Effects of voluntary isometric and isotonic activity on late transcortical reflex components in normal subjects and...

When the median nerve is stimulated at the wrist during voluntary contraction of m. abductor pollicis brevis (APB) EMG activity of this muscle contains different distinct responses: a direct muscular response (M response), a spinal reflex response (S response) and a third late reflex response, first described by Upton et al. (1971) as a second volitional reflex response (V2). Comparison of this very likely long-loop reflex involving pathways to and from the cortical level (therefore labelled C response) under isometric (hold) and isotonic (move) contraction revealed a strong facilitation of the response under the latter condition. Repetitive trains of stimuli had stronger facilitatory effect on the occurrence of this late reflex response than single stimuli. Cutaneous stimuli of the median nerve afferents did not exhibit M and S responses but definite late excitatory responses with similar latencies to the C responses. In cases of hemiparesis (mild to severe loss of sensorimotor control of finger movements) the C response was diminished, delayed or absent. These findings together with recent primate and human experimental work support the hypothesis of a transcortical stretch reflex. On the premise that muscular as well as cutaneous afferents contribute to the afferent cortical input the late C response can be interpreted as the efferent component. The C response may be a valuable aid in the diagnosis of small lesions of the sensorimotor cortex and internal capsule.