Enlarged motor units resulting from partial denervation of cat hindlimb muscles

1. It was the aim of this study to determine the extent to which a mammalian motoneuron can sprout in a partially denervated muscle, which motor unit types are involved in sprouting, and whether polyneuronal innervation exists between sprouted units. 2. The fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus were partially denervated by unilateral section of the L7 ventral root in 12-wk-old kittens. After approximately 100 days single motor units were isolated, and their isometric contractile characteristics were determined. FDL units were also tested for their resistance to fatigue and categorized as fast-twitch, fatiguing fibers (FF), fast-twitch, fatigue-resistant fibers (FR), and slow-twitch, fatigue-resistant fibers (S). The presence of polyneuronal innervation was investigated between pairs of like and unlike units. 3. The extent of the original denervation was variable and was estimated from the distribution of motor axons innervating the muscle via the L7 and S1 (soleus) or L6 and L7 (FDL) ventral roots on the contralateral side. In soleus, denervations ranged from 75 to 98%; in FDL, 60 to 97% (denervations less than 60% were not investigated). In general, motor-unit force increased in proportion to the extent of the denervation. 4. Within soleus, unit force increased to over 2 N, which was about 16 times greater than the average for a normal muscle (133 mN). However, most units increased in force to between five and 12 times normal. 5. Within FDL, the force development of type S units was unaffected by partial denervation. Type FF units increased by up to 11 times (4.3 N) compared with normal FF units (395 mN) with most increasing between two and four times. FR units exhibited the greatest relative increase in force [up to 19 times (4.3 N) compared with normal (225 mN)]. Most units were two to seven times the normal. 6. A few FDL units were glycogen depleted, the muscles frozen, and cross sections prepared for histochemical analysis. This indicated that the largest units contained approximately 5,000 fibers, and there was little fiber hypertrophy. In the extensively denervated soleus muscle, large numbers of small, presumably denervated fibers were observed. The innervation ratio of several large units was determined indirectly using mean fiber area. This gave estimates of 3,000-4,000 fibers for the largest units. Again, fiber hypertrophy contributed little to the increase in unit force. It was concluded that the increased force of units in both muscles was largely attributable to terminal and axonal sprouting of the intact motor axons. 7. No evidence for polyneuronal innervation was found in either FDL or soleus muscle.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tetrodotoxin prevents motor unit enlargement after partial denervation in rat hindlimb muscles

Findings that increased neuromuscular activity significantly reduced sprouting in partially denervated muscles prompted this present study to determine if the converse is true, namely that reduced activity promotes sprouting and motor unit (MU) enlargement. Partial denervation of rat hindlimb muscles by either the L4 or L5 spinal root avulsion resulted in extensive denervation (> 80%) in tibialis anterior (TA) and medial gastrocnemius (MG) muscles, and moderate denervation (∼50%) in soleus (SOL) and plantaris (PL) muscles. The partially denervated muscles were then subjected to a 4 week programme of normal caged activity or TTX-induced neuromuscular inactivity. At 1 month, measurement of MU enlargement and quantification of sprouting were evaluated, respectively, by electrophysiological and histochemical means. Analysis of electrophysiological data showed that MU forces were significantly increased in both extensively and moderately denervated muscles 1 month after partial denervation and normal cage activity and that neuromuscular activity blockade by TTX completely abolished the MU enlargement in these partially denervated muscles. Histochemical analysis of sprouting revealed that the number of sprouts was significantly increased after partial denervation and normal cage activity, particularly after extensive denervation. TTX-induced neuromuscular inactivity dramatically reduced the number of sprouts and increased the number of free endplates in the extensively but not the moderately denervated muscles. These data demonstrate that a reduction in neuromuscular activity mediated by presynaptic blockade of neural action potentials reduces MU enlargement in partially denervated muscles by reducing axonal sprouting.     

