Enlarged motor units resulting from partial denervation of cat hindlimb muscles

1988 ◽  
Vol 59 (5) ◽  
pp. 1377-1394 ◽  
Author(s):  
A. R. Luff ◽  
D. D. Hatcher ◽  
K. Torkko
Keyword(s):  
Motor Unit ◽  
Soleus Muscle ◽  
Motor Units ◽  
Motor Axons ◽  
Unit Force ◽  
Single Motor Units ◽  
Partial Denervation ◽  
Slow Twitch ◽  
Fast Twitch ◽  
Polyneuronal Innervation

1. It was the aim of this study to determine the extent to which a mammalian motoneuron can sprout in a partially denervated muscle, which motor unit types are involved in sprouting, and whether polyneuronal innervation exists between sprouted units. 2. The fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus were partially denervated by unilateral section of the L7 ventral root in 12-wk-old kittens. After approximately 100 days single motor units were isolated, and their isometric contractile characteristics were determined. FDL units were also tested for their resistance to fatigue and categorized as fast-twitch, fatiguing fibers (FF), fast-twitch, fatigue-resistant fibers (FR), and slow-twitch, fatigue-resistant fibers (S). The presence of polyneuronal innervation was investigated between pairs of like and unlike units. 3. The extent of the original denervation was variable and was estimated from the distribution of motor axons innervating the muscle via the L7 and S1 (soleus) or L6 and L7 (FDL) ventral roots on the contralateral side. In soleus, denervations ranged from 75 to 98%; in FDL, 60 to 97% (denervations less than 60% were not investigated). In general, motor-unit force increased in proportion to the extent of the denervation. 4. Within soleus, unit force increased to over 2 N, which was about 16 times greater than the average for a normal muscle (133 mN). However, most units increased in force to between five and 12 times normal. 5. Within FDL, the force development of type S units was unaffected by partial denervation. Type FF units increased by up to 11 times (4.3 N) compared with normal FF units (395 mN) with most increasing between two and four times. FR units exhibited the greatest relative increase in force [up to 19 times (4.3 N) compared with normal (225 mN)]. Most units were two to seven times the normal. 6. A few FDL units were glycogen depleted, the muscles frozen, and cross sections prepared for histochemical analysis. This indicated that the largest units contained approximately 5,000 fibers, and there was little fiber hypertrophy. In the extensively denervated soleus muscle, large numbers of small, presumably denervated fibers were observed. The innervation ratio of several large units was determined indirectly using mean fiber area. This gave estimates of 3,000-4,000 fibers for the largest units. Again, fiber hypertrophy contributed little to the increase in unit force. It was concluded that the increased force of units in both muscles was largely attributable to terminal and axonal sprouting of the intact motor axons. 7. No evidence for polyneuronal innervation was found in either FDL or soleus muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term persistence of enlarged motor units in partially denervated hindlimb muscles of cats

1990 ◽  
Vol 64 (4) ◽  
pp. 1261-1269 ◽  
Author(s):  
A. R. Luff ◽  
K. Torkko
Keyword(s):  
Motor Units ◽  
Contralateral Side ◽  
Cross Sectional Area ◽  
Type I ◽  
Sectional Area ◽  
Motor Axons ◽  
Cross Sectional ◽  
Single Motor Units ◽  
Slow Twitch ◽  
Fast Twitch

1. It was the aim of this study to determine the effect that regenerating motor axons would have on enlarged or sprouted motor units that had been established for a relatively long time. 2. The fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus were partially denervated by unilateral section of the L7 ventral root in 12-wk-old kittens. After 200+ days single motor units were isolated, and their isometric contractile properties were determined. FDL units were also tested for their resistance to fatigue and categorized as fast-twitch-fatigable (FF), fast-twitch-fatigue-resistant (FR), and slow-twitch-fatigue-resistant (S). 3. It had been established previously that regenerating motor axons via L7 returned to the experimental muscles by approximately 100 days. Therefore from 100 to 200 days it was assumed that the sprouted units would be in competition with the regenerating axons. The extent of the original denervations was variable and was estimated from the contralateral side. In soleus most denervations ranged from 83 to 99%; in FDL, from 37 to 81%. 4. In both soleus and FDL there was no evidence that the motor units had sprouted to any less extent than found previously. Within some soleus muscles the largest motor units were developing substantially more force than was expected. However, determination of mean fiber cross-sectional area from muscles frozen, sectioned, and prepared for histochemical analysis showed that this was attributable to increased mean cross-sectional area of the type I fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

scholarly journals Motor unit composition has little effect on the short-range stiffness of feline medial gastrocnemius muscle

