Anatomical Substrate: Microanatomy of the Iris
Pupil diameter depends on the tone of two antagonistic muscles located within the iris. In cross-section (from outside to inside / anterior to posterior), the following structures are identified:
- Iris stroma — the anterior layer containing blood vessels and pigment cells.
- Sphincter pupillae (m. sphincter pupillae) — circular muscle fibers located near the pupillary margin. Their contraction causes pupil constriction (miosis).
- Dilator pupillae (m. dilatator pupillae) — radially arranged muscle fibers located within the body of the iris. Their contraction leads to pupil dilation (mydriasis).
- Pigment epithelium — the posteriormost (innermost) dark layer that prevents light scattering.
How Pupil Constriction is Regulated
Physiological miosis is the efferent response of the pupillary light reflex. Regulation is mediated by the parasympathetic division of the autonomic nervous system.
Afferent pathway (sensory component):
- The optic nerve (n. opticus) carries light impulses from the retina.
- Fibers pass through the optic chiasm (chiasma opticum).
- From the diencephalon, impulses are directed to the pretectal nucleus (nucleus pretectalis) in the midbrain, from which they are relayed to parasympathetic centers.
Efferent parasympathetic pathway (causes miosis):
- Center: Edinger-Westphal nucleus (accessory oculomotor nucleus) located in the midbrain.
- Preganglionic fibers: travel within the oculomotor nerve (n. oculomotorius, cranial nerve III).
- Ganglion: synaptic relay occurs in the ciliary ganglion (ganglion ciliare).
- Postganglionic fibers: run directly to the circular muscle—the sphincter pupillae—triggering its contraction.
The Role of the Sympathetic System
The sympathetic nervous system exerts the opposite effect—it dilates the pupil. The central pathway descends from the hypothalamus to the lateral gray horns of the spinal cord at the Th1 segment (the ciliospinal center of Budge). Preganglionic fibers ascend via the sympathetic trunk (truncus sympathicus) and synapse in the superior cervical ganglion (ganglion cervicale superius). Postganglionic fibers form a plexus around the internal carotid artery (a. carotis interna) and reach the dilator pupillae.
If sympathetic innervation is interrupted at any point along this route, paralysis of the dilator muscle ensues. As a result, parasympathetic sphincter tone predominates, clinically presenting as miosis.
Clinical Significance and Pathological Miosis
Assessment of pupillary light reaction and diameter is an essential component of a neurological examination. Pathological miosis is observed in several conditions:
- Horner syndrome: caused by damage to the sympathetic fibers (ciliospinal center, stellate ganglion, or perivascular plexuses around the neck vessels). It presents with a classic ipsilateral triad: miosis, ptosis (drooping of the upper eyelid), and anhidrosis (absence of sweating on one half of the face). Causes may include a Pancoast tumor (bronchogenic carcinoma of the lung apex) or a life-threatening internal carotid artery dissection.
- Cluster headache: severe headache attacks accompanied by marked autonomic symptoms. During an attack, ipsilateral autonomic manifestations occur on the pain side: lacrimation, rhinorrhea, as well as ptosis and miosis (manifestations of Horner syndrome).
- Acute poisoning and substance use disorder: pinpoint miosis combined with bradypnea is a hallmark of opioid intoxication. During opioid withdrawal (abstinence syndrome), the opposite reaction occurs—the pupils dilate.