Axons and synapses mediating startle-like responses evoked by electrical stimulation of the reticular formation in rats: symmetric and asymmetric collision effects.
A new method for determining the locations, directions of transmission and transmission times of synapses mediating electrically evoked responses is proposed here. Electrical stimulation of pontine or medullary reticular formation with one 0.1-ms pulse evokes a short-latency startle-like response. Two pulses were delivered to single sites at various interpulse intervals and the currents required to evoke a criterion startle response were measured. The results suggest that the startle-evoking substrates have absolute refractory periods that range from 0.25-0.6 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 from +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 interpulse interval of 0, suggest that electrically evoked startle is mediated by fast axons that pass longitudinally through medulla. When one pulse was delivered to the rostral pons and a second pulse to the ipsilateral medulla, however, required currents decreased sharply as interpulse 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 effects suggest that strong synapses in the caudal pons, transmitting from pons to medulla, mediate electrically evoked startle. The 0.3-ms asymmetry suggests that the transmission time (i.e., from presynaptic stimulus to postsynaptic action potential) averaged 0.3 ms via monosynaptic connections. The short duration of collision (0.7 ms) suggests that only one postsynaptic action potential was produced with high probability for each presynaptic action potential. From the localization of these effects and the short refractory periods, we estimate that < 60 giant cells on each side of the ventral pontine reticular formation mediate the startle reflex in the rat.
Yeomans
J S
JS
Department of Psychology, University of Toronto, Ont., Canada.
Hempel
C M
CM
Chapman
C A
CA
D000818:Animals; D001369:Axons / Q000502:physiology*; D003214:Conditioning, Classical / Q000502:physiology*; D004553:Electric Conductivity / Q000502:physiology; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000502:physiology; D008297:Male; D051381:Rats; D011930:Reaction Time / Q000502:physiology; D013216:Reflex, Startle / Q000502:physiology*; D012154:Reticular Formation / Q000502:physiology*; D013569:Synapses / Q000502:physiology*