The role of the gamma-system for improving information transmission in populations of Ia afferents

Improved proprioceptive function by application of subsensory electrical noise: Effects of aging and task-demand

The application of subsensory noise stimulation over the lower limbs has been shown to improve proprioception and postural control under certain conditions. Whereas the effect specificity seems to depend on several factors, studies are still needed to determine the appropriate method for training and rehabilitation purposes. In the current study, we investigated whether the application of subsensory electrical noise over the legs improves proprioceptive function in young and older adults. We aimed to provide evidence that stronger and age-related differential effects occur in more demanding tasks. Proprioceptive function was initially assessed by testing the detection of passive ankle movement (kinesthetic perception) in twenty-eight subjects (14 young and 14 older adults). Thereafter, postural control was assessed during tasks with different sensory challenges: i) by removing visual information (eyes closed) and; ii) by moving the visual scene (moving room paradigm). Tests performed with the application of electrical noise stimulation were compared to those performed without noise. The results showed that electrical noise applied over the legs led to a reduction in the response time to kinesthetic perception in both young and older adults. On the other hand, the magnitude of postural sway was reduced by noise stimulation only during a more challenging task, namely, when the optical flow was changing in an unpredictable (nonperiodic) manner. No differential effects of stimulation between groups were observed. These findings suggest that the relevance of proprioceptive inputs in tasks with different challenges, but not the subjects' age, is a determining factor for sensorimotor improvements due to electrical noise stimulation.

Rehabilitation strategies for wrist sensorimotor control impairment: From theory to practice

This clinical review discusses the organization, neuroanatomy, assessment, clinical relevance, and rehabilitation of sensorimotor (SM) control impairment after wrist trauma. The wrist SM control system encompasses complex SM pathways that control normal wrist active range of motion and mediate wrist joint neuromuscular stability for maintaining joint function. Among various known assessment methods of wrist SM control impairment, the active wrist joint position sense test is determined to be a clinically meaningful and responsive measure for wrist SM control impairment after wrist fracture. Wrist trauma may involve significant soft tissue injury (ie, skin, ligament, muscle), which could disrupt the generation and transmission of adequate proprioceptive input from wrist mechanoreceptors, thus leading to significant joint SM impairment. Various clinical examples of wrist trauma (eg, distal radius fracture, scapholunate joint injury) along with known prognostic factors (eg, pain) that may influence wrist SM control impairment recovery are discussed to illustrate this point. This article proposes promising rehabilitation strategies toward restoring wrist joint conscious and unconscious SM control impairments, integrating current research evidence with clinical practice. These strategies require more rigorous evaluation in clinical trials.

LEVEL OF EVIDENCE

5.

Noise-enhanced kinaesthesia: a psychophysical and microneurographic study

We first explored whether the ability of subjects to detect the direction of slow ramp imposed movements may be improved by the application of mechanical noise to muscle tendons. Movements were plantar/dorsal flexion of the ankle at 0.04°/s, and the amplitude was just sub-threshold for each subject. A white noise signal (random vibration), low-pass filtered to 100 Hz and distributed uniformly in amplitude, was applied to both the extensor and the flexor ankle muscle tendons with four different mean amplitudes (20, 30, 100, 280 μm). The population of subjects was observed to exhibit clear stochastic-type behaviour: their ability to determine the direction of sub-threshold movements significantly increased when the two lower levels of noise were added and subsequently decreased when the noise magnitude was enhanced. Second, using microneurography, we explored the response of 9 primary muscle spindle afferents and 8 cutaneous afferents to the same imposed movements with and without noise application. While these conditions of ankle mobilisation were too small to induce a response in most of the recorded afferents, two muscle afferents exhibited responses that were characteristic of aperiodic stochastic resonance behaviour: the unit movement response was either triggered or improved by the application of an optimal level of noise. All cutaneous afferents were unresponsive to the imposed movements with or without noise application. We conclude that ankle movement sense can be significantly improved by adding an optimal level of mechanical noise to ankle muscle tendons and discuss the optimisation of the response of movement-encoding receptors that may account for this improvement. The application of a mechanical noise on ankle muscle tendons may constitute a means of improving postural stability in subjects with sensory deficits.

