Ia presynaptic inhibition in human wrist extensor muscles: effects of motor task and cutaneous afferent activity

Effects of a compression garment on sensory feedback transmission in the human upper limb

Compression apparel is popular in both medical and sport performance settings. Perceived benefits are suggested to include changes in sensory feedback transmission caused by activation of mechanoreceptors. However, little is known about effects of compression apparel on sensorimotor control. Our purpose was to mechanistically examine whether compression apparel modulates sensory feedback transmission and reaching accuracy in the upper limb. Two experiments were completed under CONTROL and COMPRESSION (sleeve applied across the elbow joint) conditions. M-waves and H-reflexes were elicited by stimulating the median nerve and were recorded via surface electromyography (EMG). In experiment 1, H-reflexes and M-H recruitment curves were assessed at REST, during wrist flexion (10% EMG_max), and during a cutaneous conditioning of the superficial radial (SR) or distal median (MED) nerve. Cutaneous reflexes were elicited during 10% wrist flexion via stimulation of SR or MED. In experiment 2, unconditioned H-reflex measures were assessed at rest, during arm cycling, and during a discrete reaching task. Results indicate that compression apparel modulates spinal cord excitability across multiple sensory pathways and movement tasks. Interestingly, there was a significant improvement in reaching accuracy while wearing the compression sleeve. Taken together, the compression sleeve appears to increase precision and sensitivity around the joint where the sleeve is applied. Compression apparel may function as a "filter" of irrelevant mechanoreceptor information allowing for optimal task-related sensory information to enhance proprioception. NEW & NOTEWORTHY Wearing a customized compression sleeve was shown to alter the excitability of multiple pathways within the central nervous system regardless of conditioning input or movement task and was accompanied by improved accuracy of reaching movements and determination of movement end point. Compression apparel may assist as a type of "filter function" of tonic and nonspecific mechanoreceptor information leading to increased precision and movement sensitivity around the joint where compression is applied.

Nerve-Specific Input Modulation to Spinal Neurons during a Motor Task in the Monkey

If not properly regulated, the large amount of reafferent sensory signals generated by our own movement could destabilize the CNS. We investigated how input from peripheral nerves to spinal cord is modulated during behavior. We chronically stimulated the deep radial nerve (DR; proprioceptive, wrist extensors), the median nerve (M; mixed, wrist flexors and palmar skin) and the superficial radial nerve (SR; cutaneous, hand dorsum) while four monkeys performed a delayed wrist flexion-extension task. Spinal neurons putatively receiving direct sensory input were defined based on their evoked response latency following nerve stimulation. We compared the influence of behavior on the evoked response (responsiveness to a specific peripheral input) and firing rate of 128 neuron-nerve pairs based on their source nerve. Firing rate increased during movement regardless of source nerve, whereas evoked response modulation was strikingly nerve-dependent. In SR (n = 47) and M (n = 27) neurons (cutaneous or mixed input), the evoked response was suppressed during wrist flexion and extension. In contrast, in DR neurons (n = 54, pure proprioceptive input), the evoked response was facilitated exclusively during movements corresponding to the contraction of DR spindle-bearing muscles (i.e., wrist extension). Furthermore, modulations of firing rate and evoked response were uncorrelated in SR and M neurons, whereas they tended to be positively comodulated in DR neurons. Our results suggest that proprioceptive and cutaneous inputs to the spinal cord are modulated differently during voluntary movements, suggesting a refined gating mechanism of sensory signals according to behavior.SIGNIFICANCE STATEMENT Voluntary movements produce copious sensory signals, which may overwhelm the CNS if not properly regulated. This regulation is called "gating" and occurs at several levels of the CNS. To evaluate the specificity of sensory gating, we investigated how different sources of somatosensory inputs to the spinal cord were modulated while monkeys performed wrist movements. We recorded activity from spinal neurons that putatively received direct connections from peripheral nerves while stimulating their source nerves, and measured the evoked responses. Whereas cutaneous inputs were suppressed regardless of the type of movement, muscular inputs were specifically facilitated during relevant movements. We conclude that, even at the spinal level, sensory gating is a refined and input-specific process.

