The primary action of the neurotransmitter Nitric oxide (NO) in the gastrointestinal tract is:
Vasodilatation
Vasoconstriction
Gastrointestinal smooth muscle relaxation
Gastrointestinal slow smooth muscle contraction
Nitric oxide (NO) is an inhibitory neurotransmitter of enteric motor neurons that mediates Gastrointestinal smooth muscle relaxation. Unlike most neurotransmitters and hormones, NO does not act a membrane bound receptor. Instead NO readily diffuses into adjacent target cells to directly activate guanylcyclase and mediate smooth muscle relaxation. Ref: Review of Medical Physiology by William ganong, 22nd edn/page 598-9