The contribution of transcortical pathways to long-latency stretch and tactile reflexes in human hand muscles

Long-latency electromyographic (EMG) responses can be evoked in the first dorsal interosseous muscle (FDI) by unexpected slips of an object (skin stretch) held between the index and thumb, or by forcible adduction of the metacarpophalangeal joint (muscle stretch). The former type of response is due to stimulation of tactile afferents in the skin of the digits, whereas the latter also activates muscle receptors. Previous studies have provided good evidence that long-latency reflex responses to stretch of distal muscles involve activity in a transcortical reflex pathway. The present experiments examined whether cutaneous reflexes also utilise a transcortical route. Transcranial magnetic or electrical stimuli were given over the motor cortex to evoke EMG activity during the period of the long-latency reflex response. When evoked by muscle stretch the responses to magnetic stimulation were facilitated more than those to electric stimulation. In contrast, facilitation was equal during the long-latency reflex elicited by cutaneous stimulation. Because of the different ways in which electrical and magnetic stimuli are believed to activate the motor cortex, we interpret these results to mean that the long-latency response to skin stretch is not mediated by a transcortical mechanism in the majority of subjects, whereas that following muscle stretch is. However, these are average data. In a few individual subjects, the opposite results were obtained. We suggest that there may be differences between subjects in the transcortical contribution to long-latency reflex responses. The implication is that, under normal circumstances, several pathways may contribute to these responses. If so, the relative roles of the pathways may change during different tasks, and in pathological states lesions in one system may well be accompanied by compensatory changes in other systems.