Facilitation of monosynaptic reflexes by voluntary contraction of muscle in remote parts of the body. Mechanisms involved in the Jendrassik Manoeuvre.

The facilitation of the tendon reflex of quadriceps, induced by voluntary contraction of a muscle group in the upper limb, evolves in the three phases and depends on several factos. These include the strength and type (ballistic or ramp) of the contraction. In the absence of any electromyographic activity in the conditioning muscle (from a lesion of its nerve trunk or by blockage of conduction by injection of xylocaine), a voluntary attempt to contract it causes only a moderate facilitation of the quadriceps motor nucleus; while reflex activation of the conditioning muscles, by vibration or sudden stretching. causes a more marked facilitation. Thus, it is concluded that at least two distinct mechanisms act sequentially. First, a general motor facilitation of supraspinal origin induces the first moderate phase of facilitations (phase I). This early facilitation precedes the onset of electromyographic activity in the conditioning muscle. Secondly, the more marked facilitation (phase II) is related to the stimulation of afferents originating from the conditioning muscles, either by fusimotor excitation in voluntary contraction or by manoeuvres which stimulate the spindle receptors mechanically. To investigate the transmission pathways of these facilitations, the latencies of these two phases were compared at two distant motor nuclei, the trigeminal motor nucleus and the quadriceps nucleus, following voluntary contractions of the tibialis anterior. Since the facilitations of the trigeminal motor nucleus precede those of the quadriceps motor nucleus, a slowly-travelling rostro-caudal facilitation is postulated during phase II when the facilitation is related to afferents coming from the contracting muscle. A long loop, therefore, is postulated to interpret the second phase of the facilitation curves.