Tendon Organs Research Articles

Regulation of motor pattern frequency by reversals in proprioceptive feedback

Proprioceptive sensory feedback has important functions for motor pattern generation in which phasic negative and positive feedback is used to coordinate neural and musculoskeletal dynamics. Whether and how feedback sign regulates the motor patterns in behaviorally relevant closed-loop conditions has not been fully elucidated. We characterized the feedback provided by the anterior gastric receptor (AGR), a muscle tendon organ in the stomatogastric nervous system of the crab Cancer pagurus, to the gastric mill motor pattern in intact animals. AGR innervates the protractor muscles and was activated either during the protraction or retraction phase of the rhythm. Experiments with neuromuscular preparations imply that this was due to isometric contractions of the protractor muscles and their passive stretch by the antagonistic retractor muscles. As AGR excited the protractors and inhibited the retractors independently of the timing of its activation, the timing switch changed AGR feedback from positive to negative. We tested the effects of this change in feedback sign on the motor pattern in the isolated nervous system by activating AGR at the corresponding phases of the rhythm, using intracellular current injection. When AGR was activated during the protractor phase and provided positive feedback, it prolonged the burst activities of protractor and retractor neurons and slowed ongoing rhythms. When activated during the retraction phase and thus provided negative feedback, burst durations decreased and the rhythm cycle frequency increased. Our study thus shows that the cycle frequency of centrally generated activity patterns can be regulated by switching the sign of phasic proprioceptive feedback.

The inverted pendulum model of bipedal standing cannot be stabilized through direct feedback of force and contractile element length and velocity at realistic series elastic element stiffness

Control of bipedal standing is typically analyzed in the context of a single-segment inverted pendulum model. The stiffness K (SE) of the series elastic element that transmits the force generated by the contractile elements of the ankle plantarflexors to the skeletal system has been reported to be smaller in magnitude than the destabilizing gravitational stiffness K ( g ). In this study, we assess, in case K (SE) + K ( g ) < 0, if bipedal standing can be locally stable under direct feedback of contractile element length, contractile element velocity (both sensed by muscle spindles) and muscle force (sensed by Golgi tendon organs) to alpha-motoneuron activity. A theoretical analysis reveals that even though positive feedback of force may increase the stiffness of the muscle-tendon complex to values well over the destabilizing gravitational stiffness, dynamic instability makes it impossible to obtain locally stable standing under the conditions assumed.

Motor Unit Recruitment and Proprioceptive Feedback Decrease the Common Drive

It has been documented that concurrently active motor units fire under the control of a common drive. That is, the firing rates show high correlation with near-zero time lag. This degree of correlation has been found to vary among muscles and among contractions performed at different force levels in the same muscle. This study provides an explanation indicating that motor units recruited during a contraction cause an increase in the variation (SD) and a decrease in the degree (amplitude) of the correlation of the firing rates. The degree of correlation is lower in muscles having greater spindle density. This effect appears to be mediated by the proprioceptive feedback from the spindles and possibly the Golgi tendon organs. Muscle spindles in particular respond to the mechanical excitation of the nonfused muscle fibers and provide a discordant excitation to the homonymous motoneurons, resulting in a decrease in the correlation of the firing rates of motor units. The implication of this work is that the decreased correlation of the firing rates in some muscles is not necessarily an indication of a decreased common drive from the CNS, but rather an inhibitory influence of the proprioceptive feedback from the peripheral nervous system. This explanation is useful for understanding various manifestations of the common drive reported in the literature.