2007 ◽  
Vol 103 (3) ◽  
pp. 796-802 ◽  
Author(s):  
Lei Cui ◽  
Eric J. Perreault ◽  
Thomas G. Sandercock
Keyword(s):  
Motor Unit ◽  
Short Range ◽  
Gastrocnemius Muscle ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Single Motor Units ◽  
Slow Twitch ◽  
Slow Twitch Fibers ◽  
Fast Twitch ◽  
Medial Gastrocnemius Muscle

Studies on skinned fibers and single motor units have indicated that slow-twitch fibers are stiffer than fast-twitch fibers. This suggests that skeletal muscles with different motor unit compositions may have different short-range stiffness (SRS) properties. Furthermore, the natural recruitment of slow before fast motor units may result in an SRS-force profile that is different from electrical stimulation. However, muscle architecture and the mechanical properties of surrounding tissues also contribute to the net SRS of a muscle, and it remains unclear how these structural features each contribute to the SRS of a muscle. In this study, the SRS-force characteristics of cat medial gastrocnemius muscle were measured during natural activation using the crossed-extension reflex, which activates slow before fast motor units, and during electrical activation, in which all motor units are activated synchronously. Short, rapid, isovelocity stretches were applied using a linear puller to measure SRS across the range of muscle forces. Data were collected from eight animals. Although there was a trend toward greater stiffness during natural activation, this trend was small and not statistically significant across the population of animals tested. A simple model, in which the slow-twitch fibers were assumed to be 30% stiffer than the fast-twitch fibers, was used to simulate the experimental results. Experimental and simulated results show that motor unit composition or firing rate has little effect on the SRS property of the cat MG muscle, suggesting that architectural features may be the primary determinant of SRS.

Changes in diaphragm motor unit EMG during fatigue

1990 ◽  
Vol 68 (5) ◽  
pp. 1917-1926 ◽  
Author(s):  
G. C. Sieck ◽  
M. Fournier
Keyword(s):  
Motor Unit ◽  
Fatigue Test ◽  
Motor Units ◽  
Evoked Force ◽  
Slow Twitch ◽  
The Right ◽  
Fast Twitch ◽  
Multiple Units ◽  
Motor Unit Emg ◽  
Stimulation Of

Fatigue-related changes in the waveform and root-mean-square (rms) values of evoked motor unit electromyographic (EMG) responses were studied in the right sternocostal region of the cat diaphragm. Motor units were isolated by microdissection and stimulation of C5 ventral root filaments and then classified as fast-twitch fatigable (FF), fast-twitch fatigue intermediate (FInt), fast-twitch fatigue resistant (FR), or slow-twitch (S) based on standard physiological criteria. The evoked EMG responses of S and FR units showed very little change during the fatigue test. The evoked EMG waveform and rms values of FF and FInt units displayed variable changes during the fatigue test. When changes were observed, they typically included a prolongation of the EMG waveform, a decrease in peak amplitude, and a decrease in rms value. The changes in EMG amplitude and rms values were not correlated. In more fatigable units, the decrease in force during the fatigue test generally exceeded the decrease in EMG rms values. Changes in the evoked force and EMG responses of multiple units innervated by C5 or C6 ventral roots were also examined during the fatigue test. The decrease in diaphragm force during the fatigue test closely matched the force decline predicted by the proportionate contribution of different motor unit types. However, the observed reduction in diaphragm EMG rms values during the fatigue test exceeded that predicted based on the aggregate contribution of different motor unit types. It was concluded that changes in EMG do not reflect the extent of diaphragm fatigue.

Synchronization of Motor Units in Human Soleus Muscle During Standing Postural Tasks

2005 ◽  
Vol 94 (1) ◽  
pp. 62-69 ◽  
Author(s):  
G. Mochizuki ◽  
T. D. Ivanova ◽  
S. J. Garland
Keyword(s):  
Motor Unit ◽  
Soleus Muscle ◽  
Motor Units ◽  
Eyes Closed ◽  
Single Motor Units ◽  
Postural Tasks ◽  
Eyes Open ◽  
Low Incidence ◽  
To Receive ◽  
Common Function

