Anatomy and Structure
The sphincter pupillae is part of the iris, the anterior component of the vascular tunic (uvea) of the eye. The muscle lies within the anterior connective tissue stroma of the iris, directly adjacent to the pupillary border.
Its structure has a unique neuroglial origin. At the cellular level, the myocyte cell bodies reside within the epithelium and are non-contractile; contractile processes extend from them into the iris stroma. The sphincter fibers are arranged circularly (parallel to the pupillary margin), which differentiates them from the radial fibers of the dilator pupillae muscle.
Innervation
Innervation is supplied by the parasympathetic division of the autonomic nervous system. The efferent pathway involves the following steps:
- Center — Edinger-Westphal nucleus (accessory parasympathetic nucleus), located in the midbrain.
- Preganglionic fibers — travel within the oculomotor nerve (n. oculomotorius, CN III), which enters the orbit via the superior orbital fissure.
- Ganglion — synapse occurs in the ciliary ganglion (ganglion ciliare).
- Postganglionic fibers — short ciliary nerves leave the ganglion and travel directly to the sphincter pupillae within the eyeball.
Function
The primary function of the muscle is pupillary constriction (miosis). This process occurs reflexively or voluntarily in specific scenarios:
- Pupillary light reflex: upon illumination, the sphincter contracts to protect the retina from excessive light flux. Due to bilateral pathways in the central nervous system, shining a light in one eye causes both pupils to constrict (direct and consensual responses).
- Accommodation and convergence: when focusing on a near object, the pupil constricts. This acts like a camera aperture, increasing the depth of field and ensuring a sharp image on the retina.
Clinical Significance and Lesions
Disruptions in sphincter pupillae function typically stem from neural pathway pathology:
- Mydriasis (pupillary dilation): occurs due to lesions of the efferent parasympathetic pathway (oculomotor nerve or ciliary ganglion). Impulse conduction to the sphincter is interrupted, and the pupil on the affected side fails to constrict to light. Mydriasis can also occur during severe pain or emotional arousal due to sympathetic overactivity.
- Anisocoria: a pathologic inequality in pupil diameter. It may indicate an uncal herniation or other intracranial space-occupying lesion compressing the oculomotor nerve.
- Pharmacologic effects: topical eye drops can cause drug-induced anisocoria. Such agents should be avoided in comatose patients to prevent confounding neurological assessments.