Absence of force-feedback regulation in soleus muscle of the decerebrate cat

Non-uniform upregulation of the autogenic stretch reflex among hindlimb extensors following lateral spinal lesion in the cat

Successful propagation throughout the step cycle is contingent on adequate regulation of whole-limb stiffness by proprioceptive feedback. Following spinal cord injury (SCI), there are changes in the strength and organization of proprioceptive feedback that can result in altered joint stiffness. In this study, we measured changes in autogenic feedback of five hindlimb extensor muscles following chronic low thoracic lateral hemisection (LSH) in decerebrate cats. We present three features of the autogenic stretch reflex obtained using a mechanographic method. Stiffness was a measure of the resistance to stretch during the length change. The dynamic index documented the extent of adaptation or increase of the force response during the hold phase, and the impulse measured the integral of the response from initiation of a stretch to the return to the initial length. The changes took the form of variable and transient increases in the stiffness of vastus (VASTI) group, soleus (SOL), and flexor hallucis longus (FHL), and either increased (VASTI) or decreased adaptation (GAS and PLANT). The stiffness of the gastrocnemius group (GAS) was also variable over time but remained elevated at the final time point. An unexpected finding was that these effects were observed bilaterally. Potential reasons for this finding and possible sources of increased excitability to this muscle group are discussed.

What muscle variable(s) does the nervous system control in limb movements?

To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements.

Morton's hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.

The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

Heterogenic feedback between hindlimb extensors in the spontaneously locomoting premammillary cat

Electrophysiological studies in anesthetized animals have revealed that pathways carrying force information from Golgi tendon organs in antigravity muscles mediate widespread inhibition among other antigravity muscles in the feline hindlimb. More recent evidence in paralyzed or nonparalyzed decerebrate cats has shown that some inhibitory pathways are suppressed and separate excitatory pathways from Golgi tendon organ afferents are opened on the transition from steady force production to locomotor activity. To obtain additional insight into the functions of these pathways during locomotion, we investigated the distribution of force-dependent inhibition and excitation during spontaneous locomotion and during constant force exertion in the premammillary decerebrate cat. We used four servo-controlled stretching devices to apply controlled stretches in various combinations to the gastrocnemius muscles (G), plantaris muscle (PLAN), flexor hallucis longus muscle (FHL), and quadriceps muscles (QUADS) during treadmill stepping and the crossed-extension reflex (XER). We recorded the force responses from the same muscles and were therefore able to evaluate autogenic (intramuscular) and heterogenic (intermuscular) reflexes among this set of muscles. In previous studies using the intercollicular decerebrate cat, heterogenic inhibition among QUADS, G, FHL, and PLAN was bidirectional. During treadmill stepping, heterogenic feedback from QUADS onto G and G onto PLAN and FHL remained inhibitory and was force-dependent. However, heterogenic inhibition from PLAN and FHL onto G, and from G onto QUADS, was weaker than during the XER. We propose that pathways mediating heterogenic inhibition may remain inhibitory under some forms of locomotion on a level surface but that the strengths of these pathways change to result in a proximal to distal gradient of inhibition. The potential contributions of heterogenic inhibition to interjoint coordination and limb stability are discussed.

Evidence for force-feedback inhibition in chronic stroke

The presence of force-feedback inhibition was explored during reflex responses in five subjects with known incidence of stroke. Using constant velocity stretches, it was previously found that after movement onset, active reflex force progressively increases with increasing joint angle, at a rate proportional to a fractional exponent of the speed of stretch. However, after the reflex force magnitude exceeds a particular level, it begins rolling off until maintaining a steady-state value. The magnitudes of these force plateaus are correlated with the speed of stretch, such that higher movement speeds result in higher steady-state forces. Based upon these previous studies, we hypothesized that force plateau behavior could be explained by a force-feedback inhibitory pathway. To help facilitate an understanding of this stretch reflex force roll off, a simple model representing the elbow reflex pathways was developed. This model contained two separate feedback pathways, one representing the monosynaptic stretch reflex originating from muscle spindle excitation, and another representing force-feedback inhibition arising from force sensitive receptors. It was found that force-feedback inhibition altered the stretch reflex response, resulting in a force response that followed a sigmoidal shape similar to that observed experimentally. Furthermore, simulated reflex responses were highly dependent on force-feedback gain, where predicted reflex force began plateauing at decreasing levels with increases in this force-feedback gain. The parameters from the model fits indicate that the force threshold for force-sensitive receptors is relatively high, suggesting that the inhibition may arise from muscle free nerve endings rather than Golgi tendon organs. The experimental results coupled with the simulations of elbow reflex responses suggest the possibility that after stroke, the effectiveness of force-feedback inhibition may increase to a level that has functional significance. Practical implications of these findings are discussed in relation to muscle weakness commonly associated with stroke.

