Electrically evoked responses of the human external oblique abdominal muscle.

An electric shock delivered to a mixed intercostal nerve of a standing human subject evokes two responses in the external oblique abdominal EMG, namely an M-wave and an electrically elicited reflex. The reflex latency is very close to that of the mechanically elicited phasic stretch reflex of the external oblique, suggesting that it is a monosynaptic spinal reflex. However, several of its characteristics make it very different from the mechanically evoked reflex and the H-responses of calf or jaw muscles. Unlike the mechanically evoked reflex, the electrically elicited reflex is neither modulated by respiration nor attenuated when the subject assumes a supine posture. Unlike the H-response of calf or jaw muscles, the electrically elicited reflex is extremely reproducible from trial to trial, displays no antidromic occlusion in response to an increase in shock strength, and is neither suppressed nor potentiated by abdominal muscle vibration. A silent period, i.e., an indicator of alpha-motoneuron suppression, follows an electrically elicited reflex only when the subject rotates his trunk away from the recording site. Presumably, inputs from external oblique muscle receptors inhibit their homonymous motoneurons effectively when the muscle is functioning as the chief power generator, but not when it is functioning as an agonist. Using single motor unit analysis, the electrically elicited reflex was shown to be comprised of monosynaptic, oligosynaptic, and multisegmental components with the major component being monosynaptic. These unique characteristics of the EER are presumably a reflection of the external oblique's multisegmental innervation, multilayered anatomy, short neuronal pathways, and diverse functions.