Neural mechanisms underlying the clasp-knife reflex in the cat. II. Stretch-sensitive muscular-free nerve endings

1. The goal of this study was to determine the contribution of muscular free nerve endings to the clasp-knife reflex by comparing their response properties and reflex actions to the clasp-knife reflex. 2. The responses of single muscle afferents were examined in anesthetized cats using stretch and isometric contraction of ankle extensor muscles identical to those that evoked clasp-knife inhibition in decerebrated and dorsal spinal-hemisectioned cats. 3. Fifty-three stretch-sensitive mechanoreceptor afferents were identified as free nerve ending afferents based on their conduction velocities, location within the muscle, uniformity of response, and dissimilarity to other muscle proprioceptors. The afferent conduction velocities were in both the group III (56%) and group II (44%) range, including five fast-conducting group II afferents (greater than 55 m/s). 4. The stretch response of stretch-sensitive, free nerve endings (SSFNEs) showed several characteristic features: 1) afferents were excited only by large stretches that produced significant passive force; 2) afferent activity began after a brief delay and exhibited segmentation of discharge during ramp stretch, a maximum at the end of ramp stretch, and rapid and complete decay during static stretch, and 3) afferent response adapted to repeated stretches. These properties match those of clasp-knife inhibition described in the companion paper, except that the SSFNE segmentation and maximum were more pronounced and their decay during maintained stretch was more rapid. 5. Isometric contraction produced by electrical stimulation of the muscle nerve, which induced force-evoked inhibition in decerebrated and dorsal hemisectioned cats, also consistently excited SSFNEs. Stretch evoked greater excitation than contraction, indicating that both length and force contribute to SSFNE activity. 6. Stimulation of free nerve endings by squeezing the achilles tendon in cats exhibiting the clasp-knife reflex evoked powerful, homonymous inhibition and a flexion-withdrawal pattern of reflex action--that is, inhibition of extensor and excitation of flexor muscles throughout the hindlimb, which parallels the spatial divergence of the clasp-knife reflex. 7. Intrathecal application of capsaicin, which preferentially blocks the reflex actions of small afferent fibers, blocked clasp-knife inhibition in decerebrated, dorsal hemisectioned cats. 8. The similarities between the reflex actions and response properties of SSFNEs and the properties of the clasp-knife reflex suggest that SSFNEs mediate clasp-knife inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Wipe and flexion withdrawal reflexes display different EMG patterns prior to movement onset in the spinalized frog

We investigated the hypotheses (1) that the initial flexion part of the wipe reflex elicited in the spinalized frog has the same EMG pattern for wipes to different target locations (Berkinblit et al. 1986), thereby reducing the complexity of the control of this task, and (2) that this initial flexion is the same as occurs in the flexion withdrawal reflex (Easton 1972). The activities of seven muscles of the hindlimb of the spinal frog were recorded via intramuscular electromyograms (EMGs) during the wipe reflex to two target locations and during the flexion withdrawal reflex. The EMGs were analyzed during the interval between stimulus placement and movement onset for mean integrated EMG and duration from EMG onset to movement onset. This analysis revealed significant differences (p less than 0.0001) in the EMG patterns that preceded the initial flexion posture for all three movements. These findings suggest that the spinal circuitry coordinating the initial flexion part of the wipe reflex to different target locations and the flexion withdrawal reflex may not be uniformly shared.

Strategies for muscle activation during isometric torque generation at the human elbow

1. We studied the patterns of electromyographic (EMG) activity in elbow muscles of 14 normal human subjects. The activity of five muscles that act in flexion-extension and forearm supination-pronation was simultaneously recorded during isometric voluntary torque generation, in which torques generated in a plane orthogonal to the long axis of the forearm were voluntarily coupled with torques generated about the long axis of the forearm (i.e., supination-pronation). 2. When forearm supination torques were superimposed on a background of elbow flexion torque, biceps brachii activity increased substantially, as expected; however, brachioradialis and brachialis EMG levels decreased modestly, a less predictable outcome. The pronator teres was also active during pure flexion and flexion coupled with mild supination (even though no pronation torque was required). This was presumably to offset inappropriate torque contributions of other muscles, such as the biceps brachii. 3. When forearm supination torque was superimposed on elbow extension torque, again the biceps brachii was strongly active. The pronator teres also became mildly active during extension with added pronation torque. These changes occurred despite the fact that both the pronator and biceps muscles induce elbow flexion. 4. In these same elbow extension tasks, triceps brachii activity was also modulated with both pronation or supination loads. It was most active during either supination or pronation loads, again despite the fact that it has no mechanical role in producing forearm supination-pronation torque. 5. Recordings of EMG activity during changes in forearm supination-pronation angle demonstrated that activation of the biceps brachii followed classic length-tension predictions, in that less EMG activity was required to achieve a given supination torque when the forearm was pronated (where biceps brachii is relatively longer). On the other hand, EMG activity of the pronator teres did not decrease when the pronator was lengthened. Triceps EMG was also more active when the forearm was supinated, despite its having no direct functional role in this movement. 6. Plots relating EMG activity in biceps brachii, brachialis, and brachioradialis at three different forearm positions revealed that there was a consistent positive near-linear relationship between brachialis and brachioradialis and that biceps brachii is often most active when brachioradialis and brachialis are least active. 7. We argue that, for the human elbow joint at least, fixed muscle synergies are rather uncommon and that relationships between muscle activities are situation dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence of shared, direct input to motoneurons supplying synergist muscles in humans

