Changes in properties of the medial gastrocnemius motor units in aging rats

1989 ◽  
Vol 61 (4) ◽  
pp. 737-746 ◽  
Author(s):  
K. Kanda ◽  
K. Hashizume
Keyword(s):  
Muscle Fibers ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Aged Rats ◽  
Fatigue Index ◽  
Middle Aged ◽  
Old Rats ◽  
The Mean ◽  
Slow Twitch ◽  
Fast Twitch

1. The properties of motor units were investigated in the medial gastrocnemius (MG) of old rats [27.5 +/- 1.6 (SD) mo old, n = 18]. Individual motor units were functionally isolated by ventral root fiber splitting and grading stimulus intensity. The muscle-unit portion of the motor unit was identified by the glycogen depletion method. The physiological properties of 77 motor units in 6 animals and the histological results of 7 slow-twitch (type S) muscle units were compared with data from motor units in the same muscle of middle-aged rats (12.8 +/- 1.6 mo old, n = 33). 2. The motor units were classified into four types of categories [FF (fast-twitch motor units with a fatigue index less than or equal to 0.5), FI (fast-twitch motor units with a fatigue index greater than 0.5 but less than 0.75), FR (fast-twitch motor units with a fatigue index greater than or equal to 0.75), S (slow-twitch motor units with a fatigue index greater than 0.75)] using the same criteria (i.e., presence or absence of the "sag" property and fatigability) used for middle-aged rats. No significant difference in the relative distributions of these unit types was detected, although the MG muscle in old rats exhibited a relatively high proportion of type S units and fewer type FR units. 3. The mean tetanic tensions for type FF + FI and FR units were significantly smaller than those in the middle-aged rats. On the other hand, type S motor units produced more tension than in the middle-aged rats. 4. The conduction velocity of motor axons was considerably slower in any unit type of old motor units, and the most marked change was found in type FR units. 5. The general morphological features of the old rat MG were fiber-type grouping, disseminated atrophic or angulated fibers, a decrease in the total number of muscle fibers, and an increase in the number of type I muscle fibers. The major distribution patterns of fibers of different types were the same as those in the middle-aged MG. 6. Seven type S units that produced large tetanic tension were depleted of glycogen in the muscle-unit portions. These units had a large innervation ratio compared with those in the middle-aged rats, whereas the mean cross-sectional area of muscle fibers and the calculated specific tension remained unaltered.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of self-reinnervated motor units of medial gastrocnemius of cat. II. Axotomized motoneurons and time course of recovery

1986 ◽  
Vol 55 (5) ◽  
pp. 947-965 ◽  
Author(s):  
R. C. Foehring ◽  
G. W. Sypert ◽  
J. B. Munson
Keyword(s):  
Electrical Properties ◽  
Motor Unit ◽  
Fatigue Resistance ◽  
Time Course ◽  
Muscle Fibers ◽  
Motor Units ◽  
Input Resistance ◽  
Medial Gastrocnemius ◽  
Muscle Properties ◽  
Fast Twitch

This study tested the hypothesis that functional connection to muscle is necessary for expression of normal motoneuron electrical properties. Also examined was the time course of self-reinnervation. Properties of individual medial gastrocnemius (MG) motor units were examined following section and reanastomosis of the MG nerve. Stages examined were 3-5 wk (prior to reinnervation, no-re), 5-6 wk (low-re), 9-10 wk (med-re), and 9 mo (long-re, preceding paper) after nerve section. Motor units were classified on the basis of their mechanical response 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) (11, 24). Motoneuron electrical properties were measured. Muscle fibers were classified using histochemical methods as type fast glycolytic (FG), fast oxidative glycolytic (FOG), or slow oxidative (SO) (60). Prior to functional reinnervation, MG motoneurons exhibited increased input resistance, decreased rheobase, decreased rheobase/input resistance, and decreased axonal conduction velocity. There was no change in mean afterhyperpolarization (AHP) half-decay time. Normal relationships between motoneuron electrical properties were lost. These data are consistent with dedifferentiation of motoneuron properties following axotomy (35, 47). At 5-6 wk after reanastomosis, motor-unit tensions were small, and motoneuron membrane electrical properties were unchanged from the no-re stage. There were no differences in motoneuron electrical properties between cells that elicited muscle contraction and those that did not. Motor-unit types were first recognizable at the med-re stage. The proportions of fast and slow motor units were similar to normal MG. Within the fast units, there were fewer type-FF units and more type-FI and type-FR units than normal, reflecting a general increase in fatigue resistance at this stage. Neither motoneuron membrane electrical properties nor muscle contractile properties had reached normal values, although both were changed in that direction from the low-re stage. Normal relationships between muscle properties, between motoneuron properties, and between motoneuron and muscle properties were re-established. The correspondence between motor-unit type and motoneuron type was similar to normal or 9 mo reinnervated MG. Muscle-unit tetanic tensions became larger with time after reinnervation. Most of the increase in muscle tension beyond the med-re stage could be accounted for by increase in muscle fiber area. There was an increased proportion of SO muscle fibers observed in the med-re muscles, as at the long-re stage.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Size and Proportions of Slow-Twitch and Fast-Twitch Muscle Fibers in Human Costal Diaphragm

