Muscle Spindle Sensitivity Research Articles

Sensorimotor disturbances in neck disorders affecting postural stability, head and eye movement control

The receptors in the cervical spine have important connections to the vestibular and visual apparatus as well as several areas of the central nervous system. Dysfunction of the cervical receptors in neck disorders can alter afferent input subsequently changing the integration, timing and tuning of sensorimotor control. Measurable changes in cervical joint position sense, eye movement control and postural stability and reports of dizziness and unsteadiness by patients with neck disorders can be related to such alterations to sensorimotor control. It is advocated that assessment and management of abnormal cervical somatosensory input and sensorimotor control in neck pain patients is as important as considering lower limb proprioceptive retraining following an ankle or knee injury. Afferent information from the cervical receptors can be altered via a number of mechanisms such as trauma, functional impairment of the receptors, changes in muscle spindle sensitivity and the vast effects of pain at many levels of the nervous system. Recommendations for clinical assessment and management of such sensorimotor control disturbances in neck disorders are presented based on the evidence available to date.

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.

Vibration sensitivity of human muscle spindles and golgi tendon organs

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

Aging affects passive stiffness and spindle function of the rat soleus muscle

Aging affects many motor functions, notably the spinal stretch reflexes and muscle spindle sensitivity. Spindle activation also depends on the elastic properties of the structures linked to the proprioceptive receptors. We have calculated a spindle efficacy index, SEI, for old rats. This index relates the spindle sensitivity, deduced from electroneurograms recording (ENG), to the passive stiffness of the muscle. Spindle sensitivity and passive incremental stiffness were calculated during ramp and hold stretches imposed on pseudo-isolated soleus muscles of control rats (aged 4 months, n = 12) and old rats (aged 24 months, n = 16). SEI were calculated for the dynamic and static phases of ramp (1–80 mm/s) and for hold (0.5–2 mm) stretches imposed at two reference lengths: length threshold for spindle afferents discharges, L n (neurogram length) and slack length, L s. The passive incremental stiffness was calculated from the peak and steady values of passive tension, measured under the stretch conditions used for the ENG recordings, and taking into account the muscle cross-sectional area. The pseudo-isolated soleus muscles were also stretched to establish the stress–strain relationship and to calculate muscle stiffness constant. The contralateral muscle was used to count muscle spindles and spindle fibers (ATPase staining) and immunostained to identify MyHC isoforms. L n and L s lengths were not significantly different in the control group, while L n was significantly greater than L s in old muscles. Under dynamic conditions, the SEI of old muscles was the same as in controls at L s, but it was significantly lower than in controls at L n due to increased passive incremental stiffness under the stretch conditions used to analyze the ENG. Under static conditions, the SEI of old muscles was significantly lower than control values at all the stretch amplitudes and threshold lengths tested, due to increased passive incremental stiffness and decreased spindle sensitivity at L s. The muscle stiffness constant values were greater in old muscles than in controls, confirming the changes in elastic properties under passive conditions due to aging. Aging also altered the intrafusal fibers: it increased the mean number of intrafusal fibers and the contents in the slow, neonatal and developmental isoforms intrafusal of MyHC have been modified. These structural modifications do not seem great enough to counteract the loss of the spindle sensitivity or the spindle efficacy under passive conditions and after the nerve was severed. However, they may help to maintain the spindle afferent message under natural conditions and under fusimotor control.

Functional Adaptability of Jaw-muscle Spindles after Bite-raising

In a previous experiment, we found that masseter muscle spindles show functional plasticity after 5 to 15 days under increased occlusal vertical dimension (iOVD). In the present study, we hypothesized that spindle function would eventually recover if longer observation periods were allowed. Therefore, in this study we investigated changes in masseter muscle spindle function over periods of 1 day to 8 weeks. Masseter muscle-spindle responses to ramp-and-hold jaw stretches were recorded from the mesencephalic trigeminal nucleus in 35 barbiturate-anesthetized female Wistar rats. The rats were previously divided into Control and iOVD groups, and those in the iOVD group received a 2.0-mm composite resin build-up to the maxillary molars. In this condition, there were no statistically significant differences in masseter muscle spindle sensitivity between Control and iOVD in the six- and eight-week subgroups. Our results further indicate a high degree of adaptability in masseter muscle spindle function following changes in OVD.

