Characteristics of motor units in muscles of rats grafted with nerves intact

1986 ◽  
Vol 250 (6) ◽  
pp. C828-C833 ◽  
Author(s):  
C. Cote ◽  
J. A. Faulkner
Keyword(s):  
Total Cross Section ◽  
Linear Relationship ◽  
Cross Section ◽  
Entire Range ◽  
Extensor Digitorum Longus ◽  
Motor Units ◽  
Axonal Sprouting ◽  
Single Motor ◽  
Single Motor Units ◽  
The Mean

Extensor digitorum longus (EDL) muscles of rats were grafted with the nerves intact. Sixty days after grafting, single motor units were isolated and characterized in terms of fatigability and morphology. The distribution of fatigue indexes for motor units in control muscles revealed two main peaks, whereas in grafts no clear peaks were observed, fatigue indexes being distributed more evenly over the entire range. This difference in the distribution of fatigue indexes of motor units may explain the increased resistance to fatigue observed for whole grafts compared with whole EDL muscles. An inverse linear relationship was found between maximum tetanic tensions and the fatigue indexes of motor units from control and grafted EDL muscles. The distribution of fiber areas for single motor units was broader in grafts than in control EDL muscles, but the mean innervation ratio was not different. In approximately 20% of the units mapped in grafts, fibers were clustered tightly within a small portion of the total cross section, suggesting axonal sprouting during regeneration.

Physiological and histochemical characteristics of motor units in cat tibialis anterior and extensor digitorum longus muscles

1980 ◽  
Vol 43 (6) ◽  
pp. 1615-1630 ◽  
Author(s):  
R. P. Dum ◽  
T. T. Kennedy
Keyword(s):  
Motor Unit ◽  
Tibialis Anterior ◽  
Extensor Digitorum Longus ◽  
Motor Units ◽  
Input Resistance ◽  
Histochemical Staining ◽  
Glycogen Depletion ◽  
Single Motor Units ◽  
The Mean ◽  
Tetanic Tension

1. Intracellular recording and stimulation techniques were used to study the normal motor-unit population of tibialis anterior (TA) and extensor digitorum longus (EDL) muscles in the cat. Histochemical staining of the whole muscle and glycogen depletion of single motor units were performed. These results may be compared to those of their extensor antagonist, medial gastrocnemius (MG), as reported in studies by Burke and co-workers (7, 11, 13). 2. On the basis of two physiological properties, “sag” and fatigue resistance, the motor units in both TA and EDL could be classified into the same categories (types FF, F(int), FR, and S) as in MG (11). In contrast to MG, TA and EDL had nearly twice as many type-FR motor units and only half as many type-S motor units. 3. Glycogen depletion of representative single motor units of types FF and FR suggests a close correspondence between the physiological classification and a unique histochemical profile. No type-S units were depleted. 4. On the basis of histochemical staining, the muscle fibers in TA were presumed to belong to type-FF, -FR, or -S motor units. TA had a higher proportion of type-FR and a lower proportion of type-S muscle fibers than are found in MG. A striking feature was the variation in the proportion of each fiber type in different regions of TA. The anterolateral portion had mostly types FF and FR, while the posteriomedial portion had more types FR and S. 5. The twitch time to peak (TwTP) of isometric motor-unit contractions was generally quite fast with none having TwTP greater than 55 ms. The mean TwTP (not in EDL) and the mean tetanic tension of each motor-unit type were significantly different from each other. Most of the motor units exhibited significant postetanic potentiation of twitch tension and a corresponding lengthening of half-relaxation time and to a lesser degree, twitch contraction time. 6. There was a significant relationship between the inverse of motoneuronal input resistance and either tetanic tension or twitch contraction time. These relationships were not apparent when axonal conduction velocity rather than input resistance was used as an index of motoneuron size. The mean input resistances of the three major motor-unit types were significantly different while the mean conduction velocities of types FF and FR were nearly identical. A weak positive correlation was observed between the TwTP and the afterhyperpolarization of TA and EDL motoneurons. 7. In general, the mechanical characteristics and intrinsic motoneuronal properties of TA and EDL appear to parallel the organization of their extensor antagonist, MG, with some important quantitative differences that may reflect their different functional roles.

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.