2016 ◽  
Vol 2016 ◽  
pp. 1-6 ◽  
Author(s):  
Marija Meznaric ◽  
Erika Cvetko
Keyword(s):  
Vastus Lateralis ◽  
Muscle Fibers ◽  
Hybrid Fibers ◽  
Cross Sectional ◽  
Myosin Heavy Chain Isoform ◽  
Diaphragmatic Muscle ◽  
Fast Twitch Muscle ◽  
The Mean ◽  
Slow Twitch ◽  
Fast Twitch

Smaller diaphragmatic motor unit potentials (MUPs) compared to MUPs of limb muscles lead to the hypothesis that diaphragmatic muscle fibers, being the generators of MUPs, might be also smaller. We compared autopsy samples of costal diaphragm and vastus lateralis of healthy men with respect to fibers’ size and expression of slow myosin heavy chain isoform (MyHC-1) and fast 2A isoform (MyHC-2A). Diaphragmatic fibers were smaller than fibers in vastus lateralis with regard to the mean minimal fiber diameter of slow-twitch (46.8 versus 72.2 μm,p<0.001), fast-twitch (45.1 versus 62.4 μm,p<0.001), and hybrid fibers (47.3 versus 65.0 μm,p<0.01) as well as to the mean fiber cross-sectional areas of slow-twitch (2376.0 versus 5455.9 μm2,p<0.001), fast-twitch (2258.7 versus 4189.7 μm2,p<0.001), and hybrid fibers (2404.4 versus 4776.3 μm2,p<0.01). The numerical proportion of slow-twitch fibers was higher (50.2 versus 36.3%,p<0.01) in costal diaphragm and the numerical proportion of fast-twitch fibers (47.2 versus 58.7%,p<0.01) was lower. The numerical proportion of hybrid fibers did not differ. Muscle fibers of costal diaphragm have specific characteristics which support increased resistance of diaphragm to fatigue.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. II. Motoneuron firing-rate modulation

1996 ◽  
Vol 75 (1) ◽  
pp. 38-50 ◽  
Author(s):  
K. E. Tansey ◽  
B. R. Botterman
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Firing Rates ◽  
Rate Modulation ◽  
The Mean ◽  
Fast Twitch ◽  
The Relationship

1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.

Progress of Age-Related Changes in Properties of Motor Units in the Gastrocnemius Muscle of Rats

2004 ◽  
Vol 92 (3) ◽  
pp. 1357-1365 ◽  
Author(s):  
Miho Sugiura ◽  
Kenro Kanda
Keyword(s):  
Gastrocnemius Muscle ◽  
Muscle Tension ◽  
Muscle Fibers ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Motor Nucleus ◽  
Twitch Contraction ◽  
Age Related ◽  
Old Rats ◽  
Whole Muscle

