The maintenance of partial neuromuscular blocking during general anesthesia for neurosurgery is essential for intraoperative motor-evoked potential monitoring. However, the precise strategy of administering neuromuscular blocking agent for obtaining that goal has not been established. Therefore, the investigators tried to find the optimal initial dose of vecuronium infusion and determine the adequate goal of neuromuscular blocking as guided by neuromuscular transmission module (M-NMT Module, Datex-Ohmeda Inc, Helsinki, Finland). Previously, one to two counts of response to TOF stimulation has been considered to be primary goal of partial neuromuscular blocking needed for intraoperative motor evoked potential monitoring. However, the visualization of twitch height of response to TOF stimulation has been possible with the help of NMT module. For adequate motor evoked potential monitoring, twitch height of T1 or T2 is also as important as simple count of TOF stimulation. The investigators tried to establish a vecuronium infusion strategy as guided by not only count of TOF stimulation but also twitch height of T1 or T2.