Interneuronal Activity in Reflex Pathways from Group II Muscle Afferents Is Monitored by Dorsal Spinocerebellar Tract Neurons in the Cat

The main aim of the study was to investigate whether group II muscle afferents contribute to the inhibition of dorsal spinocerebellar tract (DSCT) neurons and thereby modulate information provided by these neurons in the cat. In intracellular recordings, we found disynaptic IPSPs from group II afferents in the majority of DSCT neurons, most often in parallel with IPSPs evoked from group I afferents. In an attempt to identify interneurons that mediate these IPSPs, the second aim of the study, laminas IV-VII in midlumbar segments were searched for interneurons antidromically activated by stimuli applied within Clarke's column. Such interneurons were found in regions in which focal field potentials were evoked by group I and II afferents, or ventral to them, and most were coexcited by these afferents. The input to these interneurons and their location indicate that they belonged to the previously identified population of premotor interneurons in disynaptic pathways between group I and II afferents and hindlimb motoneurons. The study leads thus to the conclusion that inhibitory actions of group II afferents on DSCT neurons are collateral to actions on motoneurons and that DSCT neurons monitor inhibitory actions of group II afferents on motoneurons as closely as they monitor actions of group I afferents. The results also indicate that interneurons mediating disynaptic reflex actions from tendon organ (group Ib) afferents and those mediating disynaptic actions from secondary muscle spindle (group II) afferents to motoneurons may be parts and parcel of the same interneuronal population rather than constitute distinct interneuronal populations.

(120) Physiological relevance of a rodent model of degenerative disc disease

The American Pain Society estimates that 45% of the U.S. population seeks medical help for chronic pain at some point in their lives. Those affected include the 15% of Americans with persistent back pain. There are many potential sources including degenerative disc disease (DDD), which is one of the primary causes of chronic low back pain (LBP). Natural age related degeneration of lumbar intervertebral discs (IVDs) is common, but abnormal degeneration can occur, often resulting in pain, instability, and biomechanical movement disadvantages. IVDs are articular joints between vertebra and they act to distribute loads placed on the spine through hydrostatic pressures. The disc is composed of the exterior annulus fibrosus surrounded by ligaments and the central nucleus pulposus. The presence of nerve fibres and nerve endings in the lumbar IVDs and in the adjacent longitudinal ligaments has been demonstrated by many authors. In healthy discs, innervation is mostly limited to the posterior and anterior longitudinal ligaments and the superficial layers of the annulus fibrosus. Human IVDs are supplied by a variety of nerve fiber types including C- and A-delta fibers (substance P and CGRP), A-beta fibers (myelin basic protein) and sympathetic fibers entering from the sympathetic nerve truck. Specialized sensory nerve endings have also been described (i.e. pacinian corpuscles and golgi tendon organs). The goal of this study is to assess if the rat is of potential use to develop a relevant, rodent model of DDD-related LBP. Lumbar IVD innervation will be assessed in rats by immunohistochemistry to examine total innervation and fiber type(s). Preliminary results suggest that IVD innervation in rat and human are similar. Supported by grants from the Canadian Institute for Health Research (LSS) and the American Pain Society (MM). The American Pain Society estimates that 45% of the U.S. population seeks medical help for chronic pain at some point in their lives. Those affected include the 15% of Americans with persistent back pain. There are many potential sources including degenerative disc disease (DDD), which is one of the primary causes of chronic low back pain (LBP). Natural age related degeneration of lumbar intervertebral discs (IVDs) is common, but abnormal degeneration can occur, often resulting in pain, instability, and biomechanical movement disadvantages. IVDs are articular joints between vertebra and they act to distribute loads placed on the spine through hydrostatic pressures. The disc is composed of the exterior annulus fibrosus surrounded by ligaments and the central nucleus pulposus. The presence of nerve fibres and nerve endings in the lumbar IVDs and in the adjacent longitudinal ligaments has been demonstrated by many authors. In healthy discs, innervation is mostly limited to the posterior and anterior longitudinal ligaments and the superficial layers of the annulus fibrosus. Human IVDs are supplied by a variety of nerve fiber types including C- and A-delta fibers (substance P and CGRP), A-beta fibers (myelin basic protein) and sympathetic fibers entering from the sympathetic nerve truck. Specialized sensory nerve endings have also been described (i.e. pacinian corpuscles and golgi tendon organs). The goal of this study is to assess if the rat is of potential use to develop a relevant, rodent model of DDD-related LBP. Lumbar IVD innervation will be assessed in rats by immunohistochemistry to examine total innervation and fiber type(s). Preliminary results suggest that IVD innervation in rat and human are similar. Supported by grants from the Canadian Institute for Health Research (LSS) and the American Pain Society (MM).