During standing posture, the soleus muscles acts to control sway in the anteroposterior (AP) direction. The soleus muscles bilaterally share a common function during standing tasks. We sought to determine whether common descending inputs, as evidenced by the synchronization of bilateral motor unit pairs, were employed as a strategy to control this common function. Single motor units were recorded from the soleus muscles in subjects who stood on adjacent force platforms for 5 min with their eyes open or closed. While standing with the eyes open, only 4/39 bilateral motor unit pairs showed significant synchronization. Similarly, only 3/36 motor unit pairs were significantly synchronized during the eyes closed task. The low incidence of synchronization was observed despite a high correlation in the amount of sway in the AP direction between legs in both the eyes open and eyes closed tasks (ρ = 0.80 and ρ = 0.83, respectively). When the extent of synchronization was assessed between pairs of motor units within the same leg with the eyes open, 10/12 pairs were synchronized. Furthermore, when pairs of soleus motor units were recorded both bilaterally and unilaterally during voluntary isometric ankle plantarflexion, only 4/30 bilateral pairs showed significant synchronization, whereas 19/24 unilateral pairs had significant synchronization. In this study, there was little evidence of the existence of synchronization between bilateral soleus motor unit pairs in either postural tasks or voluntary isometric contractions. In cases in which bilateral synchronization was observed, it was considerably weaker than the synchronization of motor units within a single soleus muscle. The results of this study reveal that it is rather uncommon for bilateral soleus motoneurons to receive common descending synaptic inputs, whereas two motoneurons within a single soleus muscle do.

Organization of single motor units in feline sartorius

1994 ◽  
Vol 72 (4) ◽  
pp. 1885-1896 ◽  
Author(s):  
E. Smits ◽  
P. K. Rose ◽  
T. Gordon ◽  
F. J. Richmond
Keyword(s):  
Motor Unit ◽  
Cross Section ◽  
Cross Sections ◽  
Muscle Fibers ◽  
Motor Units ◽  
Single Motor Unit ◽  
Single Motor ◽  
Medial Head ◽  
Single Motor Units ◽  
Fast Twitch

1. We depleted single motor units in feline sartorius muscles of glycogen by stimulating their motoneurons intracellularly. We mapped the intramuscular distribution of depleted fibers by inspecting histological cross-sections throughout the length of sartorius. 2. We selected ten depleted motor units for detailed study and quantitative analysis. Nine motor units were located in the anterior head of sartorius. One was located in a muscle whose distal half appeared to have been damaged some time before the acute experiment. A single motor unit was located in the medial head of sartorius. 3. Five motor units were composed of fast-twitch glycolytic (FG) muscle fibers, two of fast-twitch oxidative glycolytic (FOG) muscle fibers, and three of slow-twitch oxidative (SO) muscle fibers. Estimates of the numbers of depleted fibers in motor units of anterior sartorius indicated that FG motor units were larger (mean 566 fibers) than FOG and SO motor units (SO mean 190, FOG mean 156 fibers). The SO motor unit in the damaged muscle had 550 fibers. One motor unit depleted in the medial head of sartorius had 270 fibers with FG profiles. 4. Muscle fibers belonging to each anterior motor unit were never distributed throughout the whole cross-section of anterior sartorius at any proximodistal level. Furthermore, fibers were distributed nonuniformly along the proximodistal axis of the muscle. In most muscles at least a few depleted fibers were found at all proximodistal levels. However, in one normal muscle and the damaged muscle, depleted fibers were confined to the proximal end. 5. The fibers in the medial motor unit were confined to a strip that did not extend across the whole cross-section of the muscle head. Fibers within this strip were scattered quite evenly from origin to insertion. This medial FG motor unit occupied a smaller territory and contained fewer fibers than anterior motor units of the same histochemical type. 6. These results show that sartorius motor units are not distributed uniformly in the mediolateral plane; those in anterior sartorius were distributed asymmetrically in the proximodistal axis as well. This finding has important functional implications for the way in which we model force development and transmission in sartorius and other long muscles.

Properties of motor units after self-reinnervation of the cat superior oblique muscle

1995 ◽  
Vol 74 (6) ◽  
pp. 2309-2318 ◽  
Author(s):  
R. F. Waldeck ◽  
E. H. Murphy ◽  
M. J. Pinter
Keyword(s):  
Motor Unit ◽  
Muscle Fiber ◽  
Motor Units ◽  
Oblique Muscle ◽  
Unit Force ◽  
Cross Sectional ◽  
Conduction Velocities ◽  
Polyneuronal Innervation ◽  
Superior Oblique Muscle ◽  
Superior Oblique

