ON THE SILENT PERIOD AND GOLGI TENDON ORGANS OF THE SOLEUS MUSCLE OF THE CAT

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.

Quadriceps muscle stimulation evokes heteronymous inhibition onto soleus with limited Ia activation compared to femoral nerve stimulation

Heteronymous excitatory feedback from muscle spindles and inhibitory feedback from Golgi tendon organs and recurrent inhibitory circuits can influence motor coordination. The functional role of inhibitory feedback is difficult to determine, because nerve stimulation, the primary method used in humans, cannot evoke inhibition without first activating the largest diameter muscle spindle axons. Here, we tested the hypothesis that quadriceps muscle stimulation could be used to examine heteronymous inhibition more selectively when compared to femoral nerve stimulation by comparing the effects of nerve and muscle stimulation onto ongoing soleus EMG held at 20% of maximal effort. Motor threshold and two higher femoral nerve and quadriceps stimulus intensities matched by twitch evoked torque magnitudes were examined. We found that significantly fewer participants exhibited excitation during quadriceps muscle stimulation when compared to nerve stimulation (14-29% vs. 64-71% of participants across stimulation intensities) and the magnitude of heteronymous excitation from muscle stimulation, when present, was much reduced compared to nerve stimulation. Muscle and nerve stimulation resulted in heteronymous inhibition that significantly increased with increasing stimulation evoked torque magnitudes. This study provides novel evidence that muscle stimulation may be used to more selectively examine inhibitory heteronymous feedback between muscles in the human lower limb when compared to nerve stimulation.

Effects of different modalities of afferent stimuli of the lumbo-sacral area on control of lumbar paravertebral muscles

Somatosensory feedback to the central nervous system is essential to plan, perform and refine spine motor control. However, the influence of somatosensory afferent input from the trunk on the motor output to trunk muscles has received little attention. The objective was to compare the effects of distinct modalities of afferent stimulation on the net motoneuron and corticomotor excitability of paravertebral muscles. Fourteen individuals were recruited. Modulation of corticospinal excitability (motor-evoked potential [MEP]) of paravertebral muscles was measured when afferent stimuli (cutaneous noxious and non-noxious, muscle contraction) were delivered to the trunk at 10 intervals prior to transcranial magnetic stimulation. Each peripheral stimulation was applied alone, and subsequent EMG modulation was measured to control for net motoneuron excitability. MEP modulation and MEP/EMG ratio were used as measures of corticospinal excitability with and without control of net motoneuron excitability, respectively. MEP and EMG modulation were smaller after evoked muscle contraction than after cutaneous noxious and non-noxious stimuli. MEP/EMG ratio was not different between stimulation types. Both MEP and EMG amplitudes were reduced after evoked muscle contraction, but not when expressed as MEP/EMG ratio. Noxious and non-noxious stimulation had limited impact on all variables. Distinct modalities of peripheral afferent stimulation of the lumbo-sacral area differently modulated responses of paravertebral muscles, but without an influence on corticospinal excitability with control of net motoneuron excitability. Muscle stimulation reduced paravertebral activity and was best explained by spinal mechanisms. The impact of afferent stimulation on back muscles differs from the effects reported for limb muscles.

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.

EMG-Torque Dynamics Change With Contraction Bandwidth

An accurate model for ElectroMyoGram (EMG)-torque dynamics has many uses. One of its applications which has gained high attention among researchers is its use, in estimating the muscle contraction level for the efficient control of prosthesis. In this paper, the dynamic relationship between the surface EMG and torque during isometric contractions at the human ankle was studied using system identification techniques. Subjects voluntarily modulated their ankle torque in dorsiflexion direction, by activating their tibialis anterior muscle, while tracking a pseudo-random binary sequence in a torque matching task. The effects of contraction bandwidth, described by torque spectrum, on EMG-torque dynamics were evaluated by varying the visual command switching time. Nonparametric impulse response functions (IRF) were estimated between the processed surface EMG and torque. It was demonstrated that: 1) at low contraction bandwidths, the identified IRFs had unphysiological anticipatory (i.e., non-causal) components, whose amplitude decreased as the contraction bandwidth increased. We hypothesized that this non-causal behavior arose, because the EMG input contained a component due to feedback from the output torque, i.e., it was recorded from within a closed-loop. Vision was not the feedback source since the non-causal behavior persisted when visual feedback was removed. Repeating the identification using a nonparametric closed-loop identification algorithm yielded causal IRFs at all bandwidths, supporting this hypothesis. 2) EMG-torque dynamics became faster and the bandwidth of system increased as contraction modulation rate increased. Thus, accurate prediction of torque from EMG signals must take into account the contraction bandwidth sensitivity of this system.

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.

Perception of Movement Patterns. Recognition from Visual Arrays of Distorted Patterns

A review of literature indicates that the perception and memory of movement patterns with kinaesthesis might be expected to differ depending on whether a movement pattern is actively commanded or passively induced. In addition, it has been suggested that kinaesthesis is a gross sense in the absence of vision. An attempt was made to demonstrate differences between the perception and memory of actively commanded movement patterns, passively induced movement patterns, and visual patterns formed by a moving light. A visual recognition test was employed. While it was apparent that the recognition task was a sensitive one for testing the accuracy of pattern perception, no significant difference was demonstrated between the three movement conditions.

