Illustrations of the mouse brainstem including approximative position of optic fiber and depiction of the light spread across tissue. Using a 3D Monte Carlo simulation platform designed by Liu and colleagues (Liu et al., 2015), we identified blue circle regions of the brain where power is >5 mW/mm2 when the power at the tip of the optical fiber is 5 mW. Power is >5 mW/mm2 inside a sphere centered at the tip of the optical fiber with the following dimensions ventral = 470 µm, dorsal = 230 µm, lateral = 400 µm, caudal = 470 µm, and rostral = 470 µm. (a) Rostral section including the facial nucleus (7 n). In the 7 n, expression of ChETA-eYFP can be found, but these transfected cells are out of reach of the laser. (b) Rostral medullary sections of the brainstem with eYFP (ChETA, green) and Tac1 neurons (red) including the Bötzinger Complex and the rostral ventrolateral medulla (RVLM). Some Tac1 neurons of the Bötzinger Complex contain ChETA and are located within the circle of power 1 mW/mm2. (c) In a section located in the region of the preBötC, Tac1-ChETA co-expression was observed in the preBötC and the CVLM. (d) In caudal section, Tac1-ChETA co-expression was found in the CVLM but outside the effective range of the laser. DAPI (blue) was used to define the brain structures as reference. 7 N: facial nucleus, preBötC: preBötzinger Complex, Na: nucleus ambiguus, RVLM; rostro-ventral lateral medulla. CVLM, caudal-ventro-lateral medulla. C1A1, area C1A1.