Logan Steele
The pupil is a fascinating feature of the eye, and its size is controlled by two muscles found in the iris: the sphincter pupillae and the dilator pupillae. The sphincter pupillae, also known as the iris sphincter muscle, is located in the anterior part of the iris and functions to constrict the pupil in bright light or during accommodation. On the other hand, the dilator pupillae, also known as the iris dilator muscle, enlarges the pupil by pulling the iris radially. These muscles work together to regulate the amount of light that reaches the retina, and their functions are beyond our conscious control.
| Characteristics | Values |
|---|---|
| Muscle type | Smooth muscle |
| Muscle function | Constriction and dilation of the pupil |
| Muscle location | Iris (coloured part of the eye) |
| Muscle structure | Sphincter pupillae and dilator pupillae |
| Muscle innervation | Parasympathetic and sympathetic |
| Muscle control | Autonomic (involuntary) |
| Muscle dimensions | About 1 mm wide and 0.15 mm thick |
| Muscle reflexes | Pupillary light reflex, accommodative reflex |
| Muscle antagonists | Muscarinic antagonists (e.g., atropine) |
| Muscle receptors | Muscarinic M2 and M3 receptors |
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What You'll Learn
The iris sphincter muscle
The sympathetic nervous system, in opposition to the parasympathetic system, helps promote sphincter muscle relaxation, which is crucial for low-light conditions or night vision. Sympathetic innervation from the superior cervical ganglion projects to the dilator muscle of the iris, controlling pupil dilation.
The vertebrate eye is made up of tissue from various embryonic origins. The lens and cornea originate from the surface ectoderm, while the retina and epithelial layers of the iris and ciliary body originate from the anterior neural plate. Within the stroma of the iris, the sphincter pupillae and dilator pupillae muscles develop from the optic cup neuroectoderm.
In terms of clinical relevance, Adie syndrome, or Holmes-Adie syndrome, is a neuro-ophthalmological disorder characterised by a lack of response to light and a tonically dilated pupil. It results from damage to the postganglionic fibres of the parasympathetic innervation to the eye, impairing the ability to constrict the iris sphincter muscle.
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The iris dilator muscle
The sympathetic fibres that innervate the iris dilator muscle arise from the superior cervical ganglion. These fibres travel through the internal carotid artery and into the cavernous sinus before passing through the superior orbital fissure. They then travel with the ophthalmic artery and form long ciliary nerves that supply the iris dilator muscle.
In certain contexts, such as in the application of parasympatholytic agents or the use of belladonna by sixteenth-century Italian courtesans, the iris dilator muscle can be influenced to produce dilation of the pupil. This dilation effect can also be observed with the use of stimulants or sympathomimetic drugs.
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How light affects pupil size
The pupil is an opening in the eye that lets light in. The pupil is surrounded by the iris, which is the coloured part of the eye. The iris controls the amount of light that reaches the back of the eye, or the retina. The pupil appears black because most of the light entering the eye does not escape.
The pupil changes size to control how much light enters the eye. In dim light, the pupil expands to let more light in, and in bright light, it contracts. The pupil can range in diameter from 1.5 mm to more than 8 mm. The size of the pupil is controlled by the iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae. This muscle encircles the pupil and constricts it in response to bright light via the pupillary light reflex. The sphincter muscle fibres are located near the pupillary margin and slightly anterior to the pigmented epithelium of the iris.
The iris sphincter muscle is controlled by parasympathetic fibres that travel from the Edinger-Westphal nucleus with the oculomotor nerve (cranial nerve III) to the ciliary ganglion, where they synapse. The sphincter muscle contracts the pupil in a circular motion, while the dilator muscles, which are sympathetically innervated, enlarge the pupil by pulling the iris radially. The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupil size.
The pupil's response to light is an involuntary reflex, similar to the knee-jerk reflex. The pupil's size can also change in response to other stimuli, such as emotions, medications, and viewing distance.