Physiological characteristics of identified motor units in the mouse extensor digitorum longus muscle: an in vitro approach

1998 ◽  
Vol 76 (1) ◽  
pp. 68-71
Author(s):  
D M Zardini ◽  
D J Parry
Keyword(s):  
Extensor Digitorum Longus ◽  
Fiber Type ◽  
Motor Units ◽  
Ventral Root ◽  
Extensor Digitorum ◽  
Physiological Characteristics ◽  
Glycogen Depletion ◽  
Single Motor ◽  
Single Motor Units

Physiological, histochemical, and morphometric properties of fast-twitch single motor units were studied in mouse extensor digitorum longus muscles in an in vitro ventral root - nerve - muscle preparation. Single motor units were functionally isolated by microdissection of the ventral root, and the glycogen depletion technique was used to demonstrate the component muscle fibers. Monoclonal antibodies were used to identify their myosin heavy chain composition. The technique allows one to correlate physiological characteristics of single motor units with fiber type but is less useful for morphological assessment of motor unit size as a result of failure to deplete glycogen from all fibers of motor units containing fibers with high oxidative capacity.Key words: fiber type, IIx motor units, fatigue index, glycogen depletion.

Contractile properties of single motor units in human toe extensors assessed by intraneural motor axon stimulation

1996 ◽  
Vol 75 (6) ◽  
pp. 2509-2519 ◽  
Author(s):  
V. G. Macefield ◽  
A. J. Fuglevand ◽  
B. Bigland-Ritchie
Keyword(s):  
Stimulus Frequency ◽  
Motor Units ◽  
Extensor Digitorum ◽  
Force Frequency ◽  
Contraction Time ◽  
Single Motor ◽  
Single Motor Units ◽  
Extensor Muscles ◽  
Stimulus Train ◽  
The Mean

1. Single motor axons innervating human toe extensor muscles were selectively stimulated through a tungsten microelectrode inserted percutaneously into the peroneal nerve. Twitch and tetanic forces were measured from a strain gauge over the proximal phalanx of the toe generating the greatest force. Twitch data were obtained from 19 single motor units in nine subjects: 8 motor units supplied extensor hallucis longus (EHL), 5 motor units supplied extensor digitorum longus (EDL), and 6 motor units supplied extensor digitorum brevis (EDB). Unpotentiated twitch forces ranged from 6.3 to 78.1 mN (20.0 +/- 4.0 mN, mean +/- SE), with the distribution highly skewed toward small forces. Twitch contraction and half-relaxation times were 74.8 +/- 3.9 and 78.6 +/- 6.0 ms, respectively. Compared with motor units in human thenar muscles, those in human toe extensor muscles were stronger but slower. However, as in thenar motor units, twitch force and contraction time were not related. 2. Force-frequency relationships were determined for 13 units (5 EDL, 5 EHL, 3 EDB) by stimulating each unit with short trains (1.0-5.0 s) of constant frequency (2-100 Hz). Peak force was related to stimulus frequency in a sigmoid fashion. The steep region of the curve extended from 5.5 +/- 0.7 (SE) Hz to 16.3 +/- 1.1 Hz for all units, and the stimulus frequency required to generate half-maximal force (9.6 +/- 0.6 Hz) was close to the center of the steep range. This frequency, which was inversely related to twitch contraction time, was lower than the frequency required to develop half-maximal force of human thenar motor units (12 +/- 4 Hz, mean +/- SD). The slopes of the regression lines relating force to frequency, computed over the steep range for each unit, were also lower for the toe extensors (3.7 +/- 0.7 mN/Hz) than for the thenar muscles (6 +/- 1 mN/Hz). 3. Maximal tetanic forces ranged from 29.9 to 188.1 mN (89.0 +/- 16.5 mN, mean +/- SE), and were generated at stimulus frequencies from 15 to 100 Hz (median 50 Hz). The stimulation frequency required for fused tetani (absence of noticeable force fluctuation) was generally less than that required for maximum tetanic force. The mean twitch-tetanus ratio, calculated for unpotentiated twitches, was 0.22 +/- 0.02 (range 0.15-0.41). This ratio was higher than for human thenar motor units (0.14 +/- 0.06, mean +/- SE). After twitch potentiation of 10 units, the mean twitch-tetanus ratio increased to 0.28 +/- 0.04. 4. The effects of preceding each stimulus train with a short interstimulus interval (10 ms) on force production at each frequency were examined in nine motor units. Peak forces at the onset of each contraction were higher when such an “initial doublet” preceded stimulus trains of < or = 20 Hz, but the mean force at the end of each stimulus train was not significantly affected at any frequency. 5. Eight units were stimulated with a train that increased in frequency continuously from 2 to 80 Hz, and then decreased symmetrically. This pattern resulted in peak forces that were higher on the descending limb of the stimulus train, the force-frequency relationship tracing a hysteresis loop. Hysteresis was exhibited because damping in the neuromuscular system causes the mechanical output of muscle to lag behind neural input. Thus, in non-steady-state conditions (as in most forms of natural activity), somewhat higher firing rates may be required to attain a particular level of force; once attained, force output will be transiently unresponsive to diminution of firing rate. 6. We conclude that there are differences in the contractile properties of single motor units in human toe extensor muscles (involved in posture and locomotion) and thenar muscles (involved in prehension and manipulation). Twitch-tetanus ratios were greater for motor units in the toe extensors, and this property accounted for the lower force sensitivity of these units to increases in frequency. (ABSTRACT TRUNCATED)