Cross-correlation techniques were used to test for the presence of shared, direct input to motoneurons innervating different synergist elbow flexor muscles in man. Motor unit activity was recorded intramuscularly from two elbow flexor muscles during steady isometric elbow flexion in normal and paretic subjects. To increase the probability of detecting weak synchrony, one of the intramuscular needles was positioned to record multiunit activity. Significant correlogram peaks were obtained in 25/57 runs in normal subjects, and the features of the correlograms were similar to those previously reported based on cross-correlation of two single units within the same muscle. Further, the characteristics of discharge synchrony measured in paretic stroke patients are consistent with other reports on the effects of stroke on synchrony among motoneurons belonging to the same pool, i.e. narrow correlogram peaks were rare in paretic subjects and significant correlogram peaks often had longer than normal durations.

Spastic hypertonia: mechanisms and measurement

Spastic hypertonia has been defined as a motor disorder characterized by a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex, as one component of the upper motor neuron syndrome. Heightened muscle tone may be the result of changes intrinsic to the muscle or to altered reflex properties. Increased motoneuronal excitability and/or enhanced stretch-evoked synaptic excitation of motoneurons are mechanisms that might enhance stretch reflexes. Two distinct parameters may be altered in the pathologic stretch reflex--the "set point," or angular threshold of the stretch reflex, and the reflex "gain," or the amount of force required to extend the limb in proportion to the increasing joint angle. Earlier studies fail to dissociate the contributions of reflex threshold and reflex gain. Recent investigations suggest that spastic hypertonia may be the result of a decrease in stretch reflex threshold without significant increase in reflex gain, as was previously believed. Various clinical scales, biomechanical paradigms, pendulum models, and electrophysiologic studies have been used to quantify spastic hypertonia. Biomechanical methods seem to correlate most closely with the clinical state. Spastic hypertonia is but one component of the upper motor neuron syndrome, whose features also include loss of dexterity, weakness, fatigability, and various reflex release phenomena. These other features of the upper motor neuron syndrome may well be more disabling to the patient than changes in muscle tone.

Abnormal spatial patterns of elbow muscle activation in hemiparetic human subjects

The patterns of electromyographic (EMG) activity in spastic-paretic and contralateral elbow muscles of 10 hemiparetic human subjects were compared during a sequence of graded voluntary isometric contractions against 4 different-sized loads. These loads were orientated successively at 8 different angles over a 360 degree range, referenced to a plane at the wrist orthogonal to the long axis of the forearm. Comparisons of EMG activity recorded from normal and paretic limbs revealed that there were marked differences in the torque angles which evoked significant EMG activity, in the angular range of EMG, in the angle of peak EMG, and in the scaling of EMG magnitude with increasing isometric loads. In severely impaired limbs, there was a marked shift in both the peak EMG angle and the angular domain of EMG activity for both biceps and triceps muscle groups, away from the normal elbow flexion-extension axis towards external humeral rotation and shoulder girdle elevation. The extent of the disturbance in the spatial patterns of EMG activity was closely correlated with the clinical severity of the spastic-paretic disability, which was quantified using a functional scale patterned after that described by Fugl Meyer et al. (1975). The observed patterns of EMG activity in paretic flexor muscles do not conform with established synergistic patterns, such as might be released by excitation of the flexor reflex in a normal limb. Possible origins for the anomalous EMG patterns are discussed.

