Vascular tone is generally controlled by both the humoral component and the neuro-vegetative component. Regarding the second one, catecholaminergic sympathetic innervation of blood vessels is almost a rule in the body[1,2].
It is known that the iris is sympathetically inner-vated, both at the level of the iris dilator muscle[1,2] and at the level of iris blood vessels. Cathecolamines are responsible for vasoconstriction and, in some vascular beds, vasodilation. [...]