Muscle spindle traffic in functionally unstable ankles during ligamentous stress

CONTEXT

Ankle sprains are common in athletes, with functional ankle instability (FAI) developing in approximately half of cases. The relationship between laxity and FAI has been inconclusive, suggesting that instability may be caused by insufficient sensorimotor function and dynamic restraint. Research has suggested that deafferentation of peripheral mechanoreceptors potentially causes FAI; however, direct evidence confirming peripheral sensory deficits has been elusive because previous investigators relied upon subjective proprioceptive tests.

OBJECTIVE

To develop a method for simultaneously recording peripheral sensory traffic, joint forces, and laxity and to quantify differences between healthy ankles and those with reported instability.

DESIGN

Case-control study.

SETTING

University laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 29 participants (age = 20.9 ± 2.2 years, height = 173.1 ± 8.9 cm, mass = 74.5 ± 12.7 kg) stratified as having healthy (HA, n = 19) or unstable ankles (UA, n = 10).

INTERVENTION(S)

Sensory traffic from muscle spindle afferents in the peroneal nerve was recorded with microneurography while anterior (AP) and inversion (IE) stress was applied to ligamentous structures using an ankle arthrometer under test and sham conditions.

MAIN OUTCOME MEASURE(S)

Laxity (millimeters or degrees) and amplitude of sensory traffic (percentage) were determined at 0, 30, 60, 90, and 125 N of AP force and at 0, 1, 2, 3, and 4 Nm of IE torque. Two-factor repeated-measures analyses of variance were used to determine differences between groups and conditions.

RESULTS

No differences in laxity were observed between groups (P > .05). Afferent traffic increased with increased force and torque in test trials (P < .001). The UA group displayed decreased afferent activity at 30 N of AP force compared with the HA group (HA: 30.2% ± 9.9%, UA: 17.1% ± 16.1%, P < .05).

CONCLUSIONS

The amplitude of sensory traffic increased simultaneously with greater ankle motion and loading, providing evidence of the integrated role of capsuloligamentous and musculotendinous mechanoreceptors in maintaining joint sensation. Unstable ankles demonstrated diminished afferent traffic at low levels of force, suggesting the early detection of joint loading may be compromised.

Visual, motor and attentional influences on proprioceptive contributions to perception of hand path rectilinearity during reaching

We examined how proprioceptive contributions to perception of hand path straightness are influenced by visual, motor and attentional sources of performance variability during horizontal planar reaching. Subjects held the handle of a robot that constrained goal-directed movements of the hand to the paths of controlled curvature. Subjects attempted to detect the presence of hand path curvature during both active (subject driven) and passive (robot driven) movements that either required active muscle force production or not. Subjects were less able to discriminate curved from straight paths when actively reaching for a target versus when the robot moved their hand through the same curved paths. This effect was especially evident during robot-driven movements requiring concurrent activation of lengthening but not shortening muscles. Subjects were less likely to report curvature and were more variable in reporting when movements appeared straight in a novel "visual channel" condition previously shown to block adaptive updating of motor commands in response to deviations from a straight-line hand path. Similarly, compromised performance was obtained when subjects simultaneously performed a distracting secondary task (key pressing with the contralateral hand). The effects compounded when these last two treatments were combined. It is concluded that environmental, intrinsic and attentional factors all impact the ability to detect deviations from a rectilinear hand path during goal-directed movement by decreasing proprioceptive contributions to limb state estimation. In contrast, response variability increased only in experimental conditions thought to impose additional attentional demands on the observer. Implications of these results for perception and other sensorimotor behaviors are discussed.