Stretch reflex dynamics in spastic elbow flexor muscles

Previous studies of stretch reflexes in patients with spastic hypertonia have emphasized the dynamic character of stretch reflex output. In contrast, our own studies of stretch reflex dynamics in spastic elbow flexor muscles of 14 hemiparetic human subjects have shown that stretch-evoked torque displays a relatively weak dependence on stretch velocity, and there is generally no preferential enhancement of dynamic as compared with static reflex output. Moreover, stretch reflex dynamics are broadly similar in voluntarily activated spastic and normal elbow flexor muscles. These findings support our hypothesis that spastic hypertonia results primarily from a decrease in stretch reflex threshold. The strong velocity dependence of stretch-evoked electromyographic activity in initially inactive spastic muscles could be due to a decrease in reflex threshold with increasing stretch velocity, rather than an abnormal velocity-dependent increase in stretch reflex responsiveness.

Effects of acute dorsal spinal hemisection on motoneuron discharge in the medial gastrocnemius of the decerebrate cat

1. The discharge of single alpha-motoneuron axons was recorded from small cut filaments of the medial gastrocnemius (MG) muscle nerve in the decerebrated cat preparation before and after a dorsal hemisection of the thoracic spinal cord. The remainder of the MG muscle nerve was left intact, and muscle force and multiunit electromyographic (EMG) activity were recorded along with alpha-motoneuron discharge, while motor output was varied by manual stimulation of the contralateral hindlimb. 2. We recorded activity in 32 motoneurons before and after the spinal lesion, and pre- and postlesion recruitment forces and minimum firing rates were determined for 30 of these. Postlesion decreases in minimum firing rates were observed in 25/30 motoneurons, and decreases in recruitment force were seen in 21/30 motoneurons. The remaining motoneurons, which generally had low presection recruitment forces and minimum rates, exhibited postlesion increases in both parameters (see below). 3. The effects of the spinal lesion on the recruitment force and minimum firing rate of a motoneuron were related to the prelesion values of these parameters; the largest postlesion decreases were seen in motoneurons with the highest prelesion rates and recruitment forces. Spinal lesions thus acted to shift and compress the range of recruitment forces and minimum firing rates, so that after the lesion all motoneurons tended to exhibit discharge behavior typical of that seen only in the lowest threshold motoneurons before the lesion. In addition, motoneurons with low prelesion recruitment forces (less than 1.0 N of active force) generally showed an increase in recruitment force after the lesion, indicating that the lesion may have led to changes in the prelesion recruitment order. Direct evidence of recruitment reversals was obtained in 4/14 experiments where two or more motoneurons were followed pre- and postlesion. 4. The lesion-induced changes in motoneuron discharge characteristics were associated with changes in the relations between muscle force, rectified EMG, and motoneuron rate. Postlesion discharge rates were always significantly lower than the prelesion rates when compared over the same range of EMG levels. This postlesion drop in discharge rates was generally associated with inefficient force production, as evidenced by a significant drop in muscle force for matched EMG levels. 5. The degree of discharge synchrony in MG motoneurons was assessed by calculating a spike-triggered average (STA) between axonal discharge and multiunit rectified EMG. Significant STA peaks were rare before the lesion (4/32 motoneurons) but were quite common after the lesion (29/32 motoneurons).(ABSTRACT TRUNCATED AT 400 WORDS)

Increased inhibitory effects on close synergists during muscle fatigue in the decerebrate cat

We compared the magnitude of reflex inhibition induced in the soleus muscle by contraction or stretch of the medial gastrocnemius (MG), before, during, and after electrically induced fatigue of the MG. Our findings are that MG fatigue is accompanied by a substantial increase in soleus inhibition, which then recovers with MG rest. This increased inhibition may explain, at least in part, the decline in motoneuron discharge rate that has been described in fatiguing human muscle.

Quantitative relations between hypertonia and stretch reflex threshold in spastic hemiparesis

The relative contributions of variations in stretch reflex threshold and total joint stiffness to changes in stretch-evoked torque were assessed in the spastic elbow muscles of 14 hemiparetic spastic subjects. For a given subject, variations in torque, measured after a constant angular deflection, were mediated largely by changes in stretch reflex threshold, rather than by changes in reflex stiffness. Between-subject comparisons were sensitive to stiffness differences between limbs, but reflex thresholds were still broadly correlated with torque magnitude, suggesting that reductions in stretch reflex threshold are uniformly present in spastic muscles. These findings, coupled with the apparent similarity of reflex stiffness estimates in voluntarily activated spastic and normal muscles, suggest that the central disturbance in spasticity is a reduction in the threshold of the stretch reflex, without a significant enhancement of reflex gain.

