Fig. 2. Isoflurane minimum alveolar concentration (MAC) after different lesions of the brainstem and inactivation of the midbrain locomotor region (MLR) by lidocaine. Data are shown as mean and SEM (A ) Mean isoflurane MAC values. Dashed lines  denote transections. Precollicular decerebration did not significantly change MAC. MAC was slightly but significantly reduced to 90% of control (+, P < 0.006) after mid-collicular transections that began to encroach on or removed part of the pedunculopontine nucleus (PPN) associated with the MLR. Inactivation of the MLR with bilateral lidocaine microinjections (shaded ) decreased MAC to 68% of control (*P < 0.001), whereas transections ranging from the rostral pons to the pontomedullary junction reduced MAC to 60% (**P < 0.0006) of control (all pontine transection data pooled in figure). Mid-medullary transection only reduced MAC to 90% of control (% P < 0.03). Caudal medullary transections at the level of the obex caused MAC to decrease to approximately 50% of control ($ P < 0.0006) (B ) The same sagittal template as in (A ) depicting the actual range of transection levels (shaded ) in each group, verified histologically. 1 = mid-collicular transection range; 2 = rostral pons transection range; 3 = pontomedullary transection range; 4 = mid-medullary transection range. Precollicular decerebrations (gray dashed line ) and obex-level transections (black dashed line ) were made by viewing the superior colliculus and obex, respectively, but they were not histologically examined. CN = cuneiform nucleus; 7 = facial nucleus; Vc = trigeminal subnucleus caudalis. *= significantly different from intact MAC (paired t  test).