Myofascial syndrome and pain: A neurophysiological approach

It has been debated whether muscle spindles have a role in myofascial pain or not. We present a number of arguments for the former hypothesis. It was hypothesized that firing of intrafusal muscle fibres, i.e. fusimotor activity can be observed as "end plate spikes" (EPSs) in electromyography (EMG). The EPSs may be found in local active spots of muscle, often associated with miniature end plate potentials (MEPPs). Insertion of EMG needle electrodes into an active spot is painful, indicating nociception in the muscle spindle. Myofascial syndrome patients have taut bands with active trigger points (TrPs) in painful muscles. End plate activity (EPSs and MEPPs) is a significantly more common finding in TrPs of myofascial pain than in control points of the muscle, indicating the presence of muscle spindles. However, some control sites may show EPSs of normal muscle spindles. Increased amount of inflammatory metabolites have been observed in active TrPs. Muscle spindle is a capsulated gel-filled container, where inflammatory and contraction metabolites may be heavily concentrated during sustained fusimotor activation. Thus the intrafusal chemosensitive pain mediating III- and IV-afferents are sensitized and activated. Intrafusal inflammation causes further reflex activation of the fusimotor and skeletofusimotor systems via sensitized III- and IV-afferents. The taut band itself may be a contracture (rigor) of local skeletofusimotor (beta) units caused by sustained reflex drive by the given muscle spindles. In EMG this may be seen as complex repetitive discharges. We conclude that TrPs of myofascial pain are related to painful muscle spindles in taut bands.

Computational motor control: feedback and accuracy

Speed/accuracy trade-off is a ubiquitous phenomenon in motor behaviour, which has been ascribed to the presence of signal-dependent noise (SDN) in motor commands. Although this explanation can provide a quantitative account of many aspects of motor variability, including Fitts' law, the fact that this law is frequently violated, e.g. during the acquisition of new motor skills, remains unexplained. Here, we describe a principled approach to the influence of noise on motor behaviour, in which motor variability results from the interplay between sensory and motor execution noises in an optimal feedback-controlled system. In this framework, we first show that Fitts' law arises due to signal-dependent motor noise (SDN(m)) when sensory (proprioceptive) noise is low, e.g. under visual feedback. Then we show that the terminal variability of non-visually guided movement can be explained by the presence of signal-dependent proprioceptive noise. Finally, we show that movement accuracy can be controlled by opposite changes in signal-dependent sensory (SDN(s)) and SDN(m), a phenomenon that could be ascribed to muscular co-contraction. As the model also explains kinematics, kinetics, muscular and neural characteristics of reaching movements, it provides a unified framework to address motor variability.

Changes in human muscle spindle sensitivity during a proprioceptive attention task

The aim of the present study was to test whether fusimotor control of human muscle spindle sensitivity changed when attention was selectively directed to the recognition of an imposed two-dimensional movement in the form of a written symbol. The unitary activities of 32 muscle spindle afferents (26 Ia, 6 II) were recorded by microneurography at the level of the common peroneal nerve. The patterns of firing rate in response to passive movements of the ankle, forming different letters or numbers, were compared in two conditions: control and recognition. No visual cues were given in either condition, but subjects had to recognize and name the character in one condition compared with not paying attention in the control condition. The results showed that 58% of the tested Ia afferents presented modified responses to movements when these had to be recognized. Changes in Ia afferent responses included decreased depth of modulation, increased variability of discharge, and changes in spontaneous activity. Not all changes were evident in the same afferent. Furthermore, the percentage of correctly recognized movements amounted to 63% when changes were observed, but it was only 48% when the primary ending sensitivity was unaltered. The responses of group II afferents were only weakly changed or unchanged. It is suggested that the altered muscle spindle sensitivity is because of selective changes in fusimotor control, the consequence of which might be to feed the brain movement trajectory information that is more accurate.

Sensory-specific balance training in older adults: effect on position, movement, and velocity sense at the ankle

BACKGROUND AND PURPOSE

Age-related changes in proprioception contribute to impairments in postural control and increased fall risk in older adults. The purpose of this randomized controlled trial was to examine the effects of balance exercises on proprioception.

SUBJECTS

The participants were 36 older people and 24 younger people who were healthy.