1. The mechanical properties of motor units of the cat superior oblique muscle and axonal conduction velocities of trochlear motoneurons have been studied at several postoperative times after intracranial axotomy of the trochlear nerve. 2. Whole muscle twitch forces were generally within the normal range by approximately 4 mo postoperative, indicating that reinnervation is complete at this time. 3. Among animals studied 3.5-4.5 months after trochlear axotomy, average motor-unit tetanic forces were increased by a factor of approximately 2.5 compared with units studied in normal superior oblique muscle. Average motor-unit tetanic forces in animals studied 14.5-23 mo after axotomy were also increased relative to normal, but the difference was not significant. Among all reinnervated motor units, there was a tendency for increased twitch time-to-peak relative to control. Reinnervated motor-unit fatigue properties were similar to normal. 4. Average trochlear motoneuron conduction velocities for animals at all postoperative intervals remained significantly lower than the average conduction velocities from three of four normal animals. 5. Counts of Nissl-stained cell bodies in axotomized and control, contralateral trochlear nuclei showed that some cell loss had occurred, averaging approximately 17% 3.5-4.5 mo postoperative and 24% 14.5-23 mo postoperative. Associated with this loss was an increase (10%) of axotomized motoneuron soma cross-sectional area. 6. Muscle fiber cross-sectional areas (CSA) were measured in reinnervated superior oblique muscles and compared with CSAs from contralateral, control muscles. Average CSA was significantly decreased in all reinnervated muscles, with the relative decreases ranging from approximately 10 to 28%. 7. The results are discussed in terms of factors that determine motor-unit force; muscle fiber CSA, specific force, and innervation ratio. We conclude that the increases of average motor-unit force in short-term reinnervated superior oblique muscles are most likely related to polyneuronal innervation of muscle fibers and that the return of these forces to normal levels in long-term muscles is related to synapse elimination. Our results are compared with those of other self-reinnervation studies, and the potential role played by the time muscle remains denervated in determining the persistence of polyneuronal innervation after reinnervation is considered.

Spontaneous Electromyographic Activity in Adult Rat Soleus Muscle

1998 ◽  
Vol 80 (1) ◽  
pp. 365-376 ◽  
Author(s):  
Torsten Eken
Keyword(s):  
Motor Unit ◽  
Soleus Muscle ◽  
Motor Units ◽  
Firing Frequency ◽  
Electromyographic Activity ◽  
Single Motor Unit ◽  
Single Motor ◽  
Adult Rat ◽  
Single Motor Units ◽  
Rat Soleus Muscle

Eken, Torsten. Spontaneous electromyographic activity in adult rat soleus muscle. J. Neurophysiol. 80: 365–376, 1998. Single-motor-unit and gross electromyograms (EMG) were recorded from the soleus muscle in six unrestrained rats. The median firing frequencies of nine motor units were in the 16–25 Hz range, in agreement with previous studies. One additional motor unit had a median firing frequency of 47 Hz. This unit and one of the lower-frequency units regularly fired doublets. Motor-unit firing frequency was well correlated to whole-muscle EMG during locomotion. Integrated rectified gross EMG revealed periods of continuous modulation, phasic high-amplitude events, and tonic low-amplitude segments. The tonic segments typically were caused by a small number of motor units firing at stable high frequencies (20–30 Hz) for extended periods of time without detectable activity in other units. This long-lasting firing in single motor units typically was initiated by transient mass activity, which recruited many units. However, only one or a few units continued firing at a stable high frequency. The tonic firing terminated spontaneously or in conjunction with an episode of mass activity. Different units were active in different tonic segments. Thus there was an apparent dissociation between activity in different single motor units and consequently between single-motor-unit activity and whole-muscle EMG. It is proposed that the maintained tonic motor-unit activity is caused by intrinsic motoneuron properties in the form of depolarizing plateau potentials.

Myosin phenotype and SDH enzyme variability among motor unit fibers

1996 ◽  
Vol 80 (6) ◽  
pp. 2179-2189 ◽  
Author(s):  
G. C. Sieck ◽  
M. Fournier ◽  
Y. S. Prakash ◽  
C. E. Blanco
Keyword(s):  
Motor Unit ◽  
Heavy Chain ◽  
Motor Units ◽  
Tibialis Posterior ◽  
Type I ◽  
Glycogen Depletion ◽  
Mixed Composition ◽  
Slow Twitch ◽  
Fast Twitch ◽  
Intermediate Units