Physiological recruitment of motor units by high-frequency electrical stimulation of afferent pathways

Neuromuscular electrical stimulation (NMES) is commonly used in rehabilitation, but electrically evoked muscle activation is in several ways different from voluntary muscle contractions. These differences lead to challenges in the use of NMES for restoring muscle function. We investigated the use of low-current, high-frequency nerve stimulation to activate the muscle via the spinal motoneuron (MN) pool to achieve more natural activation patterns. Using a novel stimulation protocol, the H-reflex responses to individual stimuli in a train of stimulation pulses at 100 Hz were reliably estimated with surface EMG during low-level contractions. Furthermore, single motor unit recruitment by afferent stimulation was analyzed with intramuscular EMG. The results showed that substantially elevated H-reflex responses were obtained during 100-Hz stimulation with respect to a lower stimulation frequency. Furthermore, motor unit recruitment using 100-Hz stimulation was not fully synchronized, as it occurs in classic NMES, and the discharge rates differed among motor units because each unit was activated only after a specific number of stimuli. The most likely mechanism behind these observations is the temporal summation of subthreshold excitatory postsynaptic potentials from Ia fibers to the MNs. These findings and their interpretation were also verified by a realistic simulation model of afferent stimulation of a MN population. These results suggest that the proposed stimulation strategy may allow generation of considerable levels of muscle activation by motor unit recruitment that resembles the physiological conditions.

Task- and time-dependent modulation of Ia presynaptic inhibition during fatiguing contractions performed by humans

Presynaptic modulation of Ia afferents converging onto the motor neuron pool of the extensor carpi radialis (ECR) was compared during contractions (20% of maximal force) sustained to failure as subjects controlled either the angular position of the wrist while supporting an inertial load (position task) or exerted an equivalent force against a rigid restraint (force task). Test Hoffmann (H) reflexes were evoked in the ECR by stimulating the radial nerve above the elbow. Conditioned H reflexes were obtained by stimulating either the median nerve above the elbow or at the wrist (palmar branch) to assess presynaptic inhibition of homonymous (D1 inhibition) and heteronymous Ia afferents (heteronymous Ia facilitation), respectively. The position task was briefer than the force task (P = 0.001), although the maximal voluntary force and electromyograph for ECR declined similarly at failure for both tasks. Changes in the amplitude of the conditioned H reflex were positively correlated between the two conditioning methods (P = 0.02) and differed between the two tasks (P < 0.05). The amplitude of the conditioned H reflex during the position task first increased (129 ± 20.5% of the initial value, P < 0.001) before returning to its initial value (P = 0.22), whereas it increased progressively during the force task to reach 122 ± 17.4% of the initial value at failure (P < 0.001). Moreover, changes in conditioned H reflexes were associated with the time to task failure and force fluctuations. The results suggest a task- and time-dependent modulation of presynaptic inhibition of Ia afferents during fatiguing contractions.

Presynaptic modulation of Ia afferents in young and old adults when performing force and position control

The present work investigated presynaptic modulation of Ia afferents in the extensor carpi radialis (ECR) when young and old adults exerted a wrist extension force either to support an inertial load (position control) or to achieve an equivalent constant torque against a rigid restraint (force control) at 5, 10, and 15% of the maximal force. H reflexes were evoked in the ECR by stimulating the radial nerve above the elbow. A conditioning stimulus was applied to the median nerve above the elbow to assess presynaptic inhibition of homonymous Ia afferents (D1 inhibition) or at the wrist (palmar branch) to assess the ongoing presynaptic inhibition of heteronymous Ia afferents that converge onto the ECR motor neuron pool (heteronymous Ia facilitation). The young adults had less D1 inhibition and greater heteronymous Ia facilitation during the position task (79 and 132.1%, respectively) compared with the force task (69.1 and 115.1%, respectively, P < 0.05). In contrast, the old adults exhibited no difference between the two tasks for either D1 inhibition ( approximately 72%) or heteronymous Ia facilitation ( approximately 114%). Contraction intensity did not influence the amount of D1 inhibition or heteronymous Ia facilitation for either group of subjects. The amount of antagonist coactivation was similar between tasks for young adults, whereas it was greater in the position task for old adults (P = 0.02). These data indicate that in contrast to young adults, old adults did not modulate presynaptic inhibition of Ia afferents when controlling the position of a compliant load but rather increased coactivation of the antagonist muscle.

