Compound Projections from Extensor Carpi Radialis Longus to Biceps Brachii Motoneurons in Humans

Abstract

Neural projections from the extensor carpi radialis longus (ECRL) muscle to the ipsilateral biceps brachii (BB) motoneurons were studied in 10 healthy human subjects using a post-stimulus time histogram, in which a conditioning stimulus was triggered by the BB motor-unit discharge. Out of 100 BB motor units, electrical stimulation to the ECRL nerve using intramuscular needle electrodes induced inhibition in 15 units (15%) and facilitation in 10 units (10%). Both inhibition and facilitation were also evoked at intensities well below the motor threshold. As a reference for latency, a monosynaptic Ia facilitation was also induced in the BB in each subject by stimulating the median nerve trunk using surface electrodes at the distal end of the medial intermuscular septum of the arm. Compared to this latency, the evoked modulations by ECRL nerve stimulation were regarded as projections of mono- or di-synaptic connections.