Effects of Age and ACL Reconstruction on Quadriceps Gamma Loop Function

Both aging and anterior cruciate ligament (ACL) reconstruction are associated with strength deficits, which can in turn influence performance of activities of daily living. Thus it is informative to understand mechanisms underlying strength deficits. Age-related declines in strength follow reductions in muscle fiber numbers and size, whereas strength deficits following ACL reconstruction may be caused by the loss of intraligamentous mechanoreceptors. A common link between these conditions is the gamma spindle system, or the gamma loop. Appropriately applied vibration can affect the gamma loop by causing disruption of afferent feedback to a muscle and result in decreased force capabilities. We investigated the effect of age and ACL reconstruction on gamma loop function. Maximal isometric strength (MVC) and electromyography (EMG) of the quadriceps were quantified before and after vibration stimulation of the infrapatellar tendon of 3 groups: young healthy (n=4; mean age=23.8 yrs), young ACL reconstructed (n=7; mean age=22.4 yrs), and older healthy (n=4; mean age=66.1 yrs) individuals. The quadriceps MVC, vastus lateralis EMG, vastus medialis EMG, and rectus femoris EMG declined significantly in the young healthy group following vibration stimulation to the infrapatellar tendon, which indicated an intact gamma loop. There were no changes in these variables for the old healthy and ACL reconstructed groups. Gamma loop function was impaired in both the older and ACL reconstructed groups possibly due to either decreased muscle spindle sensitivity or the loss of mechanoreceptors.

Effect of quadriceps contraction on upper limb position sense errors in humans

Short-length muscle contraction (hold-short conditioning) causes limb position sense errors after the muscle returns to its intermediate length; this is due to intrafusal muscle thixotropy, which raises the muscle spindle sensitivity. In humans, contraction of muscles in the upper body (referred to as the Jendrassik manoeuvre) reinforces tendon reflexes in the lower limbs. However, it is unclear whether such a reinforcement manoeuvre enhances thixotropic position sense errors. We studied the effect of quadriceps contraction on upper limb position sense errors induced by hold-short conditioning of the biceps in 12 healthy men. Quadriceps contraction increased the tonic vibration reflex of the biceps, suggesting that quadriceps contraction has a reinforcing effect similar to that of the Jendrassik manoeuvre. After hold-short conditioning of the right biceps, subjects perceived that the conditioned forearm was placed in a more extended position than it actually was. Such position sense errors were enhanced during quadriceps contraction and the degree of error was increased with the intensity of the quadriceps contraction. These results suggest that limb position sense is affected by remote muscle contraction.

Lumbar position sense acuity during an electrical shock stressor

BackgroundOptimal motor control of the spine depends on proprioceptive input as a prerequisite for co-ordination and the stability of the spine. Muscle spindles are known to play an important role in proprioception. Animal experiments suggest that an increase in sympathetic outflow can depress muscle spindle sensitivity. As the muscle spindle may be influenced by sympathetic modulation, we hypothesized that a state of high sympathetic activity as during mental stress would affect the proprioceptive output from the muscle spindles in the back muscles leading to alterations in proprioception and position sense acuity. The aim was to investigate the effect of mental stress, in this study the response to an electrical shock stressor, on position sense acuity in the rotational axis of the lumbar spine.MethodsPassive and active position sense acuity in the rotational plane of the lumbar spine was investigated in the presence and absence of an electrical shock stressor in 14 healthy participants. An electrical shock-threat stressor lasting for approximately 12 minutes was used as imposed stressor to build up a strong anticipatory arousal: The participants were told that they were going to receive 8 painful electrical shocks however the participants never received the shocks. To quantify the level of physiological arousal and the level of sympathetic outflow continuous beat-to-beat changes in heart rate (beats*min-1) and systolic, diastolic and mean arterial blood pressure (mmHg) were measured. To quantify position sense acuity absolute error (AE) expressed in degrees was measured. Two-way analysis of variance with repeated measurements (subjects as random factor and treatments as fixed factors) was used to compare the different treatments.ResultsSignificant increases were observed in systolic blood pressure, diastolic blood pressure, and heart rate during the stress sessions indicating elevated sympathetic activity (15, 14 and 10%, respectively). Despite pronounced changes in the sympathetic activity and subjective experiences of stress no changes were found in position sense acuity in the rotational plane of the lumbar spine in the presence of the electrical shock stressor compared to the control period.ConclusionThe present findings indicate that position sense acuity in the rotational plane of the spine was unaffected by the electrical shock stressor.