Influence of load type on presynaptic modulation of Ia afferent input onto two synergist muscles

The present work was designed to investigate presynaptic modulation of Ia afferents in the extensor (ECR) and flexor carpi radialis (FCR) when the two muscles acted as synergists during radial deviation to either support an inertial load (position task) or exert an equivalent constant torque against a rigid restraint (force task). H reflexes were evoked in the ECR and FCR by stimulating at the elbow level (1-ms duration) the radial and median nerves, respectively. Conditioning stimulation was applied to the median and radial nerves at the elbow level to assess presynaptic inhibition of homonymous Ia afferent input (D1 inhibition) from the ECR and FCR, respectively. The ongoing presynaptic inhibition of heteronymous Ia afferents that converges onto ECR and FCR motor neuron pools (heteronymous Ia facilitation) was assessed by stimulating the median nerve at the wrist level (palmar branch) prior to the stimulus applied over the radial or median nerve. The heteronymous monosynaptic Ia facilitation was greater (P < 0.05) during the position task (ECR 121%; FCR 147%) compared with the force task (ECR 115%; FCR 132%), and was paralleled by the depression of D1 inhibition (P < 0.05) during the position task (ECR 75.4%; FCR 79.0%) compared with force task (ECR 58.7%; FCR 58.8%). These data indicate that Ia presynaptic inhibition is reduced during the position task relative to the force task. Such differential modulation of Ia afferent input onto the motor neuron pool likely reflects the requirement to heighten reflex responsiveness during the unstable task of maintaining limb position.

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

OBJECTIVE

This study compared reflex responsiveness of the first dorsal interosseus muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net muscle torque.

METHODS

Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered.

RESULTS

The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load.

CONCLUSIONS

Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects.

SIGNIFICANCE

Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus muscle.

Training-induced adaptive plasticity in human somatosensory reflex pathways

This paper reviews evidence supporting adaptive plasticity in muscle and cutaneous afferent reflex pathways induced by training and rehabilitative interventions. The perspective is advanced that the behavioral and functional relevance of any intervention and the reflex pathway under study should be considered when evaluating both adaptation and transfer. A cornerstone of this concept can be found in acute task-dependent reflex modulation. Because the nervous system allows the expression of a given reflex according to the motor task, an attempt to evaluate the training adaptation should also be evoked under the same conditions as training bearing in mind the functional role of the pathway under study. Within this framework, considerable evidence supports extensive adaptive plasticity in human muscle afferent pathways in the form of operant conditioning, strength training, skill training, and locomotor training or retraining. Directly comparable evidence for chronic adaptation in cutaneous reflex pathways is lacking. However, activity-dependent plasticity in cutaneous pathways is documented particularly in approaches to neurological rehabilitation. Overall, the adaptive range for human muscle afferent reflexes appears bidirectional (that is, increased or reduced amplitudes) and on the order of 25-50%. The adaptive range for cutaneous pathways is currently uncertain.