METHODS

Older participants were randomly assigned to a balance exercise group (n=17) or a falls prevention education group (n=19). Baseline, postintervention, and 8-week follow-up measurements of 3 proprioceptive measures (threshold to perception of passive movement, passive joint position sense, and velocity discrimination) were obtained at the ankle. For comparative purposes, younger participants underwent a one-time assessment of the 3 proprioceptive measures.

RESULTS

Postintervention improvements in velocity discrimination were found in the balance exercise group when compared with values at baseline and in the falls prevention education group. Age-related differences found at baseline were reduced in the balance exercise group after intervention. Improvements were not maintained at the 8-week follow-up. Threshold to perception of passive movement and passive joint position sense did not change as a function of the exercise intervention.

DISCUSSION AND CONCLUSION

The results suggest that short-term improvements in velocity sense, but not movement and position sense, may be achieved following a balance exercise intervention.

Estimation of muscle spindle information rate by pattern matching and the effect of gamma system activity on parallel spindles

The information transmission properties of ensembles of MSs and the effect of the gamma system on these properties were studied. Three converging lines of research were taken: (1) the development of information theoretic estimation tools, and the formulation of an "operational" interpretation for the information rate; (2) animal experiments in which the mutual information rate was estimated and the effect of the gamma system was quantified; (3) simulation of a muscle spindle model with gamma activation in order to corroborate the results of the animal experiments. The main hypothesis was that the gamma system will enhance information theoretic measures that quantify the quality of the sensory neural channel comprised from an ensemble of primary muscle spindle afferents. A random stimulus was applied to a muscle in the hind limb of a cat, while spike trains from several primary MS afferents were recorded simultaneously. The stimulus was administered twice, with an operative and a disconnected gamma system. The mutual information rate between the stimulus and spike trains, as well as other information theoretic measures, was estimated. The information rate of ensembles of MSs increased with increasing ensemble size. However, with an operative gamma system the "ensemble effect" was much higher. In addition, the ensemble effect was influenced by the stimulus spectrum. A muscle spindle population model with gamma activation was simulated with stimuli that were identical to that of the animal experiments. The simulation results supported the experimental results and corroborated the main hypothesis. The results indicate that the gamma system has an important role in enhancing information transmission from ensembles of MSs to the spinal cord.

Effect of sex on preactivation of the gastrocnemius and hamstring muscles

BACKGROUND

The reason for the higher incidence of anterior cruciate ligament injury from non-contact mechanisms in female athletes is not known. Stability of the joint from dynamic restraints occurs through proprioceptive and kinaesthetic mechanisms providing a flexion moment. Reflexive muscle activation is different between the sexes, but it is unclear if sex differences exist in the ability to dynamically stabilise joints through a neuromuscular feed forward process as measured by preactivation of the muscles.

OBJECTIVE

To determine if the level of preactivation of the gastrocnemius and hamstring muscles during dynamic activity is affected by sex.

METHODS

Thirty four healthy active subjects, evenly grouped by sex, participated in the study. Maximum voluntary contraction normalised electromyographic (EMG) activity of the quadriceps, hamstrings, and gastrocnemius muscles was recorded during downhill walking (0.92 m/s) and running (2.08 m/s) on a 15 degrees declined treadmill. Preactivation of the EMG signal was calculated by setting a mark 150 milliseconds before foot strike, as indicated by a footswitch. Multiple t tests for sex differences of preactivity mean percentage (M-EMG%) during the downhill activities were performed.

RESULTS

The female subjects had a higher M-EMG% for the medial hamstrings than the male subjects (31.73 (9.89) and 23.04 (8.59) respectively; t((2,32)) = 2.732, p = 0.01) during walking. No other muscles exhibited a sex difference in M-EMG% during either activity.

CONCLUSION

The female subjects in this study showed higher medial hamstring preactivation. However, this may be because they were not injured, indicating their propensity for joint stabilisation. A long term prospective study is required to eliminate this potential explanation. No sex difference in gastrocnemius preactivation was seen, adding to the controversy about whether this muscle contributes to feed forward joint stability. Further research of preactivation of the musculature of the leg is required.

