Motor unit physiology: Some unresolved issues

Enoka, Roger M.; Fuglevand, Andrew J.

doi:10.1002/1097-4598(200101)24:1<4::aid-mus13>3.0.co;2-f

Cited by 317 publications

(249 citation statements)

References 160 publications

(199 reference statements)

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“…However, the changes in contraction force occurring during the larval-larval molt were not due to simple disruption of the rhythmic output of the FG. We also failed to detect changes in motorneuron firing frequency or spike number typically associated with the modulation of muscle contraction force (Morris and Hooper, 1997;Enoka and Fuglevand, 2001). Also, our recordings of the kinetics of the muscle EJP activity revealed no differences between earlymolt and intermolt stage larvae (Tables 1-3).…”

Section: Mechanism Of Foregut Modulation During the Moltmentioning

confidence: 69%

Modulation of foregut synaptic activity controls resorption of molting fluid during larval molts of the mothManduca sexta

Bestman

Booker

2003

Journal of Experimental Biology

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The regulation and organization of a variety of stereotyped behaviors are essential for many aspects of animal development. One such collection is a series of ecdysial-related motor programs that lead to the shedding of an insect's old cuticle during a molt. Like all insects, the growth of larval Manduca sexta moths is limited by its inelastic cuticle. Consequently, growing larvae undergo a series of molts in which the old cuticle is shed and replaced by a new, larger cuticle. The two ecdysial-related motor patterns that have been the focus of much attention are the pre-ecdysis and ecdysis motor programs, which serve to loosen and then propel the old cuticle off the insect at the end of the molt, respectively (Copenhaver and Truman, 1982;Miles and Weeks, 1991; Ž itňan and Adams, 2000). The analysis of these two simple motor programs has served as a model system for the role of hormones in coordinating stereotyped behaviors. However, pre-ecdysis and ecdysis behaviors define the end of a series of ecdysial-related motor patterns that can stretch over several days. The first external sign that a Manduca larva has entered the molt cycle is apolysis, or the detachment of the old and new cuticles from one another as molting fluid (MF), a cocktail of digestive enzymes, is secreted by the epidermis into the exuvial space (the space between the old and new cuticles) where it degrades and weakens the old cuticle. Once degraded, the digested components of the exuvia and the MF are absorbed, leaving behind a thin, papery outer cuticle to be shed during ecdysis.The secretion of MF and its resorption many hours later are among a series of molt-related events that must be precisely coordinated during the molt cycle. While the removal of the MF prior to ecdysis is a feature common to all insect molts, there is no consensus on the mechanisms used to remove the MF from the exuvial space. It has often been reported that MF is resorbed across the newly formed cuticle and epidermis (Jungreis, 1979;Reynolds and Samuels, 1996;Wigglesworth, 1972). This conclusion is based on the studies of the larval molt of the bug Rhodnius prolixus (Wigglesworth, 1933(Wigglesworth, , 1972 and the pupal-adult molt of the silkworm Hyalophora cecropia (Lensky et al., 1970;Passonneau and Williams, 1953). These studies concluded that MF is absorbed into the hemocoel by passing through pores in the new cuticle.There is also evidence suggesting that the gut serves as a route for the removal of MF. Wachter (1930) We examined the role of the foregut in the resorption of molting fluid (MF) from the exuvial space during the last larval-larval molt of the moth Manduca sexta. In intermolt larvae, the activity of the foregut is characterized by robust peristaltic contractions. With the onset of the molt, MF is secreted into the exuvial space where it digests and weakens the old cuticle. The appearance of MF in the exuvial space is accompanied by a dramatic reduction in the amplitude of the foregut contractions. Foregut peristalsis returned about halfway through ...

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Section: Mechanism Of Foregut Modulation During the Moltmentioning

confidence: 69%

Modulation of foregut synaptic activity controls resorption of molting fluid during larval molts of the mothManduca sexta

Bestman

Booker

2003

Journal of Experimental Biology

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“…Moreover, referring to a compilation of MUAP rates in a review article by Enoka and Fuglevand (2001) they assumed a MUAP rate of 25/s per motor unit. For the ensemble of all contributing motor units they consequently estimated a MUAP rate between 1250/s and 2500/s.…”

