2. Sympathetic cholinergic vasodilator fibres

These originate in the cerebral cortex and have connections in the hypothalamus. Although these are sympathetic fibres they release acetycholine from their nerve endings which acts on muscarinic cholinoceptors on vascular smooth muscle cells causing relaxation and vasodilatation.

These fibres may give rise to the transient increase in blood flow in preparation for exercise though there is some doubt as to their existence in humans. These fibres show no tonic activity.