Effects Of Age And ACL Injury On Quadriceps Gamma Loop Function

Significant muscular strength deficits are recognizable in the quadriceps muscles of both aging and ACL reconstructed individuals. These strength deficits can have a significant negative impact on functional independence, so it is both clinically and theoretically important to fully understand the possible mechanisms responsible for strength decrements associated with increasing age and ACL reconstructions. It appears that the common link between the conditions of aging and ACL reconstruction may lie in the gamma spindle system, or the gamma loop. Purpose Therefore, the purpose of this study was to investigate the effects of age and ACL reconstruction on gamma loop function. Methods Differences in isometric maximal voluntary contraction (MVC) forces and quadriceps EMG activity (vastus lateralis, vastus medialis, rectus femoris) of young healthy (n = 14; mean age 23.8 yrs), young ACL reconstructed (n = 7; mean age 22.4 yrs), and older healthy individuals (n = 14; mean age 66.1 yrs) were quantified both before and after vibration stimulation application to the infrapatellar tendon using a percussion hammer (50 Hz, 30 N, 20 min). A two-way ANOVA with repeated measures was used to evaluate group and time main effects with a Bonferroni post hoc analyses. Results The young healthy group's MVC force (p = 0.001), vastus lateralis EMG (p = 0.012), vastus medialis EMG (p = 0.002), and rectus femoris EMG (p = 0.003) declined following vibration stimulation to the infrapatellar tendon, indicating an intact gamma loop, whereas both the old healthy and ACL reconstructed groups showed no decline in either force or EMG activity. Conclusion Gamma loop function was impaired in both the older and ACL reconstructed groups possibly due to the loss of mechanoreceptors in the ACL reconstructed group and decreased muscle spindle sensitivity in the older group.Table

Effects Of Age And ACL Injury On Quadriceps Gamma Loop Function

Significant muscular strength deficits are recognizable in the quadriceps muscles of both aging and ACL reconstructed individuals. These strength deficits can have a significant negative impact on functional independence, so it is both clinically and theoretically important to fully understand the possible mechanisms responsible for strength decrements associated with increasing age and ACL reconstructions. It appears that the common link between the conditions of aging and ACL reconstruction may lie in the gamma spindle system, or the gamma loop. Purpose Therefore, the purpose of this study was to investigate the effects of age and ACL reconstruction on gamma loop function. Methods Differences in isometric maximal voluntary contraction (MVC) forces and quadriceps EMG activity (vastus lateralis, vastus medialis, rectus femoris) of young healthy (n = 14; mean age 23.8 yrs), young ACL reconstructed (n = 7; mean age 22.4 yrs), and older healthy individuals (n = 14; mean age 66.1 yrs) were quantified both before and after vibration stimulation application to the infrapatellar tendon using a percussion hammer (50 Hz, 30 N, 20 min). A two-way ANOVA with repeated measures was used to evaluate group and time main effects with a Bonferroni post hoc analyses. Results The young healthy group's MVC force (p = 0.001), vastus lateralis EMG (p = 0.012), vastus medialis EMG (p = 0.002), and rectus femoris EMG (p = 0.003) declined following vibration stimulation to the infrapatellar tendon, indicating an intact gamma loop, whereas both the old healthy and ACL reconstructed groups showed no decline in either force or EMG activity. Conclusion Gamma loop function was impaired in both the older and ACL reconstructed groups possibly due to the loss of mechanoreceptors in the ACL reconstructed group and decreased muscle spindle sensitivity in the older group.Table

Functional sparing of intrafusal muscle fibers in muscular dystrophies

In a previous study, we showed that patients with muscular dystrophies (MDs) perceive passive movements, experience sensations of illusory movement induced by muscle tendon vibration, and have proprioceptive-regulated sways in response to vibratory stimulation applied to the neck and ankle muscle tendons. These findings argue for preserved proprioceptive functions of muscle spindles. However, it is unclear whether the function of intrafusal muscle fibers is spared, i.e., whether they retain their ability to contract when submitted to a fusimotor drive. To answer this question, we analyzed the effects of reinforcement maneuvers (mental computation and the Jendrassik maneuver) that are known to increase muscle spindle sensitivity via fusimotor drive in healthy subjects. Nine patients with different MDs participated in the study. Reinforcement maneuvers increased both the mean amplitude of the Achilles tendon reflex (187 +/- 52.9% of the mean control amplitude) and the sensitivity of muscle spindle afferents to imposed movements of the ankle. The same reinforcement maneuvers failed to alter the amplitude of the Hoffmann reflex in the triceps surae muscle. These results suggest that the intrafusal muscle fibers preserve their contractile abilities in slowly progressive MDs. The reasons for a differential impairment of intrafusal and extrafusal muscle fibers and the clinical implications of the present results are discussed.

