Mechanical arrangement and transducing properties of Golgi tendon organs

Regional Phenotypic Differences of the Opener Muscle in Procambarus clarkii: Sarcomere Length, Fiber Diameter, and Force Development

The opener muscle in the walking legs of the crayfish (Procambarus clarkii) has three distinct phenotypic regions although innervated by only one excitatory motor neuron. These regions (distal, central, and proximal) have varied biochemistry and physiology, including synaptic structure, troponin-T levels, fiber diameter, input resistance, sarcomere length, and force generation. The force generated by the central fibers when the excitatory neuron was stimulated at 40 Hz was more than the force generated by the other regions. This increase in force was correlated with the central fibers having longer sarcomeres when measured in a relaxed claw. These data support the idea that the central fibers are tonic-like and that the proximal fibers are phasic-like. The addition of serotonin directly on the fibers was hypothesized to increase the force generated by the central fibers more than in the other regions, but this did not occur at 40-Hz stimulation. We hypothesized that the central distal fibers would generate the most force due to the arrangement on the apodeme. This study demonstrates how malleable the motor unit is with modulation and frequency of stimulation.

Neurophysiological Mechanisms Underpinning Stretch-Induced Force Loss

It is well known that prolonged passive muscle stretch reduces maximal muscle force production. There is a growing body of evidence suggesting that adaptations occurring within the nervous system play a major role in this stretch-induced force reduction. This article reviews the existing literature, and some new evidence, regarding acute neurophysiological changes in response to passive muscle stretching. We discuss the possible contribution of supra-spinal and spinal structures to the force reduction after passive muscle stretch. In summary, based on the recent evidence reviewed we propose a new hypothesis that a disfacilitation occurring at the motoneuronal level after passive muscle stretch is a major factor affecting the neural efferent drive to the muscle and, subsequently, its ability to produce maximal force.

Human-inspired feedback synergies for environmental interaction with a dexterous robotic hand

Effortless control of the human hand is mediated by the physical and neural couplings inherent in the structure of the hand. This concept was explored for environmental interaction tasks with the human hand, and a novel human-inspired feedback synergy (HFS) controller was developed for a robotic hand which synchronized position and force feedback signals to mimic observed human hand motions. This was achieved by first recording the finger joint motion profiles of human test subjects, where it was observed that the subjects would extend their fingers to maintain a natural hand posture when interacting with different surfaces. The resulting human joint angle data were used as inspiration to develop the HFS controller for the anthropomorphic robotic hand, which incorporated finger abduction and force feedback in the control laws for finger extension. Experimental results showed that by projecting a broader view of the tasks at hand to each specific joint, the HFS controller produced hand motion profiles that closely mimic the observed human responses and allowed the robotic manipulator to interact with the surfaces while maintaining a natural hand posture. Additionally, the HFS controller enabled the robotic hand to autonomously traverse vertical step discontinuities without prior knowledge of the environment, visual feedback, or traditional trajectory planning techniques.

Proprioception and tension receptors in crab limbs: student laboratory exercises

The primary purpose of these procedures is to demonstrate for teaching and research purposes how to record the activity of living primary sensory neurons responsible for proprioception as they are detecting joint position and movement, and muscle tension. Electrical activity from crustacean proprioceptors and tension receptors is recorded by basic neurophysiological instrumentation, and a transducer is used to simultaneously measure force that is generated by stimulating a motor nerve. In addition, we demonstrate how to stain the neurons for a quick assessment of their anatomical arrangement or for permanent fixation. Staining reveals anatomical organization that is representative of chordotonal organs in most crustaceans. Comparing the tension nerve responses to the proprioceptive responses is an effective teaching tool in determining how these sensory neurons are defined functionally and how the anatomy is correlated to the function. Three staining techniques are presented allowing researchers and instructors to choose a method that is ideal for their laboratory.

Mathematical models of proprioceptors. II. Structure and function of the Golgi tendon organ

We developed a physiologically realistic mathematical model of the Golgi tendon organ (GTO) whose elements correspond to anatomical features of the biological receptor. The mechanical interactions of these elements enable it to capture all salient aspects of GTO afferent behavior reported in the literature. The model accurately describes the GTO's static and dynamic responses to activation of single motor units whose muscle fibers insert into the GTO, including the different static and dynamic sensitivities that exist for different types of muscle fibers (S, FR, and FF). Furthermore, it captures the phenomena of self- and cross-adaptation wherein the GTO dynamic response during motor unit activation is reduced by prior activation of the same or a different motor unit, respectively. The model demonstrates various degrees of nonlinear summation of GTO responses resulting from simultaneous activation of multiple motor units. Similarly to the biological GTO, the model suggests that the activation of every additional motor unit to already active motor units that influence the receptor will have a progressively weaker incremental effect on the GTO afferent activity. Finally, the proportional relationship between the cross-adaptation and summation recorded for various pairs of motor units was captured by the model, but only by incorporating a particular type of occlusion between multiple transduction regions that were previously suggested. This occlusion mechanism is consistent with the anatomy of the afferent innervation and its arrangement with respect to the collagen strands inserting into the GTO.