The spinal reflex cannot be perceptually separated from voluntary movements

Both voluntary and involuntary movements activate sensors in the muscles, skin, tendon and joints. As limb movement can result from a mixture of spinal reflexes and voluntary motor commands, the cortical centres underlying conscious proprioception might either aggregate or separate the sensory inputs generated by voluntary movements from those generated by involuntary movements such as spinal reflexes. We addressed whether healthy volunteers could perceive the contribution of a spinal reflex during movements that combined both reflexive and voluntary contributions. Volunteers reported the reflexive contribution in leg movements that were partly driven by the knee-jerk reflex induced by a patellar tendon tap and partly by voluntary motor control. In one condition, participants were instructed to kick back in response to a tendon tap. The results were compared to reflexes in a resting baseline condition without voluntary movement. In a further condition, participants were instructed to kick forwards after a tap. Volunteers reported the perceived reflex contribution by repositioning the leg to the perceived maximum displacement to which the reflex moved the leg after each tendon tap. In the resting baseline condition, the reflex was accurately perceived. We found a near-unity slope of linear regressions of perceived on actual reflexive displacement. Both the slope value and the quality of regression fit in individual volunteers were significantly reduced when volunteers were instructed to generate voluntary backward kicks as soon as they detected the tap. In the kick forward condition, kinematic analysis showed continuity of reflex and voluntary movements, but the reflex contribution could be estimated from electromyography (EMG) recording on each trial. Again, participants' judgements of reflexes showed a poor relation to reflex EMG, in contrast to the baseline condition. In sum, we show that reflexes can be accurately perceived from afferent information. However, the presence of voluntary movement significantly impairs reflex perception. We suggest that perceptual separation between voluntary and reflex movement is poor at best. Our results imply that the brain has no clear marker for perceptually separating voluntary and involuntary movement. Attribution of body movement to voluntary or involuntary motor commands is surprisingly poor when both are present.

Presynaptic inhibition evoked by muscle contraction

1. Contractions of flexor and extensor muscles of the knee and ankle were used to investigate presynaptic inhibition at the spinal level.2. Contractions evoked dorsal root potentials, and increased the excitability of the central terminals of group Ia, Ib and low threshold cutaneous primary afferent fibres.3. The monosynaptic reflexes recorded in response to stimulation of flexor or extensor muscle nerves were depressed, in the presence of strychnine hydrochloride 0.1 mg/kg I.V., by the contractions.4. It is suggested that these presynaptic inhibitory effects are largely due to the activation of Golgi tendon organs by contraction.

Muscle spindles and Golgi tendon organs in bovine calf extraocular muscle studied by means of double-fluorescent labeling, electron microscopy, and three-dimensional reconstruction

In the present study muscle spindles (MSps) and Golgi tendon organs (GTOs) in bovine extraocular muscles (EOMs) were analyzed in detail. The innervation pattern of these proprioceptors was investigated with transmission electron microscope and confocal laser scanning microscope after double-fluorescent labeling. Three-dimensional (3D) reconstructions were performed of GTOs. Muscle spindles. MSps are numerous, each containing two nuclear bag fibers and up to eight nuclear chain fibers. In the equatorial region and paraequatorial region thin axons enwrapping the intrafusal muscle fibers form numerous nerve contacts on the muscle fiber surface. Double staining of such nerve terminals with synaptophysin and alpha-bungarotoxin and their fine structural features confirm their sensory nature. In the encapsulated part of the polar region neuromuscular contacts have structural features of motor nerve terminals and stain positively with alpha-bungarotoxin. Golgi tendon organs. GTOs are numerous in bovine EOMs. Each GTO contains collagen bundles but frequently also intracapsular muscle fibers. Intracapsular muscle fibers either terminate inside the GTO in collagen bundles or pass through the proprioceptor. GTOs are richly supplied with sensory nerve terminals which intermingle with the collagen bundles. Nerve terminals on intracapsular muscle fibers exhibit fine structural characteristics of motor nerve terminals and are alpha-bungarotoxin positive. The 3D images of GTOs show the detailed spatial arrangement of the GTO tissue components. These new insights in the complex and specific morphology of MSps and GTOs in bovine EOMs indicate that we deal with highly developed proprioceptors. These are supposed to provide important information for EOM innervation.

Give proprioceptors a chance

The theme of this review is that it is inappropriate to regard proprioceptors as general purpose transducers of system variables associated with movements. We should not try to describe their properties by general expressions derived by testing with a wide range of externally applied disturbances, in the way that is customary in engineering practice. Instead, if study is concentrated on their behaviour during natural active movements such as locomotion, then the significance of the signals which they feed back to the CNS is much easier to understand. This idea is developed briefly for tendon organs, and then in more detail for muscle spindles in locomotion.

The transduction properties of intercostal muscle mechanoreceptors

BACKGROUND

Intercostal muscles are richly innervated by mechanoreceptors. In vivo studies of cat intercostal muscle have shown that there are 3 populations of intercostal muscle mechanoreceptors: primary muscle spindles (1 degrees ), secondary muscle spindles (2 degrees ) and Golgi tendon organs (GTO). The purpose of this study was to determine the mechanical transduction properties of intercostal muscle mechanoreceptors in response to controlled length and velocity displacements of the intercostal space. Mechanoreceptors, recorded from dorsal root fibers, were localized within an isolated intercostal muscle space (ICS). Changes in ICS displacement and the velocity of ICS displacement were independently controlled with an electromagnetic motor. ICS velocity (0.5 - 100 microm/msec to a displacement of 2,000 microm) and displacement (50-2,000 microm at a constant velocity of 10 microm/msec) parameters encompassed the full range of rib motion.

