Anatomical substrate: microanatomy of the iris
Pupil size depends on the tone of two muscles located in the iris. In an iris section (from outside inwards / anterior to posterior), the following structures are distinguished:
- Stroma of the iris — the anterior layer containing blood vessels and pigment cells.
- Sphincter pupillae (m. sphincter pupillae) — muscle fibers arranged circularly around the pupillary margin. Their contraction causes pupil constriction (miosis).
- Dilatator pupillae (m. dilatator pupillae) — muscle fibers arranged radially within the thickness of the iris. Their contraction leads to pupil dilation (mydriasis).
- Pigment epithelium is the posteriormost (internal) dark layer that prevents light scattering.
How is pupillary constriction regulated?
Physiological miosis is an efferent response within the pupillary reflex. Regulation is carried out by the parasympathetic division of the autonomic nervous system.
Afferent pathway (sensory link):
- 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 where they are transmitted to parasympathetic centers.
Efferent parasympathetic pathway (causes miosis):
- Center: Edinger-Westphal nucleus (accessory oculomotor nucleus) located in the midbrain.
- Preganglionic fibers: travel as part of the oculomotor nerve (n. oculomotorius, cranial nerve III).
- Ganglion: synaptic switching occurs in the ciliary ganglion (ganglion ciliare).
- Postganglionic fibers: proceed directly to the circular muscle — sphincter pupillae, causing its contraction.
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 horns of the spinal cord at the Th1 segment (Budge's ciliospinal center). 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 dilatator pupillae.
If sympathetic innervation is interrupted at any point, paralysis of the dilator occurs. As a result, the tone of the parasympathetic sphincter begins to predominate, which clinically manifests as miosis.
Clinical significance and pathological miosis
Assessment of pupillary reaction and size is a mandatory component of a neurological examination. Pathological miosis is observed in a number of conditions:
- Horner's syndrome: occurs when sympathetic fibers are damaged (ciliospinal center, stellate ganglion, or plexuses around neck vessels). It manifests with a classic triad strictly on the affected side (ipsilaterally): miosis, ptosis (drooping of the upper eyelid), and anhidrosis (loss of sweating on one half of the face). Causes may include Pancoast bronchogenic apex tumor or life-threatening internal carotid artery dissection.
- Cluster headache: severe headache attacks accompanied by marked psychomotor agitation. During an attack, autonomic manifestations occur on the affected side: lacrimation, rhinorrhea, as well as ptosis and miosis (as manifestations of Horner's syndrome).
- Acute poisoning and substance abuse: marked miosis combined with bradypnea is a characteristic sign of opioid intoxication. With the development of withdrawal syndrome (abstinence), the reverse reaction is observed — the pupils dilate.
Class and exam questions
Why does miosis occur in Horner's syndrome?
Due to damage to the sympathetic pathways, paralysis of the pupillary dilator occurs. As a result, the tone of the parasympathetically innervated sphincter pupillae begins to predominate, leading to its constriction.
How do pupils change with opiate use?
In a state of opioid intoxication, marked miosis is observed. Against the background of withdrawal syndrome (upon drug cessation), a reverse autonomic reaction occurs — pupil dilation (mydriasis).
Where is the center of parasympathetic innervation that constricts the pupil located?
The center is the accessory oculomotor nucleus (Edinger-Westphal nucleus), which is located in the midbrain.
What else to study on this topic
- Pupillary reflex and its afferent pathways
- Horner's syndrome
- Autonomic innervation of the eye
- Mydriasis (pupil dilation)
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