Effect of spinal manipulation duration on low threshold mechanoreceptors in lumbar paraspinal muscles: a preliminary report

STUDY DESIGN

Electrophysiologic recordings were obtained from low threshold primary afferent neurons innervating lumbar multifidus and longissimus muscles in the anesthetized cat.

OBJECTIVE

The purpose of this study was to classify sensory nerve endings in lumbar paraspinal muscles and characterize their responses to biomechanical loads applied over a range of durations that encompass those occurring during spinal manipulation.

SUMMARY OF BACKGROUND DATA

Neural responses arising from the mechanical input during spinal manipulation are thought to contribute to this maneuver's therapeutic effects. Because manual therapies are distinguished to a large extent on the basis of the speed with which they are applied, it is important to understand how their rate of application affects the signaling properties of primary afferent neurons innervating paraspinal tissues. If alterations in sensory input do contribute to the mechanism of spinal manipulation's therapeutic effect, it seems reasonable to expect that these primary afferents would respond to spinal manipulation in some unique fashion.

METHODS

Experiments were performed on 6 adult cats. A L4-L5 laminectomy was performed and the L6 dorsal roots exposed. The L6-L7 vertebrae and associated paraspinal tissues remained intact bilaterally, including lumbodorsal fascia, multifidus, longissimus, iliocostalis muscles, and deeper tissues. Forceps were clamped tightly onto the lateral surfaces of the L6 spinous process through a thin narrow, slit in the lumbodorsal fascia. Single unit afferent activity was recorded from fine filaments teased from the L6 dorsal root. Instantaneous discharge frequency was calculated. Afferents were classified based on von Frey threshold, conduction velocity, and responses to direct muscle stimulation and to succinylcholine injection. Spinal manipulative-like loads were applied to the L6 vertebra (posterior to anterior) using a programmable electronic feedback control system. Force-time profiles were half-sine waves with durations of 25, 50, 100, 200, 400, and 800 milliseconds delivered at constant magnitudes of 33%, 66%, or 100% body weight.

RESULTS

The 6 afferents were classified as low threshold mechanoreceptors based on von Frey thresholds being less than 6 g. Five afferents were Group I or II muscle proprioceptors and one afferent was a Group III muscle mechanoreceptor. The receptive field for 2 of the 6 afferents was in the multifidus muscle and the receptive field of the remaining 4 afferents was in the longissimus muscle. In general, the mean instantaneous discharge frequency for all 6 afferents increased abruptly as the duration of the impulse approached 100 milliseconds. An increase in loading magnitude (33% vs. 66% vs. 100% body weight) did not appear to systematically affect the discharge from the 6 low threshold mechanoreceptors.

CONCLUSIONS

This preliminary report suggests that abrupt changes in neural discharge (instantaneous frequency) of low threshold muscle mechanoreceptors of the lumbar spine occur as the duration of a biomechanical load approaches that typically used during spinal manipulation. These changes could comprise part of the mechanism contributing to this intervention's physiologic effects. Further studies are warranted to better understand the signaling properties of a wider range of sensory receptors as well as determine the central effects of these high frequency discharges.

Dynamic knee stability: current theory and implications for clinicians and scientists

We will discuss the mechanisms by which dynamic knee stability may be achieved and relate this to issues that interest clinicians and scientists concerned with dynamic knee stability. Emphasis is placed on the neurophysiologic evidence and theory related to neuromuscular control. Specific topics discussed include the ensemble firing of peripheral mechanoreceptors, the potential for muscle stiffness modulation via force and length feedback, postural control synergies, motor programs, and the neural control of gait. Factors related to answering the difficult question of whether or not knee ligament injuries can be prevented during athletic activities are discussed. Prevention programs that train athletes to perform their sport skills in a safe fashion are put forth as the most promising prospect for injury prevention. Methods of assessing neuromuscular function are reviewed critically and the need for future research in this area is emphasized. We conclude with a brief review of the literature regarding neuromuscular training programs.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Topography of muscle spindles and Golgi tendon organs in shoulder muscles of "Monodelphis domestica"

