Co-contraction of the pronator teres and extensor carpi radialis during wrist extension movements in humans

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.

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.

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.

A Cadaver Study of Median-to-Radial Nerve Transfer for Radial Nerve Injuries

PURPOSE

To assess the anatomic feasibility of a median-to-radial nerve transfer in cadaver limbs and to quantify the number of axons present in the cut ends of the involved donor and recipient nerves.

METHODS

Ten fresh frozen cadaveric upper limbs were dissected. We investigated whether the flexor carpi radialis (FCR) branch/flexor digitorum superficialis (FDS) branch (donor nerve) reached the posterior interosseous nerve (PIN)/extensor carpi radialis brevis (ECRB) branch (recipient nerve) without tension. We also investigated the length of the transected supinator fascia for FCR-posterior interosseous nerve transfer and the FDS-ECRB positional relationship using the epicondyle line and the midline of the forearm as axes. The findings were used for these 2 types of nerve transfer with evaluation closer to the target muscles. The distance between the point at which the FDS and ECRB branches met and the point at which the ECRB branch entered the muscle was measured. After nerve coaptation, the axon number was determined by histological evaluation.

RESULTS

In all limbs, the FCR and FDS branches reached the PIN and the ECRB branch without tension. The transected supinator fascia was 17 (3-25) mm long. The point at which the FDS branch reached the ECRB branch [corrected] was 48 (23-65) mm distal to the epicondyle line and approximately 23 (18-27) mm radial to the midline of the forearm. The distance between the point at which the FDS and ECRB branches met and the point at which the ECRB branch entered the muscle was 27 (17-40) mm. The mean axon numbers were FCR, 1501 (932-3022); PIN, 5162 (4325-7732); FDS, 885 (558-962); and ECRB, 548 (433-723).

CONCLUSIONS

The FCR branch could be transferred to the PIN [corrected] and the FDS to the ECRB branch in all limbs without tension.

CLINICAL RELEVANCE

We provide anatomical and histological information for median-to-radial nerve transfer.

Strict actions of the human wrist flexors: A study with an electrical neuromuscular stimulation method

In order to elucidate strict actions of the human wrist flexors, motion and force produced by electrical neuromuscular stimulation (ENS) to each of musculus (m.) flexsor carpi radialis (FCR) and m. flexsor carpi ulnaris (FCU) with the prone, semiprone, and supine forearm were studied in ten healthy human subjects. Abduction, extension, adduction, and flexion directions were represented by, respectively, 0°, 90°, 180°, and 270°. ENS to FCR and FCU produced motion in direction of, respectively, 273° (mean) and 265° with the prone, 249° and 232° with the semiprone, and 242° and 229° with the supine forearm to the maximal range. Direction/strength (Nm) of force by ENS to FCR and FCU were, respectively, 298°/1.16 and 239°/1.70 with the prone, 279°/1.30 and 241°/1.62 with the semiprone, and 267°/1.24 and 227°/2.04 with the supine forearm. ENS to FCR exhibited force of 20-29% of maximal flexion and 7-15% of maximal abduction or 1-4% of maximal adduction and that to FCU force of 24-28% of maximal flexion and 15-25% of maximal adduction. The force study results suggest that FCU is a flexor rather than an adductor with every forearm position. FCR should be a flexor rather than an abductor with the prone and semiprone and a flexor with the supine forearm. The action of FCR as the abductor should diminish with supinating the forearm.

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.

Nerve transfers from branches to the flexor carpi radialis and pronator teres to reconstruct the radial nerve

PURPOSE

To present our method and results for transferring branches of the median nerve for radial nerve palsy or posterior cord lesions.

METHODS

We transferred 1 branch to the pronator teres to the branch to the extensor carpi radialis longus muscle and transferred the branch to the flexor carpi radialis to the posterior interosseous nerve. We carried out these transfers in 6 patients with radial nerve palsy or posterior cord lesions. We reviewed functional outcomes, Disabilities of the Arm, Shoulder and Hand scores, and Patient Evaluation Measure scores.

RESULTS

After 20 months of follow-up evaluation, all patients had recovered extensor carpi radialis longus activity of M4. Activity of the extensor carpi ulnaris was M3 in 2 patients and M4 in 4 patients. Extensor pollicis longus activity was M4 in all 6 cases. Metacarpophalangeal extension was M4 in 4 cases and M3 in 2 cases. The mean Disabilities of the Arm, Shoulder, and Hand score was 26 (range, 7-43), and the mean Patient Evaluation Measure score was 34 (range, 24-53).

CONCLUSIONS

Selective independent synergistic transfer of median nerve fascicles to the radial nerve branches has shown excellent results in the treatment of severe lesions of the radial nerve.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic IV.

Transfer of median and ulnar nerve fascicles for lesions of the posterior cord in infraclavicular brachial plexus injury: report of 2 cases

In infraclavicular lesions of brachial plexus, severe lesions of the posterior cord often occur when medial and lateral cord function is preserved to a greater or lesser extent. In these cases, shoulder function may be preserved by activity of the muscles innervated by the suprascapular nerve, but complete paralysis exists in the deltoid, triceps, and brachioradialis, and all wrist and finger extensors. Classical reconstruction procedures consist of nerve grafts, but their results in adults are disappointing. We report an approach transferring: (1) an ulnar nerve fascicle to the motor branch of the long portion of the triceps brachii muscle, (2) a median nerve branch from the pronator teres to the motor branch of the extensor carpi radialis longus, and (3) a median nerve branch from the flexor carpi radialis to the posterior interosseous nerve. We describe the procedure and report 2 clinical cases showing the effectiveness of this technique for restoring extension of the elbow, wrist, and fingers in the common infraclavicular lesions of the brachial plexus affecting the posterior cord.