Activity and motor unit size in partially denervated rat medial gastrocnemius

1994 ◽  
Vol 76 (6) ◽  
pp. 2663-2671 ◽  
Author(s):  
L. J. Einsiedel ◽  
A. R. Luff
Keyword(s):  
Motor Unit ◽  
Gastrocnemius Muscle ◽  
Motor Units ◽  
Ventral Root ◽  
Treadmill Walking ◽  
Medial Gastrocnemius ◽  
Unit Size ◽  
Partial Denervation ◽  
Motoneuron Activity ◽  
Medial Gastrocnemius Muscle

The aim of the study was to determine whether increased motoneuron activity induced by treadmill walking would alter the extent of motoneuron sprouting in the partially denervated rat medial gastrocnemius muscle. An extensive partial denervation was effected by unilateral section of the L5 ventral root, and it is very likely that all units remaining in the medial gastrocnemius were used in treadmill walking. Rats were trained for 1.5 h/day and after 14 days were walking at least 1 km/day. Motor unit characteristics were determined 24 days after the partial denervation and were compared with units from partially denervated control (PDC) animals and with units from normal (control) animals. In PDC rats, force developed by slow, fast fatigue-resistant, and fast intermediate-fatigable motor units increased substantially compared with control animals; that of fast-fatigable units did not increase. In partially denervated exercised animals, force developed by slow and fast-fatigue-resistant units showed no further increase, but fast-intermediate- and fast-fatigable units showed significant increases compared with those in PDC animals. The changes in force were closely paralleled by changes in innervation ratios. We concluded that neuronal activity is an important factor in determining the rate of motoneuron sprouting.

Effect of velocity and mechanical history on the forces of motor units in the cat medial gastrocnemius muscle

1992 ◽  
Vol 68 (5) ◽  
pp. 1503-1515 ◽  
Author(s):  
C. J. Heckman ◽  
J. L. Weytjens ◽  
G. E. Loeb
Keyword(s):  
Motor Unit ◽  
Gastrocnemius Muscle ◽  
Motor Behavior ◽  
Isometric Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Stimulation Rate ◽  
Force Velocity ◽  
Isometric Forces ◽  
Medial Gastrocnemius Muscle

1. Two fundamental aspects of the dynamic behavior of motor units of the cat medial gastrocnemius (MG) muscle were measured. Force-velocity (FV) relationships were measured with the use of constant velocity shortening and lengthening movements. Effects of mechanical history were assessed via comparisons of forces immediately after or during slow movements with standard isometric forces. Isometric force-length (FL) relations were also measured, and the effect of different stimulation rates on both FV and FL data was assessed. 2. Prior or concurrent movement greatly potentiated motor-unit force, but this movement potentiation was highly dependent on the amplitude of the unit's force. The smallest twitch forces of type S units (< 10 mN) were potentiated more than threefold, but no potentiation occurred for unit forces > 200 mN. It was tentatively concluded that movement potentiation may play little role in normal movements because it does not occur at forces > 1% of maximal isometric force of the MG. 3. During shortening, the normalized FV relations of type S units were relatively steeper than those of type FR or FF units. For lengthening, there was no evident relation between FV steepness and motor-unit type. 4. Stimulation rate affected both the FV and FL relationships of the motor units. The peak of the FL relationship (Lo) clearly shifted to shorter muscle lengths as stimulation rate was increased. The steepness of the FV relationship for shortening was decreased by increasing stimulation rate, but this effect was modest. 5. The shift in motor-unit Lo and the differences in motor-unit FV relationships were hypothesized to play significant roles during normal motor behavior. Realistic computer simulations of FL and FV functions for a population of motor units undergoing normal steady-state recruitment and rate modulation supported these hypotheses. As the level of simulated neural drive increased, the population Lo shifted to considerably shorter lengths, and the normalized FV function became much less steep. The significance of these results for models of muscle are discussed.

scholarly journals A model of the rat medial gastrocnemius muscle based on inputs to motoneurons and on an algorithm for prediction of the motor unit force

