Anatomy & Organs

Musculus sphincter pupillae – structure, function & diseases

Pupil sphincter muscle

The sphincter pupillae muscle is one of the inner eye muscles and has the task of narrowing the pupil . This so-called miosis occurs reflexively when there is light and is also part of the close-up triad. The sphincter pupillae muscle can be artificially stimulated to contract using substances such as miotics.

Was ist the sphincter muscle of the pupil?

The eye muscles are responsible for all movements of the eyes including the functional adjustment of the eyes. In addition to six external muscles, humans have three internal eye muscles. The inner eye muscles carry smooth muscle and are subject to control by the autonomic nervous system . All inner eye muscles serve to change the size of both pupils. This process is also known as adaptation.

In addition to adaptation, the inner eye muscles are responsible for regulating refractive power and thus controlling visual acuity. The sphincter pupillae muscle is one of the inner eye muscles. The muscle is a circular muscle that can constrict the pupils. Like all circular muscles in the human body, the sphincter pupillae muscle has a circular fiber course. Its fibers lie ring-like around the pupil and form the posterior part of the iris stroma.

Because of its functions, the muscle is also called the constrictor pupillae muscle in medical literature. Nerve fibers from the ciliary ganglion innervate the circular muscle parasympathetically. The musculus dilatator pupillae is considered to be the antagonist of the musculus sphincter pupillae.

Anatomy & Structure

The individual fibers for the innervation of the latticed sphincter pupillae muscle originate from the Edinger-Westphal nucleus and run via the oculomotor nerve to the ciliary ganglion . The Edinger-Westphal nucleus is a portion of the midbrain and corresponds to the nucleus controlling the pupillary reflex or eye adaptation.

The nucleus receives afferents via the optic nerve and the tractus opticus, which project directly into the epithalamus and are switched in the nucleus pretectalis to so-called interneurons with a bilateral connection to the Edinger-Westphal nucleus.

The efferents of the nucleus reach the pupillary constrictor and the ciliary muscle via the ciliary ganglion. The fibers of the musculus sphincter pupillae come from the nucleus accessorius n. oculomotorii, the nucleus of the III. cranial nerves. In the ciliary ganglion, there is a connection from the pre- to the post-ganglionic neuron. From there, the fibers traverse in the form of the Nn. ciliares breves the whites of the eyes and pull towards the inside of the eyes.

Function & Tasks

The sphincter pupillae muscle is involved in the adaptation of the eyes by constricting the pupils through contractions. The sphincter muscle receives the commands to contract via efferents (descending pathways) from the midbrain in the form of bioelectrical excitation and then sets the so-called miosis in motion. Based on the average eye hole diameter, this constriction of the pupils can be of varying severity.

Not only the active contraction of the musculus sphincter pupillae, but also the failure or limitation of its antagonist musculus dilatator pupillae initiates a miosis. Physiologically, parasympathetic nerve fibers mediate the constriction of the pupils. The incidence of light and the close-up triad of close-up fixation, accommodation and convergence movement automatically determine the adaptation movement. Specifically, in miosis, the nerve fibers originating from the accessory nucleus of the oculomotor nerve are interconnected in the ciliary ganglion. Via the Nervi ciliares breves they reach the Musculus sphincter pupillae.

The reflex arc starts at the retina , from where it is wired on both sides via the optic nerve in the pretectural area. The main task of the sphincter pupillae muscle is therefore a reflex movement that is primarily initiated in response to light stimuli. With a unilateral light stimulus, both pupils narrow. This is also referred to as a consensual or indirect light reflex. In contrast, pupillary constriction in accommodative lens curvature enhancement occurs whenever close objects are in focus.

Diseases

The contraction of the sphincter pupillae muscle in the sense of a miosis can be brought about by opiates or opioids . Pathologically constricted pupils are therefore often interpreted as a sign of intoxication. Pharmacological agents such as miotics (pilocarpine) can also cause constriction of the pupils.

These active ingredients are usually administered in a therapeutic or diagnostic context. The therapy steps are used, for example, in the case of glaucoma or for the differential diagnostic clarification of pharmacodynamic pupillotonia. A pronounced miosis even improves the visual acuity of people without lenses. The narrowing of the visual hole increases the depth of field and has a similar effect to a stenopic gap. By stimulating the pupil-constricting muscle, miotics therefore increase visual acuity.

Unlike the substances mentioned, mydriatics such as atropine do not stimulate the sphincter pupillae muscle, but paralyze the sphincter muscle. By administering the funds, miosis can be prevented for a limited period of time. Drugs such as parasympatholytics in turn lead to a complete loss of accommodation, which is due to a temporary paralysis of the parasympathetically innervated ciliary muscle part. Paralysis of the sphincter pupillae muscle is gaining clinical relevance not only in the context of diagnostics and therapy. Paralysis of the muscle that occurs suddenly is usually expressed in rigid pupils with an inability to accommodate.

Traumatic and inflammatory lesions of the supplying nerves as well as nerve compressions by tumors are possible causes for this phenomenon . In the case of paralysis of the sphincter pupillae muscle, miosis is hardly possible or not possible at all. On the other hand, pathological narrowing of the pupils occurs in the case of disturbances in the sympathetic supply, for example in the context of Horner’s syndrome or Argyll-Robertson’s syndrome .

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.