Model-Based Sensorimotor Integration for Multi-Joint Control: Development of a Virtual Arm Model

An integrated, sensorimotor virtual arm (VA) model has been developed and validated for simulation studies of control of human arm movements. Realistic anatomical features of shoulder, elbow and forearm joints were captured with a graphic modeling environment, SIMM. The model included 15 musculotendon elements acting at the shoulder, elbow and forearm. Muscle actions on joints were evaluated by SIMM generated moment arms that were matched to experimentally measured profiles. The Virtual Muscle (VM) model contained appropriate admixture of slow and fast twitch fibers with realistic physiological properties for force production. A realistic spindle model was embedded in each VM with inputs of fascicle length, gamma static (gamma(stat)) and dynamic (gamma(dyn)) controls and outputs of primary (I(a)) and secondary (II) afferents. A piecewise linear model of Golgi Tendon Organ (GTO) represented the ensemble sampling (I(b)) of the total muscle force at the tendon. All model components were integrated into a Simulink block using a special software tool. The complete VA model was validated with open-loop simulation at discrete hand positions within the full range of alpha and gamma drives to extrafusal and intrafusal muscle fibers. The model behaviors were consistent with a wide variety of physiological phenomena. Spindle afferents were effectively modulated by fusimotor drives and hand positions of the arm. These simulations validated the VA model as a computational tool for studying arm movement control. The VA model is available to researchers at website http://pt.usc.edu/cel .

Characterization of sensory deficits in TrkB knockout mice

The sensory deficit in TrkB deficient mice was evaluated by counting the neuronal loss in lumbar dorsal root ganglia (DRG), the absence of sensory receptors (cutaneous--associated to the hairy and glabrous skin - muscular and articular), and the percentage and size of the neurocalcin-positive DRG neurons (a calcium-binding protein which labels proprioceptive and mechanoceptive neurons). Mice lacking TrkB lost 32% of neurons, corresponding to the intermediate-sized and neurocalcin-positive ones. This neuronal lost was accomplished by the absence of Meissner corpuscles, and reduction of hair follicle-associated sensory nerve endings and Merkel cells. The mutation was without effect on Pacinian corpuscles, Golgi's organs and muscle spindles. Present results further characterize the sensory deficit of the TrkB-/- mice demonstrating that the intermediate-sized neurons in lumbar DRG, as well as the cutaneous rapidly and slowly adapting sensory receptors connected to them, are under the control of TrkB for survival and differentiation. This study might serve as a baseline for future studies in experimentally induced neuropathies affecting TrkB positive DRG neurons and their peripheral targets, and to use TrkB ligands in the treatment of neuropathies in which cutaneous mechanoreceptors are primarily involved.

NMClab, a model to assess the contributions of muscle visco-elasticity and afferent feedback to joint dynamics

The dynamic behavior of a neuromusculoskeletal system results from the complex mechanical interaction between muscle visco-elasticity resulting from (co-)contraction and afferent feedback from muscle spindles and Golgi tendon organs. As a result of the multiple interactions the individual effect of each of the structures to the overall dynamics is hard to recognize, if not impossible. Here a neuromuscular control (NMC) model is developed to analyze the functional contribution of the various physiological structures on the mechanical behavior of a limb. The dynamics of a joint are presented in admittances, i.e. the dynamic relation between input force (or torque) and the output displacement, which can be represented by either frequency or impulse response functions. With the model it can be shown that afferent feedback reduces, while muscle visco-elasticity increases, the stability margins. This implicates that there is a delicate balance between muscle co-contraction and afferent feedback, which depends on the joint specific physiological properties. The main application of the model is educational; it is implemented in a graphical user interface allowing users to explore the role of the various physiological structures on joint dynamics. Other applications of the model are more experimental, e.g. to elucidate experimentally measured admittances and to compare the quantified parameter values with the theoretically optimal ones. It is concluded that the NMC model is a useful and intuitive tool to investigate human motor control, in a theoretical as well as an experimental way.