RESULTS

Both 1 degrees and 2 degrees muscle spindles were found evenly distributed within the ICS. GTOs were localized along the rib borders. The 1 degrees spindles had the greatest discharge frequency in response to displacement amplitude followed by the 2 degrees afferents and GTOs. The 1 degrees muscle spindles also possessed the greatest discharge frequency in response to graded velocity changes, 3.0 spikes x sec(-1)/microm x msec(-1). GTOs had a velocity response of 2.4 spikes x sec(-1)/microm x msec(-1) followed by 2 degrees muscle spindles at 0.6 spikes x sec(-1)/microm x msec(-1).

CONCLUSION

The results of this study provide a systematic description of the mechanosenitivity of the 3 types of intercostal muscle mechanoreceptors. These mechanoreceptors have discharge properties that transduce the magnitude and velocity of intercostal muscle length.

Muscle spindle-derived neurotrophin 3 regulates synaptic connectivity between muscle sensory and motor neurons

Ia afferents induce the formation of muscle spindles prenatally and maintain them postnatally. To address whether spindles, in turn, regulate the function of Ia afferents, we examined Egr3-null mutant mice (Egr3-/-), in which muscle spindles degenerate progressively after birth. Egr3-/- mice develop gait ataxia, scoliosis, resting tremors, and ptosis, suggesting a defect in proprioception. Despite the normal morphological appearance of peripheral and central sensory projections, we observed a profound functional deficit in the strength of sensory-motor connections in Egr3-/- mice. Muscle spindles in Egr3-/- mice do not express NT3. Intramuscular injections of NT3 to Egr3-/- mice during the postnatal period restored sensory-motor connections. Thus, NT3 derived from muscle spindles regulates the synaptic connectivity between muscle sensory and motor neurons.

ETS gene Er81 controls the formation of functional connections between group Ia sensory afferents and motor neurons

The connections formed between sensory and motor neurons (MNs) play a critical role in the control of motor behavior. During development, the axons of proprioceptive sensory neurons project into the spinal cord and form both direct and indirect connections with MNs. Two ETS transcription factors, ER81 and PEA3, are expressed by developing proprioceptive neurons and MNs, raising the possibility that these genes are involved in the formation of sensory-motor connections. Er81 mutant mice exhibit a severe motor discoordination, yet the specification of MNs and induction of muscle spindles occurs normally. The motor defect in Er81 mutants results from a failure of group Ia proprioceptive afferents to form a discrete termination zone in the ventral spinal cord. As a consequence there is a dramatic reduction in the formation of direct connections between proprioceptive afferents and MNs. ER81 therefore controls a late step in the establishment of functional sensory-motor circuitry in the developing spinal cord.

On the adequate stimulus for rectal mechanoreception and perception: a study in cat and humans

The adequate stimulus that is specific for both rectal mechanoreceptor excitation and rectal perception is still undefined. Using a visual analogue scale, healthy male volunteer subjects rated the intensity of the non-noxious 'pressure sensation evoked by slow balloon-induced distensions of the rectum. In a parallel study, the responses of spinal afferents originating from intramural mechanoreceptors of the rectum to the same stimulus were recorded in decerebrate cats. Both receptor activity and sensation intensity were linearly related to the diameter of the rectum, which is in turn a linear function of the tangential length of the rectal wall. In contrast, both saturated when expressed as a function of intrarectal pressure or rectal wall tension. It is concluded that the perception associated with rectal dis tensions in the non-noxious range is mediated by intramural mechanoreceptors that linearly encode tangential wall length, and that the underlying information is linearly transmitted throughout the CNS.

Load-regulating mechanisms in gait and posture: comparative aspects

How is load sensed by receptors, and how is this sensory information used to guide locomotion? Many insights in this domain have evolved from comparative studies since it has been realized that basic principles concerning load sensing and regulation can be found in a wide variety of animals, both vertebrate and invertebrate. Feedback about load is not only derived from specific load receptors but also from other types of receptors that previously were thought to have other functions. In the central nervous system of many species, a convergence is found between specific and nonspecific load receptors. Furthermore, feedback from load receptors onto central circuits involved in the generation of rhythmic locomotor output is commonly found. During the stance phase, afferent activity from various load detectors can activate the extensor part in such circuits, thereby providing reinforcing force feedback. At the same time, the flexion is suppressed. The functional role of this arrangement is that activity in antigravity muscles is promoted while the onset of the next flexion is delayed as long as the limb is loaded. This type of reinforcing force feedback is present during gait but absent in the immoble resting animal.

Two groups of Golgi tendon organs in cat tibial anterior muscle identified from the discharge frequency recorded under a ramp-and-hold stretch

Twenty Golgi tendon organs (GTOs) from the tibial anterior muscle of the cat were investigated under a ramp-and-hold stretch of the passive muscle. The stretch rate was varied between 1 and 100 mm/s, the stretch amplitude between 0.1 and 7 mm, the prestretch of the muscle between 0 and 12 mm. The action potential sequences of the GTOs were recorded, and discharge patterns derived from them. The basic discharge frequencies, namely the initial frequency, the peak dynamic discharge, the maximum static value, and the final static value, were read from each discharge pattern. The tension of the muscle was determined at the same points in time at which one of the basic discharge frequencies was read from a discharge pattern. The static and dynamic properties of the GTOs were determined from the basic discharge frequencies. Two groups of GTOs were identified. Four GTOs discharged with an initial frequency and at the same time had static properties of small magnitude. Sixteen GTOs showed no initial activity and had static properties of large magnitude. The two groups of GTOs did not differ in their dynamic properties. The number of alpha-fibers activating a single GTO was determined from a further 11 GTOs. Eight GTOs without initial activity were activated by a mean number of 9.7 alpha-fibers. Three GTOs discharging with initial activity were activated by a mean number of 15.3 alpha-fibers. The two mean values were significantly different (p=0. 02). The identification of two groups of GTOs is explained by the GTOs being positioned differently within the muscle and its tendon.

