3. Parasympathetic vasodilator fibres

These originate in the brainstem and supply vascular smooth muscle cells in arterioles of gastrointestinal tract, external genitalia and salivary glands. Postganglionic nerves release acetylcholine at their nerve endings which causes vasodilatation and increased blood flow. The restricted sites of innervation means that these nerves are not as important as sympathetics in regulating peripheral resistance. Some of these fibres do give us lots of fun in life!