Stretch reflexes in human abdominal muscles

Homonymous and heteronymous reflex connections of the abdominal muscles were investigated by the application of a tap to the muscle belly and observation of surface electromyographic responses. Reflex responses of the following abdominal muscles were investigated both ipsilateral and contralateral to the tap: rectus abdominis (RA), external oblique (EO) and internal oblique (IO). Reflexes were evoked in each of the homonymous muscles with latencies and estimated conduction velocities compatible with being evoked by Ia muscle afferents and having a monosynaptic component. Short latency heteronymous excitatory reflex connections were also observed in muscles on both ipsilateral and contralateral sides in response to the same stimulus. The latencies of the crossed responses were only marginally longer than responses evoked in the respective ipsilateral muscle. Moreover, the reflexes evoked in the IO muscle from ipsilateral and contralateral IO muscle afferents were of comparable amplitude, as were those reflexes evoked in ipsilateral and contralateral EO and RA muscles when tapping IO. These similarities in the reflex characteristics on the ipsilateral and contralateral sides suggest that abdominal muscle afferents activate similar pathways to muscles on both sides of the body. It follows that if the homonymous stretch reflex of abdominal muscles have a monosynaptic component, then a similar monosynaptic pathway activates synergistic motoneurones, not only ipsilaterally but also contralaterally.