Pupillary response
Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.
A constriction response, is the narrowing of the pupil, which may be caused by scleral buckles or drugs such as opiates/opioids or anti-hypertension medications. Constriction of the pupil occurs when the circular muscle, controlled by the parasympathetic nervous system, contracts.
A dilation response, is the widening of the pupil and may be caused by adrenaline, anti-cholinergic agents or drugs such as MDMA, cocaine, amphetamines, dissociatives and some hallucinogenics. Dilation of the pupil occurs when the smooth cells of the radial muscle, controlled by the sympathetic nervous system, contract.
The responses can have a variety of causes, from an involuntary reflex reaction to exposure or inexposure to light—in low light conditions a dilated pupil lets more light into the eye—or it may indicate interest in the subject of attention, or sexual stimulation. The pupils contract immediately before REM sleep begins. A pupillary response can be intentionally conditioned as a Pavlovian response to some stimuli.
The latency of pupillary response increases with age. Use of central nervous system stimulant drugs and some hallucinogenic drugs can cause dilation of the pupil.
In ophthalmology, intensive studies of pupillary response are conducted via videopupillometry.
Anisocoria is the condition of one pupil being more dilated than the other.
| Constriction | Dilation | |
| Muscular mechanism | Relaxation of iris dilator muscle, activation of iris sphincter muscle | Activation of iris dilator muscle, relaxation of iris sphincter muscle |
| Cause in pupillary light reflex | Increased light | Decreased light |
| Other physiological causes | Accommodation reflex | Fight-or-flight response |
| Corresponding non-physiological state | Miosis | Mydriasis |