Disturbances of motor output in a cat hindlimb muscle after acute dorsal spinal hemisection

Force and electromyogram (EMG) responses of the medial gastrocnemius muscle were assessed during isometric contractions in 8 decerebrate cat preparations, before and after acute dorsal hemisection of the spinal cord at the T12 level. The measures derived included the relation between static force and mean rectified EMG, the EMG amplitude distribution, EMG power spectral density, and force power spectral density. Our findings were that the spinal lesion induced modifications in the shape of the EMG amplitude distribution, a substantial increase in mean rectified EMG per unit force, and increases in EMG spectral power and force spectral power over a broad band of frequencies. In 7/8 preparations, there was disproportionate enhancement of EMG spectral power below 40 Hz, with a commensurate reduction in the EMG mean power frequency (MPF) in 6 of these 7 cases. Recordings of motoneuron discharge from 9 decerebrate preparations taken before and after the spinal hemisection revealed that the lesion-induced changes in EMG and force power spectra were accompanied by lower mean discharge rates, and by a compression of the range of recruitment force. These changes in motoneuron rate and recruitment were probably responsible for the changes in EMG and force measures, especially for the relative increase in low-frequency EMG power. If these acute disturbances of motoneuron rate and recruitment persist in chronic human neurological disorders, they represent an important and largely unrecognized source of muscular weakness and increased fatigability.

Absence of stretch reflex gain enhancement in voluntarily activated spastic muscle

Static and dynamic stiffnesses of voluntarily activated elbow muscles were compared in spastic and contralateral arms of 15 subjects with spastic hemiparesis. Stiffnesses were estimated from the positional deflections induced by applying load perturbations to each forearm. In 11/15 subjects (73%), stiffness were comparable on the two sides. In the remaining 4/15 subjects (27%), stiffness were consistently greater on the spastic side, however, EMG recordings from these spastic muscles were of much smaller amplitude than those of the contralateral muscles, indicating that this increase was probably caused by changes in the mechanical properties of elbow muscles, rather than by stretch reflex enhancement. We conclude that for voluntarily activated muscles of spastic hemiparetic subjects, reflex stiffness (and presumably stretch reflex gain), of spastic and contralateral limbs is not significantly different. These findings impose important constraints upon theories attempting to explain spastic hypertonia, and they also provide guidelines for clinical quantification of spasticity.

Neural compensation for muscular fatigue: evidence for significant force regulation in man

We have investigated the role of reflex regulation of muscle force in normal human subjects by comparing changes in the stretch-evoked increments in elbow joint flexor electromyogram (EMG) and elbow joint torque before and after fatigue. Elbow flexor muscle fatigue was induced by repetitive voluntary isometric contractions. To assess the appropriateness of the EMG signal as an index of neural excitation of muscle under fatiguing conditions, we examined the time course of recovery of joint torque and EMG power spectrum following fatigue. Fatigue-related changes in the EMG power spectra recovered within 5-10 min after fatiguing exercise was terminated, yet the muscle weakness induced by the exercise lasted greater than 7 h and was substantial in magnitude. The decoupling of torque and EMG recovery allowed us to compare pre- and postfatigue EMG stretch responses without adjusting for differences in EMG spectral content. Torque and EMG responses to stretch were quantified by time-averaging over 250-ms "isometric" and "steady-state" periods, just before and just after a ramp angular stretch of the elbow joint, respectively. The torque increment elicited by stretch was lower following fatigue in seven of eight experiments. However, the average decrease of 20.13 +/- 14.42% in these seven subjects was somewhat smaller than the corresponding average shift in the slope of the isometric EMG-torque relationship of 85.84 +/- 90.29% (n = 8). Furthermore, the stretch-induced EMG increment was larger following fatigue in all eight sessions (average of 56.14 +/- 28.96%, n = 8), with six of the shifts reaching statistical significance for alpha = 0.05. Because the pattern of torque and EMG responses before and after fatigue suggested the presence of an active force regulator, we used a simple model of the neuromuscular system to estimate a loop gain value for each session. When pre- and postfatigue responses were matched by isometric background torque level, an average loop gain value of 7.9 was computed, whereas for responses matched by average prestretch EMG level, the loop gain estimates averaged 2.1. Although our assessment of force regulation was essentially static and derived from the responses to a single type of perturbation, the change in the incremental torque and EMG stretch responses indicates that meaningful neural compensation for fatigue occurred. Moreover, the loop gain estimates derived from these responses are an order of magnitude larger than those previously reported in animal models, suggesting that force regulation may be important in the control of human muscle contraction.