The mechanical properties of individual motor units in the medial gastrocnemius muscle, as well as the whole muscle properties and innervating motor nucleus, were investigated in dietary-restricted, male Fischer 344/DuCrj rats at ages of 4, 7, 12, 21/22, 27, 31, and 36 mo. The tetanic tension of the type S units continuously increased until the age of 36 mo. Those of type FF and FR units declined from 21/22 to 27 mo of age but did not change further while the whole muscle tension decreased greatly. The atrophy of muscle fibers, the decline in motoneuron number and axonal conduction velocity, and the decrease in the posttetanic potentiation of twitch contraction of motor units seemed to start after 21/22 mo of age and were accelerated with advancing age. Prolongation of twitch contraction time was evident for only type S and FR units in 36-mo-old rats. The fatigue index was greatly increased for type FF units in 36-mo-old rats. These findings indicated that the progress of changes in various properties occurring in the senescent muscle was different in terms of their time course and degree and also dependent on the types of motor unit. The atrophy and decrease in specific tension of muscle fibers affected the decline in tension output of motor units. This was effectively compensated for by the capture of denervated muscle fibers over time.

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)

Effects of age at cordotomy and subsequent exercise on contraction times of motor units in the cat

1993 ◽  
Vol 75 (6) ◽  
pp. 2683-2688
Author(s):  
L. A. Smith ◽  
E. Eldred ◽  
V. R. Edgerton
Keyword(s):  
Motor Units ◽  
Medial Gastrocnemius ◽  
Fatigue Index ◽  
High Fatigue ◽  
Contraction Times ◽  
The Mean

The contraction times (CTs) of functionally isolated motor units (MUs) in the soleus (SOL) and medial gastrocnemius (MG) muscles were determined in cats that had been spinalized at ages 2 (n = 15) or 12 (n = 9) wk and then either subjected to exercise on a treadmill or simply given manipulative care of the hindlimbs. The MUs were tested approximately 12 wk after the low-thoracic cordotomy, and comparisons were made with data from control animals. The CT of 50.9 ms obtained for SOL units (n = 163) in the spinal cats was 22% shorter than the mean of 65.0 ms for MUs (n = 57) from control cats (n = 4). Contrary to expectation, the CT in animals spinalized at 12 wk was significantly shorter than that in the 2-wk group. The CT for MG units (n = 105) in spinal cats was also significantly shorter (11%) than that in controls cats (n = 66, 6 cats), and those units identified by their high fatigue index as being of slow or fatigue-resistant type had a shorter CT than units with a low index. No distinction in CT of exercised and nonexercised groups was detected for either muscle. These findings are discussed in relation to the bearing influences of supraspinal and segmental origin have on CT duration in SOL and MG muscles during growth of the kitten. A slight, significant decrease (6%) in the fatigue index of SOL MUs (n = 144) was detected, but the values remained high (mean 0.87).

Motor-unit stimulation patterns during fatiguing contractions of constant tension

1988 ◽  
Vol 60 (4) ◽  
pp. 1198-1214 ◽  
Author(s):  
B. R. Botterman ◽  
T. C. Cope
Keyword(s):  
Motor Units ◽  
Medial Gastrocnemius ◽  
Fatigue Properties ◽  
Endurance Time ◽  
Stimulation Rate ◽  
Wide Range ◽  
Constant Tension ◽  
Slow Twitch ◽  
Fast Twitch ◽  
Tetanic Tension