2018 ◽  
Vol 120 (4) ◽  
pp. 1973-1987 ◽  
Author(s):  
R. Raikova ◽  
J. Celichowski ◽  
S. Angelova ◽  
P. Krutki
Keyword(s):  
Motor Unit ◽  
Input Data ◽  
Gastrocnemius Muscle ◽  
Muscle Force ◽  
Motor Units ◽  
Contractile Properties ◽  
Medial Gastrocnemius ◽  
Physiological Data ◽  
Excitation Signal ◽  
Medial Gastrocnemius Muscle

The muscle force is the sum of forces of multiple motor units (MUs), which have different contractile properties. During movements, MUs develop unfused tetani, which result from summation of twitch-shape responses to individual stimuli, which are variable in amplitude and duration. The aim of the study was to develop a realistic muscle model that would integrate previously developed models of MU contractions and an algorithm for the prediction of tetanic forces. The proposed model of rat medial gastrocnemius muscle is based on physiological data: excitability and firing frequencies of motoneurons, contractile properties, and the number and proportion of MUs in the muscle. The MU twitches were modeled by a six-parameter analytical function. The excitability of motoneurons was modeled according to a distribution of their rheobase currents measured experimentally. Processes of muscle force regulation were modeled according to a common drive hypothesis. The excitation signal to motoneurons was modeled by two form types: triangular and trapezoid. The discharge frequencies of MUs, calculated individually for each MU, corresponded to those recorded for rhythmic firing of motoneurons. The force of the muscle was calculated as the sum of all recruited MUs. Participation of the three types of MUs in the developed muscle force was presented at different levels of the excitation signal to motoneurons. The model appears highly realistic and open for input data from various skeletal muscles with different compositions of MU types. The results were compared with three other models with different distribution of the input parameters.NEW & NOTEWORTHY The proposed mathematical model of rat medial gastrocnemius muscle is highly realistic because it is based strictly on experimentally determined motor unit contractile parameters and motoneuron properties. It contains the actual number and proportion of motor units and takes into consideration their different contributions to the whole muscle force, depending on the level of the excitation signal. The model is open for input data from other muscles, and additional physiological parameters can also be included.

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.

The effect of long-term immobilization on the motor unit population of the cat medial gastrocnemius muscle

Neuroscience ◽  
1981 ◽  
Vol 6 (4) ◽  
pp. 725-739 ◽  
Author(s):  
R.F. Mayer ◽  
R.E. Burke ◽  
J. Toop ◽  
J.A. Hodgson ◽  
K. Kanda ◽  
...  
Keyword(s):  
Motor Unit ◽  
Gastrocnemius Muscle ◽  
Medial Gastrocnemius ◽  
Medial Gastrocnemius Muscle

Fast-to-Slow Conversion Following Chronic Low-Frequency Activation of Medial Gastrocnemius Muscle in Cats. II. Motoneuron Properties

1997 ◽  
Vol 77 (5) ◽  
pp. 2605-2615 ◽  
Author(s):  
John B. Munson ◽  
Robert C. Foehring ◽  
Lorne M. Mendell ◽  
Tessa Gordon
Keyword(s):  
Gastrocnemius Muscle ◽  
Low Frequency ◽  
Motor Units ◽  
Input Resistance ◽  
Medial Gastrocnemius ◽  
Excitatory Postsynaptic Potential ◽  
Chronic Stimulation ◽  
General Hypothesis ◽  
Slow Type ◽  
Medial Gastrocnemius Muscle

