The spinal cord, anesthesia and immobility: A re-examination

There has been much recent emphasis on the spinal cord as an important site of anesthetic action with respect to immobility. For example, decerebration or spinal cord transection in rats appears to have little effect on minimum alveolar anesthetic concentration (MAC) needed to produce immobility. Likewise, selective delivery of isoflurane, halothane or thiopental to the brain circulation necessitates increased anesthetic concentrations to provide immobility. However, we have observed that chronic spinal cord transection or acute spinal cooling results in lower MAC and reduced force of limb movement. Others have shown that discrete drug microinjections at various brain sites (gabazine in tuberomammillary nucleus or pentobarbital in the pontine tegmentum) alter anesthetic requirements or produce anesthesia. Collectively, these data suggest that, while the spinal cord is a major site of action for the immobilizing properties for anesthetics, supraspinal sites still play an important role in determining MAC.