Spinal and supraspinal effects of activity in ligament afferents

In this paper available knowledge on effects from joint and ligament afferents on spinal neurones and pathways are briefly reviewed, and possible functional implications discussed. Ligament afferents may contribute to joint stability, muscle coordination and proprioception through direct polysynaptic reflex effects onto ascending pathways and skeletomotoneurones, and/or indirectly via reflex actions on the gamma-muscle spindle system. Theoretical and experimental evidence indicate that ligament afferents, together with afferents from other joint structures, muscles and the skin, provide the CNS with information on movements and posture through ensemble coding mechanisms, rather than via modality specific private pathways. The existence and functional relevance of ligamentomuscular protective reflexes, that are triggered when the ligament is threatened by potentially harmful loads, has been seriously questioned. It seems more likely that peripheral sensory inputs from ligament afferents participate in a continuous control of the muscle activity through feedforward, or preprogramming, mechanisms. In line with these ideas it has been suggested that ligament mechanoreceptors have an important role in muscle coordination and in the reflex regulation of the functional joint stability, by contributing to the preprogramming of the muscle stiffness through reflex modulation of the gamma-muscle spindle system.

Increased intra-articular concentration of bradykinin in the temporomandibular joint changes the sensitivity of muscle spindles in dorsal neck muscles in the cat

The aim of the present study was to investigate to what extent activation of bradykinin-sensitive nerve endings of the temporomandibular joint (TMJ) might induce changes in the muscle spindles output from neck muscles through reflex effects on cervical fusimotor neurones. To this end, 26 muscle spindle afferents (MSAs) emanating from the trapezius and splenius muscles of the anaesthetised cat (alpha-chloralose, initial dosage 60 mg/kg) were recorded during injection of Bradykinin (BK) (12.5-50 microg/ml) in the ipsilateral TMJ. Fifteen (58%) MSAs exhibited statistically significant fusimotor effects to injection of BK into the TMJ. Of the 15, ten MSAs showed a response related to activation of static fusimotor neurones, three MSAs showed a response related to an activation of both static and dynamic fusimotor neurones and two MSAs showed a inhibition of both static and dynamic fusimotor neurones. The control experiments suggests that the effects observed were due to activation of BK sensitive nerve endings in the TMJ. It seems possible that the reflex connections between TMJ nociceptors and the fusimotor-muscle spindle system of dorsal neck muscles might be involved in the pathophysiological mechanisms responsible for the sensory-motor disturbances in the neck region often found on patients with temporomandibular disorders.

Alteration of the position sense at the ankle induced by muscular fatigue in humans

PURPOSE

The purpose of this study was to investigate how an isometric exhaustion test affects the position sense at the ankle using an active matching task.

METHODS

Eight male subjects with a mean age 24.6 yr participated in the study. Subjects' ability to match the right ankle with the position of the left reference ankle position-determined using signed and absolute errors and variability-was investigated. This task was realized for four ankle angular values (in degrees), two in dorsiflexion (P(-20), P(-10)) and two in plantarflexion (P(20), P(10)) and for two experimental conditions: 1) normal and 2) fatigue. Muscular fatigue was induced in tibialis anterior (TA) of the right leg (ankle dorsiflexor) by using an isometric test. Subjects were instructed to maintain a workload of 70% of their maximal voluntary contraction in series of 40 s.

RESULTS

With fatigue, subjects produced ankle movements characterized by greater absolute errors for movements of large amplitude in dorsiflexion and for movements of small amplitude in plantarflexion. In addition, using a "worst-case scenario" analysis, results showed that errors were significantly greater than the normal functioning range for 20 degrees dorsiflexion and 10 degrees plantarflexion positioning.

CONCLUSIONS

The acuity of the position sense at the ankle is reduced subsequent to a fatigue protocol. The influence and the potential deleterious effects of muscular fatigue on position sense are discussed.