Munson, John B., Robert C. Foehring, Lorne M. Mendell, and Tessa Gordon. Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. II. Motoneuron properties. J. Neurophysiol. 77: 2605–2615, 1997. Chronic stimulation (for 2–3 mo) of the medial gastrocnemius (MG) muscle nerve by indwelling electrodes renders the normally heterogeneous MG muscle mechanically and histochemically slow (type SO). We tested the hypothesis that motoneurons of MG muscle thus made type SO by chronic stimulation would also convert to slow phenotype. Properties of all single muscle units became homogeneously type SO (slowly contracting, nonfatiguing, nonsagging contraction during tetanic activation). Motoneuron electrical properties were also modified in the direction of type S, fatigue-resistant motor units. Two separate populations were identified (on the basis of afterhyperpolarization, rheobase, and input resistance) that likely correspond to motoneurons that had been fast (type F) or type S before stimulation. Type F motoneurons, although modified by chronic stimulation, were not converted to the type S phenotype, despite apparent complete conversion of their muscle units to the slow oxidative type (type SO). Muscle units of the former type F motor units were faster and/or more powerful than those of the former type S motor units, indicating some intrinsic regulation of motor unit properties. Experiments in which chronic stimulation was applied to the MG nerve cross-regenerated into skin yielded changes in motoneuron properties similar to those above, suggesting that muscle was not essential for the effects observed. Modulation of group Ia excitatory postsynaptic potential (EPSP) amplitude during high-frequency trains, which in normal MG motoneurons can be either positive or negative, was negative in 48 of 49 chronically stimulated motoneurons. Negative modulation is characteristic of EPSPs in motoneurons of most fatigue-resistant motor units. The general hypothesis of a periphery-to-motoneuron retrograde mechanism was supported, although the degree of control exerted by the periphery may vary: natural type SO muscle appears especially competent to modify motoneuron properties. We speculate that activity-dependent regulation of the neurotrophin-(NT) 4/5 in muscle plays an important role in controlling muscle and motoneuron properties.

scholarly journals Fast-to-Slow Conversion Following Chronic Low-Frequency Activation of Medial Gastrocnemius Muscle in Cats. I. Muscle and Motor Unit Properties

1997 ◽  
Vol 77 (5) ◽  
pp. 2585-2604 ◽  
Author(s):  
T. Gordon ◽  
N. Tyreman ◽  
V. F. Rafuse ◽  
J. B. Munson
Keyword(s):  
Motor Unit ◽  
Time Course ◽  
Gastrocnemius Muscle ◽  
Low Frequency ◽  
Contractile Force ◽  
Medial Gastrocnemius ◽  
Normal Range ◽  
Chronic Stimulation ◽  
Time To Peak ◽  
Medial Gastrocnemius Muscle

Gordon, T., N. Tyreman, V. F. Rafuse, and J. B. Munson. Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. I. Muscle and motor unit properties. J. Neurophysiol. 77: 2585–2604, 1997. This study of cat medial gastrocnemius (MG) muscle and motor unit (MU) properties tests the hypothesis that the normal ranges of MU contractile force, endurance, and speed are directly associated with the amount of neuromuscular activity normally experienced by each MU. We synchronously activated all MUs in the MG muscle with the same activity (20 Hz in a 50% duty cycle) and asked whether conversion of whole muscle contractile properties is associated with loss of the normal heterogeneity in MU properties. Chronically implanted cuff electrodes on the nerve to MG muscle were used for 24-h/day stimulation and for monitoring progressive changes in contractile force, endurance, and speed by periodic recording of maximal isometric twitch and tetanic contractions under halothane anesthesia. Chronic low-frequency stimulation slowed muscle contractions and made them weaker, and increased muscle endurance. The most rapid and least variable response to stimulation was a decline in force output of the muscle and constituent MUs. Fatigue resistance increased more slowly, whereas the increase in time to peak force varied most widely between animals and occurred with a longer time course than either force or endurance. Changes in contractile force, endurance, and speed of the whole MG muscle accurately reflected changes in the properties of the constituent MUs both in extent and time course. Normally there is a 100-fold range in tetanic force and a 10-fold range in fatigue indexes and twitch time to peak force. After chronic stimulation, the range in these properties was significantly reduced and, even in MU samples from single animals, the range was shown to correspond with the slow (type S) MUs of the normal MG. In no case was the range reduced to less than the type S range. The same results were obtained when the same chronic stimulation pattern of 20 Hz/50% duty cycle was imposed on paralyzed muscles after hemisection and unilateral deafferentation. The findings that the properties of MUs still varied within the normal range of type S MUs and were still heterogeneous despite a decline in the variance in any one property indicate that the neuromuscular activity can account only in part for the wide range of muscle properties. It is concluded that the normal range of properties within MU types reflects an intrinsic regulation of properties in the multinucleated muscle fibers.

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