Tone burst-evoked myogenic potentials in rat neck extensor and flexor muscles.

Vestibular evoked myogenic potential (VEMP) has been used to test the vestibulocollic reflex. This study establishes a stable recording of VEMPs of animals, and presents useful parameters for vestibular ability. In an acute experiment, rats were decerebrated, and myogenic potential from neck extensor muscles was recorded. The myogenic potentials elicited by a tone-burst stimulus showed a biphasic response in the ipsilateral muscle, and the mean latency of the response was 3.56 ms, the positive peak appearing at 4.63 ms. The onset latencies of the response lengthen as the stimulus becomes weaker; this is the most suitable parameter of vestibular ability. The latencies of the monophasic response from the spinal cord were shorter than those of muscle. After injection of a muscle relaxant, myogenic potentials disappeared immediately, but the spinal cord response remained. We succeeded in recording responses not only from acute experimental animals but also from free-moving animals for the first time. These myogenic potentials were similar to VEMPs in humans; because the threshold of the response was higher than the auditory brainstem response threshold by 40-45 dB, the response could only be recorded with very high spontaneous muscle activity and the latency was shorter than the startle reflex.