Role of Occlusal Vertical Dimension in Spindle Function

Several studies have suggested the jaw-muscle spindle as the receptor responsible for regulating and maintaining the occlusal vertical dimension (OVD). However, to challenge this assumption, we hypothesized that long-term changes in OVD could affect the sensory inputs from jaw-muscle spindles. In this study, we investigated changes in masseter muscle spindle function under an increased OVD (iOVD) condition. Responses of primary and secondary endings of masseter muscle spindles to cyclic sinusoidal stretches were investigated. Twenty barbiturate-anesthetized female Wistar rats were divided into control and iOVD groups. Rats in the iOVD group received a 2.0-mm composite resin build-up to the maxillary molars. After iOVD, masseter muscle spindle sensitivity gradually decreased. Primary and secondary spindle endings were affected differently. We conclude that iOVD caused reduction in masseter muscle spindle sensitivity. This result suggests that peripheral sensory plasticity may occur following changes in OVD. Such changes may provide a basis for physiological adaptation to clinical occlusal adjustments.

Effects of effort and EMG levels on short-latency stretch reflex modulation after varying background muscle contractions

It is known that the short-latency stretch reflex (SLSR) is modulated by the background muscle activity when it is elicited at matched torque levels. This study was designed to examine the effects of muscle contraction types before a stretch perturbation on SLSR in the human soleus muscle (SOL) when SLSR was elicited at the same levels of effort and at matched electromyographic (EMG) activity levels. A mechanical stretch perturbation was applied to the calf muscles when the ankle joint reached a ninety degree tibio-tarsal joint angle after the muscles performed an isometric (pre-ISO), shortening (pre-SHO) and lengthening contraction (pre-LEN). Subjects were seated on an ankle ergometer chair and developed 0%, 10%, 20%, 30%, 40%, 50%, 60% and 70% ankle joint torque (AJT) of maximum voluntary isometric plantar flexion contraction at 80° in pre-SHO, at 90° in pre-ISO and at 100° in pre-LEN. After that, isometric or dynamic contractions started, and the subjects were asked to maintain effort levels as, needed, to maintain the target torque levels until the end of the stretch. They relaxed their muscles fully after the stretch. This chain of processes was consecutively repeated 10 times. EMG signals obtained from SOL were averaged after they were high-pass filtered and full-wave rectified. Some major findings resulted: (1) there were no differences in SLSR area in the active muscle between pre-ISO and pre-SHO, whereas its waveform was steeper in pre-ISO than in pre-SHO. (2) SLSR p-to-p amplitude and waveform were larger and steeper in the active muscle than in the relaxed one in all conditions, whereas they were independent of the effort levels once the muscle was activated. This led to steady SLSR modulation in response to the background muscle contraction in the active muscle regardless of whether the SLSR was elicited at matched AJT or EMG activity levels. These findings suggest that SLSR is closely related to the muscle spindle sensitivity influenced by the following factors: (1) the background muscle contraction type, and (2) gamma motoneuron activity set by CNS based on the effort level.

Sympathetic outflow enhances the stretch reflex response in the relaxed soleus muscle in humans

Animal experiments suggest that an increase in sympathetic outflow can depress muscle spindle sensitivity and thus modulate the stretch reflex response. The results are, however, controversial, and human studies have failed to demonstrate a direct influence of the sympathetic nervous system on the sensitivity of muscle spindles. We studied the effect of increased sympathetic outflow on the short-latency stretch reflex in the soleus muscle evoked by tapping the Achilles tendon. Nine subjects performed three maneuvers causing a sustained activation of sympathetic outflow to the leg: 3 min of static handgrip exercise at 30% of maximal voluntary contraction, followed by 3 min of posthandgrip ischemia, and finally during a 3-min mental arithmetic task. Electromyography was measured from the soleus muscle with bipolar surface electrodes during the Achilles tendon tapping, and beat-to-beat changes in heart rate and mean arterial blood pressure were monitored continuously. Mean arterial pressure was significantly elevated during all three maneuvers, whereas heart rate was significantly elevated during static handgrip exercise and mental arithmetic but not during posthandgrip ischemia. The peak-to-peak amplitude of the short-latency stretch reflex was significantly increased during mental arithmetic (P < 0.05), static handgrip exercise (P < 0.001), and posthandgrip ischemia (P < 0.005). When expressed in percent change from rest, the mean peak-to-peak amplitude increased by 111 (SD 100)% during mental arithmetic, by 160 (SD 103)% during static handgrip exercise, and by 90 (SD 67)% during posthandgrip ischemia. The study clearly indicates a facilitation of the short-latency stretch reflex during increased sympathetic outflow. We note that the enhanced stretch reflex responses observed in relaxed muscles in the absence of skeletomotor activity support the idea that the sympathetic nervous system can exert a direct influence on the human muscle spindles.

