Facilitation from flexor digitorum superficialis to extensor carpi radialis in humans

Abstract

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 Ia afferents from brachioradialis to triceps brachii motor neurons in humans

“…The PSTH method was employed to evaluate the effects of a reflex arc on each individual motor neuron and to estimate the central synaptic delay of the reflex arc . PSTHs (bin width 0.1 ms) of the discharge of a voluntarily activated TB motor unit (firing interval: about 100 ms) were constructed for the period from 10 to 50 ms after the conditioning stimulation.…”

“…For motor units receiving inhibition, to estimate the central synaptic delay of the inhibitory process, latencies of the troughs and the monosynaptic peaks in PSTH were compared following stimuli to the respective BR and homonymous afferents (radial nerve; Fig. ) . The homonymous TB afferents were stimulated at the proximal part of the medial intermuscular septum (axilla) or the distal part of the lateral intermuscular septum of the arm with surface electrodes.…”

“…The EMG‐A method was employed to evaluate effects of the reflex arcs on excitability of the motor neuron pool and to examine effects of tonic vibration stimulation (TVS) on the transmission of group Ia afferents . The EMG‐A technique used in this study has been described previously . The conditioning stimulation was delivered during recording of EMGs of TB exhibiting steady tonic contraction.…”

“…To examine effects of TVS on the inhibition, TVS (100 H Z ) was delivered to the BR muscle belly by using a portable vibrator (MD‐011; Daito Electric Machine, Higashi‐Osaka, Japan) . The head of a vibrator was firmly covered by a thermoplastic board (Ezeform splinting material; Sakai Medical, Tokyo, Japan), to which a steel screw bar (0.5‐cm diameter, 5.0‐cm length) was installed.…”

“…Spinal reflex arcs mediated by low‐threshold afferents (group I afferents, Ia fibers from muscle spindles and Ib fibers from Golgi tendon organs) among muscles in the human upper limb have been extensively studied . Among human elbow flexors and extensors, oligosynaptic inhibition mediated by group Ia afferents has been demonstrated between the biceps brachii (BB) and the triceps brachii (TB) muscles, which are elbow flexors and extensors, respectively, and between the BB and the brachioradialis (BR) muscles, both of which are elbow flexors .…”