Motor units in cat diaphragm and tibialis posterior muscles were classified physiologically as slow-twitch, fast-twitch, fatigue-resistant, fast-twitch fatigue-intermediate, or fast-twitch fatigable. Motor unit fibers were then identified by glycogen depletion and classified as type I, IIa, IIb, or IIx on the basis of myofibrillar adenosinetriphosphatase-staining profiles and immunoreactivity for myosin heavy-chain (MHC) isoforms. In both muscles, slow-twitch and fast-twitch fatigue-resistant units comprised type I and IIa fibers expressing MHC-slow and MHC-2A isoforms, respectively. Fast-twitch fatigue-intermediate and fast-twitch fatigable units comprised type IIx fibers expressing the MHC-2X isoform. Some fast-twitch fatigue-intermediate units had a mixed composition with a few fibers (approximately 10%) expressing the MHC-2A isoform. Motor unit fiber succinate dehydrogenase (SDH) activity was quantified, and variability was estimated by the interquartile range, which was lower among motor unit fibers than in adjacent fibers of the same histochemical type but comparable to that along the length of individual fibers. We conclude that, despite the mixed-MHC phenotype of some diaphragm and tibialis posterior motor units, SDH activity is relatively uniform. This supports the hypothesis that motoneurons exert a predominant influence on muscle fiber SDH activity.

Maximum shortening speed of motor units of various types in cat lumbrical muscles

1993 ◽  
Vol 69 (2) ◽  
pp. 442-448 ◽  
Author(s):  
J. Petit ◽  
M. Chua ◽  
C. C. Hunt
Keyword(s):  
Motor Unit ◽  
Inverse Correlation ◽  
Motor Units ◽  
Maximum Speed ◽  
Single Motor Units ◽  
Maximal Speed ◽  
Contraction Times ◽  
Isometric Twitch ◽  
Lumbrical Muscles ◽  
Speed Of Shortening

1. Isotonic shortening of cat superficial lumbrical muscles was studied during maximal tetanic contractions of single motor units of identified types. For each motor unit, the maximal speed of contraction, Vmax, was determined by extrapolating to zero the hyperbolic relation between applied tension and speed of shortening. 2. The maximal speeds of shortening of motor units formed a continuum with the highest velocities observed for the fast fatigable motor units and the lowest for the slow motor units. 3. On average, the maximum speed of shortening increased with the tetanic tension developed by the motor units. 4. In motor units with isometric twitch contraction times less than 35 ms, these times showed a significant inverse correlation with Vmax. Progressively longer contraction times were associated with rather small changes in Vmax. 5. The implications of these findings on the speed of muscle shortening during motor-unit recruitment are discussed.

Rectification is required to extract oscillatory envelope modulation from surface electromyographic signals

2014 ◽  
Vol 112 (7) ◽  
pp. 1685-1691 ◽  
Author(s):  
Christopher J. Dakin ◽  
Brian H. Dalton ◽  
Billy L. Luu ◽  
Jean-Sébastien Blouin
Keyword(s):  
Motor Unit ◽  
Stimulus Frequency ◽  
Motor Units ◽  
Surface Emg ◽  
Medial Gastrocnemius ◽  
Single Motor Unit ◽  
Single Motor ◽  
Single Motor Units ◽  
Emg Signal ◽  
Envelope Modulation

Rectification of surface electromyographic (EMG) recordings prior to their correlation with other signals is a widely used form of preprocessing. Recently this practice has come into question, elevating the subject of EMG rectification to a topic of much debate. Proponents for rectifying suggest it accentuates the EMG spike timing information, whereas opponents indicate it is unnecessary and its nonlinear distortion of data is potentially destructive. Here we examine the necessity of rectification on the extraction of muscle responses, but for the first time using a known oscillatory input to the muscle in the form of electrical vestibular stimulation. Participants were exposed to sinusoidal vestibular stimuli while surface and intramuscular EMG were recorded from the left medial gastrocnemius. We compared the unrectified and rectified surface EMG to single motor units to determine which method best identified stimulus-EMG coherence and phase at the single-motor unit level. Surface EMG modulation at the stimulus frequency was obvious in the unrectified surface EMG. However, this modulation was not identified by the fast Fourier transform, and therefore stimulus coherence with the unrectified EMG signal failed to capture this covariance. Both the rectified surface EMG and single motor units displayed significant coherence over the entire stimulus bandwidth (1–20 Hz). Furthermore, the stimulus-phase relationship for the rectified EMG and motor units shared a moderate correlation ( r = 0.56). These data indicate that rectification of surface EMG is a necessary step to extract EMG envelope modulation due to motor unit entrainment to a known stimulus.

Sign in / Sign up

Export Citation Format

Share Document