ROLE OF FORELIMB MORPHOLOGY IN MUSCLE SENSORIMOTOR FUNCTIONS DURING LOCOMOTION IN THE CAT.

Previous studies established strong links between morphological characteristics of mammalian hindlimb muscles and their sensorimotor functions during locomotion. Less is known about the role of forelimb morphology in motor outputs and generation of sensory signals. Here, we measured morphological characteristics of 46 forelimb muscles from 6 cats. These characteristics included muscle attachments, physiological cross-sectional area (PCSA), fascicle length, etc. We also recorded full-body mechanics and EMG activity of forelimb muscles during level overground and treadmill locomotion in 7 and 16 adult cats of either sex, respectively. We computed forelimb muscle forces along with force- and length-dependent sensory signals mapped onto corresponding cervical spinal segments. We found that patterns of computed muscle forces and afferent activities were strongly affected by the muscle's moment arm, PCSA, and fascicle length. Morphology of the shoulder muscles suggests distinct roles of the forelimbs in lateral force production and movements. Patterns of length-dependent sensory activity of muscles with long fibers (brachioradialis, extensor carpi radialis) closely matched patterns of overall forelimb length, whereas the activity pattern of biceps brachii matched forelimb orientation. We conclude that cat forelimb muscle morphology contributes substantially to locomotor function, particularly to control lateral stability and turning, rather than propulsion.

Reduced inhibition from quadriceps onto soleus after acute quadriceps fatigue suggests Golgi tendon organ contribution to heteronymous inhibition.

Heteronymous inhibition between lower limb muscles is primarily attributed to recurrent inhibitory circuits in humans but could also arise from Golgi tendon organs (GTOs). Distinguishing between recurrent inhibition and mechanical activation of GTOs is challenging because their heteronymous effects are both elicited by stimulation of nerves or a muscle above motor threshold. Here, the unique influence of mechanically activated GTOs was examined by comparing the magnitude of heteronymous inhibition from quadriceps (Q) muscle stimulation onto ongoing soleus electromyographic at five Q stimulation intensities (1.5-2.5× motor threshold) before and after an acute bout of stimulation-induced Q fatigue. Fatigue was used to decrease Q stimulation evoked force (i.e., decreased GTO activation) despite using the same pre-fatigue stimulation currents (i.e., same antidromic recurrent inhibition input). Thus, a decrease in heteronymous inhibition after Q fatigue and a linear relation between stimulation-evoked torque and inhibition both before and after fatigue would support mechanical activation of GTOs as a source of inhibition. A reduction in evoked torque but no change in inhibition would support recurrent inhibition. After fatigue, Q stimulation-evoked knee torque, heteronymous inhibition magnitude and inhibition duration were significantly decreased for all stimulation intensities. In addition, heteronymous inhibition magnitude was linearly related to twitch-evoked knee torque before and after fatigue. These findings support mechanical activation of GTOs as a source of heteronymous inhibition along with recurrent inhibition. The unique patterns of heteronymous inhibition before and after fatigue across participants suggest the relative contribution of GTOs, and recurrent inhibition may vary across persons.

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.

Redistribution of inhibitory force feedback between a long toe flexor and the major ankle extensor muscles following spinal cord injury.

Inhibitory pathways from Golgi tendon organs project widely between muscles crossing different joints and axes of rotation. Evidence suggests that the strength and distribution of this intermuscular inhibition is dependent on motor task and corresponding signals from the brainstem. The purpose of the present study was to investigate whether this sensory network is altered after spinal cord hemisection as a potential explanation for motor deficits observed after spinal cord injury (SCI). Force feedback was assessed between the long toe flexor and ankle plantarflexor (flexor hallucis longus), and the three major ankle extensors, (combined gastrocnemius, soleus, and plantaris muscles) in the hind limbs of unanesthetized, decerebrate, female cats. Data were collected from animals with intact spinal cords (control) and lateral spinal hemisections (LSHs) including chronic LSH (4-20 weeks), subchronic LSH (2 weeks), and acute LSH. Muscles were stretched individually and in pairwise combinations to measure intermuscular feedback between the toe flexor and each of the ankle extensors. In control animals, three patterns were observed (balanced inhibition between toe flexor and ankle extensors, stronger inhibition from toe flexor to ankle extensor, and vice versa). Following spinal hemisection, only strong inhibition from toe flexors onto ankle extensors was observed independent of survival time. The results suggest immediate and permanent reorganization of force feedback in the injured spinal cord. The altered strength and distribution of force feedback after SCI may be an important future target for rehabilitation.

Adaptation to slope in locomotor-trained spinal cats with intact and self-reinnervated lateral gastrocnemius and soleus muscles.

