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 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.

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.

Facilitation from flexor digitorum superficialis to extensor carpi radialis in humans.

Effects of low-threshold afferents from the flexor digitorum superficialis (FDS) to the extensor carpi radialis (ECR) motoneurons were examined using a post-stimulus time-histogram (PSTH) and electromyogram-averaging (EMG-A) methods in eight healthy human subjects. In the PSTH study in five of the eight subjects, electrical conditioning stimuli (ES) to the median nerve branch innervating FDS with the intensity below the motor threshold induced excitatory effects (facilitation) in 39 out of 92 ECR motor units. In 11 ECR motor units, the central synaptic delay of the facilitation was -0.1 ± 0.3 ms longer than that of the homonymous facilitation of ECR. Mechanical conditioning stimuli (MS) to FDS with the intensity below the threshold of the tendon(T)-wave-induced facilitation in 51 out of 51 ECR motor units. With the EMG-A method, early and significant peaks were produced by ES and MS in all the eight subjects. The difference between latencies of the peaks by ES and MS was almost equivalent to that of the Hoffmann- and T-waves of FDS by ES and MS. The peak was diminished by tonic vibration stimuli to FDS. These findings suggest that a facilitation from FDS to ECR exists in humans and group Ia afferents mediate the facilitation through a monosynaptic path.

Facilitation between extensor carpi radialis and pronator teres in humans: a study using a post-stimulus time histogram method.

Group I muscle afferents modulate the excitability of motor neurons through excitatory and inhibitory spinal reflexes. Spinal reflex relationships between various muscle pairs are well described in experimental animals but not in the human upper limb, which exhibits a fine control of movement. In the present study, spinal reflexes between the extensor carpi radialis (ECR) and pronator teres (PT) muscles were examined in healthy human subjects using a post-stimulus time histogram method. Electrical stimulation of low-threshold afferents of ECR nerves increased the motor neuron excitability in 31 of 76 PT motor units (MUs) in all eight subjects tested, while stimulation of low-threshold afferents of PT nerves increased the motor neuron excitability in 36 of 102 ECR MUs in all 10 subjects. The estimated central synaptic delay was almost equivalent to that of homonymous facilitation. Mechanical stimulation (MS) of ECR facilitated 16 of 30 PT MUs in all five subjects tested, while MS of PT facilitated 17 of 30 ECR MUs in all six subjects. These results suggest excitatory reflex (facilitation) between PT and ECR. Group I afferents should mediate the facilitation through a monosynaptic path.

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.

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.

Oligosynaptic inhibition of group I afferents between the brachioradialis and flexor carpi radialis in humans.

Spinal reflex arcs mediated by low threshold afferents between the brachioradialis (BR) and flexor carpi radialis (FCR) were studied in eleven healthy human subjects using a post-stimulus time-histogram method. Electrical conditioning stimuli (ES) to the radial nerve branch innervating BR with the intensity below the motor threshold (MT) induced an early and significant trough (inhibition) in 32/85 FCR motor units (MUs) in 9/9 subjects. Such inhibition was never provoked by cutaneous stimulation. The central synaptic delay (CSD) of the inhibition was approximately 1.1ms longer than that of the homonymous FCR facilitation. ES to the median nerve branch innervating FCR with the intensity below MT induced an inhibition in 27/71 BR-MUs in 10/10 subjects. CSD of the inhibition was about 1.1ms longer than that of the homonymous BR facilitation. These findings suggest that inhibition between BR and FCR exists in humans. Group I afferents seem to mediate the inhibition through an oligo(di or tri)-synaptic path.

Modulation of brachioradialis motoneuron excitabilities by group I fibers of the median nerve in humans.

Group I fibers from muscle spindles and Golgi tendon organs modulate motoneuron excitabilities to coordinate smooth movements. In this study, to elucidate the effects of group I fibers of the median nerve (MN) on the excitabilities of the brachioradialis (BR), we evaluated the changes in the firing probability of a BR motor unit after electrical conditioning stimulation (CS) to MN with a post-stimulus time-histogram technique in six healthy human subjects. We tested 171 motor units: in 72 of them CS to MN at the elbow with the intensity just below the threshold of alpha motor fibers (MT) produced a facilitatory effect (facilitation), while in 43 of them it produced inhibitory one (inhibition). The facilitation and inhibition were not produced by electrical stimulation of the skin overlaying MN. The central synaptic delays of the facilitation and inhibition were on average -0.13 and 0.13 msec, respectively, longer than those of the homonymous facilitation mediated by a monosynaptic path. The thresholds of the facilitation and inhibition were less than 0.7-0.8 and 0.7-0.9 times MT, respectively. CS to MN of hand muscles produced facilitatory effects and that of the pronator teres, palmaris longus, and flexor carpi radialis inhibitory effects. The facilitatory and inhibitory effects were compatible, for latency, with the facilitation and inhibition. These findings suggest that BR motoneurons receive monosynaptic facilitation and oligosynaptic inhibition from MN in humans. Group I fibers of the hand and forearm muscles should mediate the facilitation and inhibition, respectively, to coordinate movements of the hand, forearm, and elbow.