Positive force feedback control of muscles

This study was prompted by recent evidence for the existence of positive force feedback in feline locomotor control. Our aim was to establish some basic properties of positive force feedback in relation to load compensation, stability, intrinsic muscle properties, and interaction with displacement feedback. In human subjects, muscles acting about the wrist and ankle were activated by feedback-controlled electrical stimulation. The feedback signals were obtained from sensors monitoring force and displacement. The signals were filtered to mimic transduction by mammalian tendon organ and muscle spindle receptors. We found that when muscles under positive force feedback were loaded inertially, they responded in a stable manner with increased active force. The activation attenuated the muscle stretch (yield) that would otherwise occur in the absence of feedback. With enough positive force feedback gain, yield could actually reverse. This behavior, which we termed the affirming reaction, was reminiscent of the mammalian positive supporting reaction, a postural response elicited by contact of the foot with the ground. Muscles under positive force feedback remained stable, even when the loop gain (Gf) was set at levels of 2 or 3. In a linear system, if Gf > 1, instability occurs when the loop is closed. On further investigation, we found that Gf changed with joint angle: it declined as the load-bearing muscle actively shortened. We inferred that in closed-loop operation, the active muscles always shortened until Gf approached unity. In other words, the length-tension curve of active muscle ensures stability even when force-related excitation of motoneurons is very large. Concomitant negative displacement feedback reinforced and stabilized load compensation up to a certain gain, beyond which instability occurred. In further trials we included delays of up to 40 ms in the positive force feedback pathway, to model the delays recently described for tendon organ reflexes in cat locomotion. Contrary to expectations, this did not destabilize the loop. Indeed, when instability was deliberately evoked by setting displacement feedback gain high, delays in the positive force feedback pathway actually stabilized control. The stabilization of positive force feedback by inherent properties of the neuromuscular system increases the functional scope to be expected of feedback from force receptors in biological motor control. Our results provide a rationale for the delayed excitatory action of Ib heteronymous input on extensor motoneurons in cat locomotion.

Implications of positive feedback in the control of movement

In this paper we review some theoretical aspects of positive feedback in the control of movement. The focus is mainly on new theories regarding the reflexive role of sensory signals from mammalian tendon organ afferents. In static postures these afferents generally mediate negative force feedback. But in locomotion there is evidence of a switch to positive force feedback action. Positive feedback is often associated with instability and oscillation, neither of which occur in normal locomotion. We address this paradox with the use of analytic models of the neuromuscular control system. It is shown that positive force feedback contributes to load compensation and is surprisingly stable because the length-tension properties of mammalian muscle provide automatic gain control. This mechanism can stabilize control even when positive feedback is very strong. The models also show how positive force feedback is stabilized by concomitant negative displacement feedback and, unexpectedly, by delays in the positive feedback pathway. Other examples of positive feedback in animal motor control systems are discussed, including the beta-fusimotor system, which mediates positive feedback of displacement. In general it is seen that positive feedback reduces the sensitivity of the controlled extremities to perturbations of posture and load. We conclude that positive force feedback can provide stable and effective load compensation that complements the action of negative displacement and velocity feedback.

Autogenetic inhibition from contraction receptors in the decerebrate cat

1. Autogenetic inhibition from contraction receptors was measured by eliciting contractions of the soleus muscle in the decerebrate cat. Inhibitory feedback was detected when the tension increment f, produced by stimulating motor fibres in the presence of a background reflex contraction, was smaller than the tension d elicited by the same stimulus in the absence of reflex action. Tendon vibration was applied throughout to clamp primary spindle afferents at a constant firing rate, thereby preventing spindle unloading from disfacilitating the reflex contraction. 2. The reduction in tension d--f varied roughly linearly with the size of the tension stimulus f. Feedback gain was proportional to d--f/f, i.e. the ratio of inhibited tension to stimulus tension. It was computed by averaging over several measurements obtained with stimuli of different sizes, and ranged between 0 and 0.88 in ten animals. The average gain, 0.39, implies that voluntary muscle force is reduced by approximately 27% through the direct inhibition of alpha-motoneurones from homonymous contraction receptors. 3. Inhibitory feedback gain did not appear to co-vary with the background reflex contraction. When measured without vibration, however, a positive covariance did emerge, suggesting that this is due to unloading of muscle spindles, either by extrafusal muscle shortening or by inhibition of fusimotor neurones. 4. Inhibited tension varied linearly with the estimated increment in Ib afferent firing. On the assumption that group Ib afferents carried the entire inhibitory signal, inhibitory feedback gain measured with vibration was used to predict the size of the gain if vibration had not been applied. Feedback gain calculated in this way was reduced by still did not vary with reflex tension. 5. In one animal with signs of brain stem trauma, feedback gain was increased to around six. It is argued that inhibitory feedback in the intact animal can rise to comparable values, as a result both of convergence of signals from different muscles and of supraspinal facilitation.

