last updated 03-08-2021

Control of pupil size

Control of pupil size

Light is a strong controller of pupil size. High light levels lead to pupil constriction (miosis) and low light levels cause the pupil to dilate (mydriasis). The smooth muscle cells of the iris perform this effect.

The parasympathetic part of the autonomic nervous system maintains the muscle tone of the iris and causes miosis when stimulated. The sympathetic part causes mydriasis.

The pupillary dilator (the radial smooth muscle of the iris) is innervated by sympathetic nerves via an adrenergic effect on α1-adrenoreceptors. Upon receptor stimulation, these radial muscles contract, causing pupil dilation.

The pupillary constrictor (the sphincteric smooth muscle of the iris) is innervated by parasympathetic nerves via a cholinergic effect on muscarinic receptors. Stimulation of the muscarinic receptors causes sphincter muscle contraction and thus pupil constriction.