Modeling of One-Degree-of-Freedom Finger-Grasp System with Tendon Organ Reflex and Stretch Reflex

One of the most crucial issues in the force feedback micromanipulator is the dynamics of the nerve-musculoskeletal system of fingers. Micromanipulator is now actively applied in the medical field such as a brain surgery operation, a cancer test, etc. In this study, a mathematical model of the human finger grasp system was proposed on the basis of physiological findings as part of studies on the force feedback micromanipulator. Specifically, we considered the nerve-musculoskeletal control system of the fingers to be a mixed system of a length-control system using the muscle spindle and a tension-control system using the tendon organ. We proposed model of how these two reflex systems are mingling. Then, the model was evaluated through an experiment. The results revealed that the model in this study could reproduce the operation of the finger more faithfully than the conventional compliance model for fingers. Moreover, this study revealed that the refractory period characteristics of muscle in a nerve pulse transfer mechanism influence final value of contraction force.

2P1-G02 筋の生体力学的特性を考慮した神経筋骨格ネットワークのモデル化及び同定(脳科学・神経科学とロボティクス)

In this paper, we build a whole-body neuromusculoskeletal network model including somatic reflex, and identify its parameters through non-invasive measurements and statistical analysis. Our neuromuscular model consists of two parts. The first part models the neuromuscular network to represent the relationships between spinal nerve signals and muscle activities, which are then converted to muscle tensions using a physiological muscle dynamics model. The second part includes the feedback loops from muscle spindles and Golgi tendon organs to spinal nerves that represent the somatic reflex using muscle length, velocity, and tension information. We demonstrate the consistency of the model by showing that a forward dynamics simulation of somatic reflex yields a motion similar to the actual response of human.

Strabismus and eye muscle function

Studies of external eye muscle morphology and physiology are reviewed, with respect to both motor and sensory functions in concomitant strabismus. The eye muscles have a more complex fibre composition than other striated muscle, and they are among the fastest and most fatigue-resistant muscles in the body. However, it is not generally believed that concomitant strabismus is due to a primary abnormality of the eye muscles or the ocular motor system. The gross anatomy of eye muscles, including the shape and position of the eye muscle pulleys, was not changed in strabismus. The histology of the eye muscle fibres was also basically the same, but changes have been observed in the cellular and biochemical machinery of the fibres, most notably in the singly innervated orbital fibres. Functionally, this was seen as slower contractions and reduced fatigue resistance of eye muscles in animals with strabismus and defects of binocular vision. Most likely the changes represented an adaptation to modified visual demands on the ocular motor control, because of the defects of binocular vision in strabismus from an early age. Adaptation of eye muscle function to visual demands could be seen also in the adult human ocular motor system, but here the effects could be reversed with treatment in some conditions. External eye muscles in the human have sensory organs, muscle spindles and tendon organs, responding to changes in muscle force and length. It is not known how these proprioceptors are used more specifically in ocular motor control, and there is no stretch reflex in the external eye muscles. However, a clear influence on space localization and eye position can be demonstrated with vibratory stimulation of the eye muscles, presumably activating muscle spindles. Different effects were observed in normal subjects and in adult patients with strabismus, which would indicate that the proprioceptive input from one eye of strabismic patients could be suppressed by the other eye, similar to visual suppression in concomitant strabismus. Such an interaction would most likely occur in the visual cortex, and not in the ocular motor system. Further studies of proprioceptive mechanisms, during the postnatal developmental stage and in adult concomitant strabismus may shed light on the mechanisms of childhood strabismus and may, in this respect, be a more fruitful avenue for further research than eye motor studies.

