Relative strength of synaptic input from short-latency pathways to motor units of defined type in cat medial gastrocnemius

1976 ◽  
Vol 39 (3) ◽  
pp. 447-458 ◽  
Author(s):  
R. E. Burke ◽  
W. Z. Rymer
Keyword(s):  
Motor Unit ◽  
Motor Behavior ◽  
Motor Units ◽  
Relative Strength ◽  
Short Latency ◽  
Medial Gastrocnemius ◽  
Unit Type ◽  
Ia Afferents ◽  
Ia Epsp ◽  
Thoracic Cord

1. Intracellular recording and stimulation techniques were used in anesthetized cats to study the interrelations between amplitudes of PSPs produced by electrical stimulation of several short-latency pathways to MG alpha motoneurons and the mechanical properties of muscle units innervated by the same cells. Motor-unit types were identified by muscle-unit properties.2. The maximum amplitudes of monosynaptic EPSPs produced in MG motoneurons by activation of homonymous (MG) and heteronymous (LGS) group Ia afferents were clearly related to motor-unit type, being, on the average, largest in type S units, somewhat smaller in type FR and F(int) units, and smallest in type FF units. Correspondingly, group Ia EPSP amplitudes were inversely correlated with muscle-unit tension production and directly correlated with resistance to fatigue. The same input distribution was true for disynaptic IPSPs produced by group Ia afferents from antagonist ankle flexors.3. The amplitudes of monosynaptic EPSPs produced by fibers descending in the ipsilateral ventral funiculi of the low thoracic cord were not clearly related to MG motor-unit type or (therefore) to muscle-unit properties.4. A quantitative input-output model of the MG motor-unit pool, based in part on the present results, suggests that overall characteristics of MG motor units, and their relative numbers in the MG pool, reflect functional specializations determined by specific mechanical demands placed on the MG muscle by the usual motor behavior of the animal.

Synaptic organization of defined motor-unit types in cat tibialis anterior

1980 ◽  
Vol 43 (6) ◽  
pp. 1631-1644 ◽  
Author(s):  
R. P. Dum ◽  
T. T. Kennedy
Keyword(s):  
Motor Unit ◽  
Tibialis Anterior ◽  
Muscle Afferents ◽  
Medial Gastrocnemius ◽  
Synaptic Organization ◽  
Ia Afferents ◽  
Group I ◽  
Ia Epsp ◽  
Group I Afferents ◽  
Stimulation Of

1. Synaptic potentials were recorded intracellularly in tibialis anterior (TA) motoneurons following stimulation of a descending brain stem pathway, the medial longitudinal fasciculus (MLF), and three segmental inputs, the homonymous and heteronymous group Ia afferents, the group I afferents from the antagonist, and the cutaneous and muscle afferents. Intracellular stimulation of the motoneurons was used to classify them, based on the properties of the innervated muscle units, into types FF, F(int), FR, and S (6, 16). 2. The sum of the monosynaptic EPSP amplitudes resulting from stimulation of homonymous and heteronymous group Ia afferents (summed group Ia EPSP) was inversely related to motoneuron size, as assessed by motoneuron input resistance, and was inversely related to motor-unit tetanic tension. Type-FF, -FR, and -S motoneurons showed significant differences in the mean amplitude of their summed group Ia EPSPs. 3. The amplitudes of disynaptic IPSPs resulting from stimulation of group I afferents in the antagonist muscle also showed an inverse relationship to motoneuron size. The observed relationships between motoneuron size and the monosynaptic group Ia EPSP amplitude or the disynaptic group I IPSP amplitude are compatible with the “size principle” of motor-unit recruitment (26). 4. The amplitudes of the monosynaptic EPSPs evoked in TA motoneurons by stimulation of the MLF were distributed rather randomly among all types of TA motoneurons. A slight tendency of larger monosynaptic EPSPs to occur in motoneurons with larger tetanic tensions was observed. 5. The polysynaptic effects from cutaneous and muscle afferents in sural and gastrocnemius-soleus nerves were frequently excitatory on type-FF motoneurons, but were primarily inhibitory on type-FR and -S motoneurons. Clearly, the polysynaptic cutaneous and muscle inputs and the monosynaptic MLF input onto TA motoneurons show a different pattern of synaptic organization than the group I inputs. 6. In general, the synaptic organization of the TA motor nucleus is similar to that of its extensor antagonist, medial gastrocnemius (MG) (2--5, 7, 8), when analogous neural circuits are compared. This parallel organization suggests a commonality of motor-control systems for both flexor and extensor muscles.