The mixed nerve silent period is prolonged during a submaximal contraction sustained to failure

The purpose of this study was to determine the effect of a sustained contraction of vastus lateralis on the silent period (SP) in the surface electromyogram (EMG) following direct neural stimulation. Five men and 5 women performed isometric knee extension at 30% maximal voluntary contraction (MVC) to the limit of endurance. During the contraction, EMG increased, and superimposed twitch amplitude and time to peak tension decreased, but the SP duration did not change. After 10 min of recovery, MVC had returned to its initial value, and the potentiated twitch amplitude was 70% of initial value, but the SP was now 11% shorter. Based on these results, we hypothesize that during a sustained contraction of 30% MVC, the increase in central drive may have been offset by inhibitory input from the periphery, but after 10 min of recovery the SP was shortened because of increased central drive. This aspect of the SP's behavior should be taken into account whenever it is employed as a diagnostic tool.

Topography of mechanoreceptors in the shoulder joint region--a computer-aided 3D reconstruction in the laboratory mouse

BACKGROUND

We investigated the pattern of distribution of corpuscular sensory nerve endings in the shoulder region of the laboratory mouse in relation to their functional properties.

METHODS

Twelve adult female white NMRI-F2-mice were used. The topography of sensory nerve endings in the shoulder joint region was reconstructed by three-dimensional image processing by using serial silver-stained sections of paraffin-embedded samples. Semithin sections obtained from additional samples were used for light microscopy.

RESULTS

Within the fibrous layer of the joint capsule, three types of mechanoreceptors were identified: small lamellated corpuscles of the Pacini type, Ruffini corpuscles, and Golgi tendon organs. Intracapsular small lamellated corpuscles of the Pacini type (in an average number of 29/joint) were found mainly in three areas: in the predominantly flaccid tissue of the axillary region, in the denser ventromedial parts of the capsule, close to the scapula, and in the tight texture of the fiber bundles near the glenoid labrum. Ruffini corpuscles were identified only in small numbers (2/joint) in the ventral aspect of the articular capsule of two animals. Golgi tendon organs (14 or 15 receptors/joint) were discovered predominantly in close vicinity to the joint capsule at the muscle tendon junction of the inserting rotator cuff muscles and in the biceps brachii and triceps brachii muscles.

CONCLUSIONS

In view of their location in the shoulder joint capsule and the glenoid labrum, corpuscular mechanoreceptors evidently play an important role in joint control by inducing protective reflex actions in phases of extreme or abnormal movement. The density of sensory receptors in distinct areas of the shoulder joint capsule appears to be related to zones that are subjected to increased biomechanical stress during physical activity.

The sensory innervation of the shoulder joint of the mouse

The ultrastructure and location of sensory nerve endings in the shoulder-joint capsule, its tendinous reinforcements and in the periarticular connective and muscle tissue have been studied by means of light and electron microscopy in adult female white NMRI-F2 laboratory mice, aged 2.5-13 months. Most of the sensory nerve endings were detected in the fibrous layer of the joint capsule or in the inserting tendons. The identified lamellated corpuscles of the Pacini type are small and sometimes associated with Golgi tendon-organs. Large Vater-Pacini corpuscles were not detected. Ruffini corpuscles are found in small numbers only in the moderately dense connective tissue of the joint capsule. Golgi tendon organs were found mainly at the muscle-tendon junction of the muscles surrounding the joint. Muscle spindles have been identified mainly in periarticular muscles close to the muscle-tendon junctions. The number and distribution of the different types of mechanoreceptors investigated in the present study suggest that periarticular corpuscular sensory nerve endings play an important role in shoulder-joint control and mobility. The occurrence of small uniformly shaped lamellated corpuscles of the Pacini type in qualitatively different areas of surrounding tissue implies that they are susceptible to different kinds of mechanical stimuli.

Coding of pulsatile motor output by human muscle afferents during slow finger movements

1. Impulse activities of thirty-eight muscle spindle and tendon organ afferents from the finger extensor muscles were recorded in the radial nerve of human subjects while the subjects performed voluntary flexion and extension finger movements at a single metacarpophalangeal joint. 2. The afferent firing was analysed in relation to the 8-10 Hz discontinuities which previously have been shown to characterize these movements. Spike-triggered averaging and frequency domain analyses demonstrated that all Ia muscle spindle afferents and a large proportion of group II spindle afferents responded in close association with local peaks in the joint acceleration. During muscle lengthening the impulses appeared during phases of rapid muscle stretch, whereas they appeared during the phase of minimal speed during muscle shortening. 3. The Golgi tendon organ (Ib) afferents displayed a reverse pattern of activity in relation to the discontinuities, i.e. the impulses tended to appear in the phase of minimal speed during lengthening movements and close to maximal shortening speed during shortening movements. Hence, their firing often coincided with the phasic increases of the parent muscle activity which account for the 8-10 Hz discontinuities. 4. A close analysis of the time relations between spindle firing and the kinematics of the 8-10 Hz discontinuities revealed that the population spindle response was too delayed and too dispersed to support the hypothesis that the discontinuities are accounted for by the stretch reflex. 5. If, as suggested in a previous paper, the 8-10 Hz discontinuities are produced by a pulsatile descending motor command, the coding of the periodic but tenuous kinematic events by the population of proprioceptors may have a role in relation to an alleged pulsatile command generator.

