Contractile Properties of Human Motor Units: Is Man a Gat?

1998 ◽  
Vol 4 (4) ◽  
pp. 240-249 ◽  
Author(s):  
Brenda Bigland-Ritchie ◽  
Andrew J. Fuglevand ◽  
Christine K. Thomas
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Contractile Properties ◽  
Medial Gastrocnemius ◽  
Force Output ◽  
Rate Coding ◽  
Human Motor ◽  
Human Muscles ◽  
Human Motor Units ◽  
Individual Human

A major goal in neuroscience is to understand how the CNS controls posture and movement in humans. This requires an understanding of individual human motor unit properties and how they interact within the muscle to perform different tasks. This article describes differences and similarities between the contractile properties of human motor units and those of the cat prototype medial gastrocnemius (MG) muscle, on which so many studies have been conducted. The article describes the methods available for measuring human motor unit properties and their limitations, and it discusses how far the behavior of whole muscles can be predicted from their histochemistry. It questions the extent to which human motor units conform to the conventional criteria by which S (slow, fatigue resistant), FR (fast but fatigue resistant) and FF (fast, fatigable) unit types are usually classified. An important difference between human and cat MG data is that weak human motor units are not necessarily slow, nor strong ones fast; that is, generally, human unit force is not correlated with contractile speed. Also, unlike cat MG, the few human muscles studied so far contain few if any FF units but a high proportion of units with intermediate fatigue resistance (Flnt). These apparently aberrant human properties, however, are also found in other cat and rat muscles. Thus, cat MG may not be the best model for motor unit behavior generally. Finally, the influence of human motor unit properties on force output by recruitment and/or rate coding is discussed.

Properties of self-reinnervated motor units of medial gastrocnemius of cat. I. Long-term reinnervation

1986 ◽  
Vol 55 (5) ◽  
pp. 931-946 ◽  
Author(s):  
R. C. Foehring ◽  
G. W. Sypert ◽  
J. B. Munson
Keyword(s):  
Electrical Properties ◽  
Muscle Contraction ◽  
Motor Unit ◽  
Fatigue Resistance ◽  
Muscle Fibers ◽  
Motor Units ◽  
Contractile Properties ◽  
Medial Gastrocnemius ◽  
Unit Type ◽  
Fast Twitch

This work tested whether the membrane electrical properties of cat motoneurons, the contractile properties of their muscle units, and the normal relationships among them would be restored 9 mo after section and resuture of their muscle nerve. Properties of medial gastrocnemius (MG) motor units were examined 9 mo following section and resuture of the MG nerve in adult cats. Motoneuron electrical properties and muscle-unit contractile properties were measured. Motor units were classified on the basis of their contractile properties as type fast twitch, fast fatiguing (FF), fast twitch with intermediate fatigue resistance (FI), fast twitch, fatigue resistant (FR), or slow twitch, fatigue resistant (S) (8, 20). Muscle fibers were classified as type fast glycolytic (FG), fast oxidative glycolytic (FOG), or slow oxidative (SO) on the basis of histochemical staining for myosin adenosine triphosphatase, nicotinamide adenine dinucleotide diaphorase, and alpha-glycerophosphate dehydrogenase (48). Following 9 mo self-reinnervation, the proportions of each motor-unit type were the same as in normal control animals. Motoneuron membrane electrical properties [axonal conduction velocity, afterhyperpolarization (AHP) half-decay time, rheobase, and input resistance] also returned to control levels in those motoneurons that made functional reconnection with the muscle (as determined by ability to elicit measurable tension). The relationships among motoneuron electrical properties were normal in motoneurons making functional reconnection. Approximately 10% of MG motoneurons sampled did not elicit muscle contraction. These cells' membrane electrical properties were different from those that did elicit muscle contraction. Contractile speed and fatigue resistance of reinnervated muscle units had recovered to control levels at 9 mo postoperation. Force generation did not recover fully in type-FF units. The reduced tensions were apparently due to failure of recovery of FG muscle fiber area. Following reinnervation, relationships between motoneuron electrical and muscle-unit contractile properties were similar to controls. This was reflected in a degree of correspondence between motor-unit type and motoneuron type similar to normal units (84 vs. 86%, as defined by Ref. 61). There was a significantly increased proportion of type-SO muscle fibers and a decrease in the fast muscle fibers (especially type FOG) in 9 mo reinnervated MG. Together with the unchanged proportions of motor-unit types, this led to an estimate of average innervation ratios being increased in type-S motor units and decreased in type-FR units.(ABSTRACT TRUNCATED AT 400 WORDS)

Motor-unit properties following cross-reinnervation of cat lateral gastrocnemius and soleus muscles with medial gastrocnemius nerve. I. Influence of motoneurons on muscle