The topography of muscle spindles and Golgi tendon organs in the rotator cuff and surrounding shoulder muscles of a small laboratory marsupial (monodelphis domestica) were studied using light microscopy of serial sections. The shoulder joint of monodelphis has a large degree of freedom of movement allowing this animal to use the upper extremities for a wide range of activities like climbing and manipulating food. Thus, similar to the situation in man the shoulder joint is mainly secured by muscles. Silver stained serial paraffin sections were examined under the light microscope and the distribution of muscle spindles and Golgi tendon organs was reconstructed using three-dimensional image processing. In the two animals examined 113 and 131 muscle spindles respectively were found within the 4 rotator cuff muscles. In addition, 76 and 40 Golgi tendon organs respectively were seen at the musculo-tendinous junctions of these muscles preferentially close to the insertion at the humerus head. Also the surrounding shoulder muscles contain both muscle spindles and Golgi tendon organs in large numbers, but the ratio of Golgi tendon organs per muscle spindle appears to be lower. Number and localization of muscle spindles and Golgi tendon organs suggest, that these receptors are important for both reflex control of shoulder muscle tone as well as monitoring of static position and movement in the shoulder joint.

The reversal of antagonists facilitates the peak rate of tension development

OBJECTIVE

The present study was designed to test the effects of the reversal of antagonists on the peak rate of tension development (dF/dt(max)) of the elbow extensors.

DESIGN

Experimental, with matched controls.

SETTING

A biomechanics research laboratory.

PARTICIPANTS

Twenty-six healthy women without a history of upper extremity injury or neurologic disorder, randomly assigned to experimental (n = 13) or control (n = 13) groups.

INTERVENTIONS

Two groups of healthy subjects followed identical exercise protocols, except that the control group performed maximal isometric contractions of the elbow extensors and the experimental group executed a maximal isometric elbow flexion contraction immediately before a maximal elbow extension contraction. Both groups performed 5 cycles of a 2-second contraction with 22-second rest periods between agonist muscle contractions and were evaluated at 4 test sessions spaced 2 weeks apart.

MAIN OUTCOME MEASURES

All measurements were done with the shoulder and elbow at 90 degrees of flexion in the sagittal plane to ensure reproducibility. A load cell was used to measure elbow extension moment and to calculate the peak rate of tension development (dF/dt(max)). Biceps and triceps brachii surface electromyographic activity was monitored concurrently. The electromyographic measures were mean spike (peak-to-peak) amplitude and mean spike frequency of the biceps and triceps brachii activity.

RESULTS

Intraclass dF/dt(max) and electromyographic reliability was good (r > or = .72) in both groups. Because biceps electromyographic measures were considerably less reliable (r < or = .53), they were not included in our analysis. While dF/dt(max) increased quadratically in both groups (p <.05), the experimental group was on average 36.1Nm. s(-1) (63%) greater across sessions 2 to 4 (p <.05). In contrast, triceps electromyographic activity did not differ significantly between groups (p >.05). The means averaged across groups exhibited a quadratic increase from session 1 to session 4: 91microV or 48% for mean spike amplitude (p <.05) and 7Hz or 16% for mean spike frequency (p <.05).

CONCLUSIONS

The greater dF/dt(max) for the experimental group was not associated with increased electromyographic activity. The experimental group appeared to use the biomechanic properties of the pretensioned extensor muscle-tendon complex, rather than neurologic biasing, to accomplish its power gains.

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.

Unit identification, sampling bias and technical issues in microneurographic recordings from muscle spindle afferents

Recordings have been made directly from human muscle spindle afferents for some 3 decades, and these have allowed assessment of fusimotor function in human subjects during normal motor acts and in patients with motor disturbances. However, inferences about fusimotor function are indirect, valid only if identification of the axon as of muscle spindle origin is secure and all extrafusal influences on the spindle have been controlled. As is discussed, the identification of spindle afferents and their classification into Group Ia and Group II are more problematic than in the cat, but these problems are probably relatively minor given the insight into normal function that can come from appropriately designed experiments in awake cooperative human subjects.

Depolarization of Ib afferent axons in the cat spinal cord during homonymous muscle contraction

1. Intra-axonal records from the intraspinal course of Ib and Ia afferent fibres innervating the gastrocnemius medialis muscle were obtained in chloralose or Nembutal-anaesthetized cats during submaximal contractions of the muscle. 2. Afferent fibres in continuity with their muscle of origin were functionally identified by their responses to muscle stretch or contraction. 3. In six out of eight Ib afferents, primary depolarizations (PADs) were recorded during contraction. They were independent of the presence of orthodromic impulses fired by tendon organs. 4. These observations support the assumption that the reduction of Ib autogenetic inhibition in homonymous and synergic motoneurones during GM contractions is due to presynaptic inhibition of transmission in Ib pathways.

