Yeomans J S, Hempel C M, Chapman C A
Department of Psychology, University of Toronto, Ont., Canada.
Brain Res. 1993 Jul 23;617(2):309-19. doi: 10.1016/0006-8993(93)91099-e.
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.
本文提出了一种新方法，用于确定介导电诱发反应的突触的位置、传播方向和传播时间。用一个0.1毫秒的脉冲对脑桥或延髓网状结构进行电刺激，会诱发一种潜伏期短的惊吓样反应。以不同的脉冲间隔向单个部位施加两个脉冲，并测量诱发标准惊吓反应所需的电流。结果表明，诱发惊吓的底物具有0.25 - 0.6毫秒的绝对不应期。当一个脉冲施加到脑桥尾侧部位，第二个脉冲施加到同侧延髓部位时，随着脉冲间隔从 +0.4毫秒增加到 +0.8毫秒或从 -0.4毫秒减少到 -0.8毫秒，所需电流会降低。这些类似碰撞的效应在脉冲间隔为0时对称，表明电诱发惊吓是由纵向穿过延髓的快速轴突介导的。然而，当一个脉冲施加到脑桥嘴侧，第二个脉冲施加到同侧延髓时，随着脉冲间隔从 +0.4毫秒增加到1.0毫秒以及从 +0.2毫秒减少到 -0.2毫秒，所需电流会急剧下降。这些不对称的类似碰撞的效应表明，脑桥尾侧从脑桥向延髓传递的强突触介导了电诱发惊吓。0.3毫秒的不对称性表明，通过单突触连接的传播时间（即从突触前刺激到突触后动作电位）平均为0.3毫秒。碰撞的短持续时间（0.7毫秒）表明，每个突触前动作电位很可能只产生一个突触后动作电位。根据这些效应的定位和短不应期，我们估计大鼠腹侧脑桥网状结构每侧 < 60个巨细胞介导惊吓反射。