Tonic differential supraspinal modulation of PAD and PAH of segmental and ascending intraspinal collaterals of single group I muscle afferents in the cat spinal cord

We compared in the anesthetized cat the effects of reversible spinalization by cold block on primary afferent depolarization (PAD) and primary afferent hyperpolarization (PAH) elicited in pairs of intraspinal collaterals of single group I afferents from the gastrocnemius nerve, one of the pairs ending in the L3 segment, around the Clarke's column nuclei, and the other in the L6 segment within the intermediate zone. PAD in each collateral was estimated by independent computer-controlled measurement of the intraspinal current required to maintain a constant probability of antidromic firing. The results indicate that the segmental and ascending collaterals of individual afferents are subjected to a tonic PAD of descending origin affecting in a differential manner the excitatory and inhibitory actions of cutaneous and joint afferents on the pathways mediating the PAD of group I fibers. The PAD-mediating networks appear to function as distributed systems whose output will be determined by the balance of the segmental and supraspinal influences received at that moment. It is suggested that the descending differential modulation of PAD enables the intraspinal arborizations of the muscle afferents to function as dynamic systems, in which information transmitted to segmental reflex pathways and to Clarke's column neurons by common sources can be decoupled by sensory and descending inputs, and funneled to specific targets according to the motor tasks to be performed.

Differential modulation of primary afferent depolarization of segmental and ascending intraspinal collaterals of single muscle afferents in the cat spinal cord

We examined primary afferent depolarization (PAD) in the anesthetized cat elicited in 109 pairs of intraspinal collaterals of single group I afferents from the gastrocnemius nerve, one of the pair ending in the L3 segment, around the Clarke's column nuclei, and the other in the L6 segment within the intermediate zone. Tests for refractoriness were made to assess whether the responses produced by intraspinal stimulation in the L3 and L6 segments were due to activation of collaterals of the same afferent fiber. PAD in each collateral was estimated by independent computer-controlled measurement of the intraspinal current required to maintain a constant probability of antidromic firing. In most fibers, stimulation of the ipsilateral posterior biceps and semitendinosus (PBSt) nerve with trains of pulses maximal for group I afferents had a qualitatively similar effect but produced a larger PAD in the L6 than in the L3 collaterals. Stimulation of cutaneous nerves (sural and superficial peroneus) with single pulses and of the posterior articular nerve, the ipsilateral reticular formation, nucleus raphe magnus and contralateral motor cortex with trains of pulses often had qualitatively different effects. They could produce PAD and/or facilitate the PBSt-induced PAD in one collateral, and produce PAH and/or inhibit the PAD in the other collateral. These patterns could be changed in a differential manner by sensory or supraspinal conditioning stimulation. In summary, the present investigation suggests that the segmental and ascending collaterals of individual afferents are not fixed routes for information transmission, but parts of dynamic systems in which information transmitted to segmental reflex pathways and to Clarke's column neurons by common sources can be decoupled by sensory and descending inputs and funneled to specific targets according to the motor tasks to be performed.

Neural control of rhythmic, cyclical human arm movement: task dependency, nerve specificity and phase modulation of cutaneous reflexes

1. The organization and pattern of cutaneous reflex modulation during rhythmic cyclical movements of the human upper limbs has received much less attention than that afforded the lower limb. Our working hypothesis is that control mechanisms underlying the modulation of cutaneous reflex amplitude during rhythmic arm movement are similar to those that control reflex modulation in the leg. Thus, we hypothesized that cutaneous reflexes would show task dependency and nerve specificity in the upper limb during rhythmic cyclical arm movement as has been demonstrated in the human lower limb. 2. EMG was recorded from 10 muscles crossing the human shoulder, elbow and wrist joints while bilateral whole arm rhythmic cyclical movements were performed on a custom-made, hydraulic apparatus. 3. Cutaneous reflexes were evoked with trains (5 x 1.0 ms pulses at 300 Hz) of electrical stimulation delivered at non-noxious intensities (approximately 2 x threshold for radiating parasthesia) to the superficial radial, median and ulnar nerves innervating the hand. 4. Cutaneous reflexes were typically modulated with the movement cycle (i.e. phase dependency was observed). There was evidence for nerve specificity of cutaneous reflexes during rhythmic movement of the upper limbs. Task-dependent modulation was also seen as cutaneous reflexes were of larger amplitude or inhibitory (reflex reversal) during arm cycling as compared to static contraction. 5. While there are some differences in the patterns of cutaneous reflex modulation seen between the arms and legs, it is concluded that cutaneous reflexes are modulated similarly in the upper and lower limbs implicating similar motor control mechanisms.