Beta-contributions to fusimotor action in triceps surae muscles of decerebrated cats

The discharge of spindle afferents from medial gastrocnemius and soleus muscles was recorded in the decerebrated cat preparation, under isometric conditions and during ramp and hold stretches. Motor output was varied systematically by manual stimulation of the contralateral hindlimb. Twenty-six of 34 afferents showed response patterns consistent with enhancement of dynamic and/or static fusimotor input with increasing muscle force. To establish whether force-related fusimotor effects were mediated at least partly by beta-input, beta-innervation to these same spindles was sought, using a ventral root stimulation protocol. Twenty-three of the 34 afferents were shown to receive beta-innervation, which was most often static in type. For two measures of fusimotor action, the slope of the afferent dynamic rate-length relation and the discharge rate measured during the last portion of ramp stretch, significant increases in the measure, which paralleled increases in muscle force, made it statistically more likely that the afferent received beta-innervation. Our measures did not successfully predict the type of beta-input (beta-static or beta-dynamic). Procaine block of gamma-fibers produced substantial reductions in fusimotor effect in seven spindle afferents (although modest residual fusimotor effects were detectable for 3/7 afferents). The severity of these reductions indicates that beta-action probably requires concurrent gamma-input to the spindle in order to be effective. In support of this possibility, the fusimotor effects of electrical stimulation of single beta-fibers were greatly reduced for five out of six afferents during procaine block of gamma-fibers, compared with the beta-effects recorded when modest levels of spontaneous gamma-activity were present. We conclude that beta-innervation to muscle spindles of triceps surae is common and that this innervation exerts significant fusimotor effects. It appears likely that beta-motoneurons are able to produce both static and dynamic effects above extrafusal threshold, but that the actions require on-going gamma-activity in order to be effective.

Characteristics of reflex excitation in close synergist muscles evoked by muscle vibration

The tonic vibration reflex evoked in the vibrated medial gastrocnemius muscle was compared with that induced simultaneously in an unvibrated close synergist, the lateral gastrocnemius muscle, in the unanesthetized decerebrated cat. The time course of the force rise and fall in the synergist muscle was found to be markedly different from that produced in the vibrated muscle. In addition the magnitude of force produced in the synergist was only weakly modulated with increasing vibration frequency, in that the synergist uniformly displayed much flatter force-frequency relations than the vibrated muscle, and even showed overt force saturation in several preparations. Comparable differences in reflex responses arose when the lateral gastrocnemius served as the unvibrated synergist. In a parallel series of experiments, low intensity electrical stimulation of the proximal end of the sectioned medial gastrocnemius muscle nerve at different frequencies weakly modulated the amount of reflex force induced in the lateral gastrocnemius muscle. High frequency electrical stimulation caused the synergist force to saturate in a similar way to that seen in the vibrated preparations. A variety of possible mechanisms are discussed, with particular emphasis on "bistable" properties of motoneurons and Ia excitatory interneuronal contributions.

Characteristics of synergic relations during isometric contractions of human elbow muscles

We studied the patterns of EMG activity in elbow muscles in three normal human subjects. The myoelectrical activity of 7-10 muscles that act across the human elbow joint was simultaneously recorded with intramuscular electrodes during isometric joint torques exerted over a range of directions. These directions included flexion, extension, varus (internal humeral rotation), valgus (external humeral rotation), and several intermediate directions. The forces developed at the wrist covered a range of 360 degrees, all orthogonal to the long axis of the forearm. The levels of EMG activity were observed to increase with increasing joint torque in an approximately linear manner. All muscles were active for ranges less than 360 degrees and most were active for less than 180 degrees. The EMG activity was observed to vary in a systematic manner with changes in torque direction and, when examined over the full angular range at a variety of torque levels, is simply scaled with increasing torque magnitude. There were no torque directions or torque magnitudes for which a single muscle was observed to be active alone. In all cases, joint torque appeared to be produced by a combination of muscles. The direction for which the EMG of a muscle reached a maximum value was observed to correspond to the direction of greatest mechanical advantage as predicted by a simple mechanical model of the elbow and relevant muscles. Muscles were relatively inactive during varus torques. This implies that the muscles were not acting to stabilize the joint in this direction and could have been allowing ligaments to carry the load. Plots of EMG activity in one muscle against EMG activity in another demonstrate some instances of pure synergies, but patterns of coactivation for most muscles are more complicated and vary with torque direction. The complexity of these patterns raises the possibility that synergies are determined by the task and may have no independent existence. Activity in two heads of triceps brachii (medial head--a single-joint muscle and long head--a two-joint muscle) covaried closely for a range of torque magnitudes and directions, though shoulder torque and hence the forces experienced by the long head of the triceps undoubtedly varied. The similarity of activation patterns indicates that elbow torque was the principal determining factor. The origins of muscle synergies are discussed. It is suggested that they are best understood on the basis of a model which encodes limb torque in premotor neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement by serotonin of tonic vibration and stretch reflexes in the decerebrate cat