1987 ◽  
Vol 57 (4) ◽  
pp. 1210-1226 ◽  
Author(s):  
R. C. Foehring ◽  
G. W. Sypert ◽  
J. B. Munson
Keyword(s):  
Motor Unit ◽  
Muscle Fibers ◽  
Motor Units ◽  
Contractile Properties ◽  
Medial Gastrocnemius ◽  
Similar Degree ◽  
Muscle Properties ◽  
Lateral Gastrocnemius ◽  
Type Distribution ◽  
Unit Type

This study addresses two questions: is reinnervation of mammalian skeletal muscle selective with respect to motor-unit type? And to what degree may muscle-unit contractile properties be determined by the motoneuron? Properties of individual motor units were examined following cross-reinnervation (X-reinnervation) of lateral gastrocnemius (LG) and soleus muscles by the medial gastrocnemius (MG) nerve in the cat. We examined animals at two postoperative times: 9-10 wk (medX) and 9-11 mo (longX). For comparison, properties of normal LG and soleus motor units were studied. Motor units were classified on the basis of their contractile response as fast contracting fatigable, fast intermediate, fast contracting fatigue resistant, or slow (types FF, FI, FR, or S, respectively) (13,29). Muscle fibers were classified on the basis of histochemical properties as fast glycolytic, fast oxidative glycolytic, or slow oxidative (types FG, FOG, or SO, respectively) (61). Reinnervation of LG and soleus was not selective with respect to motor-unit type. Both muscles were innervated by a full complement of MG motoneuron types, apparently in normal MG proportions. MG motoneurons determined LG muscle fibers' properties to a similar degree as reinnervated MG muscle fibers. In contrast, soleus muscle fibers "resisted" the influence of MG motoneurons. Thus, although longX-reinnervated LG muscle (longX LG) had a motor-unit type distribution similar to normal or self-reinnervated MG, longX soleus contained predominantly type S motor units. Overall mean values for muscle-unit contractile properties reflected this motor-unit type distribution. Muscle units in longX LG and longX soleus had contractile properties typical of the same motor-unit type in normal LG or soleus, respectively. Motor-unit types were recognizable at 10 wk X-reinnervation, although muscle-unit tensions were lower than after 10 mo. The proportions of fast and slow motor units in medX LG were similar to longX LG, although a greater proportion of fast units were resistant to fatigue at 10 wk. There were fewer fast units in medX soleus than longX soleus, which suggested that motor-unit type conversion or innervation of muscle fibers by fast motoneurons is not complete at 10 wk. We conclude that reinnervation of the LG and soleus muscles by MG motoneurons was not selective with respect to motor-unit type. MG motoneurons determined LG muscle properties to a similar degree as self-reinnervated MG muscle fibers. Soleus muscle fibers resisted the influence of MG motoneurons, representing a limit to neural determination of muscle properties.

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 Recruitment of motor units in the medial gastrocnemius muscle during human quiet standing: is recruitment intermittent? What triggers recruitment?

2012 ◽  
Vol 107 (2) ◽  
pp. 666-676 ◽  
Author(s):  
Taian M. M. Vieira ◽  
Ian D. Loram ◽  
Silvia Muceli ◽  
Roberto Merletti ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Planning Process ◽  
Discharge Rate ◽  
Motor Units ◽  
Median Number ◽  
The Body ◽  
Medial Gastrocnemius ◽  
Quiet Standing ◽  
Rate Coding ◽  
Active Motor

The recruitment and the rate of discharge of motor units are determinants of muscle force. Within a motoneuron pool, recruitment and rate coding of individual motor units might be controlled independently, depending on the circumstances. In this study, we tested whether, during human quiet standing, the force of the medial gastrocnemius (MG) muscle is predominantly controlled by recruitment or rate coding. If MG control during standing was mainly due to recruitment, then we further asked what the trigger mechanism is. Is it determined internally, or is it related to body kinematics? While seven healthy subjects stood quietly, intramuscular electromyograms were recorded from the MG muscle with three pairs of wire electrodes. The number of active motor units and their mean discharge rate were compared for different sway velocities and positions. Motor unit discharges occurred more frequently when the body swayed faster and forward (Pearson R = 0.63; P < 0.0001). This higher likelihood of observing motor unit potentials was explained chiefly by the recruitment of additional units. During forward body shifts, the median number of units detected increased from 3 to 11 ( P < 0.0001), whereas the discharge rate changed from 8 ± 1.1 (mean ± SD) to 10 ± 0.9 pulses/s ( P = 0.001). Strikingly, motor units did not discharge continuously throughout standing. They were recruited within individual, forward sways and intermittently, with a modal rate of two recruitments per second. This modal rate is consistent with previous circumstantial evidence relating the control of standing to an intrinsic, higher level planning process.

