CROSSED RETICULAR-FORMATION CONNECTIONS THAT MEDIATE THE STARTLE REFLEX IN RATS

The startle response is a bilateral response even when elicited by uni lateral acoustic or tactile stimuli. Similarly, unilateral electrical stimulation of the reticular formation also elicits a bilateral startl e-like response. To examine whether crossed reticular formation connec tions can distribute the effects of unilateral stimulation across the midline, we delivered one pulse to the caudal pontine (RPC) or medulla ry reticular formation (MRF) and a second pulse to the opposite side o f the brain, at various interpulse intervals. The symmetric collision effects33 suggest that axons which produce at least 37% (range 23-53%) of the startle response efficacy cross from RPC to RPC with a mean co nduction velocity of 13 m/s. Similar collision effects were observed b etween RPC and MRF sites but at shorter conduction times. To examine w hich axons might cause these collision effect, the axonally transporte d label Dil was injected post mortem into 37 RPC sites. Many coarse ax ons were observed to cross in fascicles between bilateral RPC sites an d then separate in the contralateral RPC. The fiber diameters and traj ectories of these DiI-labelled axons are consistent with the conductio n velocities and trajectories of the substrates mediating the startle- like response determined in collision tests.