Keller E L, McPeek R M, Salz T
Smith-Kettlewell Eye Research Institute, San Francisco, California 94115, USA.
J Neurophysiol. 2000 Sep;84(3):1303-13. doi: 10.1152/jn.2000.84.3.1303.
Direct projections from the superior colliculus (SC) to the paramedian pontine reticular formation (PPRF) have been demonstrated anatomically. The PPRF contains cells called excitatory burst neurons (EBNs) that execute the final premotoneuronal processing for saccadic eye movements, as well as other burst cells called long-lead burst neurons (LLBNs). Previous electrophysiological tests in monkey have failed to find evidence for monosynaptic connections from the SC to EBNs, but have shown that direct projections to LLBNs exist. The validity of these results has been questioned because EBNs are known to be inhibited during periods of fixation by cells called omnipause neurons (OPNs). In later experiments in cat, the stimulus in the SC was triggered during saccades (when OPNs are off) and direct connections to EBNs were found. The present experiments were conducted to determine whether direct connections from the SC to EBNs could be demonstrated in monkey. LLBNs located near EBNs were also recorded. Single-pulse stimuli were delivered at sites in the SC at current levels well above those required to evoke saccades with pulse train stimuli. The stimuli were triggered shortly after the onset of ipsilateral or contralateral saccades and also slightly after the end of saccades. A sample of 21 EBNs was recorded and none were activated by postsaccadic stimulation or during contralateral saccades. The high spontaneous discharge rates of EBNs during ipsilateral saccades made activation of these cells more difficult to detect; however, when the results were quantified by peristimulus time histograms aligned on stimulus onset, only 1/21 EBNs showed evidence of activation in the monosynaptic range of latencies (<1.6 ms), 13 EBNs were activated at di- or polysynaptic latencies, and 7 were not activated. In contrast, 15/21 LLBNs were activated with latencies in the monosynaptic range. Further evidence supporting the absence of direct connections to EBNs was obtained by realigning the peristimulus time histograms for a subset of EBNs with similar firing rates on the time of occurrence of the last spike before stimulus onset. A subset of EBNs was also studied during drowsy ipsilaterally directed eye drifts, during which these cells were firing at low spontaneous rates and OPNs were off. No evidence for direct connections to EBNs was found in this behavioral state. The variance in results obtained for cat and monkey may be due to a species difference that reflects the more complex signal processing required in the monkey's saccadic system.
上丘（SC）至脑桥旁正中网状结构（PPRF）的直接投射已在解剖学上得到证实。PPRF中含有一类被称为兴奋性爆发神经元（EBNs）的细胞，它们执行眼球扫视运动的最后一级运动前神经元处理，此外还有其他爆发细胞，即长潜伏期爆发神经元（LLBNs）。先前在猴子身上进行的电生理测试未能找到SC与EBNs之间单突触连接的证据，但已表明存在对LLBNs的直接投射。这些结果的有效性受到了质疑，因为已知在注视期间EBNs会被全暂停神经元（OPNs）抑制。在后来对猫的实验中，在扫视期间（此时OPNs处于关闭状态）触发SC中的刺激，发现了与EBNs的直接连接。进行本实验是为了确定在猴子身上是否能证实SC与EBNs之间的直接连接。还记录了位于EBNs附近的LLBNs。在SC的位点以远高于用脉冲串刺激诱发扫视所需的电流水平施加单脉冲刺激。刺激在同侧或对侧扫视开始后不久以及扫视结束后稍晚触发。记录了21个EBNs样本，在扫视后刺激或对侧扫视期间均未激活任何一个样本。EBNs在同侧扫视期间的高自发放电率使得检测这些细胞的激活更加困难；然而，当通过在刺激开始时对齐的刺激时间直方图对结果进行量化时，只有1/21的EBNs在单突触潜伏期范围（<1.6毫秒）内显示出激活的证据，13个EBNs在双突触或多突触潜伏期被激活，7个未被激活。相比之下，21个LLBNs中有15个在单突触范围内被激活。通过将具有相似放电率的一部分EBNs的刺激时间直方图在刺激开始前最后一个尖峰出现的时间重新对齐，获得了支持不存在与EBNs直接连接的进一步证据。还在同侧困倦眼漂移期间研究了一部分EBNs，在此期间这些细胞以低自发放电率放电且OPNs处于关闭状态。在这种行为状态下未发现与EBNs直接连接的证据。猫和猴子实验结果的差异可能是由于物种差异，这反映了猴子扫视系统中更复杂的信号处理过程。