Increased muscle spindle sensitivity to movement during reinforcement manoeuvres in relaxed human subjects

1. The effects of reinforcement manoeuvres, such as mental computation and the Jendrassik manoeuvre, on muscle spindle sensitivity to passively imposed sinusoidal stretching (1.5 deg, 2 Hz) in relaxed subjects were analysed. 2. The unitary activity of 26 muscle spindle afferents (23 Ia, 3 II) originating from ankle muscles was recorded using the microneurographic method. Particular care was paid to the subjects' state of physical and mental relaxation. 3. The results showed that the activity of 54 % of the Ia afferents was modified during mental computation. The modifications took the form of either an increase in the number of spikes (mean, 26 % among 11 Ia fibres) or a shortening in the latency of the response to sinusoidal stretching (mean, 13 ms among 3 Ia fibres), or both. They were sometimes accompanied by an enhanced variability in the instantaneous discharge frequency. The three secondary endings tested exhibited no change in their sensitivity to stretch during mental computation. 4. The increased sensitivity to passive movements sometimes began as soon as the instructions were given to the subjects and sometimes increased during mental computation. In addition, the increased sensitivity either stopped after the subjects gave the right answer or continued for several minutes. 5. During the performance of a Jendrassik manoeuvre, the Ia units underwent changes similar to those described above for mental computation. 6. It was concluded that muscle spindle sensitivity to movement can be modified in relaxed human subjects. The results reinforce the idea that the fusimotor system plays a role in arousal and expectancy, and contribute to narrowing the gap between human and behaving animal data.

Dynamic response of human muscle spindle afferents to stretch during voluntary contraction

1. The response of twenty-eight human muscle spindle afferents from m. extensor carpi radialis brevis to large amplitude ramp stretch and release at the wrist joint was recorded. The dynamic index was calculated as the difference in firing rate between that just before the end of stretch and that during the subsequent static phase of stretch. The value during steady voluntary contraction was compared with that during relaxation. 2. In twenty-three primary afferents, the dynamic index increased in eleven and decreased in twelve afferents with a range of -8 to +25 impulses s-1. In five secondary afferents the change was less than 2 impulses s-1. 3. The primary afferents abruptly stopped firing when the stretch was released in the relaxed muscle. This cessation was prevented during contraction in seventeen primary afferents. 4. The results suggest the presence of dynamic and static fusimotor actions on the human muscle spindles during voluntary contraction.

Alterations in information transmission in ensembles of primary muscle spindle afferents after muscle fatigue in heteronymous muscle

This study showed that fatigue of the ipsilateral medial gastrocnemius muscle caused a clear-cut reduction in the ability of ensembles of primary muscle spindle afferents from the lateral gastrocnemius muscle to discriminate between muscle stretches of varying amplitude. The results were probably caused by reflex-mediated effects from chemosensitive group III and IV afferents onto the gamma-motoneurons projecting to lateral gastrocnemius muscle spindles. The experiments were conducted on seven cats anaesthetized with alpha-chloralose and a total of 41 primary muscle spindle afferents from the lateral gastrocnemius were registered. Afferents were simultaneously recorded in ensembles of three to 10 afferents. A method based on principal component analysis and algorithms for quantification of stimulus discrimination in ensembles of muscle afferents was used prior to, immediately following and five or more minutes after muscle fatigue had been induced to the ipsilateral medial gastrocnemius muscle. It is well established that the primary muscle spindle afferents play an important role in proprioception and kinaesthesia. Therefore the decrease in the accuracy of the information transmitted by ensembles of primary muscle spindle afferents caused by fatigue in an ipsilateral muscle implies concomitant effects on proprioception and kinaesthesia.