Golgi Tendon Organs: Neuroscience Update with Relevance to Stretching and Proprioception in Dancers

This article reviews the neuroscience of the Golgi tendon organ, a type of muscle sensory receptor. The intent is also to update and correct erroneous notions regarding its functional significance, particularly related to its involvement in reflexes, proprioception, and stretching techniques used in dance training. Historical research about the function of the Golgi tendon organ (GTO) has led to four erroneous views: 1. The GTO serves a protective function by responding only to muscle tension at the extreme end-range of motion; 2. Sensations from GTOs do not reach the cerebral cortex and therefore serve no role in conscious proprioception and kines-thesia; 3. The GTO and its reflex circuit (autogenic inhibition) prevent its muscle from contracting by “turning it off”; and 4. During contract-relax stretching, the GTO autogenic inhibition reflex circuit is responsible for the muscle relaxation phase and the accompanying increased range of motion. Recent research has shown that these early views are inaccurate. There is new evidence regarding the role of the GTO. 1. A GTO responds to even weak active contractions throughout the range of motion, although GTOs are much less sensitive to passive tension than to active tension. 2. GTO sensations probably do reach the cerebral cortex, buried in the fissure between the motor and sensory cortex “strips.” This information allows dancers to know where their limbs are in space. GTOs also provide unconscious proprioception, via the cerebellum, which helps dancers to learn new motor skills and improve execution of movements. 3. The GTO and its reflex (autogenic inhibition) reduce, but do not shut off, the excitability of the motor neuron and its innervated muscle. The ultimate reflex effect depends on the sum of multiple other inputs on the motor neuron. GTO input may reduce muscle activity, but does not “turn it off.” 4. During contract-relax stretching, the GTO autogenic reflex creates inhibition that lasts only as long as the muscle's active contraction. Therefore, GTO activity is unlikely to significantly affect the subsequent relaxation phase of the stretch. Other receptors and spinal circuits likely serve this function. It is hoped that this updated information will correct several erroneous notions about GTOs that have persisted in the dance kinesiology literature. Thus, dance kinesiology teachers, researchers, and health practitioners can provide accurate information to their students, readers, and patients.

Reduced oxygen tension attenuates differentiation capacity of human mesenchymal stem cells and prolongs their lifespan

Mesenchymal stem cells (MSC) are capable of differentiating into bone, fat, cartilage, tendon and other organ progenitor cells. Despite the abundance of MSC within the organism, little is known about their in vivo properties or about their corresponding in vivo niches. We therefore isolated MSC from spongy (cancellous) bone biopsies of healthy adults. When compared with the surrounding marrow, a fourfold higher number of colony-forming units was found within the tight meshwork of trabecular bone surface. At these sites, oxygen concentrations range from 1% to 7%. In MSC cultured at oxygen as low as 3%, rates for cell death and hypoxia-induced gene transcription remained unchanged, while in vitro proliferative lifespan was significantly increased, with about 10 additional population doublings before reaching terminal growth arrest. However, differentiation capacity into adipogenic progeny was diminished and no osteogenic differentiation was detectable at 3% oxygen. In turn, MSC that had previously been cultured at 3% oxygen could subsequently be stimulated to successfully differentiate at 20% oxygen. These data support our preliminary finding that primary MSC are enriched at the surface of spongy bone. Low oxygen levels in this location provide a milieu that extends cellular lifespan and furthermore is instructive for the stemness of MSC allowing proliferation upon stimulation while suppressing differentiation.