Changes in fusimotor discharge rate provoked by isotonic fatiguing muscle contractions in decerebrate cats

Changes in discharge rate of 26 fusimotor neurones to medial gastrocnemius muscle were studied during isotonic fatiguing contractions of lateral gastrocnemius and soleus muscles in decerebrate cats. Muscle contractions were elicited by either continuous or repetitive electrical stimulation of the muscle nerves. The muscles were considered to be fatigued when, against a load equal to one third of the tension developed at the onset of an isometric contraction, i.e. to the tension indicating isometrically-induced fatigue, they returned to the length at which the isometric contraction was elicited. At the onset of muscle contraction an increase in discharge rate, lasting for 5-220 s, occurred in all except one of the neurones. In 73% of the units a late increase developed in addition in parallel with muscle fatigue outlasting the contraction for 5-180 s. All but one of the remaining neurones exhibited a short lasting burst of spike discharges coincident with the end of contraction. Enhancement of the late increase by muscle ischaemia indicates contribution of chemosensitive small-diameter muscle afferents, while the short lasting burst is supposed to be elicited rather by the mechanosensitive units sensitized by metabolic products liberated during contraction and/or fatigue. Differences of the fusimotor reflex responses to isotonic vs. isometric contraction and/or fatigue and their possible functional role are discussed.

Static sensitivity of tendon organs to tetanic contraction of in-series motor units in feline peroneus tertius muscle

1. The results of several studies have indicated an absence of any consistent relationship between the discharge of Ib Golgi tendon organ afferents and the steady-state tetanic tension generated by activating motor units. This question has been re-examined by recording the responses of individual tendon organs to tetanic, isometric contractions of one or more motor units from the peroneus tertius muscle of anaesthetized cats. 2. In three experiments, seventy-three individual tendon organ-motor unit interactions were recorded. The motor units were stimulated at 30-150 s-1 and for each tendon organ-motor unit pair a linear relationship was found between steady-state tetanic tension and Ib afferent discharge. The slopes of these relationships (the static sensitivities) were steepest for the weakest units. 3. When motor units were stimulated in combination, the relationship between discharge frequency and plateau tension was again linear but the static sensitivities were generally much lower than for single units and approached 1 impulse s-1 g-1. 4. Expression of these relationships in terms of the relative tensions generated revealed that the tendon organs were activated most strongly by the IIb muscle fibres, the static sensitivities being reduced by unloading effects. 5. The linear relationships observed during stimulation of single, and groups of, motor units suggest that the patterns of discharge from the tendon organs can mirror the steady-state contractile tensions within the muscle.

The effect of therapeutic muscle stretch on neural processing

Therapeutic muscle stretch is a commonly used procedure despite little evidence in support of efficacy or information about the mechanisms underlying the various methods. The purpose of this work was to compare the sequential application of static and ballistic muscle stretch with static muscle stretch alone, using the electrically elicited Hoffmann reflex (H-reflex) as a measure of excitability of homonymous motoneurons. The foot was passively dorsiflexed and either maintained in this position or rapidly and repeatedly dorsiflexed at a velocity of 1.0 radian/sec. Hoffmann reflexes were taken using established criteria under control conditions and during stretch conditions. An analysis of variance indicated a significant difference (p < 0.05) between condition means, with H-reflex amplitude reducing to 60 and 15% of the control value during static and ballistic stretch, respectively. Since reductions in alpha-motoneuron pool excitability correlate with increased flexibility, ballistic stretch applied following static stretch appears more effective than static stretch alone.

The transduction properties of diaphragmatic mechanoreceptors

The transduction properties of diaphragmatic mechanoreceptors were studied using an isolated organ preparation. Following localization of their receptive field and receptor characterization, controlled diaphragmatic stretch in 2 mm increments was performed while recording the steady-state firing frequency of these afferents. Of 31 receptors recorded, 14 could be categorized into one of 3 types: (1) muscle spindles, (2) Golgi tendon organs, and (3) pressure-sensitive mechanoreceptors. These receptors were also found to be widely distributed in the diaphragm. Four of the muscle spindles examined were shown to possess a length sensitivity of 4-8 mm with a wide range of maximal discharge. The results of this study suggest that the diaphragm contains mechanoreceptors that transduce muscle length and projects this information regarding respiratory proprioception to the CNS.

