Location and microanatomy
The pupil dilator muscle is located in the anterior part of the middle (vascular) tunic of the eye — the iris (iris). In a cross-section of the iris (from outside in / anterior to posterior), the following layers are distinguished:
- Stroma of the iris (stroma iridis) is the anterior layer containing blood vessels and pigment cells.
- Sphincter pupillae (m. sphincter pupillae) consists of circular muscle fibers near the pupillary margin.
- Dilator pupillae (m. dilatator pupillae) consists of muscle fibers arranged radially within the thickness of the iris.
- Pigment epithelium is the posteriormost (internal) dark layer that prevents light scattering.
The muscles of the iris are of neuroglial origin. The cell bodies of the myocytes are located directly within the epithelium and are incapable of contraction. The contractile structures are cellular processes that extend from the cell bodies into the thickness of the iris, parallel to its plane. The processes of the dilator are oriented perpendicularly to the pupillary margin, i.e., radially.
Function and afferent regulation
The main function of the m. dilatator pupillae is pupil dilation in response to sympathetic stimulation. The iris is a circular diaphragm with an opening in the center — the pupil. The middle vascular tunic participates in the regulation of the light flux, and pupil size is regulated by two muscles of the iris: the sphincter and the dilator.
Afferent fibers associated with the ciliospinal center pass from the retina to the hypothalamus (suprachiasmatic nucleus). Central sympathetic fibers originate from the hypothalamus. At the level of the midbrain, the pathway crosses to the opposite side, then descends through the brainstem and spinal cord to the ciliospinal center.
How is the pupil dilator innervated?
The muscle receives sympathetic innervation, which initiates its contraction. The efferent pathway consists of several stages:
- Central pathway: originates from the hypothalamus and descends down the brainstem.
- Spinal center: fibers exit the spinal cord at the level of segments C8, Th1, Th2.
- Sympathetic trunk: preganglionic fibers ascend to the superior cervical ganglion (ganglion cervicale superius), where synaptic switching occurs.
- Postganglionic fibers: travel as part of the plexus around the internal carotid artery (a. carotis interna) to target structures. The internal carotid plexus innervates the eyes, including the pupil dilator.
- Pathway into the orbit: sympathetic fibers ascend, loop onto the ophthalmic artery, and enter the orbit.
- Ciliary ganglion (ganglion ciliare): has a sympathetic root (radix sympathicus). Short ciliary nerves (nn. ciliares breves) extend from the ganglion to innervate m. dilatator pupillae.
Note: In the clinical picture of botulism and acute intestinal infections, intrinsic eye muscles, including m. dilatator pupillae, are mentioned in connection with damage to the oculomotor nerve (n. oculomotorius, cranial nerve III).
Clinical significance
Disruption of the innervation of the pupil dilator leads to characteristic neurological and ophthalmological symptoms:
- Horner's syndrome: occurs when the sympathetic pathway is damaged at any level: central sympathetic fibers, ciliospinal center, superior cervical ganglion, or postganglionic sympathetic fibers on their way to the orbit. Due to the paralysis of the pupil dilator, the parasympathetic sphincter begins to predominate, causing miosis. The syndrome is also accompanied by ptosis, narrowing of the palpebral fissure, apparent enophthalmos, anhidrosis, and vasodilation on the ipsilateral half of the face.
- Botulism and acute intestinal infections: damage to cranial nerve pairs III, IV, and VI leads to oculomotor disorders. Ocular manifestations may include anisocoria, mydriasis, accommodation and convergence impairment, strabismus, ptosis, and impaired eyeball movement.
Class and exam questions
Which muscle constricts the pupil?
The pupil is constricted by the sphincter pupillae (m. sphincter pupillae). It is located circularly at the pupillary margin, is innervated by the parasympathetic nervous system, and causes miosis upon contraction.
Why does miosis occur in Horner's syndrome?
Miosis occurs due to paralysis of the pupil dilator resulting from damage to the sympathetic pathway. As a result, the parasympathetic pupil sphincter predominates.
Where is the spinal center associated with this muscle located?
The excerpts indicate that the fibers of the sympathetic pathway to the eye exit the spinal cord at the level of segments C8, Th1, Th2; the ciliospinal center of Budge at the level of Th1 is also mentioned.
What else to study on this topic
- Iris (iris)
- Sphincter pupillae (m. sphincter pupillae)
- Ciliary ganglion (ganglion ciliare)
- Superior cervical ganglion (ganglion cervicale superius)
- Horner's syndrome
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