1. Through computer feedback control, muscle-unit tension was maintained by altering the stimulation rate of a functionally isolated motor axon. The required stimulation patterns and fatigue properties of motor units from the flexor carpi radialis (FCR), flexor digitorum longus (FDL), and medial gastrocnemius (MG) muscles of the cat were studied when tension was maintained or "clamped" at a constant average level (25% of maximum tetanic tension). 2. In each muscle, two distinct stimulation patterns were observed during constant-tension contractions, one associated with slow-twitch units and the other with fast-twitch units. Once target tension was reached, slow-twitch units required fairly constant rates in order to maintain a constant force, whereas fast-twitch units displayed a marked decline in rate during the early phases of the contraction, averaging between 42 and 54% for the three muscles. The decline in rate most likely represented potentiation of the contractile response and slowing of contractile speed. In general, slow-twitch units responded with lower mean rates (approximately 14 pps less), averaged over the course of the contraction, than fast-twitch units. 3. For fast-twitch units of each muscle, resistance to fatigue varied continuously and over a wide range. The duration that tension could be maintained at 25% of maximum, defined as endurance time, ranged between 16 and 2063 s. No categorization of fast-twitch units into groups could be made on the basis of endurance time. Of the 5 slow-twitch units followed beyond 2700 s, only one failed to maintain tension during the observation period. 4. For hindlimb fast-twitch units, endurance was independent of the stimulation rate needed to maintain tension during the contraction. By contrast, there was a significant tendency for an inverse relation between endurance time and mean stimulation rate for FCR fast-twitch units. 5. Recovery of maximum tension was evaluated at 30 s, 1 min, 2 min, and 5 min following a constant-tension contraction. After a 5-min rest, fast-twitch units were able to produce an average of 80-85% of their maximum tetanic tension. By using the median endurance time (approximately 100 s) to divide the fast-twitch population into "low" and "high" endurance groups, recovery of tension was found not to be uniform among the two groups. High endurance units were able to recover a greater percentage of their original maximum tetanic tension. No difference was found between force recovery for low and high endurance units at 30 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of compensatory hypertrophy studied in individual motor units in medial gastrocnemius muscle of the cat

1978 ◽  
Vol 41 (2) ◽  
pp. 496-508 ◽  
Author(s):  
J. V. Walsh ◽  
R. E. Burke ◽  
W. Z. Rymer ◽  
P. Tsairis
Keyword(s):  
Motor Unit ◽  
Muscle Fibers ◽  
Motor Units ◽  
Compensatory Hypertrophy ◽  
Medial Gastrocnemius ◽  
Cross Sectional ◽  
Unit Type ◽  
The Mean ◽  
Tetanic Tension ◽  
Individual Motor

1. Compensatory hypertrophy of the medial gastrocnemius (MG) muscle was produced by denervating or removing its synergists (i.e., the lateral gastrocnemius, soleus, and plantaris muscles) in adult cats. Following survival times of 14-32 wk, intracellular recording and stimulation techniques were used to study the motor-unit population in MG. The data obtained were compared with results from MG motor units in normal unoperated cats of the same body size and weight. 2. Using criteria employed for normal motor units, the units in hypertrophic MG muscles were readily classified into the same groups (types FF, F(int), FR, and S) as in normal MG. There was no detectable difference in the distribution of motor-unit types after hypertrophy. 3. When compared with a normal motor-unit sample, there was a large increase in mean tetanic tension, but no significant change in twitch tension, for each motor-unit type in the hypertrophied muscles. The most marked increase was found among the fatigue-resistant type S and type FR motor units. There was no alteration of twitch contraction times or fatigue resistance in any unit type after hypertrophy. 4. For each motor-unit type, the mean homonymous (MG) group Ia EPSP amplitude was the same in normal and hypertrophic MG populations. There was, however, a significant increase in the average conduction velocity of MG motor axons in the animals with uncomplicated MG synergist removal and maximal MG hypertrophy. 5. On the basis of histochemical staining, muscle fibers from comparable sections of hypertrophic and contralateral (unoperated) MG muscles were presumptively identified as belonging to FF, FR, or S units. There was no significant difference between hypertrophic and contralateral MG muscles in the percentage of each fiber type, although there was some variability in muscle composition from one cat to another. One muscle pair was studied in detail for fiber cross-sectional area. In this cat, with marked hypertrophy by muscle weight, there was a modest increase in the mean fiber areas of histochemical S and FR muscle fibers, but no evident change in FF fibers, on the hypertrophic side. 6. MG motor units were examined in several cats in which synergist removal resulted in scarring and marked limitation of passive ankle mobility, and no evident weight gain in MG. Motor units of all types in these animals showed a decrease in twitch tension and in mean twitch/tetanus ratios, with little alteration in mean tetanic tensions. 7. The main effect of compensatory hypertrophy under the present conditions was a large increase in tetanic tension output from individual motor units due, at least in part, to an increase in fiber cross-sectional area. There was no evidence indicating any "conversion" of motor units or of their muscle fibers from one type to another.