Resting discharge of human muscle spindles is not modulated by increases in sympathetic drive.

There is evidence in experimental animals that, in addition to receiving fusimotor drive, muscle spindles are subject to modulation by the sympathetic nervous system. We examined the validity of this idea in human subjects by recording from muscle spindles in the relaxed ankle and toe extensor muscles during a strong and sustained physiological activation of muscle sympathetic outflow. Unitary recordings were made from 20 primary and 17 secondary muscle spindle afferents via a tungsten microelectrode inserted percutaneously into the peroneal nerve in 10 awake, healthy subjects seated with the legs supported in the extended position. ECG, blood pressure, respiration and calf circumference were also recorded. The majority of the muscle spindles were spontaneously active at rest; a background discharge was induced in four silent spindles by vibrating the tendon. A sustained increase in muscle vasoconstrictor activity, an increase in calf volume and a fall in pulse pressure were produced by subjects performing a 30-40 s maximal inspiratory breath-hold. Despite this strong increase in muscle sympathetic outflow no significant changes occurred in the discharge of either primary or secondary muscle spindle afferents, measured as a change in mean frequency and variability over sequential 5 s epochs and compared with the preceding period of rest. Strong chemoreceptor-driven sympathetic bursts during sustained expiratory breath-holds also failed to modulate the firing of 14 spindle endings. We conclude that a sustained, physiological increase in muscle sympathetic activity causes no detectable change in muscle spindle firing, lending no support to the concept that the sympathetic nervous system can influence the sensitivity of human muscle spindles directly.

Sympathetic nerve activity does not reduce proprioceptive acuity in humans.

Proprioception is essential in coordinating body segments and controlling muscles to perform movements. Animal studies have reported reduced muscle spindle sensitivity in jaw muscles during stimulation of sympathetic nerves. A stress-induced distortion of proprioception has been suggested that could lead to reduced fine control of movement and inefficient muscle use. The present study investigated whether increased muscle sympathetic activation alter human proprioception. Movement detection thresholds of the right ankle were determined in eight healthy subjects, before and during two conditions known to increase muscle sympathetic activity (MSA); a cold pressor test (CPT) and oral glucose ingestion (GLUC). During a cold pressor test plasma noradrenaline (NA) and mean arterial pressure (MAP) increased significantly, compared with baseline (Ps<0.01), whereas the movement thresholds were unchanged (Ps>>0.05). After GLUC plasma NA and MAP increased significantly, compared with baseline (Ps<0.05). The movement threshold into flexion decreased (P=0.022), whereas into extension there was no change (P=0.28). The most prominent increase of plasma NA corresponded with a reduced movement detection threshold, suggesting increased muscle spindle sensitivity. The present findings contradict previous reports from animal studies and the hypothesis that sympathetic nerve activity reduces proprioceptive acuity.

Position sensitivity of human muscle spindles: single afferent and population representations.

The representation of joint position at rest and during movement was investigated in 44 muscle spindle primary afferents originating from the extensor carpi radialis brevis (ECRb) and extensor digitorum (ED) of normal human subjects. Position sensitivity was estimated for each afferent, and 43 of 44 were position sensitive. In each trial, six sequential ramp-and-hold movements (2-6 degrees, 2 degrees/s, total 24 degrees) flexed the relaxed wrist, beginning from the angle at which the afferent was just recruited. Joint position was represented by three specific features of afferent firing patterns: the steady-state firing rate during the 4-s hold period between ramps, the initial burst at the beginning of each ramp, and the ramp increase in firing rate later in the movement. The position sensitivity of the initial burst (1.27 +/- 0.90 pps/degree, mean +/- SD) was several times higher than that of the hold period (0.40 +/- 0.30 pps/degree) and not different from that of the ramp increase in firing rate (1.36 +/- 0.68 pps/degree). The wrist position sensitivities of ECRb and ED afferents were equivalent, as were their recruitment angles and angular ranges of position sensitivity. Muscle spindle afferents, both individually and as a population, were shown to represent static joint position via the hold rate and the initial burst. Afferents were recruited over the entire 110 degree range of wrist positions investigated; however, the angular range over which each feature represented joint position was extremely limited (approximately 15 degrees). The population response, based on the summed activity of the 43 afferents, was monotonically related to joint position, and it was strongly influenced by the afferent recruitment pattern, but less so by the position sensitivities of the individual afferents.

