The relationship among brain, spinal cord and anesthetic requirements.

The spinal cord is a crucial site wherein anesthetics suppress movement in response to noxious stimuli. The balance of excitatory and inhibitory influences on the spinal cord likely determines the extent of motor response, and is thus important to anesthetic requirements. When the volatile anesthetic isoflurane is selectively delivered to the in situ goat brain (with low concentrations in the torso), anesthetic requirements increase dramatically, but when low isoflurane concentrations are delivered to the brain, anesthetic requirements decrease in the torso. When high, supraclinical concentrations of isoflurane (6-10%) are delivered to the brain and not to the torso, spontaneous movement occurs. These results are best explained by a differential effect of anesthetics on spinal cord neurons and cerebral neurons (midbrain reticular formation). Examination of neurons in the dorsal horn and midbrain reticular formation, and the electromyogram, during differential delivery of isoflurane to brain and spinal cord, will test this hypothesis.