<prompt> Bronchial calibre is reduced by:Beta 2 receptor stimulationStimulation of sympathetic fibreStimulation of parasympathetic fiberAlpha 2 receptor stimulation  <response>: Ref: Katzung 14th ed. pg. 130Sympathetic and parasympathetic fibers present widely on the viscera, control their function.Sympathetic fibers stimulateconstrict heart increase in HR and inhibitRelax all other organs e.g. Bronchodilation, decreased GI motility, mydriasis, etc. Receptor of sympathetic fibres are: b1 present on heart mainly and b2 present on lungs, muscle, liver, etc. Stimulation of b2 receptor will cause bronchodilation Increased bronchial calibreParasympathetic fibers inhibitRelax heart decreased HR and stimulateconstrict all other organs bronchoconstrictionreduced bronchial caliber bronchoconstriction, increased GI motility, miosis, etc. Receptors of parasympathetic fibres are M1, M2 and M3 M1 is located mainly in gastric glands and CNS, M2 is located on heart and M3 is located on all other areas like eyes, glands, bladder, intestine, etc. 