Victor Forge
The pupil of the eye is the black hole in the middle of the iris, which is the coloured part of the eye. The iris contains two muscles that determine the size of the pupil: the sphincter pupillae and the dilator pupillae. The sphincter pupillae is a band of smooth muscle that encircles the pupil margin, while the dilator pupillae is made up of smooth muscle fibres oriented radially from the iris root toward the pupil. The sphincter pupillae muscle is responsible for constricting the pupil under the control of parasympathetic nerve fibres, while the dilator pupillae muscle facilitates pupillary dilation. These muscles work in opposition to constantly adjust the size of the pupil, which is an important part of the vision system as it controls how much light reaches the back of the eye.
| Characteristics | Values |
|---|---|
| What is the pupil? | The pupil is the black hole in the middle of your iris. |
| What is the iris? | The iris is the colored part of your eye. |
| What controls the size of the pupil? | Muscles in the iris control how big the pupil is. |
| What are the two muscles in the iris that determine the size of the pupil? | Sphincter pupillae and dilator pupillae. |
| What is the sphincter pupillae? | A band of smooth muscle encircling the pupil margin. |
| What does the sphincter pupillae do? | Constricts the pupil in bright light. |
| What is the dilator pupillae? | A circumferential, spoke-like arrangement of contractile myoepithelial cells located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. |
| What does the dilator pupillae do? | Contraction of the dilator pupillae muscle facilitates dilation of the pupil. |
| What is the ciliary muscle? | The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). |
| What does the ciliary muscle do? | Controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humor into Schlemm's canal. |
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What You'll Learn
The ciliary muscle
When the ciliary muscle contracts, it pulls itself forward and moves towards the axis of the eye. This releases the tension on the lens caused by the zonular fibres, allowing the lens to become more spherical and adapting to short-range focus. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance for long-range focus.
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The iris sphincter muscle
Adie syndrome, also known as Holmes-Adie syndrome, is a neuro-ophthalmological disorder distinguished by a lack of response to light and a pupil that is tonically dilated but able to respond to accommodation. It is usually benign and more common in women, manifesting unilaterally in 80% of cases. Adie syndrome results from damage to postganglionic fibres of the parasympathetic innervation to the eye, which results in an inability to constrict the iris sphincter muscle.
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Pupillary constriction
The pupillary light reflex is the process by which the pupil constricts in response to light. When light enters the eye, it travels through the cornea, anterior chamber, pupil, lens, and posterior chamber, eventually reaching the retina. Photoreceptor cells in the retina, called rods and cones, convert light stimuli into neuronal impulses. These signals are then sent to the brain for image recognition via the optic nerve. The optic nerve also sends signals to the iris sphincter muscle to adjust the size of the pupil, thus controlling the amount of light that reaches the retina. This process is important for maintaining clear vision and protecting the eye from bright light.
The pupil normally ranges in size from 2 to 5 millimetres, with a decrease in size of about 0.3 mm per decade of ageing. Pupillary constriction can be affected by various factors, including age, medications, and underlying medical conditions. For example, certain drugs, such as opioids and barbiturates, can cause the pupils to become very small, a condition known as pinpoint pupils. Additionally, Horner's syndrome, a condition affecting the eyes and face, can cause miosis due to disruption of the sympathetic nervous system.
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Pupillary dilation
The pupil is the black hole in the middle of the iris, which is the coloured part of the eye. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris. The sphincter muscle fibres are located near the pupillary margin and encircle the pupil of the iris. The iris controls the pupil's diameter and, in turn, the amount of light that reaches the back of the eye.
In summary, pupillary dilation is a natural response to changes in light, but it can also be caused by various factors such as drugs, medications, medical conditions, or injuries. If you notice unusual or prolonged pupillary dilation, it is important to seek medical advice to rule out any underlying conditions and receive appropriate treatment.
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The parasympathetic nervous system
The pupil of the eye is the dark aperture in the centre of the iris, which is the coloured part of the eye. Muscles in the iris control how big the pupil is, usually in response to the amount of light available. When in bright light, the pupil constricts and becomes smaller, and when in dim light, it dilates and becomes bigger. This reaction is called the pupillary light response.
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the iris. It encircles the pupil and functions to constrict it in bright light via the pupillary light reflex. By controlling the pupil's diameter, the iris controls the amount of light that reaches the retina. The iris is a circular, pigmented diaphragm that divides the space between the cornea and the lens into anterior and posterior chambers. The sphincter pupillae is a band of smooth muscle fibres encircling the pupil margin. Because of its annular structure, contraction of the sphincter pupillae leads to a reduction in the size of the pupil.
The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture. While the parasympathetic system induces constriction of the pupil, the sympathetic system promotes sphincter muscle relaxation, which is critical for periods of low light or night vision. Sympathomimetic drugs (e.g. stimulants) may act on the dilator pupillae muscle, resulting in mydriasis or pupil dilation.
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Frequently asked questions
Pupillae muscles are the muscles in the iris that determine the size of the pupil. The iris is the coloured part of the eye.
The two pupillae muscles are the sphincter pupillae muscle and the dilator pupillae muscle.
The sphincter pupillae muscle is responsible for constricting the pupil in bright light via the pupillary light reflex or during accommodation.
The dilator pupillae muscle is responsible for dilating the pupil. This increases the amount of light that reaches the retina.
