Dominic Stone
The pupil is the black circular opening in the centre of the iris, which lets light into the eye. Pupillary response refers to the physiological response that varies the size of the pupil, causing it to dilate or constrict. Pupillary constriction, also known as miosis, occurs when the circular muscle, or sphincter pupillae, contracts. This muscle is controlled by the parasympathetic nervous system (PSNS) and is located in the central parts of the iris. Pupillary constriction may be caused by scleral buckles or certain drugs, such as opiates and opioids.
| Characteristics | Values |
|---|---|
| Name of muscle | Iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibres) |
| Location | Central parts of the iris |
| Controlled by | Parasympathetic nervous system (PSNS) |
| Function | Constricts the pupil in bright light (pupillary light reflex) or during accommodation |
| Dimensions | 0.75 mm wide by 0.15 mm thick |
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What You'll Learn
The iris sphincter muscle
In lower animals, the muscle cells themselves are photosensitive, causing iris action without brain input. Some humans have the rare ability to exert direct and voluntary control over their iris sphincter muscles, allowing them to dilate and constrict their pupils on command, regardless of lighting conditions.
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Pupillary constriction and dilation
The dilation of the pupil, or mydriasis, is the widening of the pupil and may be caused by adrenaline, anticholinergic agents, stimulant drugs such as MDMA, cocaine, and amphetamines, and some hallucinogenics such as LSD. Dilation occurs when the smooth cells of the radial muscle, controlled by the sympathetic nervous system (SNS), contract, and also when the cells of the iris sphincter muscle relax.
The pupillary response can be influenced by various factors, including lighting conditions, cognitive states, and certain drugs. For example, the pupils constrict in response to direct illumination (direct response) and illumination of the opposite eye (consensual response). The pupils also constrict when the eye is focused on a near object (accommodative response). Additionally, certain drugs can cause pupillary constriction, such as opiates, opioids, and anti-hypertension medications.
In certain cases, abnormalities in pupillary control can indicate underlying medical conditions. For instance, Argyll Robertson pupils, which are constricted pupils, can be a sign of late-stage syphilis. Horner's syndrome, which causes miosis in one eye, can be indicative of ocular issues such as cataracts or nerve lesions. Furthermore, the presence of an afferent pupillary defect, or Marcus Gunn (MG) sign, can be indicative of a lesion in the prechiasmal portion of the optic nerve.
While most people have involuntary pupillary responses, a rare ability that some humans possess is voluntary control over their iris sphincter and dilator muscles, allowing them to dilate and constrict their pupils on command, regardless of lighting conditions.
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Causes of miosis
Miosis, the medical term for small, constricted pupils, can be caused by a variety of factors, including certain drugs, medical conditions, and injuries.
Drugs
Several types of drugs can cause miosis. Opioids, including fentanyl, morphine, heroin, and methadone, are known to result in pinpoint pupils. Barbiturates can also cause miosis. Additionally, myotic drugs like pilocarpine, used to treat glaucoma and dry mouth, can induce miosis. Furthermore, alpha-1-adrenergic receptor antagonist medications, which are used to treat high blood pressure and benign prostatic hyperplasia, can lead to intraoperative floppy iris syndrome (IFIS), a condition associated with small pupils during cataract surgery.
Medical Conditions
Miosis can be a symptom of certain brain and nervous system conditions. For instance, intracranial hemorrhage and brain stem stroke can cause miosis in both pupils. Miosis is also associated with late-stage syphilis, specifically when the infection progresses to the brain, known as neurosyphilis. Cluster headaches, which cause severe pain around or above the eye, can also be accompanied by miosis.
Injuries
Eye injuries can lead to miosis by causing swelling and pain, making it challenging for the pupil to change sizes. This can include inflammation of the iris, known as iridocyclitis or uveitis, which can result in miosis.
Poisoning
Poisoning from substances such as organophosphates, benzodiazepines, and clonidine can also cause miosis.
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Pupillary light reflex
The pupillary light reflex (PLR) is a reflex that controls the diameter of the pupil in response to the intensity of light that falls on the retinal ganglion cells at the back of the eye. This reflex aids in adapting vision to various light and dark conditions. When exposed to greater intensity light, the pupil constricts (miosis/myosis) to allow less light in, and when exposed to lower intensity light, the pupil dilates (mydriasis) to allow more light in. Thus, the pupillary light reflex regulates the intensity of light entering the eye.
The pupil is the dark circular opening in the centre of the iris, where light enters the eye. The iris is the coloured part of the eye that surrounds the pupil. The iris sphincter and dilator muscles are the two involuntary muscles that control the amount of light travelling to the retina. The pupil is not a muscle, but a passive opening formed by the active iris.
The pupillary light reflex pathway is similar to the visual pathway. However, the optic tract fibres involved in pupillary light reflexes terminate at the pretectal nucleus in the midbrain, not the LGN of the thalamus. The intrinsic photosensitive ganglion cells, also known as melanopsin-containing cells, influence circadian rhythms and the pupillary light reflex.
There are four types of pupillary light reflexes, based on terminology of absolute laterality (left versus right) and relative laterality (same side versus opposite side, ipsilateral versus contralateral, direct versus consensual). A direct pupillary reflex is a pupillary response to light that enters the ipsilateral (same) eye. A consensual pupillary reflex is the response of a pupil to light that enters the contralateral (opposite) eye.
The pupillary light reflex is used to assess brain stem function. Abnormal pupillary light reflex can be indicative of optic nerve injury, oculomotor nerve damage, brain stem lesions, or medications like barbiturates.
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Voluntary control of pupil constriction
The pupil has tight neurological control, and abnormalities in this control can indicate underlying diagnoses. Pupillary response is a physiological reaction that varies the size of the pupil between 1.5 mm and 8 mm, via the optic and oculomotor cranial nerve.
A constriction response (miosis) 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 (PSNS), contracts, and also to an extent when the radial muscle relaxes.
Some humans have the ability to exert direct and voluntary control over their iris sphincter muscles and dilator muscles, allowing them to dilate and constrict their pupils on command, regardless of lighting conditions and/or eye accommodation state. This ability is very rare, and its potential advantages are unclear.
A single case study has shown that an individual can dilate and constrict their pupil size drastically on command. This individual was able to dilate their pupil diameter by around 0.8 mm and constrict it by around 2.4 mm. Various indirect mechanisms were tested, including accommodation, brightness, and arousal, but none of these behavioural tests could support an indirect strategy as the mode of action. It seems that the individual had learned to gain control over the pupillary response by decoupling pupil size changes from accommodation and vergence.
Using task-based functional magnetic resonance imaging (fMRI), researchers found that changes in the left pupil size were associated with increased activation in parts of the left dorsolateral prefrontal cortex, adjacent premotor areas, and the supplementary motor area. It is still unclear where these neural signals enter the final pathway, either by directly innervating the pupil's dilator or by inhibiting the parasympathetically innervated antagonistic sphincter, and vice versa for constriction.
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Frequently asked questions
The sphincter pupillae, also known as the iris sphincter muscle, pupillary sphincter, pupillary constrictor, and circular muscle of the iris, is the muscle responsible for constricting the pupil.
The sphincter pupillae muscle is located in the central region of the iris. It encircles the pupil and constricts it in response to bright light, a phenomenon known as the pupillary light reflex.
The sphincter pupillae muscle is controlled by the parasympathetic nervous system (PSNS) and parasympathetic fibers. Acetylcholine acts as the neurotransmitter for this function.