Group Ia synaptic input to fast and slow twitch motor units of cat triceps surae

1 The characteristics of the group Ia synaptic input to triceps surae motoneurones have been examined in pentobarbitone-anaesthetized cats, using intracellular recording techniques. The mechanical properties of the muscle units innervated by the cells were determined and the motoneurone input resistance values were also measured.2. A significant positive correlation was found between the maximum amplitudes of homonymous composite (electrically evoked) monosynaptic excitatory post-synaptic potentials (EPSPs) and the motoneurone input resistance values across the entire population of units sampled. The same correlation in the case of heteronymous EPSPs was also significant although somewhat less strong.3. The distribution of the amplitudes of unitary miniature EPSPs (mEPSPs) of presumed group Ia origin, elicited by small static stretches of the homonymous muscles, were also studied. A significant positive correlation was found between the median amplitudes of the mEPSP distributions and the input resistance values in the motoneurones studied. Positive correlation was also observed between the amplitudes of the median mEPSPs and the maximum homonymous composite EPSPs in the cells for which both data points were available.4. In each of these correlations, the synaptic potential amplitudes tended to be larger in the relatively high resistance type S (slow twitch muscle unit) motoneurones from both gastrocnemius and soleus motor pools, than in the lower resistance type F (fast twitch muscle unit) gastrocnemius cells.5. Examination of the shape of the homonymous monosynaptic EPSP wave forms in different motoneurones showed that these tended to be significantly longer in duration in type S cells than in type F. This difference could not be entirely accounted for by the relatively small difference in mean time constant values found in types F and S cells.6. The results suggest that the density of group Ia synaptic terminals tends to be higher on type S motoneurones than on the type F cells. Further, cells receiving a relatively high density of group Ia input apparently tend to have a greater proportion of this input distributed to distal membrane regions than is the case in motoneurones receiving a relatively low input density.     

Sag during unfused tetanic contractions in rat triceps surae motor units.

Contractile properties and conduction velocity were studied in 202 single motor units of intact rat triceps surae muscles activated by intra-axonal (or intra-myelin) current injection in L5 or L6 ventral root to assess the factors that determine the expression of sag (i.e., decline in force after initial increase during unfused tetanic stimulation). Sag was consistently detected in motor units with unpotentiated twitch contraction times <20 ms. However, the range of frequencies at which sag was expressed varied among motor units such that there was no single interstimulus interval (ISI), with or without adjusting for twitch contraction time, at which sag could be detected reliably. Further analysis indicated that using the absence of sag as a criterion for identifying slow-twitch motor units requires testing with tetani at several different ISIs. In motor units with sag, the shape of the force profile varied with tetanic frequency and contractile properties. Simple sag force profiles (single maximum reached late in the tetanus followed by monotonic decay) tended to occur at shorter ISIs and were observed more frequently in fatigue-resistant motor units with long half-relaxation times and small twitch amplitudes. Complex sag profiles reached an initial maximum early in the tetanus, tended to occur at longer ISIs, and were more common in fatigue-sensitive motor units with long half-relaxation times and large twitch amplitudes. The differences in frequency dependence and force maximum location suggested that these phenomena represented discrete entities. Successive stimuli elicited near-linear increments in force during tetani in motor units that never exhibited sag. In motor units with at least one tetanus displaying sag, tetanic stimulation elicited large initial force increments followed by rapidly decreasing force increments. That the latter force envelope pattern occurred in these units even in tetani without sag suggested that the factors responsible for sag were expressed in the absence of overt sag. The time-to-peak force (TTP) of the individual contractions during a tetanus decreased in tetani with sag. Differences in the pattern of TTP change during a tetanus were consistent with the differences in force maximum location between tetani exhibiting simple and complex sag. Tetani from motor units that never exhibited sag did not display a net decrease in TTP during successive contractions. These data were consistent with the initial force decrement of sag resulting from a transient reduction in the duration of the contractile state.     