Sensorimotor training providing motion-dependent somatosensory feedback to spinal locomotor networks restores treadmill weight-bearing stepping on flat surfaces in spinal cats. In this study, we examined if locomotor ability on flat surfaces transfers to sloped surfaces and the contribution of length-dependent sensory feedback from lateral gastrocnemius (LG) and soleus (Sol) to locomotor recovery after spinal transection and locomotor training. We compared kinematics and muscle activity at different slopes (±10° and ±25°) in spinalized cats (n = 8) trained to walk on a flat treadmill. Half of those animals had their right hindlimb LG/Sol nerve cut and reattached before spinal transection and locomotor training, a procedure called muscle self-reinnervation that leads to elimination of autogenic monosynaptic length feedback in spinally intact animals. All spinal animals trained on a flat surface were able to walk on slopes with minimal differences in walking kinematics and muscle activity between animals with/without LG/Sol self-reinnervation. We found minimal changes in kinematics and muscle activity at lower slopes (±10°), indicating that walking patterns obtained on flat surfaces are robust enough to accommodate low slopes. Contrary to results in spinal intact animals, force responses to muscle stretch largely returned in both SELF-REINNERVATED muscles for the trained spinalized animals. Overall, our results indicate that the locomotor patterns acquired with training on a level surface transfer to walking on low slopes and that spinalization may allow the recovery of autogenic monosynaptic length feedback following muscle self-reinnervation.NEW & NOTEWORTHY Spinal locomotor networks locomotor trained on a flat surface can adapt the locomotor output to slope walking, up to ±25° of slope, even with total absence of supraspinal CONTROL. Autogenic length feedback (stretch reflex) shows signs of recovery in spinalized animals, contrary to results in spinally intact animals.

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.

Distributed force feedback in the spinal cord and the regulation of limb mechanics.

This review is an update on the role of force feedback from Golgi tendon organs in the regulation of limb mechanics during voluntary movement. Current ideas about the role of force feedback are based on modular circuits linking idealized systems of agonists, synergists, and antagonistic muscles. In contrast, force feedback is widely distributed across the muscles of a limb and cannot be understood based on these circuit motifs. Similarly, muscle architecture cannot be understood in terms of idealized systems, since muscles cross multiple joints and axes of rotation and further influence remote joints through inertial coupling. It is hypothesized that distributed force feedback better represents the complex mechanical interactions of muscles, including the stresses in the musculoskeletal network born by muscle articulations, myofascial force transmission, and inertial coupling. Together with the strains of muscle fascicles measured by length feedback from muscle spindle receptors, this integrated proprioceptive feedback represents the mechanical state of the musculoskeletal system. Within the spinal cord, force feedback has excitatory and inhibitory components that coexist in various combinations based on motor task and integrated with length feedback at the premotoneuronal and motoneuronal levels. It is concluded that, in agreement with other investigators, autogenic, excitatory force feedback contributes to propulsion and weight support. It is further concluded that coexistent inhibitory force feedback, together with length feedback, functions to manage interjoint coordination and the mechanical properties of the limb in the face of destabilizing inertial forces and positive force feedback, as required by the accelerations and changing directions of both predator and prey.

Patterns of intermuscular inhibitory force feedback across cat hindlimbs suggest a flexible system for regulating whole limb mechanics.

Prior work has suggested that Golgi tendon organ feedback, via its distributed network linking muscles spanning all joints, could be used by the nervous system to help regulate whole limb mechanics if appropriately organized. We tested this hypothesis by characterizing the patterns of intermuscular force-dependent feedback between the primary extensor muscles spanning the knee, ankle, and toes in decerebrate cat hindlimbs. Intermuscular force feedback was evaluated by stretching tendons of selected muscles in isolation and in pairwise combinations and then measuring the resulting force-dependent intermuscular interactions. The relative inhibitory feedback between extensor muscles was examined, as well as symmetry of the interactions across limbs. Differences in the directional biases of inhibitory feedback were observed across cats, with three patterns identified as points on a spectrum: pattern 1, directional bias of inhibitory feedback onto the ankle extensors and toe flexors; pattern 2, convergence of inhibitory feedback onto ankle extensors and mostly balanced inhibitory feedback between vastus muscle group and flexor hallucis longus, and pattern 3, directional bias of inhibitory feedback onto ankle and knee extensors. The patterns of inhibitory feedback, while different across cats, were symmetric across limbs of individual cats. The variable but structured distribution of force feedback across cat hindlimbs provides preliminary evidence that inhibitory force feedback could be a regulated neural control variable. We propose the directional biases of inhibitory feedback observed experimentally could provide important task-dependent benefits, such as directionally appropriate joint compliance, joint coupling, and compensation for nonuniform inertia. NEW & NOTEWORTHY Feedback from Golgi tendon organs project widely among extensor motor nuclei in the spinal cord. The distributed nature of force feedback suggests these pathways contribute to the global regulation of limb mechanics. Analysis of this network in individual animals indicates that the strengths of these pathways can be reorganized appropriately for a variety of motor tasks, including level walking, slope walking, and landing.

Musculotendon adaptations and preservation of spinal reflex pathways following agonist-to-antagonist tendon transfer.

