A technique for measuring the mechanical actions of heterogenic (intermuscular) reflexes in 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.

Evaluating intermuscular Golgi tendon organ feedback with twitch contractions

KEY POINTS

Golgi tendon organ feedback has been evaluated most frequently using electrical stimulation of peripheral nerves, which is not a physiological or selective stimulus for Golgi tendon organs. Golgi tendon organs are most responsive to active muscle contractions. This study provides evidence that muscle stimulation evoked twitches - a physiological stimulus for Golgi tendon organs - induces intermuscular effects most likely due to mechanical activation of Golgi tendon organ feedback and not direct activation of sensory axons. The results demonstrate that twitch contractions are a feasible non-invasive approach that can be used to advance understanding of the functional role of Golgi tendon organ feedback.

ABSTRACT

Force feedback from Golgi tendon organs (GTOs) has widespread intermuscular projections mediated by interneurons that share inputs from muscle spindles, among others. Because current methods to study GTO circuitry (nerve stimulation or muscle stretch) also activate muscle spindle afferents, the selective role of GTOs remains uncertain. Here, we tested the hypothesis that intramuscular stimulation evoked twitch contractions could be used to naturally bias activation of GTOs and thus evaluate their intermuscular effects in decerebrate cats. This was achieved by comparing the effects of twitch contractions and stretches as donor inputs onto the motor output of recipient muscles. Donor-recipient pairs evaluated included those already known in the cat to receive donor excitatory muscle spindle feedback only, inhibitory GTO feedback only, and both excitatory spindle and inhibitory GTO effects. Muscle stretch, but not twitch contractions, evoked excitation onto recipient muscles with muscle spindle afferent inputs only. Both donor muscle stretch and twitch contractions inhibited a recipient muscle with GTO projections only. In a recipient muscle that receives both muscle spindle and GTO projections, donor muscle stretch evoked both excitatory and inhibitory effects, whereas twitch contractions evoked inhibitory effects only. These data support the hypothesis that muscle stimulation evoked contractions can induce intermuscular effects most consistent with mechanical GTO receptor activation and not direct activation of sensory axons. We propose this approach can be used to evaluate GTO circuitry more selectively than muscle stretch or nerve stimulation and can be adapted to study GTO feedback non-invasively in freely moving cats and humans.

Self-reinnervated muscles lose autogenic length feedback, but intermuscular feedback can recover functional connectivity

In this study, we sought to identify sensory circuitry responsible for motor deficits or compensatory adaptations after peripheral nerve cut and repair. Self-reinnervation of the ankle extensor muscles abolishes the stretch reflex and increases ankle yielding during downslope walking, but it remains unknown whether this finding generalizes to other muscle groups and whether muscles become completely deafferented. In decerebrate cats at least 19 wk after nerve cut and repair, we examined the influence of quadriceps (Q) muscles' self-reinnervation on autogenic length feedback, as well as intermuscular length and force feedback, among the primary extensor muscles in the cat hindlimb. Effects of gastrocnemius and soleus self-reinnervation on intermuscular circuitry were also evaluated. We found that autogenic length feedback was lost after Q self-reinnervation, indicating that loss of the stretch reflex appears to be a generalizable consequence of muscle self-reinnervation. However, intermuscular force and length feedback, evoked from self-reinnervated muscles, was preserved in most of the interactions evaluated with similar relative inhibitory or excitatory magnitudes. These data indicate that intermuscular spinal reflex circuitry has the ability to regain functional connectivity, but the restoration is not absolute. Explanations for the recovery of intermuscular feedback are discussed, based on identified mechanisms responsible for lost autogenic length feedback. Functional implications, due to permanent loss of autogenic length feedback and potential for compensatory adaptations from preserved intermuscular feedback, are discussed.

Effects of reinnervation of the biarticular shoulder-elbow muscles on joint kinematics and electromyographic patterns of the feline forelimb during downslope walking

Full recovery of the forelimb kinematics during level and upslope walking following reinnervation of the biarticular elbow extensor suggests that the proprioceptive loss is compensated by other sensory sources or altered central drive, yet these findings have not been explored in downslope walking. Kinematics and muscle activity of the shoulder and elbow during downslope locomotion following reinnervation of the feline long head of the triceps brachii (TLo) and biceps brachii (Bi) were evaluated (1) during paralysis and (2) after the motor function was recovered but the proprioceptive feedback was permanently disrupted. The step cycle was examined in three walking conditions: level (0%), -25% grade (-14° downslope) and -50% grade (-26.6° downslope). Measurements were taken prior to and at three time points (2 weeks, and 1 and 12+ months) after transecting and suturing the radial and musculocutaneous nerves. There was an increase in the yield (increased flexion) at the elbow and less extensor activity duration of flexion during stance as the downslope grade increased. There were two notable periods of eccentric contractions (active lengthening) providing an apparent 'braking' action. Paralysis of the TLo and the Bi resulted in uncompensated alterations in shoulder-elbow kinematics and motor activity during the stance phase. However, unlike the case for the level and upslope conditions, during both paralysis and reinnervation, changes in interjoint coordination persisted for the downslope condition. The lack of complete recovery in the long term suggests that the autogenic reflexes contribute importantly to muscle and joint stiffness during active lengthening.