Stretch reflex gain in cat triceps surae muscles with compliant loads

The triceps surae (TS) stretch reflex was measured in decerebrate cats during crossed extensor stimulation (tonic contractions) and after spinalization during rhythmic locomotor activity. The TS reflex force in response to a short pulse stretch measured during tonic contractions at low level of background activity was greater than when more background activity was present at the time of application of stretch. In contrast, the reflex force measured during rhythmic contractions was very small at low level of background force (flexion phase) and increased at moderate and high levels of background activity (extension phase). Thus, even in reduced preparations, a task modulation of the stretch reflex occurs. Throughout the experimental procedure, the torque motor used to stretch the muscles behaved like a spring of a preset compliance (from isometric to very compliant). A reflex model was used to simulate the responses obtained experimentally. The gain of the stretch reflex loop was estimated for each load condition and both behavioural tasks. The reflex loop gain was significantly larger as the compliance of the external load increased for both tonic and rhythmic contractions, although to a lesser extent in the phasically activated muscles. During rhythmic locomotor contractions the gain was less than 1, assuring stability of the system. In contrast, during tonic contractions against a compliant load the gain exceeded 1, consistent with the instability (oscillations, clonus) seen at times under these load conditions. However, the high gain and instability was only transient, since repeated stretch reduced the gain. Thus, non-linearities in the system assured vigorous responses at the onset of perturbations, but then weaker responses to ongoing perturbations to reduce the chance of feedback instability (clonus).     

Receptor mechanisms underlying heterogenic reflexes among the triceps surae muscles of the cat

The soleus (S), medial gastrocnemius (MG), and lateral gastrocnemius (LG) muscles of the cat are interlinked by rapid spinal reflex pathways. In the decerebrate state, these heterogenic reflexes are either excitatory and length dependent or inhibitory and force dependent. Mechanographic analysis was used to obtain additional evidence that the muscle spindle primary ending and the Golgi tendon organ provide the major contributions to these reflexes, respectively. The tendons of the triceps surae muscles were separated and connected to independent force transducers and servo-controlled torque motors in unanesthetized, decerebrate cats. The muscles were activated as a group using crossed-extension reflexes. Electrical stimulation of the caudal cutaneous sural nerve was used to provide a particularly strong activation of MG and decouple the forces of the triceps surae muscles. During either form of activation, the muscles were stretched either individually or in various combinations to determine the strength and characteristics of autogenic and heterogenic feedback. The corresponding force responses, including both active and passive components, were measured during the changing background tension. During activation of the entire group, the excitatory, heterogenic feedback linking the three muscles was found to be strongest onto LG and weakest onto MG, in agreement with previous results concerning the strengths of heteronymous Ia excitatory postsynaptic potentials among the triceps surae muscles. The inhibition, which is known to affect only the soleus muscle, was dependent on active contractile force and was detected essentially as rapidly as length dependent excitation. The inhibition outlasted the excitation and was blocked by intravenous strychnine. These results indicate that the excitatory and inhibitory effects are dominated by feedback from primary spindle receptors and Golgi tendon organs. The interactions between these two feedback pathways potentially can influence both the mechanical coupling between ankle and knee.     

Motor unit types of cat triceps surae muscle

1. Motor units, defined as including a motoneurone (cell body, dendrites and axon) plus the muscle unit innervated, have been examined in the triceps surae motor pool of pentobarbital anaesthetized cats.

2. The technique of intracellular stimulation and recording which was used permitted measurement of the axonal conduction velocity, post-spike hyperpolarization duration and input resistance of individual motoneurones, and the correlation of these properties with the characteristics of the twitch and tetanus responses of the muscle unit innervated by the cell elicited by direct intracellular stimulation.

3. On the basis of muscle unit speed of contraction, motor units were divided into two groups: (a) fast twitch, or F, type with twitch time to peak (TwTp) less than or equal to 30 msec, and (b) slow twitch, or S, type with TwTp of 40 msec or greater. The twitch tensions (TwTen) produced by type F units were significantly larger (median value = 18 g) than the tensions generated by type S units (TwTen median value = 1·6 g). Type F muscle units had much higher tetanus fusion frequencies (median = 85 pulses/sec) than the S type (median 25 pulses/sec), and tended to have smaller tetanus to twitch tension ratios (Tet/Tw) (median = 2·6) than type S units (median = 5·4).