Maintenance of constant arm position or force: reflex and volitional components in man

1. Normal subjects, with closed eyes, attempted to keep constant either the force exerted at the wrist or the position of the wrist against an elastic load. The load was attached to the wrist 275 mm from the axis of rotation of the elbow joint. During recording, the far end of the elastic load was displaced slowly enough that it was not immediately perceived but far enough for perception to occur before its completion. 2. The over-all relation between wrist force and position for the two conditions was approximately linear and could be described in terms of effective stiffness. The effective stiffness for the constant-position task averaged 2.8 N/mm (210 N m/rad), while for the constant-force task the mean effective stiffness was -0.028 N/mm (-2.1 N m/rad), indicative of slight over-compensation. 3. Averaging the performance at the onset of the imposed disturbance indicated that the subjects' behaviour consisted of two parts: an initial, small-range response followed by a second phase over the remainder of the displacement. The transition corresponded to the subjects' threshold for detection of the disturbance. 4. The stiffness measured for the response prior to perception was taken as a measure which included the tonic stretch reflex. The stiffness was altered appropriately for the two tasks, being lower when the subjects tried to maintain the force exerted constant (average 1.1 N/mm, 83 N m/rad) than when they attempted to keep the position constant (average 2.3 N/mm, 170 N m/rad). A small degree of co-contraction occurred but could be dissociated from the stiffness changes. 5. Scaling the results allowed comparison of the initial stiffness with values for the decerebrate cat. When analysed in this way, the values recorded in man during the constant-position task were similar to those reported for short-range stiffness in the decerebrate cat. 6. The thresholds for detection of the disturbance were much lower than those reported for subjects with relaxed muscles. 7. The stretch reflex in man has a direct role in compensating for small disturbances during motor tasks. It may also function to improve detection of applied disturbances by magnifying the corresponding force change. Once the stimulus is perceived and voluntary intervention is possible, a greater contrast is seen between the subjects' performance of the two tasks.

Fusimotor reflexes in triceps surae muscle elicited by stretch of muscles in the contralateral hind limb of the cat

Experiments were performed on twenty-one cats anaesthetized with alpha-chloralose. The aim of this study was to investigate the reflex effects on triceps surae and plantaris fusimotor neurones elicited by tonic stretch of the contralateral posterior biceps and semitendinosus (p.b.s.t.) and the contralateral triceps surae and plantaris muscles, to compare these effects with the effects evoked by flexion or extension of the intact contralateral hind limb (Appelberg, Hulliger, Johansson & Sojka, 1984) and to clarify the interactions between the reflexes from contralateral and ipsilateral muscles. Activity in fusimotor neurones was studied indirectly by recording from primary muscle spindle afferents of the triceps surae and plantaris muscle. The mean rate of firing and the modulation of the afferent response to sinusoidal extension of the triceps surae and plantaris muscles was determined. Control measurements were made with the ipsilateral p.b.s.t., the contralateral p.b.s.t. and the contralateral triceps and plantaris muscles relaxed. Tests were made with tonic stretch of one of these muscles alone or with two of them simultaneously. With stretch of the contralateral p.b.s.t. ten out of eighty-four primary afferents (11.9%) showed predominantly dynamic reflexes (six out of forty-one in spinalized preparations: 14.6%), twenty-two (26.2%) showed mixed or predominantly static effects (one spinalized: 2.4%) and fifty-two units (61.9%) showed no effect (thirty-four spinalized: 83.0%). The reflex effects could be reproduced by electrical stimulation of the cut contralateral p.b.s.t. nerve either at group II or at group III strength. With stretch of the contralateral triceps and plantaris muscles seventy out of seventy-six (92.1%) primary muscle spindle afferents showed no effect and six (7.9%) mixed or predominantly static reflex effects. In general, the reflex effects were not accompanied by detectable electromyographic (e.m.g.) activity in the ipsilateral triceps and plantaris (recorded with surface or needle electrodes), indicating that the reflexes mainly involved gamma-motoneurones. The difference in efficacy between contralateral flexor (p.b.s.t.) and extensor (triceps and plantaris) muscles seems to be in accordance with the response pattern found with extension or flexion of the intact contralateral hind limb (Appelberg et al. 1984).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of triceps stretch by ankle flexion on intact afferents and efferents of gastrocnemius in the decerebrate cat

We describe the effects on group I and II afferents as well as alpha- and gamma-efferents of gastrocnemius muscles elicited by stretch and release of the triceps surae muscle performed by a dorsiflexion movement, a maintained dorsiflexion and release of the ankle joint. The experiments were made in decerebrated cats in which the neural loops between the muscles and the spinal cord were intact. Multi-unit discharges of each kind of fibre were obtained, electronically on-line, from two monopolar electrodes 4 mm apart on a small branch of the gastrocnemius lateralis or medialis nerve. Sensory and motor impulses were separated according to the opposite directions of their propagation, then both groups were further subdivided according to conduction velocity. The stretch reflex induced by ankle flexion was associated in eighteen out of nineteen experiments with reflex changes in the activity of the gamma-axon population of the gastrocnemius nerve branch. Facilitatory, depressor and biphasic (facilitatory-depressor) effects were observed, the particular type seeming to depend on the level of gamma tone and gamma-motoneurone size. Flexion release led to cessation of the alpha stretch reflex and to a rebound firing of the gamma-axon population. The lengthening reaction and the clasp-knife phenomenon occurred in certain preparations and were associated with reflex inhibition of the gamma-motoneurones. The over-all responses of group I fibres to triceps stretch showed dynamic and static components, whereas those of the group II fibres were, except in two examples, almost devoid of dynamic sensitivity. During release of stretch and as a result of the simultaneous rebound in gamma activity, the afferent discharges showed a very short pause, no pause, or even a rebound. The origins of the discharges of groups I and II muscle afferents and the factors (muscle length, active and passive muscle tension, reflex changes in fusimotor drive) influencing the activity of the receptors involved are considered as far as possible. Some aspects of the mechanisms of the fusimotor reflex are discussed.

