Small doses of barbiturates and morphine depress respiration primarily by ________.
1) Parasympathetic action
2) Inhibiting the herine-bueuer reflex
3) Rendering the aortic chemorceptor system insensitive to O2
4) Rendering the respiratory center in the brain stem less sensitive to changes in CO₂
5) A specific effect at myoneural junctions of phrenic and intercostal nerves
