Facilitation from flexor digitorum superficialis to extensor carpi radialis in humans

Mutual oligosynaptic inhibition of group Ia afferents between the anterior and posterior parts of the deltoid in humans

The anterior (DA) and posterior parts of the deltoid (DP) show alternating contraction during shoulder flexion and extension movements. It is expected that an inhibitory spinal reflex between the DA and DP exists. In this study, spinal reflexes between the DA and DP were examined in healthy human subjects using post-stimulus time histogram (PSTH) and electromyogram averaging (EMG-A). Electrical conditioning stimulation was delivered to the axillary nerve branch that innervates the DA (DA nerve) and DP (DP nerve) with the intensity below the motor threshold. In the PSTH study, the stimulation to the DA and DP nerves inhibited (decrease in the firing probability) 31 of 54 DA motor units and 31 of 51 DP motor units. The inhibition was not provoked by cutaneous stimulation. The central synaptic delay of the inhibition between the DA and DP nerves was 1.5 ± 0.5 ms and 1.4 ± 0.4 ms (mean ± SD) longer than those of the homonymous facilitation of the DA and DP, respectively. In the EMG-A study, conditioning stimulation to the DA and DP nerves inhibited the rectified and averaged EMG of the DP and DA, respectively. The inhibition diminished with tonic vibration stimulation to the DA and DP and recovered 20-30 min after vibration removal. These findings suggest that oligo(di or tri)-synaptic inhibition mediated by group Ia afferents between the DA and DP exists in humans.

Monosynaptic facilitation of motoneurons innervating intrinsic hand muscles mediated by group Ia afferents from the extensor carpi radialis in humans

The projection pattern of low-threshold afferents from the extensor carpi radialis (ECR) to motoneurons supplying intrinsic hand muscles was investigated using the post-stimulus time-histogram (PSTH) and electromyogram-averaging (EMG-A) methods. Electrical conditioning stimulation was applied to the radial nerve branch innervating the ECR. In the PSTH study, changes in the firing probability of single motor units following the stimulation were examined. An early and significant peak (facilitation) was induced in the motoneurons innervating the muscles, but the facilitation was induced infrequently. The central latency of the facilitation was equivalent to that of homonymous facilitation through monosynaptic path in the spinal cord. In the EMG-A study, changes in the rectified and averaged electromyograms following the conditioning stimulation were examined. An early and significant peak (facilitation) was also induced. The facilitation disappeared after withdrawal of the vibration to the ECR muscle belly. Cutaneous nerve stimulation overlaying ECR never induced such facilitation in the PSTH and EMG-A studies. These findings suggest that monosynaptic facilitation mediated by group Ia afferents of ECR to the motoneurons supplying intrinsic hand muscles exists in humans, but the connection seems to be weak. This weakness might allow manipulatory movements of the hand.

Monosynaptic facilitation of flexor digitorum superficialis motoneurons mediated by group Ia afferents from the extensor carpi radialis in humans

Wrist position is known to affect the grip strength. We focused on the spinal reflex arc, which would support the movement, and investigated the effects of low-threshold afferents from the extensor carpi radialis (ECR) on the excitability of the flexor digitorum superficialis (FDS) motoneurons using the post-stimulus time-histogram (PSTH) and electromyogram-averaging (EMG-A) methods. Electrical conditioning stimulation of an intensity below the motor threshold was applied to the radial nerve branch innervating the ECR. In the PSTH study, changes in the firing probability of single motor units after electrical conditioning stimulation were investigated in seven subjects. An early and significant peak (increase in the firing probability: facilitation) was recorded for 36/60 FDS motor units. The remaining 24 motor units did not show any effects. Weak mechanical conditioning stimulation of the ECR muscle belly induced a similar peak. The central latency of the facilitation was equivalent to that of the homonymous monosynaptic facilitation. In the EMG-A study, changes in the rectified and averaged electromyograms of FDS induced by conditioning stimulation were examined in 12 subjects. An early and significant peak (facilitation) was induced by both electrical and mechanical conditioning stimulations. The facilitation decreased after withdrawal of the vibration to the ECR muscle belly. The facilitation was never induced by cutaneous nerve stimulation in the PSTH and EMG-A studies. These findings suggest that group Ia afferents from the ECR increase the excitability of FDS motoneurons through a monosynaptic path in the spinal cord. These reflex arcs likely facilitate hand grasping movements.

Monosynaptic facilitation mediated by group Ia afferents from deltoid to biceps brachii motoneurons in humans

Effects of low-threshold afferents from the anterior (DA), middle (DM) and posterior parts of the deltoid (DP) on the excitability of biceps brachii (BB) motoneurons in humans were studied. We evaluated the effects on individual motor units and motoneuron pool using a post-stimulus time-histogram (PSTH) and an electromyogram-averaging (EMG-A) methods, respectively, in 11 healthy human subjects. Electrical conditioning stimulation was delivered to the axillary nerve branch innervating DA (DA nerve), DM (DM nerve) and DP (DP nerve) with the intensity below the motor threshold. In the PSTH study, stimulation to the DA, DM and DP nerves produced a significant peak (facilitation) in 26/40 (65%), 28/47 (59%) and 0/32 (0%) of BB motor units, respectively. Since the central latency of the facilitation from the DA and DM nerves was 0.1 ± 0.3 and 0.1 ± 0.2 ms (mean ± S.D.) longer than that of the homonymous monosynaptic Ia facilitation of BB, respectively, the facilitation thus being compatible with monosynaptic path. In the EMG-A study, stimulation to the DA and DM nerves produced a significant peak (facilitation) for the BB motoneuron pool in all the subjects, whereas stimulation to the DP nerve produced no effect on BB. The facilitation diminished by vibration stimulation, and the suppression lasted for 30-40 min after removal of the vibration. Therefore, group Ia afferents should be responsible for the facilitation. These findings suggest that monosynaptic facilitation mediated by group Ia afferents from the DA and DM nerves to BB motoneurons exists in humans.