Connective-tissue macromolecules in Golgi chicken tendon organs and at their interface with muscle fibers and adjoining tendinous structures

Tendon organs from leg and forearm muscles of white leghorn chickens were examined with a library of monoclonal antibodies to determine the composition of their connective-tissue framework and the types of connective-tissue macromolecules that occur at the sites where muscle fibers attach to the receptors. The capsules of the tendon organs were positive for connective-tissue macromolecules typical of basal lamina (collagen type IV, laminin, and heparin sulfate proteoglycan) and for tenascin, collagen types III and VI, and fibronectin. Connective-tissue bundles in the lumen of a receptor reacted primarily with antibodies against collagen type I and 4-chondroitin sulfate. The narrow partitions that divide each lumen into compartments stained for collagen type III. Toward its tendinous end, a receptor made few contacts with muscle fibers. Instead, the capsule and the collagenous bundles blended gradually with the intermuscular portions of tendons. At the muscular end, the connections were more complex. Muscle fibers that attached in series to tendon organs split to produce basal lamina-covered, finger-like extensions, which were separated from each other by fissures. Tongues of connective tissue containing tenascin, collagen types I and VI, and fibronectin extended into the fissures. Distally the tongues were continuous with the tenascin in the capsule and just internal to the capsule, fibronectin and basal lamina macromolecules in the capsule, and collagen type I in the collagenous bundles. The uninterrupted presence of these macromolecules around terminating muscle fibers and in the capsule and/or the intraluminal collagen bundles suggests that muscle fibers that attach in series at the muscular end exert a force during muscular contraction on the intraluminal collagen bundles and on the receptor capsule.

Unloading of tendon organ discharges by in-series motor units in cat peroneal muscles

1. The discharges from individual Golgi tendon organs of peroneus tertius and brevis muscles were recorded in anaesthetized cats during the isometric contractions of single motor units. Upon combined contractions of several motor units, two sorts of unloading effects were observed. 2. First, the contraction of a motor unit which, by itself, was without action on a tendon organ could produce a reduction in the response of the receptor to one of its activating motor units. Unloading effects exerted by such in-parallel motor units could effectively interfere with the actions of in-series motor units on the receptor. 3. Second, the contraction of a motor unit activating a tendon organ could reduce the response of this tendon organ to the contraction of another of its activating units. This new type of unloading effect, exerted by in-series motor units, was demonstrated by the fact that the simultaneous contraction of both units elicited less discharge from the receptor than the contraction of a single unit. 4. Unfused contractions of a fast-type motor unit eliciting a response in which the tendon organ discharge was driven 1:1 at the frequency of stimulation of the motor unit, could exert unloading actions on the response of the receptor to another motor unit eliciting a higher discharge frequency. 5. In-series unloading actions were exerted not only by fast-type motor units developing large forces, but also by relatively small slow-type motor units. 6. The high incidence of in-parallel and in-series unloading effects suggests that their consequences may be functionally significant. When large numbers of motor units are being recruited in a muscle, unloading effects might result in a limitation of the Ib afferent discharges from this muscle, preventing an excessive increase of autogenetic inhibition.

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

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

The innervation of tandem muscle spindles in the cat neck

Patterns of innervation were examined in tandem muscle spindles teased from silver-stained muscles of the cat neck. Each tandem spindle was composed of two or more encapsulated receptors linked in series by a shared bag2 fiber. In most tandem spindles, two different types of encapsulation were identified according to differences in their intrafusal fiber content. One type, the b1b2c unit, contained typical bag1, bag2, and chain fibers and was structurally similar to single spindles described in other cat muscles. Each b1b2c unit contained a single primary sensory ending and 1-6 secondary endings. Fusimotor innervation was supplied by many axons. Some fusimotor axons ended in trail ramifications on bag2 and chain fibers, others ended in plates on the bag1 or long chain fiber. The other type of tandem encapsulation, the b2c unit, had only bag2 and chain fibers in its intrafusal fiber bundle. The b2c unit was usually supplied by only one sensory axon that ended on the nucleated part of the intrafusal fiber bundle. This single ending had a more variable terminal morphology than the primary ending in b1b2c units. A few b2c units (3/49) were also supplied by a secondary ending. The fusimotor innervation of the b2c unit was relatively simple. A single pole of the b2c unit was usually supplied by only one to three axons, all ending in trail ramifications. No plate endings were found in b2c units. These morphological specializations suggest that b1b2c and b2c units in tandem spindles differ in both their transductive and fusimotor mechanisms. Thus, the tandem spindle is a specialized structure that may provide additional proprioceptive information beyond that available from single muscle spindles.