4. The gastrocnemius heads contained a mixture of F and S types of muscle units, the proportions found being about 3 to 1 respectively. Units encountered in the soleus muscle were uniformly of type S. The characteristics of gastrocnemius and soleus type S motor units were not identical but appeared to represent quantitative differences in units of the same qualitative type.

5. Motoneurones innervating type F muscle units had faster axonal conduction velocities, shorter post-spike hyperpolarizations and lower input resistances than those supplying type S units. However, no combination of motoneurone properties alone was sufficient to separate unambiguously types F and S motor units.

     

Distribution of heterogenic reflexes among the quadriceps and triceps surae muscles of the cat hind limb

Neural signals from proprioceptors in muscles provide length and force-related linkages among muscles of the limbs. The functions of this network of heterogenic reflexes remain unclear. New data are reported here on the distribution and magnitudes of neural feedback among quadriceps and triceps surae muscles in the decerebrate cat. The purpose of this paper was to distinguish whether inhibitory-force feedback is directed against muscles by virtue of the motor-unit composition or articulation of the muscle. These studies were carried out using controlled stretches and measurements of the resulting force responses of individual quadriceps and triceps surae muscles. Responses were evoked over a wide range of background force levels. In agreement with earlier electrophysiological studies, excitatory length feedback strongly linked the vastus muscles, but excitatory reflexes between each vastus and rectus femoris muscles were weak. We also observed a substantial excitatory linkage from the vastus muscles to the soleus muscle. In contrast, force-related inhibition was absent in the heterogenic reflexes among the vastus muscles but strong and bidirectional between each vastus muscle and the rectus femoris muscle and between triceps surae and quadriceps muscles. We conclude that short-latency feedback in the hindlimb is organized according to muscle articulation. Length feedback within muscle groups regulates joint stiffness while interjoint length feedback may compensate for the effects of nonuniform inertial properties of the limb. Force feedback is organized to regulate coupling between joints and, along with length feedback, determine the mechanical properties of the endpoint.     

Myoelectric changes in the triceps surae muscles under sustained contractions

Summary Synergistic behaviour of triceps surae muscles (medial gastrocnemius-MG, lateral gastrocnemius-LG, soleus-SOL) during sustained submaximal plantarflexions was investigated in this study. Six male subjects were asked to sustain an isometric plantar flexor effort to exhaustion at two different knee angles. Exhaustion was defined as the point when they could no longer maintain the required tension. The loads sustained at 0 and 120 degrees of knee flexion represented 50% and 36% of their maximum voluntary contraction (MVC) respectively. MVC was measured at 0 degree knee flexion. During the contractions, electromyograms (EMG) from the surface of the triceps surae muscles were recorded. Changes in the synergistic behaviour of the triceps surae were assessed via partial correlations of the average EMG (AEMG) between three muscle combinations; MG/LG, MG/SOL, LG/SOL, and correlation between SOL/MG+LG and MG/SOL+LG. The latter combinations were based on either common fibre type or innervation properties. Two types of synergisms were identified: trade-off and coactivation. Trade-off and coactivation synergies were defined by significant (p<0.05) positive and negative correlations respectively. Coactivation synergism was found to occur predominantly under conditions of high load or reduced length of the triceps surae, and increased with the duration of the contraction. Trade-off synergism was evident when the muscles were at their optimum length and the loads sustained were submaximum. Complete shutdown of one muscle activity was ruled out. It is postulated that, in the absence of voluntary strategies on the part of the subjects, changes in the syznergistic behaviour of the triceps surae muscles, manifested through trade-off and coactivation, is dependent on the load placed on the muscle and the muscle effectiveness as characterized by the force/length curve.     