Damage to different motor units from active lengthening of the medial gastrocnemius muscle of the cat

2002 ◽  
Vol 92 (3) ◽  
pp. 1104-1110 ◽  
Author(s):  
C. L. Brockett ◽  
D. L. Morgan ◽  
J. E. Gregory ◽  
U. Proske
Keyword(s):  
Gastrocnemius Muscle ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Optimum Length ◽  
Time To Peak ◽  
Slow Twitch ◽  
Fast Twitch ◽  
Whole Muscle ◽  
Medial Gastrocnemius Muscle ◽  
Anesthetized Cat

Slow-twitch motor units in the medial gastrocnemius muscle of the anesthetized cat were found to have an average optimum length for active tension that was 0.8 ± 0.5 (SE) mm longer than the whole muscle optimum. For fast-twitch units (time to peak < 50 ms), the average optimum was 1.3 ± 0.3 mm shorter than the whole muscle optimum. After the muscle had been subjected to 10 stretches while maximally activated, beginning at the whole muscle optimum length, the optimum lengths of the 27 fast-twitch motor units shifted significantly further in the direction of longer muscle lengths (mean 4.3 ± 0.3 mm) than for the eight slow-twitch units (2.1 ± 0.4 mm). A shift in the muscle's length-tension relation was interpreted as being due to sarcomere disruption. Statistical analysis showed that a motor unit's optimum length for a contraction, relative to the whole muscle optimum, was a better indicator of the unit's susceptibility to damage from active lengthenings than was motor unit type.

Comparison of physiological and histochemical properties of motor units after cross-reinnervation of antagonistic muscles in the cat hindlimb

1988 ◽  
Vol 60 (1) ◽  
pp. 365-378 ◽  
Author(s):  
T. Gordon ◽  
C. K. Thomas ◽  
R. B. Stein ◽  
S. Erdebil
Keyword(s):  
Muscle Fibers ◽  
Motor Units ◽  
Triceps Surae ◽  
Medial Gastrocnemius ◽  
Single Motor Units ◽  
Extensor Muscles ◽  
Flexor Muscles ◽  
The Cross ◽  
Triceps Surae Muscles ◽  
Fast Twitch

1. Physiological and histochemical properties of the cat ankle extensor muscles, the lateral and medial gastrocnemius, and the soleus were studied after cross-reinnervation by flexor motoneurons. 2. Tibial and common peroneal nerves were cut and cross-united in the popliteal fossa of 2- to 6-mo-old cats. Eighteen to 24 mo later, single motor units were isolated by dissection and stimulation of ventral root filaments and classified into four types: fast-twitch, fatigable (FF), fast-twitch with intermediate fatigue resistance (FI), fast-twitch, fatigue-resistant (FR), and slow, fatigue-resistant (S). Muscle fibers were classified as fast glycolytic (FG), fast, oxidative glycolytic (FOG), and slow oxidative (SO) on the basis of histochemical staining. 3. Although motor-unit force was normally well correlated with the size of the innervating motor axon in the cross-reinnervated muscles, the force of different unit types overlapped considerably. The reinnervated motor units also showed a higher than normal degree of fatigability. 4. The range of muscle unit forces in cross-reinnervated triceps surae muscles was the same as in the normally innervated triceps surae muscles. This range is 2-3 times greater than the flexor muscles, which the common peroneal nerve normally supplies. The range of contraction speed of units in the cross-reinnervated extensor muscles was comparable to that in the flexor muscles, consistent with a motoneuron-specific determination of muscle speed (28). 5. SO and FOG muscle fibers were found in all reinnervated triceps surae muscles, but FG fibers were only found in reinnervated medial gastrocnemius (MG) and lateral gastrocnemius (LG) muscles, consistent with previous findings of the resistance of soleus muscles to complete conversion (10, 16, 20, 21). Type grouping of muscle fibers was characteristic of the reinnervated muscles. 6. Reinnervated SO muscle fibers were larger than the corresponding fibers in normally innervated muscles as were the estimated number of muscle fibers innervated by slow motor axons. Nonetheless, the force generated by the S motor units remained relatively smaller than FR and FF units. The relative contributions of the number, cross-sectional area and specific tension to the force generation of reinnervated motor units are discussed.

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