Intraoperative neurophysiologic monitoring (IONM) is commonly used during complex spinal surgery to monitor the integrity of neural structures and improve the perioperative safety profile. Transcranial Motor Evoked Potentials (TcMEPs) monitor the integrity of the motor pathways and are one of the most commonly used monitoring modalities in spinal surgery. Because inhaled anesthetics can negatively affect the ability to monitor TcMEPs, anesthesiologists commonly use a combination of propofol and opioids to maintain the anesthetic state. Additionally, anesthesiologists will frequently administer intravenous infusions of medications that can decrease postoperative pain and opioid use (called opioid-sparing adjuncts) because spinal surgeries result in significant postoperative pain. Despite the increasing use of these agents, there is scant clinical data about how they may affect the integrity of TcMEP monitoring. Magnesium (Mg), a N-methyl-d-aspartate receptors (NMDA) receptor antagonist, is one of the adjuncts with robust data supporting clinical efficacy to decrease pain and opioid use on TcMEPs. Mg has been used clinically for decades. The investigators commonly utilize intravenous magnesium as a component of our spinal anesthesia protocol. However, there is only a single case report that discusses the effects of Mg on TcMEPs. Here the investigators propose a prospective clinical trial to quantitatively assess the effects of various Mg plasma levels on TcMEPs. There is a lack of literature on the pharmacokinetics of magnesium in non-pregnant patients.