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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 between 1.5 mm and 8 mm. The constriction of the pupil, also known as miosis, is caused by the contraction of the circular muscle, or the iris sphincter muscle, which is controlled by the parasympathetic nervous system. The iris sphincter muscle is also known as the pupillary sphincter, the pupillary constrictor, or the circular muscle of the iris. It encircles the pupil and constricts it in response to bright light, a response known as the pupillary light reflex.
| Characteristics | Values |
|---|---|
| Name of muscle | Iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibres) |
| Location | Central parts of the iris |
| Function | Constricts the pupil |
| Controlled by | Parasympathetic nervous system (PSNS) |
| Controlled by (other names) | Peripheral parasympathetic pathway, Edinger-Westphal nucleus (EWN) |
| Controlled by (other names, continued) | Superior cervical sympathetic ganglion |
| Muscle type | Smooth muscle |
| Dimensions | 0.75mm wide, 0.15mm thick |
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What You'll Learn
- The circular muscle, controlled by the parasympathetic nervous system, contracts to constrict the pupil
- The radial muscle, controlled by the sympathetic nervous system, relaxes to constrict the pupil
- The iris sphincter muscle is a constrictor muscle
- The dilator pupillae dilates the pupil
- The sphincter pupillae constricts the pupil
The circular muscle, controlled by the parasympathetic nervous system, contracts to constrict the pupil
The pupil is the black circular centre of the iris, which appears as a dark hole due to the way it absorbs light. The iris itself is a thin, circular structure and the pupil is simply a hole in its centre. The size of the pupil is controlled by two muscles: the dilator pupillae and the sphincter pupillae. The dilator pupillae muscle is located in the outer parts of the iris and dilates the pupil, while the sphincter pupillae muscle is located in the central parts and constricts it.
The sphincter pupillae muscle, also known as the circular muscle of the iris, is the muscle responsible for constricting the pupil. It is controlled by the parasympathetic nervous system (PSNS), which is a division of the autonomic nervous system. The parasympathetic nervous system is responsible for stimulating "rest-and-digest" activities when the body is at rest, including digestion, salivation, and constriction of the pupil. When the circular muscle contracts, it causes the pupil to constrict or become more narrow, a response known as miosis.
The constriction of the pupil is important for several reasons. Firstly, it helps to regulate the amount of light that enters the eye. In bright light conditions, the pupil constricts to allow less light into the eye, while in low light conditions, the pupil dilates to let more light in. This process is known as the pupillary light reflex. Additionally, the constriction of the pupil can also be a response to certain drugs or medical conditions. For example, opiates and opioids can cause miosis, or constriction of the pupil.
The circular muscle of the iris is under the control of the Edinger-Westphal nucleus (EWN), which is the centre of the pupillary light reflex. The EWN receives input from the superior colliculus (SC), which is involved in eye movement and orientation. The SC, in turn, receives input from the locus coeruleus (LC) neurons. This complex neural pathway allows for the rapid constriction of the pupil in response to changes in lighting conditions or other stimuli.
In summary, the circular muscle of the iris, or sphincter pupillae muscle, is responsible for constricting the pupil. It is controlled by the parasympathetic nervous system, which stimulates constriction when the body is at rest or in response to changes in lighting conditions or other stimuli. This process is essential for maintaining optimal vision and protecting the eye from excessive light exposure.
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The radial muscle, controlled by the sympathetic nervous system, relaxes to constrict the pupil
The pupil is the black circular opening in the centre of the iris, which gives entry to light. The iris sphincter muscle, also known as the circular muscle, is a muscle in the iris that encircles the pupil. This muscle is responsible for constricting the pupil, and it is controlled by the parasympathetic nervous system (PSNS).
The radial muscle, controlled by the sympathetic nervous system (SNS), relaxes to constrict the pupil. The sympathetic nervous system is responsible for stimulating the smooth cells of the radial muscle to contract, resulting in a dilation response (mydriasis) or widening of the pupil. However, when the radial muscle relaxes, it contributes to a constriction response (miosis) or narrowing of the pupil.
The constriction response of the pupil occurs primarily due to the contraction of the circular muscle or the iris sphincter muscle. This muscle is innervated by the parasympathetic nervous system, which releases acetylcholine to act on muscarinic acetylcholine receptors in the iris sphincter muscle. This action causes the muscle to contract, resulting in the narrowing of the pupil.
While the radial muscle's relaxation is not the primary driver of pupil constriction, it contributes to a certain extent. This contribution is part of the complex interplay between the muscles and nervous system involved in maintaining optimal pupil size and function. The balance between the activation of the dilator and sphincter pupillae muscles is crucial in dictating pupil diameters.
The pupil's size and reactivity to light are important indicators of neurological health. The pupil has tight neurological control, and abnormalities in this control can indicate underlying diagnoses. Thus, the radial muscle, while not solely responsible for pupil constriction, plays a role in modulating the pupil's response, alongside the iris sphincter muscle and other physiological mechanisms.
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The iris sphincter muscle is a constrictor muscle
The iris sphincter muscle, also known as the pupillary sphincter, pupillary constrictor, or circular muscle of the iris, is a muscle located in the iris of the eye. It encircles the pupil and functions to constrict it, particularly in response to bright light, a response known as the pupillary light reflex. This reflex is triggered by the muscle's sensitivity to light, causing the pupil to constrict and limit the amount of light entering the eye.