Representation of wrist joint kinematics by the ensemble of muscle spindles from synergistic muscles

Proprioceptive information about movement is transmitted to the central nervous system by a variety of receptor types, which are widely distributed among the muscles, joints, and skin. Muscle spindles are known to be an important and reliable source of information for the perception of movement kinematics. Previous studies that focused on the characteristics of single muscle spindle firing patterns have left the impression that each receptor fires in relation to a number of kinematic variables, leaving the following question unanswered: what role is played by the ensemble of muscle spindles within the same muscle or within synergistic muscles? The study described in this paper addressed whether the perception of joint position and velocity is based on the net input of muscle spindles residing in all synergistic muscles crossing a joint. Normal human adults performed a motor coordination task that required perception of joint velocity and dynamic position at the wrist. The task was to open the left hand briskly as the right wrist was passively rotated in the flexion direction through a prescribed target angle. In randomly occurring trials, the tendons to three muscles [extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), and extensor digitorum (ED)] were vibrated either individually or in different combinations during the performance of the motor task. Tendon vibration is known to distort muscle spindle firing patterns, and consequently, kinesthesia. By comparing performance errors with and without tendon vibration, the relative influences of muscle spindles residing in ECR, ECU, and ED were quantified. Vibration of the individual ECR, ECU, or ED tendons produced systematic undershoot errors in performance, consistent with the misperception of wrist velocity and dynamic position. Performance errors were larger when combinations of, rather than individual, muscle tendons were vibrated. The error resulting from simultaneous vibration of ECR and ECU was roughly equal to the sum of the errors produced by vibration of the individual tendons. These effects of vibrating synergistic tendons at the wrist suggest that kinesthesia is derived from the integrated input of muscle spindles from all synergistic muscles.

Ago-antagonist muscle spindle inputs contribute together to joint movement coding in man

The proprioceptive feedback associated with the performance of even quite simple movements is always generated by the whole set of muscles subjected to mechanical deformation (lengthening, shortening, contraction, etc.) during that particular movement. The question was addressed here as to how muscle spindle feedbacks arising from agonist and antagonist muscles may contribute to the coding of movement parameters such as the direction and velocity. For this purpose, the activity of single muscle spindle afferents located in the lateral peroneal nerve was analysed using the microneurographic technique, in human subjects performing repetitive voluntary movements, i.e., plantar/dorsal flexions of the ankle, at three different velocities (3, 4.5 and 6 degrees/s). The data obtained suggest that in humans, the direction of a slow movement may be specified on the basis of the spindle discharge rate, which is greater in the stretched than in the shortened muscle, and that the velocity of this movement might be correlated with the difference between the spindle activity occurring in the agonist and antagonist muscles. These neurophysiological data are in agreement with the results of previous psychophysical studies showing for example that a sensation of illusory movement can be elicited only when there exists an imbalance between the agonist versus antagonist vibration-induced Ia inputs. In addition, the greater the difference between the vibration frequencies applied to the two antagonist muscles, the higher the perceived movement velocity was found to be. All in all, joint movement perception seems to result from the co-processing by the central nervous system of the multiple spindle feedbacks originating from the whole set of muscles involved in the performance of a movement.

The information transmitted by ensembles of primary spindle afferents is diminished when ketamine is used as a pre-anaesthetic

The effect of pre-anaesthetic ketamine on ensemble coding of different stimuli consisting of muscle stretches of various amplitudes was studied for ensembles of simultaneously recorded primary muscle spindle afferents (MSAs). The experiments were conducted on 8 alpha-chloralose anaesthetised cats. Three of the cats received a pre-anaesthetic dose of ketamine (25 mg/kg) injected subcutaneously (ketamine group), while the remaining five animals did not (non-ketamine group). Data for ensemble coding were collected both before and after cutting the ventral root. A method based on principal component analysis and algorithms was used to quantify stimulus discrimination and an ANOVA tested differences between groups as well as differences due to ventral root cutting. When the fusimotor supply was intact, a general trend of an increase in the ability to discriminate stimuli with increasing ensemble size was observed for both groups, however, this trend was significantly greater for the non-ketamine group as compared to the ketamine group. When the ventral root was cut, the discrimination pattern for the non-ketamine group decreased significantly (as compared to before ventral root cutting), however, no change occurred for the ketamine group. Consequently, no difference in discrimination pattern was detected between groups after ventral root cutting. The reduction in information transmitted by ensembles of primary MSAs when ketamine is used as a pre-anaesthetic may suggest that ketamine elicits an adverse affect on the fusimotor system.