The sphincter pupillae muscle is one of the internal eye muscles and is responsible for constricting the pupil. This so-called miosis takes place reflexively when light enters the eye and is also part of the near vision tria. The sphincter pupillae muscle can be artificially stimulated to contract using substances such as miotics.
What is the sphincter pupillae muscle?
The eye muscles are responsible for all movements of the eyes including functional eye adjustment. In addition to six external eye 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 of the 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 control visual acuity. The sphincter pupillae muscle is one of the inner eye muscles. The muscle is a ring muscle that can constrict the pupils. Like all ring muscles of the human body, the sphincter pupillae muscle has circular fibers. In a ring-like fashion, its fibers lie around the pupil and form the posterior part of the iris stroma. Because of its functions, the muscle is also called musculus constrictor pupillae in the medical literature. Nerve fibers from the ciliary ganglion innervate the annular muscle parasympathetically. The antagonist of the sphincter pupillae muscle is the dilatator pupillae muscle.
Anatomy and structure
The individual fibers for innervation of the lattice-like sphincter pupillae muscle originate from the Edinger-Westphal nucleus and run to the ciliary ganglion via the oculomotor nerve. The Edinger-Westphal nucleus is a portion of the midbrain and corresponds to the nucleus area controlling the pupillary reflex or eye adaptation. The nucleus receives afferents via the optic nerve and tractus opticus, which project directly to the epithalamus and are switched in the nucleus pretectalis to so-called interneurons with bilateral connections 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 sphincter pupillae muscle thus originate from the nucleus accessorius n. oculomotorii, the nucleus of the III. Cranial nerve. In the ciliary ganglion, there is an interconnection from the preganglionic to the postganglionic neuron. From there, fibers in the form of the nn. ciliares breves traverse the white membrane of the eye and move toward the interior of the eye.
Function and Tasks
The sphincter pupillae muscle is involved in the adaptation of the eyes by contracting the pupils. The sphincter muscle receives commands to contract via efferents (descending pathways) from the midbrain in the form of bioelectrical excitation and then initiates what is known as miosis. Based on the average optic disc diameter, this constriction of the pupils can vary in severity. Not only the active contraction of the sphincter pupillae muscle, but also the failure or restriction of its antagonist dilatator pupillae muscle initiates miosis. Physiologically, parasympathetic nerve fibers mediate pupillary constriction. Incidence of light as well as the near adjustment triad of near fixation, accommodation, and convergence movement automatically condition the adaptive movement. Specifically, during miosis, nerve fibers originating from the nucleus accessorius of the oculomotor nerve are interconnected in the ciliary ganglion. Via the nervi ciliares breves they reach the musculus sphincter pupillae. The reflex arc begins at the retina, from where it is connected bilaterally via the optic nerve in the area pretectalis. The main task of the sphincter pupillae muscle is therefore a reflex movement, which is initiated primarily in response to light stimuli. In response to unilateral light stimuli, both pupils constrict. This is also referred to as a consensual or indirect light reflex. In contrast, pupil constriction in accommodative lens curvature enhancement occurs whenever near objects are focused.
Diseases
Contraction of the sphincter pupillae muscle in the sense of miosis can be induced 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.The administration of these agents usually takes place in a therapeutic or diagnostic setting. The therapeutic steps are used, for example, in glaucoma or for differential diagnostic clarification of pharmacodynamic pupillotonia. A pronounced miosis even improves the visual acuity of lensless people. The narrowing of the visual aperture increases the depth of field and has an effect similar to a stenopeic gap. Miotics thus induce an increase in visual acuity by stimulating the pupil constricting muscle. Unlike the aforementioned substances, mydriatics such as atropine do not stimulate the sphincter pupillae muscle, but induce paralysis of the ring muscle. Administration of these agents can prevent miosis for a limited period of time. Agents such as parasympatholytics, on the other hand, induce a complete loss of accommodation due to a temporary paralysis of the parasympathetically innervated ciliary muscle portion. Paralysis of the sphincter pupillae muscle gains clinical relevance not only in the context of diagnosis and therapy. Sudden onset of paralysis of the muscle usually manifests as pupillary rigidity with inability to accommodate. The cause of this phenomenon may be traumatic and inflammatory lesions of the supplying nerves as well as nerve compression by tumors. Miosis is hardly or not at all possible in case of paralysis of the sphincter pupillae muscle. In contrast, pathologic pupillary constrictions occur in disorders of sympathetic supply, such as Horner syndrome or Argyll-Robertson syndrome.
