Mydriasis is an excessive dilation of the pupil of the eye due to the use of drugs or disease. Normally, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day. A mydriatic pupil will remain excessively large even in a bright environment and is sometimes colloquially referred to as a "blown pupil". The opposite, constriction of the pupil, is referred to as miosis.
Mechanism of Mydriasis
There are two types of muscle that control the size of the iris: circular muscle and radial muscle. The former is innervated by the parasympathetic nervous system, the latter by the sympathetic nervous system. Sympathetic stimulation of a1 adrenergic receptors causes the contraction of the radial muscle, and subsequent dilation of the pupil. Conversely, parasympathetic stimulation cause contraction of the circular muscle and constriction of the pupil.
The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which causes contraction of the pupil), or overactivity of the sympathetic nervous system (SNS).