Ethanol reduces motoneuronal excitability and increases presynaptic inhibition of Ia afferents in the human spinal cord

Introduction: Already low blood concentrations of ethanol acutely impair motor control and coordination. In vitro experiments have given evidence that spinal effects of ethanol contribute to this by reducing spinal excitability and enhancing presynaptic inhibition of Ia fibers. In this study, we investigated the influence of 0.7 g per kilogram of bodyweight ethanol on motoneuronal excitability and presynaptic inhibition in humans. Methods: The study was performed in 10 volunteers. Spinal excitability was measured by the maximal H-reflex of the soleus muscle normalized to the maximal muscular response (Hmax/Mmax). Presynaptic inhibition was measured by changes in heteronymous Ia-facilitation of the soleus H-reflex, which is achieved by stimulation of the femoral nerve. A decrease in facilitation can be ascribed to an increase in presynaptic inhibition. Changes of these parameters under the influence of 0.7 g per kilogram of bodyweight ethanol were assessed in comparison to control measurements before ethanol application. Results: Both parameters, Hmax/Mmax and Heteronymous facilitation, were significantly reduced under the influence of ethanol (Wilcoxon signed-rank test with Bonferroni correction for each, p < 0.01). Discussion: The increase in presynaptic inhibition by ethanol is probably caused by an increase in GABA A receptor-mediated Cl-conductance, which has been shown in spinal cord cultures. The role of presynaptic inhibition in movement is assumed to be there to control the afferent input of muscle spindles and tendon organs as a mechanism of specific input-selection. This study demonstrated that ethanol reduces spinal excitability and increases GABAergic presynaptic inhibition on Ia afferent fibers in humans.

Adaptations in the structure and innervation of follicle‐sinus complexes to an aquatic environment as seen in the Florida manatee (Trichechus manatus latirostris)

Florida manatees are large-bodied aquatic herbivores that use large tactile vibrissae for several purposes. Facial vibrissae are used to forage in a turbid water environment, and the largest perioral vibrissae can also grasp and manipulate objects. Other vibrissae distributed over the entire postfacial body appear to function as a lateral line system. All manatee vibrissae emanate from densely innervated follicle-sinus complexes (FSCs) like those in other mammals, although proportionately larger commensurate with the caliber of the vibrissae. As revealed by immunofluorescence, all manatee FSCs have many types of C, Adelta and Abeta innervation including Merkel, club, and longitudinal lanceolate endings at the level of the ring sinus, but they lack other types such as reticular and spiny endings at the level of the cavernous sinus. As in non-whisking terrestrial species, the inner conical bodies of facial FSCs are well innervated but lack Abeta-fiber terminals. Importantly, manatee FSCs have two unique types of Abeta-fiber endings. First, all of the FSCs have exceptionally large-caliber axons that branch to terminate as novel, gigantic spindle-like endings located at the upper ring sinus. Second, facial FSCs have smaller caliber Abeta fibers that terminate in the trabeculae of the cavernous sinus as an ending that resembles a Golgi tendon organ. In addition, the largest perioral vibrissae, which are used for grasping, have exceptionally well-developed medullary cores that have a structure and dense small-fiber innervation resembling that of tooth pulp. Other features of the epidermis and upper dermis structure and innervation differ from that seen in terrestrial mammals.

Wrist Splint for Upper Motor Neuron Paralysis

To the Editor: There are a number of concerns with the study published by Stroke written by Lannin et al.1 The concerns are: (1) the randomization; (2) the title; (3) the sample of stroke participants; (4) the volar splint; and (5) the splinting groups with the wrist and fingers in a “neutral” and in an “extended position.” Randomization was based on the diagnosis and time from onset, not the degree of spasticity. Therefore, it is obvious that change in flexor tendon extensibility and possible effectiveness of splinting were not significant. The randomization should be based on the degree of spasticity because it is independent of the time from onset. It was vital that the intervention groups in the study, that is “neutral” and “45° extended,” were first classified based on the degree of spasticity as …