Plasticity of medial gastrocnemius motor units following cordotomy in the cat

1986 ◽  
Vol 55 (4) ◽  
pp. 619-634 ◽  
Author(s):  
J. B. Munson ◽  
R. C. Foehring ◽  
S. A. Lofton ◽  
J. E. Zengel ◽  
G. W. Sypert
Keyword(s):  
Electrical Properties ◽  
Motor Unit ◽  
Muscle Fibers ◽  
Motor Units ◽  
Contractile Properties ◽  
Medial Gastrocnemius ◽  
Lumbar Spinal Cord ◽  
Partial Recovery ◽  
Cross Sectional ◽  
Group I

Experiments were performed in adult cats to determine the effects of lumbar cordotomy on synaptic potentials, motoneuron membrane electrical properties, muscle-unit contractile properties, and whole-muscle histochemical properties of a heterogeneous skeletal muscle. Medial gastrocnemius (MG) motor units were examined 1 wk to 7 mo following complete transection of the lumbar spinal cord (cordotomy). Motor units were classified on the basis of their contractile properties as type FF, FI, FR, or S (8, 68). Muscle fibers were classified as type FG, FOG, or SO on the basis of histochemical staining (59). Motoneuron electrical properties (axonal conduction velocity, action-potential amplitude, rheobase, input resistance, afterhyperpolarization), group I EPSPs, and muscle-unit contractile properties (unpotentiated and potentiated twitch, unfused and fused tetanus, fatigability) were measured. Reduced numbers of type FR motor units and increased numbers of types FI + FF motor units were found in electrophysiological experiments 2 wk to 7 mo following cordotomy. Corroborative data were obtained from histochemical studies of the same MG muscles. Electrical properties of the motoneurons of each motor-unit type were normal following cordotomy. The close correspondence between motoneuron electrical properties and muscle-unit contractile properties found in normal MG muscle (68) was preserved following cordotomy. Contractile strength of muscle units of all types was severely reduced following cordotomy; partial recovery occurred 4-7 mo following cordotomy. Cross-sectional area of muscle fibers was reduced at all times investigated (2 wk to 7 mo). In three cats, homonymous group Ia single-fiber-motoneuron EPSPs were studied 1 or 2 mo following cordotomy at spinal level L4-5 or L5. EPSP amplitude and afferent-to-motoneuron projection frequency were normal. In 12 other cats, composite heteronymous group I EPSPs were studied 2 wk to 7 mo following cordotomy at various levels. Amplitude of these EPSPs was increased, dependent upon level of cordotomy and postoperative time. Hypotheses concerning the influence of motoneurons on muscle, and of muscle on motoneurons, are presented as possible mechanisms whereby the close relation between motoneuron electrical and muscle-unit contractile properties is preserved in the face of redistributed motor-unit populations.

Contractile properties of human motor units in health, aging, and disease

2001 ◽  
Vol 24 (9) ◽  
pp. 1113-1133 ◽  
Author(s):  
K. Ming Chan ◽  
Timothy J. Doherty ◽  
William F. Brown
Keyword(s):  
Motor Units ◽  
Contractile Properties ◽  
Health Aging ◽  
Human Motor ◽  
Human Motor Units

Force output of cat motor units stimulated with trains of linearly varying frequency

1989 ◽  
Vol 61 (1) ◽  
pp. 208-217 ◽  
Author(s):  
S. A. Binder-Macleod ◽  
H. P. Clamann
Keyword(s):  
Motor Unit ◽  
Stimulus Frequency ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Force Frequency ◽  
Force Output ◽  
Tension Development ◽  
Fusion Frequency ◽  
Single Motor Units ◽  
Frequency Relation

1. A relation between stimulation frequency and muscle force is usually determined with stimulus trains of constant frequency and described as a single-valued sigmoid curve. This relationship fails to explain a number of features of rate coding. 2. Single motor units were isolated in medial gastrocnemius or soleus muscles of cats deeply anesthetized with pentobarbital sodium. Motor units were classified as fast or slow. Each unit was stimulated with a train whose frequency varied linearly from less than 3 pulses per second (pps) to 20% above the unit's fusion frequency and back to about 3 pps with a period of 5 s. 3. All motor units showed a marked hysteresis during frequency-varying stimulation. A greater force was produced when frequency was decreasing than when it was increasing. The force output of each unit remained nearly maximal as stimulus frequency declined from its maximum to about one-half of the unit's fusion frequency; force rapidly declined with further decreases in frequency. The force-frequency relation could change with each trial as frequency increased but was highly reproducible when frequency decreased. This suggested a strategy by which central nervous system (CNS) control could maximize the force at any discharge rate and produce a predictable force-frequency relation. 4. Posttetanic potentiation, motor unit slowing, and a preload which causes a motor unit to operate on the negatively sloping portion of the length-tension curve may each contribute to the observed hysteresis under certain circumstances. None can explain why hysteresis was consistently seen in all motor units. A time-dependent rate of tension development and decay together with a catchlike property can account for all of the properties of hysteresis and appeared to be the primary cause of hysteresis in fully potentiated motor units.

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