Changes in intracortical excitability induced by stimulation of wrist afferents in man

1. Inhibitory and facilitatory neuronal circuits may be explored in the human motor cortex by double pulse transcranial magnetic stimulation (TMS). At short interstimulus intervals (2-5 ms), conditioned motor-evoked potentials (MEPs) are reduced (intracortical inhibition, ICI), whereas they are facilitated at longer interstimulus intervals (8-25 ms; intracortical facilitation, ICF). The aim of this study was to investigate the effects of homonymous and antagonist nerve stimulation on the intracortical inhibition and facilitation in the cortical areas that control the wrist extensor and flexor radialis muscles. 2. Sixteen subjects were asked to contract either their wrist extensor or flexor muscles. The MEP evoked by a test TMS (at 1.2 x MEP threshold) and recorded in the target muscle was then conditioned by subthreshold TMS (at 0.8 x MEP threshold) 2 and 14 ms before the test TMS. The median and radial nerves were stimulated at 0.8 x motor threshold (MT). 3. In both flexor and extensor muscles, antagonist nerve stimulation 40 ms before the test TMS decreased ICI and increased ICF. In contrast, homonymous nerve stimulation had no effect on ICI and ICF. 4. The intensity of the antagonist nerve stimulation required to alter ICI and ICF was as low as 0.6 x MT, which suggests that thick diameter afferents may be involved. The nerve stimulation had to be applied 35-45 ms prior to the test TMS to alter significantly the intracortical excitability. 5. Cutaneous afferents were probably not responsible for the alterations of intracortical excitability, since cutaneous stimulation had no effect on either ICI or ICF at the investigated intervals. 6. The present data suggest that antagonist muscular afferent inputs may evoke reciprocal facilitation or disinhibition at the cortical level. This pattern of antagonist sensory afferent effects may be of significance for control of the wrist extensor and flexor muscles when used as synergists during manipulatory finger movements and gripping tasks.

Task dependence of Ia presynaptic inhibition in human wrist extensor muscles: a single motor unit study

OBJECTIVE

Task-dependent changes in the Ia presynaptic inhibition generated by flexor group I afferents were investigated in 25 identified motor units (MUs) located in human extensor carpi radialis (ECR) muscles.

METHODS

Seven subjects had to voluntarily contract their ECR muscles either alone during isometric wrist extension or concurrently with their wrist and finger flexor muscles while clenching their hand around a manipulandum. The MU reflex responses to the radial nerve stimulation (test stimulation) yielded narrow peaks in the post-stimulus time histograms (PSTH). The Ia presynaptic inhibition induced while stimulating the median nerve (conditioning stimulation) 20 and 40 ms before the radial nerve was assessed from the changes in the contents of the first 0.5 ms in the peaks.

RESULTS

With both stimulation intervals, the Ia presynaptic inhibition, as assessed from the first 0.5 ms of the PSTH peaks, was consistently weaker during hand clenching. With both motor tasks, the Ia presynaptic inhibition was strongest at the 20 ms interval, in which it showed a downward gradient, working from slow to fast contracting MUs. With both intervals, the presynaptic inhibition was consistently weaker during hand clenching. The decrease in the Ia presynaptic inhibition observed at the 40 ms conditioning-test interval was less pronounced during wrist extension.

CONCLUSION

It is suggested that the reason why Ia presynaptic inhibition was weaker during hand clenching may have been that this task involved numerous cutaneous inputs originating from the palm and finger tips. During gripping tasks, these cutaneous inputs may therefore contribute to adjusting the wrist stiffness by relieving the presynaptic inhibition.