Short-latency afferent inhibition determined by the sensory afferent volley

Short-latency afferent inhibition (SAI) is characterized by the suppression of the transcranial magnetic stimulation motor evoked potential (MEP) by the cortical arrival of a somatosensory afferent volley. It remains unknown whether the magnitude of SAI reflects changes in the sensory afferent volley, similar to that observed for somatosensory evoked potentials (SEPs). The present study investigated stimulus-response relationships between sensory nerve action potentials (SNAPs), SAI, and SEPs and their interrelatedness. Experiment 1 (n = 23, age 23 +/- 1.5 yr) investigated the stimulus-response profile for SEPs and SAI in the flexor carpi radialis muscle after stimulation of the mixed median nerve at the wrist using similar to 25%, 50%, 75%, and 100% of the maximum SNAP and at 1.2x and 2.4x motor threshold (the latter equated to 100% of the maximum SNAP). Experiment 2 (n = 20, age 23.1 +/- 2 yr) probed SEPs and SAI stimulus-response relationships after stimulation of the cutaneous digital nerve at similar to 25%, 50%, 75%, and 100% of the maximum SNAP recorded at the elbow. Results indicate that, for both nerve types, SAI magnitude is dependent on the volume of the sensory afferent volley and ceases to increase once all afferent fibers within the nerve are recruited. Furthermore, for both nerve types, the magnitudes of SAI and SEPs are related such that an increase in excitation within somatosensory cortex is associated with an increase in the magnitude of afferent-induced MEP inhibition.