Dominic Stone
The iris is a ring-shaped structure located between the cornea and the lens of the eye. It contains two sets of smooth muscles: the sphincter pupillae (circular muscles) and the dilator pupillae (radial muscles). The contraction of the sphincter pupillae muscle fibres leads to a decrease in pupil size, which is triggered by signals sent via the optic nerve to the brain when light hits the retina. This occurs in bright light to limit the amount of light entering the eye.
| Characteristics | Values |
|---|---|
| Muscle that decreases pupil size | Sphincter pupillae, or iris sphincter muscle |
| Other names | Pupillary sphincter, sphincter muscle, circular muscle |
| Location | Central parts of the iris, near the pupillary margin |
| Function | Constricts the pupil in bright light to control the amount of light entering the eye |
| Innervation | Parasympathetic, via the short ciliary nerves |
| Related conditions | Adie syndrome, iridoplegia |
| Substances causing constriction | Pilocarpine, opioids |
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What You'll Learn
The iris sphincter muscle
The iris contains two groups of smooth muscles: a circular group called the sphincter pupillae and a radial group called the dilator pupillae. When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. This is known as miosis. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract. This is known as mydriasis. The sensory pathway (rod or cone, bipolar, ganglion) is linked with its counterpart in the other eye by a partial crossover of each eye's fibres. This causes the effect in one eye to carry over to the other.
The pupil diameter of our eyes continuously fluctuates with changes in luminance, accommodation, and various brain states related to arousal, locomotion, emotion, attention, and cognitive load. The iris sphincter muscle, which is under the control of the parasympathetic pathway, is quickly modulated by the neuromodulatory system. By pharmacologically manipulating the pupil dilator and constrictor muscles of the human eye separately, it was found that the pupil constrictor muscle is a primary controller of rapid pupil dilation upon brain arousal.
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The dilator pupillae
The iris is a contractile structure, consisting mainly of smooth muscle, surrounding the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles: a circular group called the sphincter pupillae, and a radial group called the dilator pupillae.
The iris dilator muscle works in opposition to the pupillary constrictor. Pupil dilation occurs when there is insufficient light for the normal function of the eye, and during heightened sympathetic activity, for example in the fight-or-flight reflex. The dilator pupillae muscle is innervated by postganglionic sympathetic nerves arising from the superior cervical ganglion as the sympathetic root of the ciliary ganglion. From there, they travel via the internal carotid artery through the carotid canal to the foramen lacerum. They then enter the middle cranial fossa above the foramen lacerum, travel through the cavernous sinus in the middle cranial fossa, and then travel with the ophthalmic artery in the optic canal or on the ophthalmic nerve through the superior orbital fissure.
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Pupillary light reflex
The iris is a contractile structure that surrounds the pupil and consists mainly of smooth muscle. The iris contains two groups of smooth muscles: a circular group called the sphincter pupillae and a radial group called the dilator pupillae. The iris regulates the amount of light that enters the eye by controlling the size of the pupil. This is known as the pupillary light reflex.
The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye. This helps the eye adapt to various levels of lightness and darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis), allowing less light to enter the eye. A lower intensity of light causes the pupil to dilate (mydriasis/mydriasis), allowing more light to enter. Thus, the pupillary light reflex regulates the intensity of light entering the eye.
The iris sphincter muscle constricts the pupil, while the dilator pupillae muscle dilates it. The sphincter pupillae is innervated by parasympathetic nerves, while the dilator pupillae is innervated by sympathetic nerves. The parasympathetic pathway is considered the primary controller of rapid pupil dilation upon brain arousal. The sympathetic pathway is generally considered a major regulator of pupil dilation, especially in long-lasting pupil dilation.
The pupillary light reflex is also known as the pupillary response, which may be pupillary constriction or dilation. The direct pupillary reflex is the pupillary response to light that enters the ipsilateral (same) eye. The consensual pupillary reflex is the response of a pupil to light that enters the contralateral (opposite) eye. Thus, there are four types of pupillary light reflexes based on this terminology: left direct, left consensual, right direct, and right consensual.
The pupillary light reflex is used to assess brainstem function. An abnormal pupillary light reflex may be indicative of optic nerve injury, oculomotor nerve damage, brain stem lesions, or certain medications.
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The effect of light and darkness
The pupil is an opening in the iris that lets light into the eye. The iris is a contractile structure, consisting mainly of smooth muscle, that surrounds the pupil. The iris contains two groups of smooth muscles: a circular group called the sphincter pupillae and a radial group called the dilator pupillae. The iris sphincter muscle is controlled by the Edinger-Westphal nucleus (EWN) and is under the control of the parasympathetic nervous system. The iris dilator muscle is controlled by the superior cervical sympathetic ganglion and is under the control of the sympathetic nervous system.
The pupil changes size to control how much light enters the eye. In bright light, the pupil contracts to prevent aberrations of light rays and thus attain their expected acuity; in the dark, this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye. In bright light, more light creates more nerve impulses, causing the muscles to close the pupil.
The pupillary light reflex is an important test of brainstem function. The pupil will dilate if a person sees an object of interest. The pupil diameter also reflects the vigilance state of the brain, involving locus coeruleus activation and sympathetic pathways acting on the dilatory muscle. For example, alertness and effort increase pupil diameter.
Environmental light can exert potent effects on physiology and behaviour, including pupil size, vigilance and sleep. Exposure to intense red light affects vigilance during subsequent darkness. In a series of controlled experiments in healthy adult volunteers, it was found that compared to prior darkness, prior red light induced a subsequent sustained pupil dilation.
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The impact of age
The iris is a contractile structure that consists mainly of smooth muscle and surrounds the pupil. It contains two groups of muscles: the circular sphincter pupillae and the radial dilator pupillae. When the sphincter pupillae contracts, the iris decreases or constricts the size of the pupil. On the other hand, when the dilator pupillae contract, the pupil dilates.
The size of a person's pupils varies throughout the day and is influenced by various factors such as light, medications, emotions, and mental exertion. However, age is also a significant factor in pupil size. Research has shown that pupil diameter decreases with increasing age, with the most marked differences observed at low luminance levels. The smallest pupil diameters were measured for hyperopes (farsighted individuals), while the largest diameters were observed in myopes (nearsighted individuals). This variation in pupil diameter with age is critical for the design of contact lenses, particularly in the mid- and peripheral optic zone regions, to maintain optimal visual performance.
The average pupil size decreases after 25 years of age, though not at a steady rate. This decrease in pupil size with age is supported by the observation that pupils are at their largest during adolescence and gradually become smaller until around age 60. However, it is important to note that individual differences exist, and the impact of age on pupil size may be influenced by other factors such as luminance and refractive error.
Additionally, as people age, they may experience changes in their vision, which can also impact pupil size. Most adults should have their vision checked every couple of years to ensure any changes in pupil size are within a normal range and not indicative of underlying health issues.
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Frequently asked questions
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is the muscle that decreases pupil size.
The iris sphincter muscle is located in the coloured part of the eye, known as the iris. The muscle fibres are found near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.
The iris sphincter muscle encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex or during accommodation. By controlling the pupil's diameter, the iris controls the amount of light that reaches the back of the eye at the retina.
The iris consists of two types of smooth muscles: radial and circular muscles. The sphincter pupillae are the circular muscles, and the dilator pupillae are the radial muscles.