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The role of the parasympathetic nervous system
The pupil is controlled by two muscles: the iris sphincter muscle and the iris dilator muscle. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris, the coloured part of the eye. The sphincter muscle fibres are located near the pupillary margin and encircle the pupil of the iris.
The parasympathetic nervous system plays a crucial role in controlling the iris sphincter muscle. This system is responsible for the involuntary constriction of the pupil, known as miosis, in response to bright light or during accommodation. The parasympathetic fibres that control the sphincter muscle originate from the Edinger-Westphal nucleus of the oculomotor nerve (cranial nerve III). These fibres travel to the ciliary ganglion, where they synapse and then continue to the sphincter muscle.
The accommodative reflex, which is controlled by the parasympathetic nervous system, involves three actions: alteration of pupil aperture, lens shape, and convergence. When the visual focus switches from a distant object to a closer one, the parasympathetic nervous system initiates the accommodative reflex. This reflex causes the sphincter pupillae to narrow the pupil, preventing diverging light rays from creating a blurred image.
The parasympathetic nervous system works in opposition to the sympathetic nervous system to regulate the pupillary aperture. While the parasympathetic system constricts the pupil, the sympathetic system controls the iris dilator muscle, which enlarges the pupil by pulling the iris radially. This dilation allows more light to enter the eye.
The interaction between the parasympathetic and sympathetic systems ensures that the pupil dynamically adjusts its diameter to control the amount of light that reaches the retina at the back of the eye. This dynamic adjustment is essential for maintaining clear vision in varying light conditions.
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The role of the sympathetic nervous system
The pupil is controlled by two muscles: the sphincter pupillae and the dilator pupillae. The sphincter pupillae is a circular muscle located in the iris, which constricts the pupil in response to bright light or during accommodation. This muscle is controlled by the parasympathetic nervous system, which releases acetylcholine and acts on muscarinic receptors.
The dilator pupillae muscle, on the other hand, is innervated by the sympathetic nervous system. This system acts by releasing noradrenaline, which binds to α1-receptors. The activation of this muscle leads to the dilation of the pupil, allowing more light to enter the eye. This is particularly important during periods of low light or night vision.
The sympathetic nervous system plays a crucial role in the function of the dilator pupillae muscle. When an individual is presented with a threatening stimulus that activates the fight-or-flight response, the sympathetic system innervates and contracts the dilator pupillae muscle. This results in the dilation of the pupil, temporarily allowing more light to reach the retina. This response enhances the individual's ability to perceive their surroundings and react accordingly.
Additionally, the sympathetic nervous system works in conjunction with the parasympathetic system to maintain a constant adjustment of the pupillary aperture. While the sphincter pupillae constricts the pupil, the dilator pupillae, under the control of the sympathetic system, enlarges the pupil by pulling the iris radially. This reciprocal function is made possible by sympathetic innervation, which promotes sphincter muscle relaxation and dilator muscle contraction.
In summary, the sympathetic nervous system plays a vital role in the control of the pupil by regulating the dilator pupillae muscle. Its functions include dilating the pupil in response to threatening stimuli, increasing light intake during low-light conditions, and working in tandem with the parasympathetic system to adjust the pupillary aperture.
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Frequently asked questions
The muscles that control the pupil are the iris sphincter muscle (also known as the pupillary sphincter or sphincter pupillae) and the iris dilator muscle (also known as the pupillary dilator or dilator pupillae).
The iris sphincter muscle encircles the pupil and constricts it in bright light or during accommodation.
The iris dilator muscle enlarges the pupil by pulling the iris radially.
The iris sphincter muscle is controlled by parasympathetic fibers that travel from the Edinger-Westphal nucleus with the oculomotor nerve (cranial nerve 3) to the ciliary ganglion, where they synapse.
The iris dilator muscle is innervated by the sympathetic nervous system. When stimulated, the muscle contracts and dilates the pupil, allowing more light to enter the eye.
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