AXONS AND SYNAPSES MEDIATING STARTLE-LIKE RESPONSES EVOKED BY ELECTRICAL-STIMULATION OF THE RETICULAR-FORMATION IN RATS - SYMMETRICAL AND ASYMMETRIC COLLISION EFFECTS

A new method for determining the locations, directions of transmission and transmission times of synapses mediating electrically evoked resp onses is proposed here. Electrical stimulation of pontine or medullary reticular formation with one 0.1-ms pulse evokes a short-latency star tle-like response. Two pulses were delivered to single sites at variou s interpulse intervals and the currents required to evoke a criterion startle response were measured. The results suggest that the startle-e voking substrates have absolute refractory periods that range from 0.2 5-0.6 ms. When one pulse was delivered to a caudal pontine site and a second pulse was delivered to a an ipsilateral medulla site, decreases in required current were observed as interpulse interval increased fr om +0.4 to +0.8 ms or as interpulse interval decreased from -0.4 to -0 .8 ms. These collision-like effects, being symmetric around an interpu lse interval of 0, suggest that electrically evoked startle is mediate d by fast axons that pass longitudinally through medulla. When one pul se was delivered to the rostral pons and a second pulse to the ipsilat eral medulla, however, required currents decreased sharply as interpul se intervals increased from +0.4 to 1.0 ms and as interpulse intervals decreased from +0.2 to -0.2 ms. These asymmetric collision-like effec ts suggest that strong synapses in the caudal pons, transmitting from pons to medulla, mediate electrically evoked startle. The 0.3-ms asymm etry suggests that the transmission time (i.e., from presynaptic stimu lus to postsynaptic action potential) averaged 0.3 ms via monosynaptic connections. The short duration of collision (0.7 ms) suggests that o nly one postsynaptic action potential was produced with high probabili ty for each presynaptic action potential. From the localization of the se effects and the short refractory periods, we estimate that < 60 gia nt cells on each side of the ventral pontine reticular formation media te the startle reflex in the rat.