Oligosynaptic inhibition mediated by group Ia afferents from flexor digitorum superficialis to wrist flexors in humans

Effects of low-threshold afferents from the flexor digitorum superficialis (FDS) to the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU) and extensor carpi ulnaris (ECU) motoneurons were examined using a post-stimulus time-histogram (PSTH) and electromyogram-averaging (EMG-A) methods in seven healthy human subjects. Electrical conditioning stimulation to the median nerve branch innervating FDS with the intensity immediately below the motor threshold was delivered. In the PSTH study, the stimulation produced a trough (inhibition) in 19/44 (43%) of FCR and 17/41 (41%) of FCU motor units. Remaining motor units received no facilitatory and inhibitory effects. The central latency of the inhibition was 1.1 ± 0.6 ms (mean ± SD) and 0.6 ± 0.4 ms longer than that of the homonymous monosynaptic Ia facilitation of FCR and FCU, respectively. In the EMG-A study, the stimulation produced a trough (inhibition) in EMG-A of FCR and FCU in all the seven subjects. Amount of the inhibition was 14.5 ± 3.8% (FCR) and 17.9 ± 2.5% (FCU). Since the inhibition diminished after withdrawal of tonic vibration stimuli to the FDS muscle belly, group Ia afferents should be responsible for the inhibition. The stimulation did not produce facilitatory or inhibitory effect on ECU motoneurons in both the PSTH and EMG-A studies. These findings suggest that group Ia afferents from FDS inhibit excitability of motoneurons supplying FCR and FCU through an oligo (di- or tri-) synaptic path in the spinal cord. The reflex arcs would function to prevent wrist flexion during hand grasping movements.

Inhibition of Group Ia Afferents Between Brachioradialis and Flexor Carpi Radialis in Humans: A Study Using an Electromyogram-Averaging Method

PURPOSE

Our previous studies using a poststimulus time histogram method demonstrated inhibitory spinal reflex arcs (inhibition) between the brachioradialis (BR) and flexor carpi radialis (FCR) in humans. Group I afferents mediated the inhibition through an oligosynaptic path. In this study, effects of the inhibition on excitability of the motoneuron pools were examined, and we tried to clarify which afferents of group Ia or Ib are responsible for the inhibition.

METHODS

We evaluated the effects of low-threshold afferents between BR and FCR on FCR and BR motoneuron pools, respectively, using an electromyogram-averaging method in 14 healthy human subjects. Changes of rectified and averaged electromyogram of BR by electrical conditioning stimulation with the intensity below the motor threshold to the median nerve branch innervating FCR (FCR nerve) and those of FCR by the stimulation to the radial nerve branch innervating BR (BR nerve) were evaluated.

RESULTS

The stimulation to the FCR and BR nerves produced an early and significant trough of rectified and averaged electromyogram of BR and FCR, respectively, in all the subjects. The amount of inhibition of BR and FCR was 13.2 ± 3.4% (mean ± SD) and 14.2 ± 1.4%, respectively. The trough of BR and FCR diminished by tonic vibration stimuli to a respective FCR and BR. Such a trough was never provoked by cutaneous stimulation.

CONCLUSIONS

The inhibition between BR and FCR depresses excitability of the FCR and BR motoneuron pools, respectively. Group Ia afferents should mediate the inhibition.

Oligosynaptic inhibition of group Ia afferents from brachioradialis to triceps brachii motor neurons in humans

INTRODUCTION

This study examines effects of low-threshold afferents from the brachioradialis (BR) on excitability of triceps brachii (TB) motor neurons in humans.

METHODS

We evaluated the effects using a post stimulus time histogram (PSTH) and electromyogram averaging (EMG-A) methods in 13 healthy human participants. Electrical conditioning stimulation to the radial nerve branch innervating BR with the intensity below the motor threshold was delivered.

RESULTS

In the PSTH study, the stimulation produced a trough (inhibition) in 36/69 TB motor units for all the participants. A cutaneous stimulation never provoked such inhibition. The central latency of the inhibition was 1.5 ± 0.5 ms longer than that of the homonymous facilitation. In the EMG-A study, the stimulation produced inhibition in EMG-A of TB in all participants. The inhibition diminished with a tonic vibration stimulation to BR.

DISCUSSION

These findings suggest that oligosynaptic inhibition mediated by group Ia afferents from BR to TB exists in humans. Muscle Nerve 57: 122-128, 2018.

Monosynaptic facilitation of group I afferents between brachioradialis and extensor carpi radialis in humans

Spinal reflex arcs mediated by low-threshold (group I) afferents from muscle spindles and Golgi tendon organs modulate motoneuron excitabilities to coordinate smooth movements. In this study, the reflex arcs between the brachioradialis (BR) and extensor carpi radialis muscles (ECR) were examined in nine healthy human subjects using a post-stimulus time-histogram method. Electrical conditioning stimuli (ES) to the radial nerve branches innervating BR (BR nerve) and ECR (ECR nerve) with the intensity just below the motor threshold were delivered and firings of the ECR and BR motor units were recorded in 6 and 7 of the nine subjects, respectively. ES to the BR and ECR nerves induced a peak (facilitation) in 27/59 ECR and 22/68 BR motor units, respectively, in every subject. Such facilitation was never provoked by pure cutaneous stimulation. The remaining motor units received no effects by ES. The central synaptic delay of the facilitation was almost equal to that of the homonymous facilitation. These findings suggest that facilitation between BR and ECR exists in humans. Group I afferents should mediate the facilitation through a monosynaptic path in the spinal cord.