Anatomical distribution and response patterns of reticular neurons active in relation to acoustic startle.

A population of reticulospinal neurons with short latency response to startle-inducing stimuli was identified in the nucleus reticularis pontis caudalis (NRPC) and nucleus gigantocellularis (NRGC) of the medial pontomedullary reticular formation. The threshold and magnitude of response to auditory stimuli was correlated in these cells and in the muscles mediating startle. Startle-related neurons were significantly more likely to have high conduction velocity spinal projections than adjacent cells not related to startle. Startle-related cells were not 'dedicated' to startle, but were active in relation to spontaneous movements. Both the unit response of the startle-related cells and the startle response recorded in muscles were suppressed by the prior presentation of a weak prepulse. Thus, prepulse inhibition of startle occurs at, or prior to, the medial pontomedullary reticular formation. We conclude that these reticulospinal cells convey the output of the brainstem system modulating and triggering startle.