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.

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.

Presynaptic inhibition, EPSP amplitude, and motor-unit type in triceps surae motoneurons in the cat

1983 ◽  
Vol 49 (4) ◽  
pp. 922-931 ◽  
Author(s):  
J. E. Zengel ◽  
S. A. Reid ◽  
G. W. Sypert ◽  
J. B. Munson
Keyword(s):  
Motor Unit ◽  
Presynaptic Inhibition ◽  
Motor Units ◽  
Input Resistance ◽  
Triceps Surae ◽  
Medial Gastrocnemius ◽  
Absolute Amount ◽  
Epsp Amplitude ◽  
Unit Type ◽  
Fast Twitch

1. Composite group Ia excitatory postsynaptic potentials (EPSPs) produced by heteronymous nerve stimulation were recorded from triceps surae motoneurons of barbiturate-anesthetized cats. Motoneuron rheobase, input resistance, and axonal conduction velocity were measured, and motor units were classified on the basis of the mechanical responses of their muscle units. 2. The amplitude of EPSPs recorded from 33 medial gastrocnemius (MG) motoneurons ranged from 0.6 to 4.3 mV. The mean EPSP amplitude differed among the major MG motor-unit types, increasing in the order fast twitch, fast fatiguing (FF); fast twitch, fatigue resistant (FR); slow twitch, fatigue resistant (S) (FF less than FR less than S). The amplitude of EPSPs recorded from 15 soleus motoneurons ranged from 0.3 to 3.4 mV, with a mean of 1.4 mV. 3. Presynaptic inhibition of EPSPs was produced by trains of conditioning volleys in the posterior biceps-semitendinosus (PBST) nerve. In 33 MG cells PBST conditioning stimulation reduced the amplitude of EPSPs by 11-50%, with a mean inhibition of 27%. The amplitude of EPSPs in 15 soleus motoneurons was decreased by 5-84%, with a mean inhibition of 37%. 4. When the magnitude of presynaptic inhibition was expressed as percent inhibition, there was no relation between presynaptic inhibition and either motor-unit type or the amplitude of the EPSP. However, when presynaptic inhibition was expressed as the absolute amount of inhibition in millivolts, the magnitude of inhibition was highly correlated with EPSP amplitude both across the entire triceps surae population (MG, lateral gastrocnemius, soleus) as well as within each muscle population. This correlation was also significant within the MG FF and FR motor-unit populations. 5. We conclude that EPSP amplitude and not motor-unit type is the major determinant of the magnitude of presynaptic inhibition. However, because of the effect of motor-unit type on EPSP amplitude, the net effect is that presynaptic inhibition increases in the order FF less than FR less than S.

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.

Recruitment order of motoneurons in stretch reflexes is highly correlated with their axonal conduction velocity

1984 ◽  
Vol 52 (3) ◽  
pp. 410-420 ◽  
Author(s):  
P. Bawa ◽  
M. D. Binder ◽  
P. Ruenzel ◽  
E. Henneman
Keyword(s):  
Motor Unit ◽  
Conduction Velocity ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Stretch Reflexes ◽  
Decerebrate Cat ◽  
Unit Type ◽  
Axonal Conduction ◽  
Conduction Velocities ◽  
Recruitment Order

