Intrinsic eye muscles are tissues located within the human eyeball. These muscles perform involuntary actions that control how light enters the eye and how the eye focuses on objects at varying distances. They differ from the extrinsic eye muscles, which are positioned outside the eyeball and primarily move the eye itself.
The Ciliary Muscle and Visual Focus
The ciliary muscle, a ring of smooth muscle within the eye, plays a role in accommodation, the eye’s ability to change focus. This muscle is connected to the eye’s crystalline lens by suspensory ligaments (zonules). The lens is elastic and can alter its shape.
When focusing on a near object, the ciliary muscle contracts, moving forward and inward, decreasing its diameter. This contraction reduces tension on the suspensory ligaments. With less tension, the elastic lens thickens and becomes more spherical, increasing its refractive power to focus nearby images on the retina.
For distant vision, the ciliary muscle relaxes, increasing its diameter. This relaxation increases tension on the suspensory ligaments, pulling on the lens. The lens flattens and becomes thinner, decreasing its refractive power, allowing light from distant objects to converge on the retina. Accommodation is controlled by the parasympathetic nervous system via the oculomotor nerve (cranial nerve III).
Pupil Control Muscles
The iris, the colored part of the eye, contains two distinct intrinsic muscles that work antagonistically to regulate pupil size, controlling light entry. These are the sphincter pupillae and the dilator pupillae muscles. Both are smooth muscles, meaning their actions are involuntary.
The sphincter pupillae muscle is circularly arranged around the pupil. When stimulated, it contracts, causing the pupil to constrict, called miosis. This action occurs in response to bright light, reducing light entry and protecting the retina. It is controlled by the parasympathetic nervous system via short ciliary nerves from the ciliary ganglion.
Conversely, the dilator pupillae muscle has fibers arranged radially, extending from the pupil outwards. When this muscle contracts, it pulls the iris outward, leading to an increase in pupil size, called mydriasis. This dilation occurs in low-light conditions to allow more light to reach the retina, or during a “fight-or-flight” response. The dilator pupillae is innervated by the sympathetic nervous system via long ciliary nerves from the superior cervical ganglion. The coordinated action of these two muscles is evident in the pupillary light reflex, where light in one eye causes both pupils to constrict.
Common Conditions Affecting Intrinsic Muscles
Dysfunction of the intrinsic eye muscles or their controlling nerve pathways can lead to various visual impairments. A common age-related condition affecting the ciliary muscle’s function is presbyopia, known as “age-related farsightedness.” This condition begins around the fourth decade of life.
While the ciliary muscle retains its ability to contract, presbyopia arises because the crystalline lens stiffens and loses its elasticity with age. This reduced flexibility means the lens cannot easily change shape for near vision, even with ciliary muscle contraction. As a result, individuals find it difficult to focus on close-up objects, requiring reading glasses or other corrective lenses.
Issues can also affect the pupillary muscles, leading to abnormal pupil responses. For instance, a sluggish pupillary light reflex might indicate a problem with nerve signals to the sphincter pupillae or dilator pupillae. Horner’s syndrome, for example, disrupts the sympathetic nerve supply to the eye, specifically affecting the dilator pupillae muscle. This disruption results in a constricted pupil (miosis) on the affected side, as the dilator muscle’s ability to widen the pupil is compromised.