Tendon transfer surgeries are performed to restore lost motor function, but outcomes are variable, particularly those involving agonist-to-antagonist muscles. Here, we evaluated the possibility that lack of proprioceptive feedback reorganization and musculotendon adaptations could influence outcomes. Plantaris-to-tibialis anterior tendon transfer along with resection of the distal third of the tibialis anterior muscle belly was performed in eight cats. Four cats had concurrent transection of the deep peroneal nerve. After 15-20 weeks, intermuscular length and force-dependent sensory feedback were examined between hindlimb muscles, and the integrity of the tendon-to-tendon connection and musculotendon adaptations were evaluated. Three of the transferred tendons tore. A common finding was the formation of new tendinous connections, which often inserted near the original location of insertion on the skeleton (e.g., connections from plantaris toward calcaneus and from tibialis anterior toward first metatarsal). The newly formed tissue connections are expected to compromise the mechanical action of the transferred muscle. We found no evidence of changes in intermuscular reflexes between transferred plantaris muscle and synergists/antagonists whether the tendon-to-tendon connection remained intact or tore, indicating no spinal reflex reorganization. We propose the lack of spinal reflex reorganization could contribute the transferred muscle not adopting the activation patterns of the host muscle. Taken together, these findings suggest that musculotendon plasticity and lack of spinal reflex circuitry reorganization could limit functional outcomes after tendon transfer surgery. Surgical planning and outcomes assessments after tendon transfer surgery should consider potential consequences of the transferred muscle's intermuscular spinal circuit actions.

A Stretchable Microneedle Electrode Array for Stimulating and Measuring Intramuscular Electromyographic Activity.

We have developed a stretchablemicroneedle electrode array (sMEA) to stimulate andmeasure the electrical activity of muscle across multiple sites. The technology provides the signal fidelity and spatial resolution of intramuscular electrodesacross a large area of tissue. Our sMEA is composed of a polydimethylsiloxane (PDMS) substrate, conductive-PDMS traces, and stainless-steel penetrating electrodes. The traces and microneedles maintain a resistance of less than 10 [Formula: see text] when stretched up to a ~63% tensile strain, which allows for the full range of physiological motion of felinemuscle. The device and its constituent materials are cytocompatible for at least 28 days in vivo. When implanted in vivo, the device measures electromyographic (EMG) activity with clear compound motor unit action potentials. The sMEA also maintains a stable connection with moving muscle while electrically stimulating the tissue. This technology has direct application to wearable sensors, neuroprostheses, and electrophysiological studies of animals 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.

The mechanical actions of muscles predict the direction of muscle activation during postural perturbations in the cat hindlimb.

Humans and cats respond to balance challenges, delivered via horizontal support surface perturbations, with directionally selective muscle recruitment and constrained ground reaction forces. It has been suggested that this postural strategy arises from an interaction of limb biomechanics and proprioceptive networks in the spinal cord. A critical experimental validation of this hypothesis is to test the prediction that the principal directions of muscular activation oppose the directions responding muscles exert their forces on the environment. Therefore, our objective was to quantify the endpoint forces of a diverse set of cat hindlimb muscles and compare them with the directionally sensitive muscle activation patterns generated in the intact and decerebrate cat. We hypothesized that muscles are activated based on their mechanical advantage. Our primary expectation was that the principal direction of muscle activation during postural perturbations will be directed oppositely (180°) from the muscle endpoint ground reaction force. We found that muscle activation during postural perturbations was indeed directed oppositely to the endpoint reaction forces of that muscle. These observations indicate that muscle recruitment during balance challenges is driven, at least in part, by limb architecture. This suggests that sensory sources that provide feedback about the mechanical environment of the limb are likely important to appropriate and effective responses during balance challenges. Finally, we extended the analysis to three dimensions and different stance widths, laying the groundwork for a more comprehensive study of postural regulation than was possible with measurements confined to the horizontal plane and a single stance configuration.

Muscle spindle responses to horizontal support surface perturbation in the anesthetized cat: insights into the role of autogenic feedback in whole body postural control.

Intact cats and humans respond to support surface perturbations with broadly tuned, directionally sensitive muscle activation. These muscle responses are further sensitive to initial stance widths (distance between feet) and perturbation velocity. The sensory origins driving these responses are not known, and conflicting hypotheses are prevalent in the literature. We hypothesize that the direction-, stance-width-, and velocity-sensitive muscle response during support surface perturbations is driven largely by rapid autogenic proprioceptive pathways. The primary objective of this study was to obtain direct evidence for our hypothesis by establishing that muscle spindle receptors in the intact limb can provide appropriate information to drive the muscle response to whole body postural perturbations. Our second objective was to determine if spindle recordings from the intact limb generate the heightened sensitivity to small perturbations that has been reported in isolated muscle experiments. Maintenance of this heightened sensitivity would indicate that muscle spindles are highly proficient at detecting even small disturbances, suggesting they can provide efficient feedback about changing postural conditions. We performed intraaxonal recordings from muscle spindles in anesthetized cats during horizontal, hindlimb perturbations. We indeed found that muscle spindle afferents in the intact limb generate broadly tuned but directionally sensitive activation patterns. These afferents were also sensitive to initial stance widths and perturbation velocities. Finally, we found that afferents in the intact limb have heightened sensitivity to small perturbations. We conclude that muscle spindle afferents provide an array of important information about biomechanics and perturbation characteristics highlighting their potential importance in generating appropriate muscular response during a postural disturbance.