Motor units of soleus and medial gastrocnemius (MG) muscles were studied in pairs during stretch reflexes in the decerebrate cat to determine the relation between their recruitment orders and axonal conduction velocities. In 97% of soleus pairs, the motor unit with the lower axonal conduction velocity was recruited first. Since the soleus is a homogeneous muscle in the cat, differences in motor-unit type are, therefore, not a sine qua non for orderly recruitment nor is recruitment random within homogeneous populations of motor units, as recently proposed (28). In the medial gastrocnemius, a heterogeneous muscle, the same high correlation (97%) between recruitment sequence and conduction velocity was observed. Thus, the factors that determine recruitment order in heterogeneous muscles are as closely correlated with axonal diameter as they are in homogeneous muscles. Comparison of axonal conduction velocities in our sample of MG units with those in three samples of type-identified MG units studied by other investigators also suggests that motor-unit type is not the critical factor controlling the sequence of activation in heterogeneous muscles. It is concluded that the combined effects of all presynaptic and postsynaptic factors that determine susceptibility to discharge in motoneurons during stretch reflexes are strictly correlated with their axonal conduction velocities, as predicted by the size principle.

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.

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

1987 ◽  
Vol 57 (4) ◽  
pp. 1227-1245 ◽  
Author(s):  
R. C. Foehring ◽  
G. W. Sypert ◽  
J. B. Munson
Keyword(s):  
Electrical Properties ◽  
Motor Unit ◽  
Conduction Velocity ◽  
Decay Time ◽  
Motor Units ◽  
Input Resistance ◽  
Medial Gastrocnemius ◽  
Lateral Gastrocnemius ◽  
Mean Values ◽  
Axonal Conduction

We tested whether the muscle innervated may influence the expression of motoneuron electrical properties. Properties of individual motor units were examined following cross-reinnervation (X-reinnervation) of cat lateral gastrocnemius (LG) and soleus muscles by the medial gastrocnemius (MG) nerve. We examined animals at two postoperative times: 9-10 wk (medX) and 9-11 mo (longX). For comparison, normal LG and soleus motoneuron properties were also studied. Motor units were classified on the basis of their contractile responses as fast contracting fatigable, fast intermediate fast contracting fatigue resistant, and slow types FF, FI, FR, or S, respectively) (9, 21). Motoneuron electrical properties (rheobase, input resistance, axonal conduction velocity, afterhyperpolarization) were measured. After 9-11 mo, MG motoneurons that innervated LG muscle showed recovery of electrical properties similar to self-regenerated MG motoneurons. The relationships between motoneuron electrical properties were largely similar to self-regenerated MG. For MG motoneurons that innervated LG, motoneuron type (65) predicted motor-unit type in 74% of cases. LongX-soleus motoneurons differed from longX-LG motoneurons or self-regenerated MG motoneurons in mean values for motoneuron electrical properties. The differences in overall means reflected the predominance of type S motor units. The relationships between motoneuron electrical properties were also different than in self-regenerated MG motoneurons. In all cases, the alterations were in the direction of properties of type S units, and the relationship between normal soleus motoneurons and their muscle units. Within motor-unit types, the mean values were typical for that type in self-regenerated MG. Motoneuron type (65) was a fairly strong predictor of motor-unit type in longX soleus. MG motoneurons that innervated soleus displayed altered values for axonal conduction velocity, rheobase, and input resistance, which could indicate incomplete recovery from the axotomized state. However, although mean afterhyperpolarization (AHP) half-decay time was unaltered by axotomy (25), this parameter was significantly lengthened in MG motoneurons that innervated soleus muscle. There were, however, individual motoneuron-muscle-unit mismatches, which suggested that longer mean AHP half-decay time may also be due to incomplete recovery of a subpopulation of motoneurons. Those MG motoneurons able to specify soleus muscle-fiber type exhibited motoneuron electrical properties typical of that same motoneuron type in self-regenerated MG.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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