The iris sphincter muscle is under the control of the parasympathetic nervous system (PSNS) and receives input from the Edinger-Westphal nucleus (EWN), which is the centre of the pupillary light reflex. When the iris sphincter muscle contracts, it leads to the constriction of the pupil, resulting in a decrease in its diameter. This process is known as miosis and is the opposite of mydriasis, which is the dilation of the pupil.
The constriction of the pupil due to the iris sphincter muscle is influenced by various factors, including lighting conditions and brain states. In bright light, the muscle constricts the pupil to regulate the amount of light entering the eye. Additionally, the muscle's activity is modulated by neuromodulator systems, which are associated with changes in brain states. These systems control the rapid dilation of the pupil upon brain arousal, with the iris sphincter muscle acting as the primary controller.
The balance between the activation of the iris sphincter muscle and the opposing dilator muscle, the iris dilator muscle, determines the pupil's diameter at any given time. The iris dilator muscle, controlled by the sympathetic nervous system, is responsible for widening the pupil. The antagonistic actions of these two muscles allow for dynamic adjustments to the size of the pupil, ensuring the optimal amount of light enters the eye in different lighting conditions.
It is worth noting that some individuals possess the rare ability to exert voluntary control over their iris sphincter and dilator muscles, allowing them to dilate and constrict their pupils on command, regardless of lighting conditions. However, the advantages of this ability are not yet fully understood.
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The dilator pupillae dilates the pupil
The pupil is the black circular opening in the centre of the iris, which lets light into the eye. Pupillary response is a physiological reaction that varies the size of the pupil between 1.5 mm and 8 mm. The dilator pupillae is an intrinsic ocular muscle located in the outer parts of the iris. It is responsible for dilating the pupil, the widening of the pupil, which lets more light into the eye.
The dilator pupillae is controlled by the superior cervical sympathetic ganglion and the sympathetic nervous system (SNS). It is also known as the iris dilator muscle. The sympathetic nervous system is responsible for the sustained dilation of the pupil. The iris dilator muscle receives signals from the neuromodulator system with a slow latency and is involved in maintaining the dilation of the pupil.
The dilator pupillae works in conjunction with the sphincter pupillae, or the iris sphincter muscle, which constricts the pupil. The sphincter pupillae is located in the central parts of the iris. The balance between the activation of the dilator and sphincter pupillae muscles dictates pupil diameters. The iris sphincter muscle is controlled by the Edinger-Westphal nucleus (EWN) and the parasympathetic nervous system (PSNS). The EWN is the centre of the pupillary light reflex and the quick control of the activity of the iris sphincter muscle.
The dilation and constriction of the pupil are influenced by a variety of factors, including lighting conditions, emotional states, and certain drugs. For example, dilation may be caused by adrenaline, anticholinergic agents, stimulant drugs, and some hallucinogens. Constriction may be caused by scleral buckles or drugs such as opiates, opioids, or anti-hypertension medications.
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The sphincter pupillae constricts the pupil
The pupil is the black circular opening in the centre of the iris, which lets light into the eye. The iris sphincter muscle, also known as the pupillary sphincter, pupillary constrictor, or sphincter pupillae, is responsible for constricting the pupil. This muscle is located in the central region of the iris and encircles the pupil.
The sphincter pupillae is a smooth muscle, and its contraction leads to the constriction of the pupil, resulting in a decrease in its diameter. This process is known as miosis and can occur due to various physiological and pathological reasons. For example, in low light conditions, the pupil naturally constricts to let less light into the eye. This is an involuntary reflex action. On the other hand, certain drugs, such as opiates, opioids, and anti-hypertension medications, can also cause miosis.
The iris sphincter muscle is controlled by the parasympathetic nervous system (PSNS) and receives input from the Edinger-Westphal nucleus (EWN). When the parasympathetic fibres are activated, the sphincter pupillae contracts, leading to miosis. This process is essential for maintaining the appropriate amount of light entering the eye in varying lighting conditions.
In addition to the sphincter pupillae, the radial muscle also plays a role in pupil constriction. When this muscle relaxes, it contributes to a slight degree of pupil constriction. The balance between the activation of the sphincter pupillae and the dilator pupillae, its antagonist muscle, determines the diameter of the pupil at any given time.
While pupil constriction is typically involuntary, some individuals possess the rare ability to exert 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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Frequently asked questions
The iris sphincter muscle, also known as the pupillary sphincter or circular muscle of the iris, is responsible for constricting the pupil.
The iris sphincter muscle is controlled by the parasympathetic nervous system (PSNS). It contracts in response to signals from the Edinger-Westphal nucleus (EWN), which is the centre of the pupillary light reflex.
The iris sphincter muscle encircles the pupil and constricts it in response to bright light, regulating the amount of light entering the eye.
Yes, pupil constriction is also influenced by the relaxation of the radial muscle, which is controlled by the sympathetic nervous system (SNS). Additionally, certain drugs, such as opiates and opioids, can cause pupil constriction.
Pupil constriction, or miosis, refers to the narrowing of the pupil, while dilation, or mydriasis, is the widening of the pupil. Dilation is controlled by the iris dilator muscle, which is innervated by the sympathetic nervous system.