Synchronous afferent discharge from a passive muscle of the cat: significance for interpreting spike-triggered averages

Evidence is presented for the existence of synchrony between the spike trains of muscle afferents of the passive cat medial gastrocnemius muscle held at fixed length. Synchrony between the spike trains of a population of muscle afferents was quantified by means of a synchronization index (Is), derived from spike-triggered averages of the muscle-nerve neurogram and the rectified neurogram. A previously used test based solely upon the neurogram average (Watt, Stauffer, Taylor, Reinking & Stuart, 1976) is shown to be invalid. The differences between experimentally derived Is values and theoretical Is values derived for the condition of asynchrony were compared to estimated confidence limits for those differences. This comparison revealed that twenty-two of fifty-three muscle-afferent spike trains whose rectified averages satisfied certain conditions for interpreting the Is were synchronized with the discharge of other afferents. The form of the rectified averages of another eight afferents suggested that these afferents might also have been synchronized. Synchrony in the discharge of muscle afferents was found in three experiments in which the neurogram was recorded from a single nerve branch to medial gastrocnemius, as well as in the data of experiments in which the whole muscle nerve was used. The degree of synchrony was similar for Ia, spindle group II and Ib afferents. The magnitude of the synchrony found in these experiments was judged by comparison to the results of analog simulations and the increase in Is values resulting from the application of small, quick stretches to the medial gastrocnemius muscle. The degree of synchrony found on average was approximately equivalent to that of a single spike occurring once for every four discharges of the reference spike train. Simulations were performed to determine the distortion of monosynaptic excitatory post-synaptic potentials (e.p.s.p.s) obtained by spike-triggered averaging which would be produced by synchrony between the spike trains of Ia and spindle group II afferents of the magnitude found in this study. These simulations indicate that the apparent amplitude would be increased by approximately 4 microV on average. Both the 10-90% rise time and the half-width would increase, the effects being greater for smaller e.p.s.p.s. Consequently, the synchrony found in this study is of most concern in the study of small post-synaptic potentials, such as those due to spindle group II afferents.

Interactions between motor units and Golgi tendon organs in the tibialis posterior muscle of the cat

The responses of Golgi tendon organs to single motor unit contractions were studied to determine whether receptors located in the same muscle region respond to a common set of motor units. In each of five experiments we isolated a large fraction (25-65%) of the motor units of the cat tibialis posterior muscle and determined to which of the units each of several tendon organs was responsive. Each tendon organ was excited by from two to fifteen of the isolated motor units, including units which produced very small forces. However, there was a much greater probability for large force units to excite a given receptor than for small force units to do so. The number of motor units which produced either an 'unloading' or an 'off response' exceeded, on average, the number of motor units which excited the same tendon organ. The extent to which single motor units excited both of a pair of tendon organs was examined statistically in relation to the mutual proximity of the receptors within the muscle. It was found, on average, that the closer were two receptors, the greater was the number of motor units that excited both of them. These results suggest that despite the extensive territories of individual motor units, the spike trains of tendon organs may still encode information about localized muscle activity.

Distribution of sensory receptors in the flexor carpi radialis muscle of the cat

The structures and distribution of encapsulated muscle receptors were examined in serial transverse sections of flexor carpi radialis in the adult cat. Four types of receptors (muscle spindles, Golgi tendon organs, paciniform, and Pacinian corpuscles) were identified. Their structures resembled those encountered in other limb muscles. Pacinian corpuscles were rare and occurred only in the external fascial coat of the muscle near its origin. The other three receptor types were distributed in an uneven but consistent pattern throughout the muscle. As noted previously (Gonyea and Ericson, '77), spindles were largely confined to a deep muscle region comprising less than 20% of the muscle volume, located directly between the long tendon of origin and the tendon of insertion. This region contains the largest proportion of type SO muscle fibers (Gonyea and Ericson, '77). Tendon organs and paciniform corpuscles were concentrated along the tendons that lined the spindle-rich muscle region. This region appeared to be composed of extrafusal fibers that were shorter and of more oblique pinnation than those in other regions. The localization of muscle receptors to the "oxidative" core of the muscle in its direct line of pull may have functional implications for afferent input to the spinal cord which are discussed. In addition, the possibility is raised that there are more paciniform corpuscles in flexor carpi radialis (and possibly other muscles) than previously thought.