Central and proprioceptive influences on the activity of levator costae motoneurones in the cat

The role of central respiratory drive, muscle spindles and tendon organs in producing respiratory movements has been studied in the cat by recording from motoneurones supplying a set of small axial muscles inserted between each rib and the vertebra immediately rostral, the levatores costae. The levator costae muscles are active during normal inspiration but activity is progressively stronger in the muscles located in more caudal thoracic segments. Intracellular recordings from levator costae motoneurons show a characteristic central respiratory drive potential (c.r.d.p.), comprising phases of depolarization during inspiration alternating with expiratory phased hyperpolarization due to post-synaptic inhibition. Loading or unloading the levator costae muscles increases and decreases, respectively, their normal inspiratory activity. Electrophysiological and histological analysis of levator costae afferents reveals that each muscle contains three to five spindles and two to three tendon organs. By dissecting the levator costae nerve, afferent discharges from muscle spindle primary and secondary endings and those from tendon organs were recorded 'in continuity' and used to trigger an averager for analysing the synaptic potentials they evoke in levator costae motoneurones. Monosynaptic excitation and oligosynaptic inhibition originate from spindle (primary and secondary endings) and from tendon organ afferents respectively. Peculiarly a monosynaptic excitation has been evoked from a tendon organ. The action of the levator costae muscle on the respiratory movements of the ribs in the different thoracic spaces was analysed in relation to their anatomical properties and to the activity of the other respiratory muscles. The levator costae muscles, because of their relatively small size and well-defined population of proprioceptors, appear to provide a favourable preparation for studying the integrative action of the motoneurone.

The in-series and in-parallel components in rat hindlimb tendon organs

The structure of the tendon organs was studied in the shank muscles of adult rats both under the light- and electron-microscope. The rat tendon organs measure on the average about 500 microns in length and 60 microns in diameter. Most tendon organs are surrounded by muscle fibres and their short individual tendons, and insert into the aponeuroses or intramuscular tendons. Each tendon organ consists of a neurotendinous core composed of collagen bundles that represent tendons of 5-10 muscle fibres; it is innervated by a Ib sensory fibre that branches and terminates among the loose collagen fascicles of the core. Sensory terminals are oriented both transversely and longitudinally. Their position and relation to collagen bundles indicate that, during tendon organ activation, the terminals are probably depolarized both by lateral compression and elongation. The core is enclosed in a capsule that consists of about 5 lamellar layers of capsular cells and closely resembles the perineurium. The majority of the tendon organs also comprise a purely tendinous compartment in the lumen or within the capsular wall. These tendinous components remain separated from the neurotendinous core and do not come into contact with axon terminals. The collagen fibrils of the tendinous compartments are densely packed and larger in diameter than those of the neurotendinous core. The sensory terminals of the tendon organ lie in series with those muscle fibres and collagen bundles that constitute the neurotendinous core, but they are in parallel with the purely tendinous tendon organ components and their respective muscle fibres. Thus, one tendon organ may comprise both in series and in parallel components, which is apparently reflected in its function. It is suggested that the purely tendinous tendon organ compartments account for the in-parallel effects upon the function of tendon organs described in some recent electrophysiological studies.

Masseteric silent periods electrically evoked in normal subjects and patients with temporomandibular joint dysfunction

The mean latency and duration of electromyographic silent periods were compared in normal subjects and patients with symptoms of temporomandibular joint (TMJ) dysfunction. The silent periods were evoked in the masseter muscle by electrical stimulation of the mental nerve area and recorded at 100% and 50% of maximum masseter EMG activity. Two silent periods were typically observed. The mean latency of the early silent period as well as the mean duration of both the early and late silent periods were shorter at 100% than at 50% of the maximal voluntary activity. In comparing the two groups, the mean latencies were shorter and the mean durations were longer for both silent periods in the TMJ group compared with the normal group. The prolonged silent periods in the TMJ group do not appear to be caused by a difference in stimulus intensity or level of voluntary activity in the masseter muscle. These results permit the speculation that the mechanism of the prolonged silent period in the TMJ patients might be central rather than peripheral.

Presumed reflex responses of human first dorsal interosseus muscle to naturally occurring twitch contractions of physiological tremor

Surface EMG activity from the first dorsal interosseus, time-locked to twitch contractions in the same muscle, was investigated using the technique of spike-triggered averaging. A decrease in surface EMG activity was observed during the rising phase of twitch force, consistently followed by two bursts of activity with roughly constant onset latencies. One burst occurred at 45 msec from the onset of the single motor unit potential used to trigger the averager and was near the peak of the twitch contraction. The other burst occurred at 77 msec during the falling phase of the twitch contraction. If it is assumed that the chance synchronization of motor units contributes to physiological tremor [1, 5, 20], then the observed relative strengths of the two bursts of EMG activity indicate that their net action could enhance tremor.

The absence of position response in spindle afferent units from human finger muscles during accurate position holding

1. Single unit activity of muscle spindle afferents from finger extensor muscles was recorded in the radial nerve of waking human subjects. The mean discharge rate of the afferent units was determined while the receptor related finger was held at fixed angular positions of the metacarpo-phalangeal joint. 2. During a visual tracking task the subjects had to maintain specified angular positions against a load of constant torque which opposed finger extension. For each unit a comparison was made between the mean discharge rates at two angular positions which differed by 20 deg. Under such isotonic conditions the rates of afferent discharge at the two joint positions did not significantly differ, neither for the whole sample of primary, nor for that of secondary units. This was true, no matter whether the load was small or intermediate. Large loads were not tested. 3. For comparison, the passive position responses of a sample of spindle afferent units from the same muscles were studied when the finger was held in fixed positions while the muscles were voluntarily relaxed. Under these conditions a significant position sensitivity was found for both primary and secondary afferents. The mean values were 0.28 impulses/sec. deg (primaries), and 0.21 impulses/sec. deg (secondaries). 4. The absence of position response during active position holding was interpreted as a manifestation of changes in fusimotor outflow which depended on joint position and were large enough to compensate for changes in muscle length.

