Longus Muscle." Journal of Neurophysiology 81, no. 2 (February 1, 1999): 771–80. http://dx.doi.org/10.1152/jn.1999.81.2.771.

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Nonlinear tension summation of different combinations of motor units in the anesthetized cat peroneus longus muscle. The purpose of this study was to examine the linearity of summation of the forces produced by the stimulation of different combinations of type identified motor units (MUs) in the cat peroneus longus muscle (PL) under isometric conditions. The muscle was fixed at its twitch optimal length, and the tension produced by the single MU was recorded during 24- and 72-Hz stimulation. The summation analysis was first carried out for MUs belonging to the same functional group, and then different combinations of fast fatigable (FF) MUs were added to the nonfatigable slow (S) and fatigue resistant (FR) group. The tension resulting from the combined stimulation of increasing numbers of MUs (measured tension) was evaluated and compared with the linearly predicted value, calculated by adding algebraically the tension produced by the individual MUs assembled in the combination (calculated tension). Tension summation displayed deviations from linearity. S and FR MUs mainly showed marked more than linear summation; FF MUs yielded either more or less than linear summation; and, when the FF units were recruited after the S and FR MUs, less than linear summation always occurred. The magnitude of the nonlinear summation appeared stimulus frequency dependent for the fatigable FF and FI group. The relationship between measured tension and calculated tension for each MU combination was examined, and linear regression lines were fitted to each set of data. The high correlation coefficients and the different slope values for the different MU-type combinations suggested that the nonlinear summation was MU-type specific. The mechanisms of nonlinear summations are discussed by considering the consequences of internal shortening and thus the mechanical interactions among MUs and shifts in muscle fiber length to a more or less advantageous portion of single MU length-tension curves.

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40

Richardson, Patrick A., and E. Fiona Bailey. "Tonically Discharging Genioglossus Motor Units Show No Evidence of Rate Coding With Hypercapnia." Journal of Neurophysiology 103, no. 3 (March 2010): 1315–21. http://dx.doi.org/10.1152/jn.00686.2009.

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The genioglossus (GG) is considered the principle protrudor muscle of the human tongue. Unlike most skeletal muscles, GG electromyographic (EMG) activities are robustly preserved in sleep and thus may fulfill a critical role in preserving airway patency. Previous studies in human subjects also confirm that the GG EMG increases in response to chemoreceptor and mechanoreceptor stimulation. This increase occurs secondary to the recruitment of previously inactive motor units (MUs) and/or an increase in firing rate of already active MUs. Which strategy the nervous system uses when the synaptic drive onto GG motoneurons increases is not known. Here we report on GG whole muscle and tonic MU activities under conditions that mimic sleep, i.e., mild-moderate elevations in CO2 (3% inspired CO2 or the addition of a 1.0 l dead space) and elevated airway resistance. Based on previous work in rat, we hypothesized that mild hypercapnia would increase the firing rates of tonic MUs and that these effects would be further potentiated by a modest increase in airway resistance. Fine wire and tungsten microelectrodes were inserted into the GG to record whole muscle and single MU activities in 21 subjects (13 women, 8 men; 20–55 yr). Either 3% inspired CO2 or added dead space resulted in a 200–300% increase in the amplitude of both tonic and phasic components of the whole muscle GG EMG and a doubling of minute ventilation. Despite these changes, recordings obtained from a total of 84 tonically discharging GG single MUs provide no evidence of a change in firing rate under any of the conditions. On this basis we conclude that in healthy adults, the increase in the tonic component of the whole muscle GG EMG secondary to mild hypercapnia is due almost exclusively to the recruitment of previously inactive MUs.

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41

Dick, T. E., F. J. Kong, and A. J. Berger. "Correlation of recruitment order with axonal conduction velocity for supraspinally driven diaphragmatic motor units." Journal of Neurophysiology 57, no. 1 (January 1, 1987): 245–59. http://dx.doi.org/10.1152/jn.1987.57.1.245.

