Figure 4. Example of the effect of different combinations of cranial and torso isoflurane concentrations on response of WDR neuron to noxious stimulation. The upper row of the peristimulus-time histograms shows the responses of the unit to the noxious clamp stimulus delivered to the lateral hoof bulb (inset drawing) when torso isoflurane concentration was held constant at 1.3% and cranial isoflurane concentration was systematically increased (left to right). The lower row shows responses to increasing cranial isoflurane (left to right) when torso isoflurane concentration was 0.2%. Note the marked increase in spontaneous firing at 0.2% torso isoflurane (lower row of the histograms). Right inset drawings show spinal recording site and an action potential.