Cutaneous mechanoreceptors contribute to the generation of a cortical reflex in speech

Owing to the lack of muscle spindles and tendon organs in the perioral system, cutaneous receptors may contribute to speech sensorimotor processes. We have investigated this possibility in the context of upper lip reflexes, which we have induced by unexpectedly stretching the facial skin lateral to the oral angle. Skin stretch at this location resulted in long latency reflex responses that were similar to the cortical reflexes observed previously. This location reliably elicited the reflex response, whereas the skin above the oral angle and the skin on the cheek did not. The data suggest that cutaneous mechanoreceptors are narrowly tuned to deformation of the facial skin and provide kinesthetic information for rapid sensorimotor processing in speech.

Mechanotransduction and the crayfish stretch receptor

Mechanotransduction or mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates. Mechanosensitivity covers a wide spectrum of functions from osmosensing, cell attachment, classical sensory mechanisms like tactile senses in the skin, detection of sound in hair cells of the hearing apparatus, proprioceptive functions like recording of muscle length and tension in the muscle spindle and tendon organ, respectively, and pressure detection in the circulation etc. Since most development regarding the molecular aspects of the mechanosensitive channel has been made in nonsensory systems it is important to focus on mechanosensitivity of sensory organs where the functional importance is undisputed. The stretch receptor organ of the crustaceans is a suitable preparation for such studies. The receptor organ is experimentally accessible to mechanical manipulation and electrophysiological recordings from the sensory neuron using intracellular microelectrode or patch clamp techniques. It is also relatively easy to inject substances into the neuron, which also makes the neuron accessible to measurements with fluorescent techniques. The aim of the present paper is to give an up to date summary of observations made on the transducer properties of the crayfish stretch receptor ( Astacus astacus and Pacifastacus leniusculus) including some recent unpublished findings. Finally some aspects on future line of research will be presented.

Positive force feedback in human walking

The objective of this study was to determine if load receptors contribute to the afferent-mediated enhancement of ankle extensor muscle activity during the late stance phase of the step cycle. Plantar flexion perturbations were presented in late stance while able-bodied human subjects walked on a treadmill that was declined by 4%, inclined by 4% or held level. The plantar flexion perturbation produced a transient, but marked, presumably spinally mediated decrease in soleus EMG that varied directly with the treadmill inclination. Similarly, the magnitude of the control step soleus EMG and Achilles' tendon force also varied directly with the treadmill inclination. In contrast, the ankle angular displacement and velocity were inversely related to the treadmill inclination. These results suggest that Golgi tendon organ feedback, via the group Ib pathway, is reduced when the muscle-tendon complex is unloaded by a rapid plantar flexion perturbation in late stance phase. The changes in the unload response with treadmill inclination suggest that the late stance phase soleus activity may be enhanced by force feedback.

Vibration sensitivity of human muscle spindles and golgi tendon organs

The responses of the various muscle receptors to vibration are more complicated than a naïve categorization into stretch (muscle spindle primary ending), length (muscle spindle secondary endings), and tension (Golgi tendon organs) receptors. To emphasize the similarity of responses to small length changes, we recorded from 58 individual muscle afferents subserving receptors in the ankle or toe dorsiflexors of awake human subjects (32 primary endings, 20 secondary endings, and six Golgi tendon organs). Transverse sinusoidal vibration was applied to the distal tendon of the receptor-bearing muscle, while subjects either remained completely relaxed or maintained a weak isometric contraction of the appropriate muscle. In relaxed muscle, few units responded in a 1:1 manner to vibration, and there was no evidence of a preferred frequency of activation. In active muscle the response profiles of all three receptor types overlapped, with no significant difference in threshold between receptor types. These results emphasize that when intramuscular tension increases during a voluntary contraction, Golgi tendon organs and muscle spindle secondary endings, not just muscle spindle primary endings, can effectively encode small imposed length changes.