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.

Distribution of monosynaptic Ia excitatory post-synaptic potentials in the motor nucleus of the cat semitendinosus muscle

Evidence is presented for a lack of localization of monosynaptic Ia excitatory post-synaptic potentials (e.p.s.p.s) in the motor nucleus supplying the atypical cat hind limb muscle semitendinosus, which has anatomically distinct in-series compartments. Recordings were made from dorsal root filaments containing functionally isolated Ia, spindle group II and Ib axons from the proximal and distal compartments of semitendinosus. Twitch of either of these in-series compartments resulted in accelerated discharge of Ia and spindle group II fibres in the other compartment. Ib fibres of either compartment showed an in-series response to twitch of a single compartment which was weaker than twitch of the whole muscle, a finding which was consistent with the diminished force produced by twitch of either compartment alone. In addition, intracellular recordings were made from semitendinosus motoneurones in anaesthetized low-spinal cats during electrical stimulation of the nerve branches to proximal semitendinosus and distal semitendinosus. Comparison of proximal semitendinosus and distal semitendinosus motoneurones failed to reveal any difference between the two cell groups with respect to the average Ia e.p.s.p. amplitude produced by either the proximal or distal semitendinosus nerve branch. However, e.p.s.p.s due to stimulation of distal semitendinosus were approximately 65% larger, on average, than those due to stimulation of proximal semitendinosus in either motoneurone group. Analysis of cell location along the rostro-caudal axis of the spinal cord indicated that the proximal and distal semitendinosus cell groups are largely co-extensive. Recordings of volleys in the proximal and distal semitendinosus nerve branches in response to stimulation of the L6, L7 and S1 dorsal roots showed that group I afferents from the proximal semitendinosus compartment tend to have a more rostral entry point to the spinal cord than do distal semitendinosus afferents. E.p.s.p. amplitude in either cell group due to stimulation of either nerve branch showed little dependence on cell location in the spinal cord. The results are discussed with respect to the relation between muscle function and the distribution of monosynaptic Ia connexions.

Histochemical and physiological properties of cat motor units after self-and cross-reinnervation

1. This report describes selected histochemical and physiological properties of the motor units of adult cat soleus muscle approximately one year after self- and cross-reinnervation with the nerve of the heterogenous flexor hallucis longus (f.h.l.). Self-reinnervated f.h.l. motor units are also considered. Whole muscles were tested for fibre reaction to alkaline pre-incubated ATPase, alpha-glycerophosphate dehydrogenase (alpha-GPD) and reduced nicotinamide adenine dinucleotide diaphorase (NADH-D). Motor units were isolated and studied by splitting the ventral root in acute preparations.2. The histochemical fibre type profile in the self-reinnervated muscle was comparable to normal muscle as was mean twitch contraction time, twitch-tetanus ratio and fatigue index. The mean tetanic tension of the soleus self- and cross-reinnervated motor units appeared close to a normal soleus whereas the mean tetanic tension of the f.h.l. self-reinnervated units was significantly less than a normal f.h.l.3. An average of 14% of the fibres of the soleus cross-reinnervated muscles had high ATPase and a alpha-GPD staining intensity in contrast to normal and self-reinnervated soleus in which such fibres are absent. Thus alkaline lability of myofibrillar ATPase increased in some fibres of what was originally a homogeneous population. The small increase in the number of densely staining fibres for ATPase at an alkaline pH (14%) was associated with a 73% decrease in (mean) contraction time (41 +/- 11 ms) of the thirty-three cross-reinnervated muscle units studied, with no unit's contraction time greater than 60 ms. Mean contraction times for the self-reinnervated soleus and f.h.l. muscles were 78 +/- 31 ms and 27 +/- 8 ms respectively.4. All fibres of the soleus cross-reinnervated muscles showed intense reaction to NADH-D, as was true of self-reinnervated soleus. This staining pattern is typical of normal soleus. In concordance, these motor units consistently demonstrated a high resistance to fatigue when stimulated for a four-minute period.5. These results suggest that in the adult self-and cross-reinnervated soleus muscle, there is some active mechanism which regulates the eventual size of motor units as reflected by tetanic tension.6. Change in contraction time from that typical for a soleus unit to that similar to an f.h.l. unit remains incomplete one year after cross-reinnervation. Within this time this partial change in single motor units reflects incomplete neural control of this property rather than a mixture of self- and foreign-innervation.7. A greater degree of independence from neural control to conversion of the histochemically demonstrated myofibrillar ATPase activity exists than is the case for contraction time.