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.

Threshold variations of medial pterygoid single motor units during vertical or horizontal force tasks

2021 ◽  
Author(s):  
Yalda Nozad Mojaver ◽  
Paul Tawadros ◽  
Polyana Moura Ferreira ◽  
Terry Whittle ◽  
Greg M. Murray
Keyword(s):  
Motor Units ◽  
Horizontal Force ◽  
Single Motor ◽  
Single Motor Units ◽  
Medial Pterygoid

Functional Properties of Single Motor Units in the Inferior Head of Human Lateral Pterygoid Muscle: Task Firing Rates

2002 ◽  
Vol 88 (2) ◽  
pp. 751-760 ◽  
Author(s):  
I. Phanachet ◽  
T. Whittle ◽  
K. Wanigaratne ◽  
G. M. Murray
Keyword(s):  
Firing Rate ◽  
Correlation Coefficients ◽  
Motor Units ◽  
Lateral Pterygoid Muscle ◽  
Jaw Movements ◽  
Firing Rates ◽  
Single Motor ◽  
Single Motor Units ◽  
Fine Control ◽  
Contralateral Movement

The precise function of the inferior head of the human lateral pterygoid muscle (IHLP) is unclear. The aim of this study was to clarify the normal function of the IHLP. The hypothesis was that an important function of the IHLP is the generation and fine control of horizontal (i.e., anteroposterior and mediolateral) jaw movements. The activities of 50 single motor units (SMUs) were recorded from IHLP (14 subjects) during two- or three-step contralateral movement ( n = 36) and/or protrusion ( n = 33). Most recording sites were identified by computer tomography. There was a statistically significant overall increase in firing rate as the magnitude of jaw displacement increased between the holding phases (range of increments: 0.3–1.6 mm). The firing rates during the dynamic phases for each unit were significantly greater than those during the previous holding phases but less than those during the subsequent holding phases. For the contralateral step task at the intermediate rate, the cross-correlation coefficients between jaw displacement in the mediolateral axis and the mean firing rate of each unit ranged from r = 0.29 to 0.77; mean ± SD; r = 0.49 ± 0.13 (protrusive step task: r = 0.12–0.74, r = 0.44 ± 0.14 for correlation with anterior–posterior axis). The correlation coefficients at the fast rate during the contralateral step task and the protrusive step task were significantly higher than those at the slow rate. The firing rate change of the SMUs per unit displacement between holding phases was significantly greater for the lower-threshold than for the higher-threshold units during contralateral movement and protrusion. After dividing IHLP into four regions, the SMUs recorded in the superior part exhibited significantly greater mean firing rate changes per unit displacement during protrusion than for the SMUs recorded in the inferior part. Significantly fewer units were related to the protrusive task in the superior–medial part. These data support previously proposed notions of functional heterogeneity within IHLP. The present findings provide further evidence for an involvement of the IHLP in the generation and fine control of horizontal jaw movements.

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