<prompt> The primary action of the neurotransmitter Nitric oxide NO in the gastrointestinal tract is:VasodilatationVasoconstrictionGastrointestinal smooth muscle relaxationGastrointestinal slow smooth muscle contraction  <response>: 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 ednpage 598-9 