Responses of isolated Golgi tendon organs of the cat to muscle contraction and electrical stimulation

1. Responses of Golgi tendon organs isolated from cat tail muscles to contraction of muscle fibers inserting directly into the receptor (GTO-muscle fibres) as well as to pulses of electrical current applied extracellularly through the sensory axon were studied. 2. Analysis of the responses to GTO-muscle fibre contraction indicated that the active force developed by each muscle fibre constituted an equally potent input to the receptor in proportion to the developed force. 3. The sensitivity of a Golgi tendon organ remained almost constant with changes in muscle length up to a length where maximum active tension was developed (lo). Beyond lo, the sensitivity tended to decrease. 4. The absolute force threshold (passive + active) at 1o for initiating an impulse in the afferent nerve was estimated for five preparations to be 4.5-14 mg. It was also demonstrated that the contraction of a single GTO-muscle fibre may initiate impulse discharge from the receptor. 5. A constant depolarizing current applied extracellularly to a GTO through its axon initiated a train of impulses, probably originating from a site near or within the receptor capsule. Analysis of responses to constant currents of various intensities suggested that a single impulse initiation site was involved. 6. During combined stimulation, responses of a tendon organ to GTO-muscle fibre contraction simply added to the response initiated by a constant current pulse, suggesting that the impulse initiation sites activated by each mode of stimulation were identical, or situated very close to each other in the nerve terminal.

Spontaneous discharges from muscle receptors of various functional types

A study was done concerning the relationship between the presence or absence of spontaneous spike activity (SSA) and the discharge characteristics of 473 receptors located in deafferented muscles of anesthetized cats. The majority of the Golgi tendon organs, pressure receptors, and the elementary dynamic and static receptors of the muscle spindles did not produce spontaneous discharges. The SSA probability was especially high in spindle receptors of the phasic-tonic type. The absence or presence of SSA could be used for the identification of all known functional types of muscle receptors. Among universal receptors of dynamic and positional types, the probability of SSA increases in proportion to the degree of increase in the magnitude of the static response. There are fewer units in the soleus muscle which show spontaneous activity than in the gently isolated m. triceps surae. The presence or absence of SAA depends, evidently, upon the different ways in which sensory terminals are connected to intrafusal spindle fibers initially stretched in a relaxed muscle.

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.

Identification of muscle spindle afferents during in vivo recordings in man

A preamplifier for use in microelectrode recordings from human muscle afferents is described. Electrical stimuli can be delivered through the microelectrode to produce the twitch contractions necessary for afferent identification while still recording the activity of the afferent fibre.

The response of Golgi tendon organs to single motor unit contractions

1. Cross-correlation analysis has been used to quantify the responses of cat soleus tendon organs to repetitive twitch contractions of: (a) different motor units within the muscle, (b) single motor units at different muscle lengths, and (c) single motor units when the pulse-train pattern of stimulation delivered to the motor unit axon was altered. 2. Ib afferents were observed which responded to each of several hundred successive motor unit twitches with identical numbers of spikes and with relatively invariant latencies. 3. The present results show that tendon organs are sensitive to subtle alterations in motor unit twitch wave form and amplitude, and that this sensitivity is reflected in the precise timings of their afferent discharge. 4. Examination of these tendon organ responses indicates that the forces produced by single motor units couples to the receptor capsule are well above threshold. Calculations based on these results, and earlier soleus motor unit and muscle fibre data, suggest that the absolute force threshold for tendon organs may be as little as 4 mg, which is less than the estimated minimum twitch force generated by individual soleus muscle fibres. 5. Considering the number of tendon organs in a muscle, and the likelihood that every motor unit is connected with at least one receptor, the sensitivity of tendon organs ensures that every twitch of every motor unit will be reflected in the population of afferent signals projecting to the spinal cord.

Responses of isolated Golgi tendon organs of the cat

1. The responses to stretch have been studied in living, isolated Golgi tendon organs (GTOs) from tail muscles of cat. Experiments were performed in vitro and consisted of subjecting single GTOs to controlled ramp-and-hold stretch while recording the response from their sensory axons raised in oil. 2. The threshold force required for sustained afferent discharge was measured directly, and found to be between 8 and 170 dynes at 24 degrees C for nine GTOs tested. Beyond threshold, the discharge frequency is approximately proportional to applied static tension over a wide range. Sensitivy to tension varies among different GTOs and appears to be inversely correlated with mechanical stiffness. 3. With impulse activity blocked by tetrodotoxin, graded receptor potentials could be recorded whose amplitude varied in proportion to applied static tension. All GTOs examined showed in addition a dynamic response, which became larger with increasing velocity of ramp stretch. This dynamic sensitivity appears in the receptor potential and is then augmentd by an apparent accommodative process at the impulse initiating site. 4. Based on the above findings, possible mechanical models are discussed for the sensory transduction mechanism.