Enhancement of synaptic function in cat motoneurones during peripheral sensory regeneration

1. Monosynaptic excitatory post-synaptic potentials (e.p.s.p.s) were recorded from medial (m.g.) and lateral gastrocnemius (l.g.) motoneurones in the cat 2-30 weeks after crushing the m.g. nerve.2. The mean amplitudes of homonymous and heteronymous e.p.s.p.s evoked from the m.g. nerve were initially depressed (2-3 weeks after injury) and subsequently reached levels greater than normal for a period (8-12 weeks) before slowly declining to about 70% of the normal values (by week 30).3. Monosynaptic e.p.s.p.s evoked in m.g. motoneurones from the intact l.g. nerve showed neither initial depression nor subsequent alterations following crush of the m.g. nerve.4. By the eighth week after nerve crush, about 70% of Group I and Group II sensory fibres in the m.g. nerve responded to muscle stretch, about 15% had regenerated into the muscle but did not respond to muscle stretch, and the remainder failed to regenerate across the neuroma formed by the nerve crush.5. Homonymous, monosynaptic e.p.s.p.s produced by impulses in single sensory fibres responding to stretch of the m.g. muscle were recorded 8 weeks after crush of the m.g. nerve. Their amplitude distribution was indistinguishable from that obtained in normal, unoperated cats. Thus, there was no evidence that functionally reinnervated sensory fibres are responsible for the enhanced phase of composite e.p.s.p.s observed during peripheral regeneration.6. When the m.g. nerve had been sectioned and prevented from regenerating into the muscle for 8 weeks, the amplitudes of homonymous and heteronymous e.p.s.p.s evoked from the m.g. nerve were significantly smaller than those observed in control animals. Thus, there was no evidence that non-regenerating sensory fibres are responsible for the enhanced phase of composite e.p.s.p.s after nerve crush.7. It is suggested that the sensory fibres responsible for abnormally large composite e.p.s.p.s following nerve crush are those that regenerate into the muscle but do not achieve functional reinnervation. This possibility is discussed in relation to the increase in central synaptic efficacy observed after prolonged disuse of the sensory pathway.

Tendon organ discharge during voluntary movements in cats

1. The discharge activity of tendon organ afferents was recorded during voluntary movements in cats. 2. The eight tendon organ afferents in our sample all fired during isotonic movements involving active muscle shortening. 3. Firing rates usually exceeded 100 sec-1, even up to the highest muscle shortening velocity observed, 1.8 resting lengths per second (lr/sec). 4. We suggest that during voluntary, isotonic movements involving muscle shortening at velocities exceeding 0.2 lr/sec, the net action of muscle afferents on homonymous motoneurones is often inhibition. 5. These observations on tendon organs, taken together with previous findings on muscle spindles, indicate that in normal fast movements the role of the large muscle afferents is to signal dynamic functions of muscle length and force.

The discharge rate: tension relation of Golgi tendon organs

Tendon organs are preferentially sensitive to activity in a small selected number of motor units. We have isolated single tendon organ afferents and the motor units exerting effects on them in the soleus muscle of the cat. Using the method of distributed stimulation it has been possible to grade motor unit tension over a wide range and record the corresponding firing rates of the receptor. The plot of firing rate against tension was found to be highly non-linear and did not conform to the simple power function previously attributed to the relation.

Two methods of measuring muscle tone applied in patients with decerebrate rigidity

Two methods were used to measure muscle tone in patients with decerebrate rigidity. In the first method forces of square waveform were applied and the calculated compliance of the joint was used as an index of rigidity. Oscillatory transients were seen at the same frequency as the physiological tremor. The range of normal variation in compliance was large and the values measured in the patients flucuated markedly which limited the value of this index. In the second method, where forces of sinusoidal waveform were employed, the resonant frequency of the joint was measured and used as an index of rigidity. This index proved reliable and reproducible.

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

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

The relative dependence of the activity of Renshaw cells on recurrent pathways during contraction of the triceps muscle

Renshaw cell activity was recorded simultaneously with motoneuronal unit discharge during vibration and tetanic stimulation of triceps muscles in decerebrated cats. The experiments confirm that, in this preparation, the motoneurones are the main source of Renshaw cell firing during muscle stretch and vibration and when motoneuronal discharge was induced through the gamma loop. However they also show that a discharge of Renshaw cells, monosynaptically coupled with triceps motoneurones through their recurrent collaterals, could be elicited during contraction of the muscle at the time when the discharge of these motoneurones had been silenced. The recording of the stretch receptors and motoneuronal unit discharge during stretch, vibration, and ventral root stimulation gave evidence of the contribution of the withdrawal of excitation by primary endings to the occurrence of the silent period during tetanic contraction of the muscle. The measurements of the critical firing level in motoneuronal units responding reflexly to held stretch and vibration of the muscles, and silencing their discharge during muscle shortening, showed that these cells are amongst the lowest ranking in the pool For these reasons, these data suggest that Renshaw cell firing during vibration and tetanic contraction of the muscle cannot be attributed only to the alpha motoneurone excitation by the Ia fibres.