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Spontaneous activities of pairs of single diaphragmatic motor units (MUs) were recorded via two electrodes in anesthetized cats, ventilated with CO2 added to the inspired gas, which slightly enhanced respiratory drive (endtidal CO2 less than 6%). These MUs were characterized by their axonal conduction velocities (CVs) and relative onset times (defined as the time after onset of phrenic nerve activity until the MU began discharging divided by the duration of inspiration). Motor unit axonal CV was estimated by the conduction time and the distance between two points on the phrenic nerve. Results were compared from two experimental preparations: one with dorsal roots intact and the other with dorsal roots transected bilaterally between fourth (C4) and seventh (C7) cervical segments. Estimated mean CV for phrenic MUs was 46.2 m/s(n = 180 MU). Motor units were classified as early and late recruited MUs depending on their relative onset times. We correlated MU axonal CV with its relative recruitment time. A highly significant (P less than 0.0001), positive correlation between axonal CV and relative recruitment time was established for those diaphragmatic MUs recruited with this respiratory drive. Correlation coefficients were r = 0.70 for intact animals, r = 0.72 for dorsal rhizotomized animals, and r = 0.72 overall population. For pairs of MUs, the CV of the earlier recruited unit was compared with the CV of the later recruited unit. In 96% of pairs from intact animals and 92% of pairs from dorsal rhizotomized animals, the first MU had a lower CV than the MU recruited later. Difference in relative times of recruitment was directly related to difference in axonal CVs. However, a portion of the motor pool with high-axonal CVs was not sampled. Under conditions of these experiments, afferent input in cervical dorsal roots, including that from diaphragmatic receptors, did not influence the distribution of MU relative onset times. Further, a similar proportion of MU pairs wherein the earlier recruited MU had a CV lower than the later recruited unit was observed in intact and dorsal rhizotomized animals. We also cross-correlated 31 pairs of simultaneously recorded MUs to assess common input onto phrenic motoneurons. Common input was characterized by the presence of peaks having widths of greater than or equal to 3 ms in the cross-correlation histograms (CCHs) and occurring within 20 ms of the trigger event. Peaks were judged significant if the bin with the largest number of occurrences was significantly greater than base line and if neighboring bins were above base line.(ABSTRACT TRUNCATED AT 400 WORDS)

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Gallina, Alessio, and Taian Vieira. "Territory and fiber orientation of vastus medialis motor units: A Surface electromyography investigation." Muscle & Nerve 52, no. 6 (September 8, 2015): 1057–65. http://dx.doi.org/10.1002/mus.24662.

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43

Wakeling, James M., and Douglas A. Syme. "Wave properties of action potentials from fast and slow motor units of rats." Muscle & Nerve 26, no. 5 (October 24, 2002): 659–68. http://dx.doi.org/10.1002/mus.10263.

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Lukács, Miklós, László Vécsei, and Sándor Beniczky. "Fiber density of the motor units recruited at high and low force output." Muscle & Nerve 40, no. 1 (June 16, 2009): 112–14. http://dx.doi.org/10.1002/mus.21241.

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45

Smith, L. A., E. Eldred, and V. R. Edgerton. "Effects of age at cordotomy and subsequent exercise on contraction times of motor units in the cat." Journal of Applied Physiology 75, no. 6 (December 1, 1993): 2683–88. http://dx.doi.org/10.1152/jappl.1993.75.6.2683.

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The contraction times (CTs) of functionally isolated motor units (MUs) in the soleus (SOL) and medial gastrocnemius (MG) muscles were determined in cats that had been spinalized at ages 2 (n = 15) or 12 (n = 9) wk and then either subjected to exercise on a treadmill or simply given manipulative care of the hindlimbs. The MUs were tested approximately 12 wk after the low-thoracic cordotomy, and comparisons were made with data from control animals. The CT of 50.9 ms obtained for SOL units (n = 163) in the spinal cats was 22% shorter than the mean of 65.0 ms for MUs (n = 57) from control cats (n = 4). Contrary to expectation, the CT in animals spinalized at 12 wk was significantly shorter than that in the 2-wk group. The CT for MG units (n = 105) in spinal cats was also significantly shorter (11%) than that in controls cats (n = 66, 6 cats), and those units identified by their high fatigue index as being of slow or fatigue-resistant type had a shorter CT than units with a low index. No distinction in CT of exercised and nonexercised groups was detected for either muscle. These findings are discussed in relation to the bearing influences of supraspinal and segmental origin have on CT duration in SOL and MG muscles during growth of the kitten. A slight, significant decrease (6%) in the fatigue index of SOL MUs (n = 144) was detected, but the values remained high (mean 0.87).