The effects of pharmacological manipulation of serotonergic systems on spinal reflexes were determined in the unanesthetized decerebrate cat. The prolonged motor output that continues after cessation of high frequency longitudinal tendon vibration was strongly enhanced by the serotonin reuptake blocker fluoxetine and the serotonin precursor 5-hydroxytryptophan, and was decreased by the serotonin receptor antagonist methysergide. In addition, both dynamic and static stretch reflex stiffness was markedly increased by fluoxetine and 5-hydroxytryptophan, while methysergide produced a decrease in stretch reflex stiffness. These powerful effects on tonic vibration and stretch reflexes could not be explained by drug-induced alterations in muscle spindle primary afferent discharge. In light of other recent results on serotonin-mediated effects on motoneurons, we believe that the effects of these agents result from modification of an intrinsically mediated prolonged depolarization of spinal neurons. However, the possibility that these drugs modify longlasting discharge in associated interneuronal pathways cannot be ruled out.

Reflex actions of muscle afferents on fusimotor innervation in decerebrated cats: an assessment of beta contributions

The existence of beta innervation in many cat muscles raises the possibility that spindle afferent discharge will excite beta motoneurons, augmenting spindle afferent discharge and thereby closing a positive feedback loop. In order to evaluate the strength of such a loop through beta motoneurons and muscle spindles, the stretch responses of muscle spindle afferents from medial gastrocnemius (MG) and soleus (SOL) muscles were studied in decerebrated cats before and after dorsal root section. If a positive feedback loop were operational, the spindle afferent stretch response should be diminished following dorsal root section by an amount related to the magnitude of positive feedback. After dorsal root section, the static positional sensitivities of our MG spindle afferent sample were significantly reduced for 72% (13/18, p less than 0.05) of the afferents, and dynamic rate/length slopes were decreased for 88% (8/9) of a subset of the afferents studied. Similar reductions for 6 afferents from SOL were not found. To apportion these afferent changes to reflex excitation of either gamma or beta motoneurons, we recorded the stretch responses of gamma and alpha-type fibers in the same preparation. (We assume that the population of alpha-type fibers includes beta fibers). In keeping with other reports, alpha fibers were much more responsive to stretch than gamma fibers. The mean positional sensitivity for alpha fibers (1.29 +/- 0.92 pps/mm, n = 15) was greater (p less than 0.05) than that of gamma fibers (0.49 +/- 0.93 pps/mm, n = 18). Because of these differences in sensitivity, beta motoneurons are more likely (than gamma motoneurons) to be involved in a positive feedback loop, although some gamma contribution is also likely. Using equations based on a beta position regulating scheme, differences in spindle positional sensitivity were used to estimate beta loop gain. The average loop gain was estimated to be 0.41 (n = 18). The contribution of such a beta configuration to reducing the sensitivity of muscle to changes in load and muscle properties is evaluated.

Force-sensitive interneurons in the spinal cord of the cat

The input-output properties of interneurons mediating spinal reflexes were investigated by extracellularly recording the response of interneurons to excitation from muscle receptors in the ankle extensor muscles of decerebrated, spinal cats. A population ofinterneurons in the intermediate region ofthe spinal cord is potently excited by increases in muscle force. Unlike the discharge of Golgi tendon organs, which accurately encodes moment-to-moment variations in the force of a single muscle, the discharge of these interneurons depends in a dynamic and usually nonlinear way on the force in several muscles. Powerful input from unidentified mechanoreceptors in muscle, presumably free nerve endings, is at least partly responsible for these properties. These force-sensitive interneurons are more likely to mediate clasp knife-type inhibition than simple negative force feedback.

Motor-unit activation patterns in lengthening and isometric contractions of hindlimb extensor muscles in the decerebrate cat