A potential mechanism underlying the voluntary suppression of tardive dyskinesia

Many clinicians have observed that patients with tardive dyskinesia (TD) can voluntarily suppress the movements to some extent. The mechanism by which voluntary control is exerted most likely involves peripheral feedback. Quantitative analyses of isometric and isotonic jaw stability were conducted in patients with and without TD to test the general hypothesis that involuntary movements of TD may be brought under voluntary control via one of two receptor mechanisms; muscle length and/or muscle force. Results indicated that in the absence of length feedback patients with TD exhibited significantly greater instability while patients without TD performed within normal limits. This suggests that patients with TD utilized muscle length feedback to suppress motor instability but were unable to utilize muscle force feedback to suppress instability. Implications for the use of laboratory procedures in the measurement and clinical management of TD are discussed.

Instability in human forearm movements studied with feed-back-controlled electrical stimulation of muscles

1. Amplitude-modulated electrical stimulation was applied to the elbow flexors and extensors to produce movements of the forearm in normal subjects. The parameters of the modulating (command) signal were set in isometric trials so as to produce equal and opposite background torques, and equal and supportive torque modulations. 2. Bode plots relating forearm movement to command signal (modulating) frequency showed the muscle-load to have a low-pass characteristic similar to that previously described in the cat, and a slightly larger bandwidth than described previously in man. 3. The transduced forearm signals were fed back to provide the command signal to the stimulators via a filter which mimicked the transfer function of muscle spindle primary endings. In effect this replaced the neural part of the reflex arc with an accessible model, but left the muscle-load effector intact. 4. All six subjects developed forearm oscillations (tremor) when the loop gain exceeded a threshold value. The mean tremor frequency at onset was 4.4 Hz, which was similar to that of the equivalent vibration-evoked tremor (previous paper, Prochazka & Trend, 1988). 5. With the linear spindle model, oscillations tended to grow rapidly in amplitude, and the stimuli became painful. The inclusion of a logarithmic limiting element resulted in stable oscillations, without significant alterations in frequency. This allowed us to study the effect on tremor of including analog delays in the loop, mimicking those associated with peripheral nerve transmission and central reflexes. In one subject, loop delays of 0, 20, 40 and 100 ms resulted in tremor at 4.0, 3.6, 3.0 and 2.1 Hz respectively, as quantified by spectral analysis. 6. By considering separately the phase contributions of the different elements of the reflex arc, including delays, it became clear that muscle-load properties were important in setting the upper limit of tremor frequencies which could conceivably be supported by reflexes. 7. The results support the conclusion of the related vibration study (Prochazka & Trend, 1988), that for moderate levels of background co-contraction, the contribution of stretch reflexes to tremor at the elbow should be sought in the 3-5 Hz range. Exaggerated long-latency reflexes would be expected to reduce these baseline frequencies by 1 or 2 Hz.

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.

Viscoelastic properties of the wrist motor servo in man

Viscoelastic properties play an important role in posture and movement. Such properties arise from muscle mechanics and from stretch-reflex actions. We describe experiments designed to characterize both linear and nonlinear elastic and viscous properties of the wrist motor servo in human subjects. First, we describe a trial comparison method for the identification of reflex responses that are unmodified by triggered reaction-time movements. Elastic properties were studied by applying step changes in load force that stretched or released the wrist flexor and extensor muscles. The properties were basically spring-like, but there was a short-range enhancement of stiffness that gave rise to a prominent hysteresis. Viscous properties were studied by applying ramp stretches at different velocities. Both EMG and force responses showed a weak fractional-power dependence on velocity similar to that described recently for muscle spindle receptors. Consideration is given to the possible advantages of this type of nonlinear feedback in the damping of postural responses and movements.

Modification of muscle responses by spinal circuitry

Recently, we developed a model based on experimental data, which includes a pair of antagonistic muscles, a general load against which the muscles act, feedback pathways from muscle sense organs and spinal inhibitory circuits involving 1A interneurons and Renshaw cells. Descending inputs can activate the model through combinations of inputs to alpha-motoneurons, gamma-motoneurons (via intrafusal muscles and their feedback pathways) or the 1A interneurons. The role of each of these connections is analysed here in terms of its effect on the response of the muscles to impulse inputs, with particular interest in the effects on the overall stability of the systems. Increasing muscle stiffness or feedback from muscle receptors tends to produce high frequency oscillations. Coactivation of alpha- and gamma-motoneurons can lead to cancellation of oscillations, because of delays in the effects of gamma-motoneurons on contraction. Connections of 1A inhibitory interneurons onto antagonist motoneurons accentuate the oscillations. Inhibitory connections from Renshaw cells onto alpha-motoneurons tend to prevent oscillations, whereas the concentrations onto gamma-motoneurons may produce them.

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.

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.