The mesencephalic locomotor region is activated during the auditory startle response of the unrestrained rat

Abstract We describe an acoustically evoked potential in the midbrain of the rat which occurred in conjunction with the auditory startle response, ‘startle correlated potential’. This potential had a variable latency to the onset of the startle-eliciting acoustic stimuli, but was precisely coupled to the startle response in the electromyogram (EMG) of the temporal muscle which was simultaneously recorded. We tried to localize the source of this potential by recording evoked potentials at different recordng sites in individual awake and unrestrained rats using a specially constructed microdrive. The potential may be generated in part by neurons in the region of the pedunculopontibe tegmental nucleus, lying within the electrophysiologically defined mesencephalic locomotor region (MLR). We suggest, therefore, that the startle correlated potential reflects the activation of the MLR during the startle response. TTiming calculations make it unlikely that the startle correlated potential is generated by a sensorimotor relay within the primary startle circuit which produced a fast startle twitch in the temporal muscle. Instead, the startle correlated potential probably reflects the involvement of the MLR in a later secondary startle response or in response modulation, e.g. habituation. In our opinion the most interesting possibility is that the MLR could be activated during a startle response to inhibit and reset the current motor program.