1. Multiunit integrated electromyographic (EMG) signals and single-unit EMG potentials were recorded during isometric and lengthening (stretch reflex) contractions of soleus and medial gastrocnemius (MG) muscles in 20 decerebrate cats. Patterns of motor-unit recruitment and rate modulation were examined in isometric muscles and during constant-velocity stretches. 2. Analysis of multiunit EMG activity and its relationship to active force revealed a marked difference between isometric and lengthening contractions. While the force-EMG relationship for isometric contractions was characteristically linear, the relation recorded during stretch-reflex responses showed a disproportionate early EMG increase, which was most obvious at low force levels, suggesting that the efficacy of force production is reduced in lengthening muscle. 3. Single-unit recruitment patterns were found to be qualitatively similar in isometric and lengthening contractions. In each case, motor units were recruited in order of increasing spike voltage. The numbers of newly recruited units declined steeply with each successive increment in active force. For a given unit, the force at which recruitment occurred was found to be greater in lengthening contractions than in isometric contractions, and in lengthening contractions it was also found to depend on the level of initial force. 4. Two patterns of motor-unit rate modulation were observed during muscle stretch, depending on whether a given unit was firing before the beginning of stretch or whether it was recruited during the course of stretch. Motor units that were active prior to stretch were found to increase firing rate at stretch onset and to vary their rate very little thereafter. Motor units recruited in the course of stretch began firing at an initial rate proportional to their force threshold, gradually increased their firing rate with increasing force, and sometimes reached an apparent maximum rate. 5. These results are discussed in terms of the mechanical properties of lengthening muscle and reflex regulation of these properties. Each identified pattern of motor-unit recruitment and rate modulation is evaluated for its potential contribution to the regulation of muscle properties, especially the prevention of muscle yield. We conclude that at low to moderate levels of initial force, recruitment of new motor units is likely to be the most effective compensatory mechanism.

Contributions of motor-unit recruitment and rate modulation of compensation for muscle yielding

1. Subdivided portions of the cut ventral root innervation of the soleus muscle were electrically stimulated in 14 anesthetized cats. The stimulus trains imposed on these nerves simulated the recruitment and rate-modulation patterns of single motor units recorded during stretch-reflex responses in decerebrate preparations. Each activation pattern was evaluated for its ability to prevent muscle yield. 2. Three basic stimulus patterns, recruitment, step increases in stimulus rate, and doublets were imposed during the course of ramp stretches applied over a wide range of velocities. The effect of each stimulus pattern on muscle force was compared to the force output recorded without stretch-related recruitment or rate modulation. 3. Motor-unit recruitment was found to be most effective in preventing yield during muscle stretch. Newly recruited motor units showed no evidence of yielding for some 250 ms following activation, at which time muscle stiffness declined slightly. This time-dependent resistance to yield was observed regardless of whether the onset of the neural stimulus closely preceded or followed stretch onset. 4. Step increases in stimulus rate arising shortly after stretch onset did not prevent the occurrence of yield at most stretch velocities, but did augment muscle stiffness later in the stretch. Doublets in the stimulus train were found to augment muscle stiffness only when they occurred in newly recruited motor units. 5. These results suggest that at low or moderate initial forces, the prevention of yield in lengthening, reflexively intact muscle results primarily from rapid motor-unit recruitment. To a lesser extent, the spring-like character of the stretch-reflex response also derives from step increases in firing rate of motor units active before stretch onset and doublets in units recruited during the course of stretch. Smooth rate increases appear to augment muscle force later in the course of the reflex response.

Prolonged time course for vibratory suppression of stretch reflex in the decerebrate cat

We studied the effects of longitudinal tendon vibration on the stretch reflex of the soleus and gastrocnemius muscles in 11 decerebrate cats. Vibration was applied at amplitudes (40-80 micrometer) and frequencies (120-250 HZ) sufficient to provide a strong tonic vibration reflex. In keeping with previous reports, we found that during an established tonic vibration reflex, the force and emg response to superimposed ramp and hold stretch are largely suppressed. This suppression is most obvious during the dynamic phase of stretch where it gives rise to a complex force response resembling that of active areflexic muscle. If stretch initiation is delayed until after vibration is terminated, the suppressed effects of vibration persist for 5 s or more. These suppressive effects are marked in the first 200 ms, and then decay gradually over the ensuing time period, paralleling the decline in emg and force which follows vibration offset. Simultaneous recordings from homonymous Ia afferents showed that this suppression persists even though the stretch responsiveness of primary spindle endings has returned to normal immediately following the end of vibration. When stretch is initiated shortly after vibration commences, the suppressive effects are first evident at 50-100 ms latency, but are not well established until 1 s or more after vibration onset. Tests of monosynaptic transmission using small amplitude tendon taps or electrical stimulation of synergist nerves to activate Ia fibers revealed that reductions in the magnitude of the response following vibration are usually modest (12% mean reduction at 50 ms, n = 5), and they are quite sensitive to the initial level of excitation of the motoneuron pool. These reductions were also rather shortlived, being largely completed within 500 ms of vibration offset. Although the relative contributions of presynaptic and postsynaptic inhibition are not readily dissociated in this type of experiment, it is likely that the magnitude of presynaptic inhibition is quite small. We argue that the effects of vibration on the stretch reflex are best explained by invoking an excitatory autogenetic projection from Ia interneurons to extensor motor neurons, which lies in parallel with the Ia monosynaptic projection. In order to account for the vibratory suppression, we propose that these interneurons are driven to saturation by vibration. When vibration ceases, the discharge rate of these interneurons declines with a prolonged time-course that coincides with the recovery of stretch responsiveness. This recovery would contribute to the return of stretch reflex force.

