The diagrams demonstrate the effect of increasing SP and NK1 receptor expression on cholinergic nerve of airway smooth muscle. The airway smooth muscles are innervated by cholinergic and sensory nerve fibers (a). The enhancing SP associates with NK1 receptor on the surface of cholinergic neurons, which alters the sensitivity of cholinergic neurons or activates cholinergic neurons and facilitates ACh release from cholinergic nerve (b).