Section: Estimation Of the Muap Ratementioning

confidence: 99%

Modeling the vestibular evoked myogenic potential

Lütkenhöner¹,

Stoll²,

Basel³

2010

Journal of Theoretical Biology

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Measuring the vestibular evoked myogenic potential (VEMP) promises to become a routine method for assessing vestibular function, although the technique is not yet standardized. To overcome the problem that the VEMP amplitude depends not only on the inhibition triggered by the acoustic stimulation of the vestibular end organs in the inner ear, but also on the tone of the muscle from which the potential is recorded, the VEMP is often normalized by dividing through a measure of the electromyogram (EMG) activity. The underlying idea is that VEMP amplitude and EMG activity are proportional. But this would imply that the muscle tone is irrelevant for a successful VEMP recording, contradicting experimental evidence. Here, an analytical model is presented that allows to resolve the contradiction. The EMG is modeled as the sum of motor unit action potentials (MUAPs). A brief inhibition can be characterized by its equivalent rectangular duration (ERD), irrespective of the actual time course of the inhibition. The VEMP resembles a polarity-inverted MUAP under such circumstances. Its amplitude is proportional to both the ERD and the MUAP rate. The EMG activity, by contrast, is proportional to the square root of the MUAP rate. Thus, the normalized VEMP still depends on the muscle tone. To avoid confounding effects of the muscle tone, the standard deviation of the EMG could be considered. But the inhibition effect on the standard deviation is small so that the measuring time would have to be much longer than usual today.

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“…Third, the number of muscle fibers innervated by a motor unit has a skewed distribution. For example, the first dorsal interosseus muscle comprises an equal number of type I and II fibers, but ϳ84% of the motor units have slow contraction times and are fatigue resistant (7). Fourth, the conduction velocity of muscle fiber action potentials can change by ϳ20% with variation in discharge rate (24) and the range of discharge rates varies with recruitment threshold in a population of motor units (2).…”

mentioning

confidence: 99%

Counterpoint: Spectral properties of the surface emg do not provide information about motor unit recruitment and muscle fiber type

Farina

2008

Journal of Applied Physiology

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The surface electromyogram (EMG) comprises the sum of the electrical contributions made by the active motor units to the interference signal detected by electrodes placed on the skin overlying the muscle. Because it provides a global measure of motor unit activity, this signal is a valuable tool for assessing the level of muscle activation.The analysis of the surface EMG in the frequency domain has been used in some instances, such as fatiguing contractions, to provide an indirect measure of the relative changes in average muscle fiber conduction velocity (19) or in the profile of the intracellular action potential (6). In addition to these applications, the spectral characteristics of the surface EMG have been used to infer motor unit recruitment strategies and the fiber-type composition of a muscle. Higher characteristic spectral frequencies are associated with a greater proportion of type II fibers (20, 21) and changes in EMG spectral variables are used to infer the recruitment of faster or slower motor units (3,26).The capacity of surface EMG spectral properties to provide information about motor unit recruitment or the proportion of fiber types is based on the rationale that higher threshold (and type II) motor units produce surface action potentials with larger relative energy at higher frequencies than lower threshold (and type I) motor units.Because in these applications the main determinant of the frequency content of an action potential is assumed to be its conduction velocity (28), the crucial issue in this debate is the validity of two assumptions: 1) average conduction velocity of the active motor units is related to fiber-type proportions, and 2) changes in the spectral properties of the surface EMG are associated with changes in average conduction velocity.Fiber-type composition and average muscle fiber conduction velocity. There are several physiological details that confound this association. First, the two main fiber types do not have distinct conduction velocities in humans, but rather conduction velocity has a continuous distribution with a single peak (25). Second, average conduction velocity of muscle fiber action potentials can differ among populations of motor units due to differences in fiber diameter and independent of changes in fiber-type proportions (4). Third, the number of muscle fibers innervated by a motor unit has a skewed distribution. For example, the first dorsal interosseus muscle comprises an equal number of type I and II fibers, but ϳ84% of the motor units have slow contraction times and are fatigue resistant (7). Fourth, the conduction velocity of muscle fiber action potentials can change by ϳ20% with variation in discharge rate (24) and the range of discharge rates varies with recruitment threshold in a population of motor units (2). Fifth, the conduction velocity of muscle fiber action potentials varies with fiber length (15).Experimental evidence: fiber-type composition vs. muscle fiber conduction velocity. The significance of the physiological limitations discussed in...

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Motor unit physiology: Some unresolved issues

Cited by 317 publications

References 160 publications

Modulation of foregut synaptic activity controls resorption of molting fluid during larval molts of the mothManduca sexta

Modulation of foregut synaptic activity controls resorption of molting fluid during larval molts of the mothManduca sexta

Modeling the vestibular evoked myogenic potential

Counterpoint: Spectral properties of the surface emg do not provide information about motor unit recruitment and muscle fiber type

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