Startle responses measured in muscles innervated by facial and trigeminal nerves show common modulation

Electromyographic (EMG) potentials of several head muscles were recorded simultaneously in freely moving rats with chronically implanted electrodes. The startle responses of m. temporalis, m. levator auris, and m. levator labii superior were compared. All muscles showed a parallel decrease in latency and an increase in response elicitability and amplitude with an increase in stimulus intensity. A significant latency difference of about 1 ms existed between m. levator auris and m. temporalis. The shortest latency of the EMG response in m. levator auris was 5.5 ms (110 dB SPL). A common fluctuation in response amplitude and latency was found in simultaneous recordings of muscles innervated by the facial and trigeminal nerve, respectively. This shows a common modulatory input to the startle pathway to the cranial motor nuclei.