Effects of reinnervation of the triceps brachii on joint kinematics and electromyographic patterns of the feline forelimb during level and upslope walking

Nerve injury in the hindlimb of the cat results in locomotor changes, yet these findings have not been explored in a more multifunctional forelimb. Kinematics and muscle activity of the shoulder and elbow during level and upslope locomotion following reinnervation of the feline long head of the triceps brachii (TLo) were evaluated (1) during paralysis [none to minimum motor activity (short-term effects)] and (2) after the motor function was recovered but the proprioceptive feedback was permanently disrupted (long-term effects). The step cycle was examined in three walking conditions: level (0%), 25% grade (14° upslope) and 50% grade (26.6° upslope). Measurements were taken prior to and at three time points (2 weeks, 1 month and 12+ months) after transecting and suturing the radial nerve of TLo. There was less of a yield (increased flexion) at the elbow joint and more extensor activity during elbow flexion during stance (E2) as the grade of walking increased. Substantial short-term effects were observed at the elbow joint (increased flexion during E2) as well as increased motor activity by the synergistic elbow extensors, and greater shoulder extension at paw contact, leading to altered interjoint coordination during stance. Forelimb shoulder and elbow kinematics during level and upslope locomotion progressed back to baseline at 12 months. The short-term effects can be explained by both mechanical and neural factors that are altered by the functional elimination of the TLo. Full recovery of the forelimb kinematics during level and upslope walking suggests that the proprioceptive length feedback loss is compensated by other sensory sources or altered central drive.

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.

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.

Fusimotor responses to fatiguing muscle contractions in non-denervated hindlimb of decerebrate cats

Changes in discharge rate of 21 fusimotor neurons to medial gastrocnemius muscle during long-lasting fatiguing contractions of lateral gastrocnemius and soleus muscles were recorded in decerebrate cats with innervation of the same hindlimb preserved. Both the spontaneous activity and reflex responses of fusimotor neurons differed from those found previously in preparations with denervated hindlimb. Higher proportion of units fired at rest at rates above 20 impulses/s, the initial increase in discharge rate at the onset of muscle contraction was markedly prolonged, lasting in the majority of units throughout the muscle contraction, while the late increase in discharge rate developing with muscle fatigue was either absent or short-lasting. It is suggested that the increase in spontaneous firing rate occurs in dynamic fusimotor neurons being supported by afferent inflow from secondary muscle spindle endings from non-contracting muscles, the enhancement of the early responses to be primarily due to recurrent disinhibition and the differences in changes of early and late responses to reflect their partly different origin. The possibility is raised that the late reflex responses are lacking more often in static than in dynamic fusimotor neurons. From the functional point of view the differences in fusimotor reflex responses in innervated versus denervated hindlimb may indicate their susceptibility to modifications by changes in afferent inflow according, supposedly, to the current demands of motor control of the active and/or fatigued muscle.

The organization of heterogenic reflexes among muscles crossing the ankle joint in the decerebrate cat

1. Mechanical actions of heterogenic (intermuscular) reflexes arising from proprioceptors in flexor and extensor ankle muscles were measured in intercollicular and premammillary decerebrate cats. Length inputs were applied to the freed tendons of one of a pair of muscles crossing the ankle joint and resulting changes in force in both muscles were measured. Interactions between autogenic and heterogenic reflexes were studied by applying length changes to both muscles. 2. A consistent asymmetry was observed in the heterogenic inhibition between the single-joint antagonists soleus and tibialis anterior (TA). Inhibition from soleus to TA was weak or absent during the reflex activation of TA. In contrast, a strong heterogenic inhibition was consistently observed from TA to soleus during the activation of soleus by a crossed-extension reflex. The effect of this inhibition in the intact joint is to increase the apparent mechanical stiffness of soleus. 3. Mutual synergism among soleus, medial gastrocnemius (MG) and lateral gastrocnemius (LG) was demonstrated only at low to moderate forces by the observation of excitatory reflexes among them. During a naturally or electrically evoked crossed-extension reflex, however, a unidirectional inhibitory reflex from MG and LG to soleus was observed. This inhibition increased with force in MG or LG. These results suggest that the knee and ankle joints become more tightly linked mechanically at high forces since the stiffness of the biarticular gastrocnemius muscle predominates over that of the uniarticular soleus. 4. Under quiescent conditions (no resting muscle activation), mutual synergism was obeyed among the ankle extensors soleus, LG and MG and also between the pretibial flexors TA and extensor digitorum longus (EDL). Moreover, inhibition was generally observed between a pretibial flexor and an ankle extensor. Departures from this expected pattern of heterogenic reflexes occurred when the muscle groups were activated by crossed-extension and flexion reflexes. Reflexes onto soleus, TA and EDL reversed in sign or increased in magnitude. 5. The observed patterns of reflex connectivity among the ankle flexors and extensors were similar in both intercollicular and premammillary preparations, although changes in reflex strength were sometimes noted in cases where a second, lower transection was performed during the experiment. 6. It is argued from the large magnitudes of certain heterogenic reflexes that the mechanical response properties of muscles crossing the ankle joint in the intact animal are not dominated by autogenic reflexes and intrinsic mechanical properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Ensemble proprioceptive activity in the cat step cycle: towards a representative look-up chart

Analysis of the control of movement in tasks such as stepping is severely restricted by the lack of quantitative data on the ensemble activity of afferents in the numerous muscles involved. We have started to build up a quantitative "look-up-chart" of the ensemble afferent and efferent profiles in the cat step cycle. To this end, we have developed software which allows us to digitize afferent firing, muscle length and electromyogram (EMG) activity, and to align segments for averaging by choosing one or more reference points in the step cycle. The ensemble firing of triceps surae Ia afferents showed lower than expected mean and peak rates, whereas triceps group II and Ib afferents were more active than predicted. There were small but significant transients in Ia firing at foot-off and touch-down which could not be explained in terms of origin-to-insertion length lone. They were most likely caused by propagated mechanical transients or tendon compliance effects giving rise to small differences between the origin-to-insertion length and the intramuscular length "seen" by spindles. Net ensemble Ia rates, based on previous estimates of spindle populations, probably exceed 25 kilo-impulses/second (ki.p.s.) in some muscles. Inputs as large as this are likely to contribute significantly to reflex control.