The influence of increased muscle spindle sensitivity on Achilles tendon jerk and H-reflex in relaxed human subjects

Whether the fusimotor system contributes to reflex gain changes during reinforcement maneuvers is re-examined in the light of new data. Recently, from direct recordings of spindle afferent activity originating from ankle flexor muscles, we showed that mental computation increased the muscle spindle mechanical sensitivity in completely relaxed human subjects without concomitant f -motoneuron activation, providing evidence for selective fusimotor drive activation. In the present study, the effects of mental computation were investigated on monosynaptic reflexes elicited in non-contracting soleus muscle either by direct nerve stimulation (Hoffmann reflex, H) or by tendon tap (Tendinous reflex, T). The aim was to relate the time course of the changes in reflex size to the increase in spindle sensitivity during mental task in order to explore whether fusimotor activation can influence the size of the monosynaptic reflex. The results show changes in reflex amplitude that parallel the increase in muscle spindle sensitivity. When T-reflex is consistently facilitated during mental effort, the H-reflex is either depressed or facilitated, depending on the subjects. These findings suggest that the increased activity in muscle spindle primary endings may account for mental computation-induced changes in both tendon jerk and H-reflex. The facilitation of T-reflex is attributed to the enhanced spindle mechanical sensitivity and the inhibition of H-reflex is attributed to post-activation depression following the increased Ia ongoing discharge. This study supports the view that the fusimotor sensitization of muscle spindles is responsible for changes in both the mechanically and electrically elicited reflexes. It is concluded that the fusimotor drive contributed to adjustment of the size of tendon jerk and H-reflex during mental effort. The possibility that a mental computation task may also operate by reducing the level of presynaptic inhibition is discussed on the basis of H-reflex facilitation.

The effects of human ankle muscle vibration on posture and balance during adaptive locomotion

This study investigated the contribution of ankle muscle proprioception to the control of dynamic stability and lower limb kinematics during adaptive locomotion, by using mechanical vibration to alter the muscle spindle output of individuals' stance limbs. It was hypothesised that muscle length information from the ankle of the stance limb provides information describing location as well as acceleration of the centre of mass (COM) with respect to the support foot during the swing phase of locomotion. Our prediction, based on this hypothesis was that ankle muscle vibration would cause changes to the position and acceleration of the COM and/or compensatory postural responses. Vibrators were attached to both the stance limb ankle plantarflexors (at the Achilles tendon) and the opposing dorsiflexor muscle group (over tibialis anterior). Participants were required to walk along a 9-m travel path and step over any obstacles placed in their way. There were three task conditions: (1) an obstacle (15 cm in height) was positioned at the midpoint of the walkway prior to the start of the trial, (2) the same obstacle was triggered to appear unexpectedly one step in front of the participant at the walkway midpoint and (3) the subjects' walking path remained clear. The participants' starting position was manipulated so that the first step over the obstacle (when present) was always performed with their right leg. For each obstacle condition participants experienced the following vibration conditions: no vibration, vibration of the left leg calf muscles or vibration of the anterior compartment muscles of the lower left leg. Vibration began one step before the obstacle at left leg heel contact and continued for 1 s. Vibrating the ankle muscles of the stance limb during the step over an obstacle resulted in significant changes to COM behaviour [measured as displacement, acceleration and position with respect to the centre of pressure (COP)] in both the medial/lateral (M/L) and anterior/posterior planes. There were also significant task-specific changes in stepping behaviour associated with COM control (measured as peak M/L acceleration, M/L foot displacement and COP position under the stance foot during the step over the obstacle). The results provide strong evidence that the primary endings of ankle muscle spindles play a significant role in the control of posture and balance during the swing phase of locomotion by providing information describing the movement of the body's COM with respect to the support foot. Our results also provide supporting evidence for the proposal that there are context-dependent changes in muscle spindle sensitivity during human locomotion.

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.