[Does a "transcortical" loop of the long-latency muscle reflex exist?].

Abstract

EMG responses were produced by electric stimulation of the dermal branches of the nn. digilates of the index finger of the right hand and recorded from the m. interosseus dorsalis I. during its slight isometric contraction. The mean initial latency of a long-latency muscular reflex was 50.68 ms in men and 49.13 in women. The author mentions some arguments indicating the probable existence of a transcortical loop: The latency of long-latency reflexes increases gradually from proximal to distal muscles. The short-latency component of the somatosensory evoked potential N20 appears mid-time between the time of the stimulus and the onset of the long-latency reflex and thus the time interval in the long-latency reflex is sufficient for a transcortical loop. The author found a facilitation of the muscular reflex response as well as of the preparatory cortical potential during the preparatory period of voluntary flexion of the fingers during concurrent electric stimulation of the peripheral nerve. The late reflex component increases considerably and is better synchronized during the movement of the thumb instead of isometric contraction, because during kinetic innervation afferent impulses reach the precentral cortex at a higher facilitated level. These results document well the modifiable, plastic behaviour of transcortical reflexes. The method of long-latency reflexes can be used in electrophysiological diagnosis of disorders of the CNS and it is promising also in therapeutic stimulation rehabilitation and in locomotor reeducation.