Unexpected reflex response to transmastoid stimulation in human subjects during near-maximal effort

1. In human subjects, a high-voltage electrical pulse between electrodes fi xed over the mastoid processes activates descending tract axons at the leve l of the cervico-medullary junction to produce motor responses (cevicomedul lary evoked responses; CMEPs) in the biceps brachii and brachioradialis mus cles. 2. During isometric maximal voluntary contractions (MVCs) of the elbow flex ors, CMEPs in the biceps brachii and brachioradialis muscles are sometimes followed by a second compound muscle action potential. This response can be observed in single trials (amplitude of up to 60 % of the maximal M wave) and follows the CMEP by about 16 ms in both muscles. The response only occu rs during very strong voluntary contractions, 3. The second response following transmastoid stimulation appears with stim ulation intensities that are at the threshold for evoking a CMEP in the con tracting muscles. The response grows with increasing stimulus intensity, bu t then decreases in amplitude and finally disappears at high stimulation in tensities. 4. A single stimulus to the brachial plexus during MVCs can also elicit a s econd response (following the M wave) in the biceps brachii and brachioradi alis muscles. The latency of this response is 3-4 ms longer than that of th e second response observed following transmastoid stimulation. This differe nce in latency is consistent, with a reflex response to stimulation of larg e-diameter afferent,,. 5. The amplitude of the second response to transmastoid stimulation can be reduced by appropriately timed subthreshold transcranial magnetic stimuli, This result is consistent with intracortical inhibition of the response. 6. We suggest that transmastoid stimulation can elicit a large transcortica l reflex response in the biceps brachii and brachioradialis muscles. The re sponse. travels via the motor cortex but is only apparent during near-maxim al voluntary efforts.