Association between biochemical and physiological properties in single motor units

Hamm, Thomas M.; Nemeth, P. M.; Solanki, L; Gordon, Debra A.; Reinking, Robert M.; Stuart, Douglas G.

doi:10.1002/mus.880110309

Cited by 43 publications

(28 citation statements)

References 35 publications

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“…Our observation of increased fatigue resistance following immobilization is not necessarily at odds with reports of decreased oxidative capacity of muscle fibers following d i s u~e~.~'~' because the standard motor unit fatigue test used in the present work is thought to challenge excitationcontraction coupling more than oxidative capacity21,28,33 (for review, see ref. 19). However, future work is necessary to determine which features of excitation-contraction coupling might be altered by immobilization.…”

Section: Changes In Forcementioning

confidence: 93%

Immobilization‐induced changes in motor unit force and fatigability in the cat

1991

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The purpose of this study was to examine the effects of 3 weeks of immobilization on the mechanical properties of motor units in a cat hindlimb muscle. The muscle, tibialis posterior, was immobilized in a shortened position. Motor units were classified as type FF, F(int), FR, or S. Force, axonal conduction velocity, fatigability, and proportions of motor unit types were compared in control and immobilized muscles. All properties exhibited some change after immobilization, including slower axonal conduction velocities, greater twitch forces, slower twitch contraction times, and greater tetanic forces. In addition, most motor units were less fatigable after immobilization. The number of motor units that could not be included in one of the four classification categories increased significantly after immobilization; these units exhibited normal axon conductivity but failed to produce measurable force or associated EMG. Short-term immobilization induced a variety of physiological adaptations in neuromuscular processes that varied with motor unit type.

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Section: Changes In Forcementioning

confidence: 93%

Immobilization‐induced changes in motor unit force and fatigability in the cat

1991

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“…Importantly, the contractile and fatigue properties of motor units correspond with the expression of myosin heavy chain (MyHC) isoforms by the muscle fibers they comprise. Although there is considerable diversity in motor unit properties, all muscle fibers within an individual motor unit are homogeneous in their type composition (Fournier and Sieck, 1988, Hamm et al, 1988, Nemeth et al, 1986). Accordingly, diaphragm muscle fibers within a motor unit are of the same fiber type (Enad et al, 1989, Johnson et al, 1994, Sieck et al, 1989a, Sieck et al, 1996).…”

Section: Classification Of Motor Unit Typesmentioning

confidence: 99%

Convergence of Pattern Generator Outputs on a Common Mechanism of Diaphragm Motor Unit Recruitment

Mantilla

Seven

Sieck

2014

Progress in Brain Research

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Motor units are the final element of neuromotor control. In manner analogous to the organization of neuromotor control in other skeletal muscles, diaphragm motor units comprise phrenic motoneurons located in the cervical spinal cord that innervate the diaphragm muscle, the main inspiratory muscle in mammals. Diaphragm motor units play a primary role in sustaining ventilation, but are also active in other non-ventilatory behaviors, including coughing, sneezing, vomiting, defecation and parturition. Diaphragm muscle fibers comprise all fiber types. Thus, diaphragm motor units display substantial differences in contractile and fatigue properties, but importantly properties of the motoneuron and muscle fibers within a motor unit are matched. As in other skeletal muscles, diaphragm motor units are recruited in order such that motor units that display greater fatigue resistance are recruited earlier and more often than more fatigable motor units. The properties of the motor unit population are critical determinants of the function of a skeletal muscle across the range of possible motor tasks. Accordingly, fatigue-resistant motor units are sufficient to generate the forces necessary for ventilatory behaviors whereas more fatigable units are only activated during expulsive behaviors important for airway clearance. Neuromotor control of diaphragm motor units may reflect selective inputs from distinct pattern generators distributed according to the motor unit properties necessary to accomplish these different motor tasks. In contrast, widely-distributed inputs to phrenic motoneurons from various pattern generators (e.g., for breathing, coughing or vocalization) would dictate recruitment order based on intrinsic electrophysiological properties.

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“…At least since the pioneering work of Burke et al (1973), the classification of motor units into four types has proven to be a useful way of cataloging the contractile properties of muscle. In this scheme, motor units are classified into four categories on the basis of the clustering of values for selected properties into distinct groups (Botterman, Iwamoto & Gonyea, 1985;Hamm et al 1988). Each property, however, seems to have values distributed along the entire continuum of motor-unit types.…”

Section: Discussionmentioning

confidence: 99%

Motor‐unit force potentiation in adult cats during a standard fatigue test.

Gordon

Enoka

Stuart

1990

The Journal of Physiology

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1. The purpose of this study was to examine the time course of tetanic force during a standard fatigue test and to distinguish between the appearance of potentiation and fatigue among the four motor‐unit types of a cat hindlimb muscle. 2. Motor units of the tibialis posterior muscle in the adult cat were assigned to four categories (i.e. types S, FR, FI, FF) based on conventional criteria (Burke, Levine, Tsairis & Zajac, 1973). The mean (+/‐ S.D.) time course of peak force was constructed for each motor‐unit type and, within each type, for those units that potentiated (a greater than 3% increase in peak force compared to the initial value) and those that did not potentiate. 3. The average time courses of force differed between motor‐unit types. There was, however, considerable variability within each motor‐unit type. For the same relative force output, the forces exerted by slow‐twitch units were less variable than those exerted by fast‐twitch units. In addition, the variability among slow‐twitch units was relatively constant during the fatigue test while variability among fast‐twitch units either increased or decreased with time. 4. For a given motor‐unit type, the average time course of force did not depend on whether force in each tetanus was expressed as a peak value, an average peak value, or a force‐time integral. 5. Some motor units within each type exhibited potentiation. Most of the variability in the time course of the peak force for each motor‐unit type could be accounted for by the potentiating units. Motor units that exhibited only force decline (i.e. fatigue), regardless of unit type, had less variable time courses of peak force. Since potentiation was transient in some unit types, it was assumed that at least two opposing processes (i.e. fatigue and potentiation) occurred simultaneously in these units (see also, Krarup, 1981; Rankin, Enoka, Volz & Stuart, 1988; Garner, Hicks & McComas, 1989). 6. It is concluded that the expression of force potentiation throughout a fatiguing regimen is variable among motor units and that this is not related to conventional motor‐unit types. This dissociation suggests that the mechanisms that form the basis for the conventional distinction between motor‐unit types are different from those which lead to force potentiation.

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Association between biochemical and physiological properties in single motor units

Cited by 43 publications

References 35 publications

Immobilization‐induced changes in motor unit force and fatigability in the cat

Immobilization‐induced changes in motor unit force and fatigability in the cat

Convergence of Pattern Generator Outputs on a Common Mechanism of Diaphragm Motor Unit Recruitment

Motor‐unit force potentiation in adult cats during a standard fatigue test.

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