Victor Forge
The iris is the coloured, muscular part of the eye surrounding the pupil. It controls the size of the pupil by means of contracting the iris sphincter muscle and/or the iris dilator muscle. The sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is located in the coloured part of the eye and encircles the pupil of the iris. It functions to constrict the pupil in bright light via the pupillary light reflex. The dilator muscle, on the other hand, enlarges the pupil by pulling the iris radially. The iris consists of two sheets of smooth muscle with contrary actions—dilation (expansion) and contraction (constriction).
| Characteristics | Values |
|---|---|
| What is the iris? | The iris is the coloured, muscular part of the eye surrounding the pupil. |
| Location | The iris is in front of the lens and behind the cornea. |
| Function | The iris controls the size of the pupil by means of contracting the iris sphincter muscle and/or the iris dilator muscle. |
| Iris sphincter muscle | The iris sphincter muscle, also known as the pupillary sphincter, is a muscle located in the coloured part of the eye called the iris. It encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex or during accommodation. |
| Iris dilator muscle | The dilator muscle of the iris expands the opening when it contracts. |
| Dimensions | The iris sphincter muscle is about 0.75 mm wide and 0.15 mm thick. |
| Innervation | The iris sphincter muscle receives parasympathetic innervation via the short ciliary nerves, leading to pupillary constriction. |
| Effects of ghrelin | Ghrelin has been shown to interfere with the kinetics of the iris sphincter muscle, promoting relaxation. |
Explore related products
What You'll Learn
The iris sphincter muscle
The iris is the coloured, muscular part of the eye surrounding the pupil. It is located in front of the lens and behind the cornea. 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, or during accommodation.
The sphincter pupillae causes the narrowing of the pupil to prevent diverging light rays from the corneal periphery from creating a blurred image. The sphincter muscle contracts the pupil in a circular motion, while the dilator muscles enlarge the pupil by pulling the iris radially. The collarette is the thickest region of the iris, where the sphincter and dilator muscles overlap.
Walking for Abs: Is It Possible?
You may want to see also
Explore related products
The ciliary muscle
The iris consists of two sheets of smooth muscle with opposing actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and determine how much light reaches the retina. The iris sphincter muscle constricts the pupil in bright light, while the dilator muscle of the iris expands the opening when it contracts.
Sympathetic innervation from the superior cervical ganglion controls pupil dilation by projecting to the dilator muscle of the iris via long ciliary nerves. The sympathetic and parasympathetic systems work together to constantly adjust the pupillary aperture.
Swimming and Muscle: Friend or Foe?
You may want to see also
Explore related products
Pupil dilation
The iris is the coloured, muscular part of the eye surrounding the pupil. It is located in front of the lens and behind the cornea. The iris consists of two sheets of smooth muscle with opposing functions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil, determining how much light reaches the retina.
The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, while the dilator muscle of the iris expands the opening when it contracts. The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, encircles the pupil and functions to constrict it in response to bright light or during accommodation. This process is known as the pupillary light reflex. The sphincter muscle fibres are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris.
The dilator muscle of the iris, on the other hand, enlarges the pupil by pulling the iris radially. It is innervated by sympathetic fibres and controlled by the superior cervical ganglion, which projects to the dilator muscle via long ciliary nerves. The sympathetic and parasympathetic systems work together to constantly adjust the pupillary aperture. The dilator muscle is responsible for pupil dilation, allowing more light to reach the retina in low-light conditions.
The size of the pupil is influenced by various factors, including light conditions, emotional state, cognitive load, arousal, and stimulation. The pupil diameter can range from less than 2 mm to as large as 9 mm, although there is individual variation and the maximum diameter decreases with age. While most humans cannot voluntarily control their iris muscles, a rare few can dilate and constrict their pupils on command.
Treating Rigid Muscles: Effective Strategies for Relief
You may want to see also
Explore related products
Pupil constriction
The iris is the coloured, muscular part of the eye surrounding the pupil. It is located in front of the lens and behind the cornea. The iris consists of two sheets of smooth muscle with opposing functions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and, consequently, the amount of light that reaches the retina.
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, encircles the pupil of the iris. It is located near the pupillary margin and is slightly anterior to the pigmented epithelium of the iris. The sphincter muscle contracts the pupil in a circular motion, reducing its diameter. This process is known as pupillary constriction or miosis and occurs in response to bright light or during accommodation.
The accommodative reflex of the eye is an involuntary response that occurs when the visual focus shifts from a distant object to one that is closer. During accommodation, the sphincter pupillae causes the pupil to narrow, preventing diverging light rays from the corneal periphery from creating a blurred image. The accommodative reflex is controlled by the parasympathetic nervous system, which induces pupillary constriction through the release of acetylcholine.
The ciliary muscle and the pupillary dilator muscle work in conjunction with the sphincter muscle to regulate the pupil's size. The dilator muscle enlarges the pupil by pulling the iris radially, while the ciliary muscle changes the shape of the lens. The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupil's aperture, ensuring optimal vision in varying light conditions.
Unlocking Muscle Growth: The Ultimate Guide to Mashing Your Muscles
You may want to see also
Explore related products
The dilator muscle
The iris consists of two sheets of smooth muscle with opposing actions: dilation (expansion) and contraction (constriction). The dilator muscle of the iris, also known as the dilator pupillae, is responsible for expanding the pupil when it contracts, thus controlling the amount of light that reaches the retina. The dilator muscle works in opposition to the sphincter muscle, which constricts the pupil in bright light.
Radial Nerve's Pupil Muscle Impact: Understanding the Connection
You may want to see also
Frequently asked questions
The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the coloured part of the eye called 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.
The iris sphincter muscle is about 0.75 mm wide and 0.15 mm thick.
The iris sphincter muscle contracts the pupil in a circular motion, controlling the pupil's diameter and thus, the amount of light that reaches the back of the eye at the retina.