Abnormal force--EMG relations in paretic limbs of hemiparetic human subjects

The relations between surface EMG and isometric force generated by elbow flexor muscles were compared in normal and paretic limbs of 17 hemiparetic human subjects. Similar analyses were performed on both arms of 11 normal subjects. In almost half of the hemiparetic subjects examined (8/17), the slope of the relation between elbow flexion force and surface EMG, measured over the biceps-brachialis and brachioradialis muscle groups was increased in the paretic limb. A mechanism based on anomalous reductions in mean motor unit discharge rate in paretic muscles is advanced as the most likely cause of the findings.

Joint position sense: the effects of muscle contraction

The effects of isometric and isotonic force production on perceived static joint position were investigated in 12 adult subjects. The joint examined was the proximal interphalangeal joint of one index finger, and its perceived position was determined from matching movements of the equivalent joint on the other hand. When the perturbed joint was free to move, even when supporting substantial loads, the position was accurately estimated; however, when the subject was required to exert substantial isometric force against the device imposing the joint movement, significant errors occurred: these errors were in the direction of the increasing force. Similar effects were evident during increasing isometric flexion force in anaesthetized fingers. It is suggested that force-related afferent discharge from muscle, presumably originating in tendon organ receptors, contributes to static joint position sense. This force-feedback may allow the nervous system to accommodate for the effects of changing fusimotor bias, but it also appears to induce errors when afferent information of force and length provide potentially conflicting information.

Mechanisms of the clasp-knife reflex studied in an animal model

The mechanisms of the clasp-knife reflex were studied in the soleus muscle of an animal model, the decerebrate cat with a dorsal hemisection of the lower thoracic cord. The reflex is shown to be autogenetic, and to depend on muscle length in keeping with previous suggestions. However, the magnitude of the inhibition increases with increasing initial force, and the inhibition is mimicked by gentle manipulation of the muscle and tendon surface. Concurrent muscle afferent recordings showed that the electromyogram (emg) reduction was not a result of a decline in Ia afferent input and was not well related to secondary or tendon organ afferent discharge. It is now known that many group III and some group IV muscle afferents are also activated by muscle stretch and contraction, and we here report limited stretch sensitivity in four non-spindle group II afferents. Since these fiber groups each include afferents that produce inhibition of extensor motoneurons, it is proposed that the clasp-knife reflex may result from the activation of these slowly conducting afferent fibers.

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

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.

An estimate of the secondary spindle receptor afferent contribution to the stretch reflex in extensor muscles of the decerebrate cat

1. Vibration or stretch of the medial gastrocnemius muscle in the decerebrate cat each caused a significant increase in the tension of a synergist, the lateral gastrocenmius. 2. Simultaneous vibration and stretch of the medial gastrocnemius resulted in a substantial increase of lateral gastrocnemius tension which was greater that that produced by medial gastrocnemius vibration alone. The size of this force increase was proportional to the amplitude of medial gastrocnemius stretch, for the limited range of amplitudes examined. 3. Since the discharge of the medial gastrocnemius I a afferent fibres was held constant by vibration, the additional tension in lateral gastrocnemius provoked by medial gastrocnemius stretch must have resulted from the activation of an excitatory pathway separate from the I a afferent system. The secondary spindle afferent pathway was considered to be the most likely candidate. 4. The contributions of the Ia afferents and the additional stretch induced excitation to the stretch reflex were compared. The Ia potency was calculated from the ration of tonic vibration reflex force and the vibration frequency. The total Ia contribution to the stretch reflex, which was estimated from the product of this ratio and the primary ending stretch sensitivity, seemed modest, and was consistently smaller than the proposed secondary contribution. 5. The medial gastrocnemius nerve was subjected to anodal blockade at a strength sufficient to eliminate Ia transmission. Under these conditions, the lateral gastrocnemius excitation produced by medial gastrocnemius stretch or vibration was largely eliminated. When lateral gastrocnemius vibration was superimposed, the excitatory effect of medial gastrocnemius stretch was partly restored suggesting that some central facilitation by group Ia afferents may be necessary for group II pexcitatory effects to be manifested. 6. Although the additional excitatory actions of medial gastrocnemius stretch were examined exclusively in a synergist, it is suggested that similar effects are likely to occur in the homonymous stretch reflex.

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

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.