Intrinsic properties and reflex compensation in reinnervated triceps surae muscles of the cat: effect of activation level

The manner in which activation levels influence intrinsic muscular properties and contributions of the stretch reflex were studied in homogeneous soleus (SOL) and heterogeneous gastrocnemius (G) muscles in the decerebrate cat. Intrinsic mechanical properties were represented by the initial stiffness of the muscle, measured prior to reflex action, and by the tendency of the muscle to yield during stretch in the absence of the stretch reflex. Stiffness regulation by the stretch reflex was evaluated by measuring the extent to which reflex action reduces yielding and the extent to which stiffness depends on background force. Intrinsic mechanical properties were measured in muscles deprived of effective autogenic reflexes using the method of muscular reinnervation. Reinnervated muscles were recruited to force levels comparable to those achieved during natural locomotion. As force declined during crossed-extension reflexes in reinnervated and intact muscles, initial stiffness declined according to similar convex trajectories. The data did not support the hypothesis that, for a given force level, initial stiffness is greatest in populations of predominantly type I motor units. Incremental stiffness (Deltaf/Deltal) of both G and SOL increased in the presence of the stretch reflex. Yielding of SOL (ratio of incremental to initial stiffness) substantially decreased in the presence of the stretch reflex over the full range of forces. In reflexive G, yielding significantly decreased for low to intermediate forces, whereas at higher forces, yielding was similar irrespective of the presence or absence of the stretch reflex. The stretch reflex regulates stiffness in both homogeneous and heterogeneous muscles.     

Intrinsic properties and reflex compensation in reinnervated triceps surae muscles of the cat: effect of movement history

Effects of prior motion on ramp stretch responses of reflexive and areflexive muscles were measured in decerebrate cats. Soleus and gastrocnemius muscles were rendered areflexive by reinnervation a minimum of 9 mo before the terminal experiments. The introduction of a shortening phase prior to the ramp stretch increased the normalized initial stiffness of muscles and decreased the tendency to yield of the reinnervated muscles as compared with the case in which muscles contracted isometrically prior to stretch. Yielding was compensated by reflex action for all amplitudes of prior shortening in soleus and gastrocnemius muscles. The comparison of responses of untreated and reinnervated muscles indicated that the contribution of reflex action progressively declined with the amplitude of prior shortening as the extent of yielding diminished. In soleus muscle, during a variable delay period of isometric contraction interposed between shortening and lengthening force generation, initial stiffness and yielding returned to levels seen with isometric contractile history. However, these attributes recovered at different rates, suggesting that distinct processes are responsible for initial stiffness and yielding. Yielding was compensated for by reflex action regardless of the length of the interposed delay or of the amplitude of the prior shortening. These and previous findings indicate that the stretch reflex regulates muscular stiffness for a wide range of conditions. This regulation apparently arises from complementary mechanical properties of intrafusal and extrafusal muscle.     

A comparison of peripheral and rubrospinal synaptic input to slow and fast twitch motor units of triceps surae

1. Post-synaptic potentials (PSPs) evoked by electrical stimulation of a variety of input systems have been compared in triceps surae motoneurones innervating slow and fast muscle units, the speed of contraction of which was also determined.2. Stimulation of high threshold afferents in both flexor and extensor muscle nerves, and of joint afferents, evoked polysynaptic PSPs which were predominantly hyperpolarizing in both fast and slow twitch motor units.3. Volleys in cutaneous afferents in the sural and saphenous nerves evoked polysynaptic PSPs composed of mixtures of inhibitory and excitatory components. The inhibitory components were predominant in slow twitch motor units, while in fast twitch units there was a trend towards excitatory predominance.4. Repetitive stimulation of the red nucleus caused predominantly inhibitory PSPs in slow twitch units and mixed or predominantly excitatory PSPs in fast twitch units. There was a correlation in the excitatory/inhibitory balance between PSPs of cutaneous and rubrospinal origin in those motoneurones in which both types of PSPs were studied.5. The amplitudes of group Ia disynaptic inhibitory PSPs were found to be correlated with motor unit twitch type: IPSPs in slow twitch units were larger than those in fast twitch units. Rubrospinal conditioning volleys were found to facilitate group Ia IPSPs in both fast and slow twitch motor units.6. The results suggest that there may be several basic patterns of synaptic input organization to motoneurones within a given motor unit pool. In addition to quantitative variation in synaptic distribution, there is evidence that qualitative differences in excitatory to inhibitory balance also exist in the pathways conveying input from cutaneous afferents and rubrospinal systems to triceps surae motoneurones. These qualitative differences are correlated with the motor unit twitch type.