Frequency of tendon organ discharges elicited by the contraction of motor units in cat leg muscles

1. The responses elicited in individual tendon organs by the contraction of single motor units were studied in peroneus longus, peroneus brevis, tibialis anterior and soleus muscles. 2. No simple relation was found between the discharge frequency of a tendon organ and the tension produced in the muscle tendon by the contraction of individual motor units. 3. The sensitivity of a given tendon organ to contractile tension was not the same for each of the motor units which elicited its discharge. There was no correlation between the sensitivity of the receptor and the strength of the motor units. 4. Upon repetitive stimulation of a tendon-organ-activating motor unit at increasing rates, the frequency of the receptor sustained discharge reached a maximal value for rates of stimulation eliciting submaximal tetanic tension. Higher rates only produced an increase in the dynamic component of the tendon organ response. 5. These observations show that the contractile tension sensed by a tendon organ is not a simple fraction of the tension which appears at the muscle tendon. They might be accounted for as consequences of the fine structure of tendon organs and of variations in the number of muscle fibres contributed by different motor units to the bundle inserted on each receptor. The location of most tendon organs at musculo-aponeurotic junctions rather than in the tendon proper, could also be responsible for some of the observed discrepancies.

The tendon organs of cat soleus: static sensitivity to active force

The static force sensitivity of soleus tendon organ (Ib) afferents has been studied by noting their responses to graded force development produced by isometric contractions of either the whole muscle or single motor units. Data included responses of 23 Ib afferents to contraction of 8 whole muscles (8 experiments) and 16 Ib afferents to contraction of 30 motor units (5 experiments). Tendon organ responses of varying magnitude to contraction of the whole muscle or several of its individual motor units could be explained by differences in the number of muscle fibers that insert into each receptor's capsule and by differences in the contraction strength of these fibers. This finding suggests that soleus tendon organs have similar absolute sensitivities to static force development. An estimate was made of this absolute sensitivity and the value obtained (314 pps/g of force actually coupled to the receptor) is 2 orders of magnitude greater than those previously reported indices that simply relate Ib firing rate to force as measured at the tendon. The relationship between force exerted on a tendon organ's capsule and Ib firing rate is whown to be curvilinear and in keeping with a possible saturation effect that reduces the receptor's responsiveness to active contractions at relatively long muscle lengths.

The tendon organs of cat medial gastrocnemius: significance of motor unit type and size for the activation of Ib afferents

1. Histological and histochemical studies suggest that each tendon organ in a mixed mammalian muscle should be particularly responsive to the contraction of a discrete number of motor units (ca. ten to fifteen), each with differing mechanical properties. This report describes physiological experiments that demonstrate this arrangement for the tendon organs of cat medial gastrocnemius. 2. No correlations could be found between the intensity of discharge of a single tendon organ and the contraction strengths of motor units whose contraction excited the receptor. Tendon organs were found to be as responsive to contraction of small slow twitch units as they were to contraction of larger fast twitch units. Taking the data as a whole, the apparent sensitivity of the receptors during motor unit contractions (pps/force recorded at the tendon) was inversely related to the contraction strengths of the motor units. 3. These findings are discussed in relation to recent evidence on the territory of single motor units in medial gastrocnemius and the force producing capabilities of their individual muscle fibres. It is concluded that in general each motor unit, whose contraction excites a given receptor, contributes one muscle fibre to the receptor capsule. Further, it appears that the various excitatory effects of those muscle fibres inserting into a given receptor capsule are not simply related to their relative contraction strengths but also depend on the details of the mechanical coupling between each fibre and the Ib afferent receptor endings. 4. The results of an ensemble analysis show that despite the lack of correlation between the intensity of tendon organ discharge and the force developed at the tendon during contraction of different motor units, a correlation does appear when the responses of several tendon organs and the forces developed by the motor units which excite them are summed progressively. This finding has implications for the recruitment order of motor units in that the profile of the collective Ib response is shown to differ according to whether motor unit forces are summed randomly or in order of increasing contraction strengths.