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Cutsem, Michaël Van, Patrick Feiereisen, Jacques Duchateau, and Karl Hainaut. "Mechanical Properties and Behaviour of Motor Units in the Tibialis Anterior During Voluntary Contractions." Canadian Journal of Applied Physiology 22, no. 6 (November 1, 1997): 585–97. http://dx.doi.org/10.1139/h97-038.

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The present work was carried out to analyse the properties and behaviour of Tibialis anterior motor units (MUs) during voluntary contractions in humans. A total of 528 single MU mechanical properties was recorded in 10 subjects by means of the spike-triggered averaging (STA) technique. MU recruitment thresholds and discharge frequencies were recorded during linearly increasing maximal voluntary contraction (MVC). The results indicate a mean (±SD) MU torque of 25.5 ± 21.5 mN•m. and a mean time-to-peak of 45.6 ± 13.6 ms. A comparison of the average MU twitch torque with that of the muscle allowed an estimate of about 300 MUs in the Tibialis anterior. A positive linear relationship was recorded between the MU twitch torque and the recruitment threshold. The mean minimal and maximal discharge frequencies of MUs were 8.4 ± 3.0 Hz and 33.2 ± 14.7 Hz, respectively. The results of the present work indicate that MU behaviour during voluntary contractions is different in the tibialis anterior and in the adductor pollicis. Key words: discharge frequency, recruitment threshold, motor unit count

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Jakobsson, Finnbogi, Kristian Borg, Lars Edström, and Lennart Grimby. "Use of motor units in relation to muscle fiber type and size in man." Muscle & Nerve 11, no. 12 (December 1988): 1211–18. http://dx.doi.org/10.1002/mus.880111205.

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Rodriquez, Arthur A., and James C. Agre. "Correlation of motor units with strength and spectral characteristics in polio survivors and controls." Muscle & Nerve 14, no. 5 (May 1991): 429–34. http://dx.doi.org/10.1002/mus.880140508.

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Becker, Klaus, Márcio Goethel, Pedro Fonseca, João Paulo Vilas-Boas, and Ulysses Ervilha. "The Strategy of the Brain to Maintain the Force Production in Painful Contractions—A Motor Units Pool Reorganization." Cells 11, no. 20 (October 20, 2022): 3299. http://dx.doi.org/10.3390/cells11203299.

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A common symptom in neuromuscular diseases is pain, which changes human movement in many ways. Using the decomposed electromyographic signal, we investigate the strategy of the brain in recruiting different pools of motor units (MUs) to produce torque during induced muscle pain in terms of firing rate (FR), recruitment threshold (RT) and action potential amplitude (MUAPAMP). These properties were used to define two groups (G1/G2) based on a K-means clusterization method. A 2.0 mL intramuscular hypertonic (6%) or isotonic (0.9%) saline solution was injected to induce pain or act as a placebo during isometric and isokinetic knee extension contractions. While isometric torque decreases after pain induction with hypertonic solution, this does not occur in isokinetic torque. This occurs because the MUs re-organized after the injection of both solutions. This is supported by an increase in RT, in both G1 and G2 MUs. However, when inducing pain with the hypertonic solution, RT increase is exacerbated. In this condition, FR also decreases, while MUAPAMP increases only for G1 MUs. Therefore, this study proposes that the strategy for maintaining force production during pain is to recruit MUs with higher RT and MUAPAMP.

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Slawnych, Michael P., Charles A. Laszlo, and Cecil Hershler. "A review of techniques employed to estimate the number of motor units in a muscle." Muscle & Nerve 13, no. 11 (November 1990): 1050–64. http://dx.doi.org/10.1002/mus.880131108.

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