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The ciliary muscle is an intrinsic muscle of the eye that controls the shape of the lens. It is formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle contracts and relaxes to adjust the shape of the lens, enabling the eye to focus on objects at varying distances.
| Characteristics | Values |
|---|---|
| Name of the muscle | Ciliary muscle |
| Location | Middle layer of the eye, also known as the uvea or vascular layer |
| Shape | Ring of smooth muscle |
| Function | Controls the shape of the lens, enabling changes in focus for viewing objects at varying distances |
| Innervation | Parasympathetic fibers from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain |
| Layers | Longitudinal (outermost), radial (middle), and circular/annular (innermost) |
| Related Conditions | Presbyopia, open-angle glaucoma (OAG), closed-angle glaucoma (CAG) |
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What You'll Learn
The ciliary muscle
The main function of the ciliary muscle is to change the shape of the lens to accommodate viewing objects at different distances. When the ciliary muscle contracts, it loosens the zonular fibres (fibres that hold and flatten the lens), allowing the lens to become more spherical and increasing its refractive power. This is important for near vision, as it enables the creation of sharply focused images of close objects on the retina. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, pulling the lens into a flatter shape, which is necessary for focusing on distant objects.
The action of the ciliary muscle is instructed by parasympathetic fibres originating from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain. These parasympathetic fibres are part of cranial nerve V1 (Nasociliary nerve of the trigeminal nerve). The ciliary muscle also has additional functions, such as regulating the flow of aqueous humour, a transparent fluid within the eye that provides nutrients to the inner structures and maintains intraocular pressure.
The Helmholtz theory of accommodation, proposed by Hermann von Helmholtz, suggests that the ciliary muscle affects zonular fibres, enabling changes in lens shape for light focusing. According to this theory, as the ciliary muscle contracts, the zonular fibres become loose, and the lens becomes rounder. However, the Schachar theory, proposed by Dr R Schachar, offers an alternative explanation. It suggests that the zonular fibres become tighter, distorting the centre of the lens into a more steeply rounded shape. While the Helmholtz theory has been widely accepted, the mechanism remains controversial, and the debate between the two theories is still ongoing.
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Contraction and relaxation
The ciliary muscle is an intrinsic muscle of the eye that controls the lens shape. It is formed as a ring of smooth muscle in the eye's middle layer, known as the uvea or vascular layer. The ciliary muscle is responsible for accommodation, enabling us to view objects at varying distances.
When the ciliary muscle contracts, it pulls itself forward, moving the frontal region towards the axis of the eye. This action releases the tension on the lens caused by the zonular fibres, which are connected to the ciliary body and hold the lens in place. With the release of tension, the lens becomes more spherical, adapting to short-range focus. This change in shape allows for increased focusing power when viewing nearby objects.
Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, pulling the lens into a flatter shape. This flattening of the lens increases the focal distance, enabling better long-range focus. When observing distant objects, the ciliary muscle is typically relaxed, allowing the refractive power of the lens to form a clear image on the retina.
The ciliary muscle's ability to change the shape of the lens diminishes with age. Presbyopia, a condition associated with ageing, involves the deterioration of the ability to alter the lens shape. According to the Helmholtz theory, the lens becomes harder with age, making it less responsive to the ciliary muscle's action.
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How the eye focuses light
The human eye is a sense organ that enables vision by reacting to light. The cornea and the crystalline lens are important for the eye to focus light. The eye focuses light in a similar way to using a magnifying glass to concentrate the sun's rays onto a piece of paper. The distance from the magnifying lens to the paper is the focal length.
The eye's lens needs to become more rounded at the central surface to focus light rays from near objects. This ability to change focus for close-up objects is called accommodation. The crystalline lens changes shape to accommodate near or far targets. The curvature of the lens's surface can be adjusted to provide a clearer image.
The ciliary muscle, an intrinsic muscle of the eye, controls accommodation for viewing objects at varying distances. It changes the shape of the lens within the eye but not the size of the pupil. The contraction of the ciliary muscle changes the shape of the lens from concave to convex. According to the Helmholtz theory, when the ciliary muscle contracts, the zonular fibres become loose, and the lens becomes rounder. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance for long-range focus.
As people age, the ability of the ciliary muscle to change the shape of the crystalline lens decreases, resulting in a condition called presbyopia. This is characterised by a deteriorating ability to focus on close-up images while distance vision remains unaffected.
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The Helmholtz theory
According to Helmholtz's theory, the ciliary muscle, a ring of smooth muscle in the eye's middle layer, plays a crucial role in controlling the shape of the lens. When viewing distant objects, the ciliary muscle relaxes, allowing the zonules (fibers that suspend the lens) and suspensory ligaments to pull on the lens, flattening it and reducing its curvature. This flattening of the lens increases the focal distance, enabling clear long-range focus.
Conversely, when focusing on nearby objects, the ciliary muscle contracts, releasing the tension on the zonules. This contraction pulls the zonules forward, reducing their tension and allowing the lens to become thicker and more curved. The increased curvature of the lens enhances short-range focus by increasing its refractive power.
Helmholtz's theory highlights the dynamic nature of the human eye's optical system. The ability to adjust the shape and curvature of the lens through the contraction and relaxation of the ciliary muscle enables the eye to accommodate and focus on objects at different distances. This process, known as accommodation, ensures that clear retinal images can be obtained regardless of the object's distance.
In addition to his work on accommodation, Helmholtz made notable contributions to the understanding of perception and stereoscopic depth perception. He proposed that perception is an "unconscious inference," where the brain interprets symbols or representations of the physical world using information gathered from multiple senses. Furthermore, Helmholtz emphasised that stereoscopic depth perception is learned and that the invention of the stereoscope revealed the limitations of the Innate Theory of sight.
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The Schachar theory
The ciliary muscle is an intrinsic muscle of the eye that controls the lens's accommodation for viewing objects at varying distances. It changes the shape of the lens within the eye, but not the size of the pupil, which is carried out by the sphincter pupillae muscle and dilator pupillae.
The widely accepted theory of how the ciliary muscle controls the lens shape was proposed by Hermann von Helmholtz in 1855. He observed that accommodation involves pupillary constriction and anterior movement of the iris. He noted that the sagittal thickness of the lens increased and hypothesized that the equatorial diameter of the lens decreased during accommodation. Helmholtz's theory claims that as the ciliary muscle contracts, the zonules become loose, and the lens becomes rounder.
However, The Schachar Theory, proposed by Ronald Schachar in 1992, offers an alternative explanation. According to Schachar, the zonules become tighter, distorting the centre of the lens into a more steeply rounded shape. This theory is similar to that of Tscherning, who proposed that accommodation occurred through an increase in zonular tension at the lens equator with contraction of the ciliary muscle. Schachar's theory also contradicts Helmholtz's claim that the lens becomes harder as people age, making it unable to sufficiently change shape. Instead, Schachar suggests that the lens does not lose flexibility with age but that the loss of accommodation is caused by the lens's continued growth, which increases the distance between the lens and the ciliary muscle.
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Frequently asked questions
The ciliary muscle controls the shape of the lens in the eye.
The ciliary muscle contracts and relaxes to change the shape of the lens. When contracted, the lens becomes more spherical, and when relaxed, the lens flattens.
When the ciliary muscle is unable to sufficiently change the shape of the lens, a condition called presbyopia occurs, causing a deteriorating ability to focus on close objects.
