Location and Microanatomy
The pupillary dilator muscle is located in the anterior portion of the vascular tunic of the eye, the iris (iris). On a cross-section of the iris (from outside inward / anterior to posterior), the layers are:
- Iris stroma (stroma iridis) — the anterior layer containing blood vessels and pigment cells.
- Sphincter pupillae (m. sphincter pupillae) — circular muscle fibers located near the pupillary margin.
- Pupillary dilator muscle (m. dilatator pupillae) — muscle fibers arranged radially within the iris stroma.
- Pigmented epithelium — the deepest (posterior) dark layer that prevents light scattering.
The iris muscles have a neuroglial origin. The myocyte cell bodies are located directly within the epithelium and are incapable of contraction themselves. The contractile structures are cellular processes that extend from the cell bodies into the iris stroma, running parallel to its plane. The processes of the dilator are oriented perpendicularly to the pupillary margin, i.e., radially.
Function and Afferent Regulation
The primary function of the m. dilatator pupillae is to dilate the pupil in response to sympathetic stimulation. The iris acts as a circular diaphragm with an aperture in the center—the pupil. The vascular tunic helps regulate the light flux, while the pupil size is controlled by two iris muscles: the sphincter and the dilator.
Afferent fibers associated with the ciliospinal center project from the retina to the hypothalamus (suprachiasmatic nucleus). Central sympathetic fibers originate in the hypothalamus. At the midbrain level, the pathway crosses to the contralateral side, then descends through the brainstem and spinal cord to the ciliospinal center.
Innervation of the Pupillary Dilator Muscle
The muscle receives sympathetic innervation, which triggers its contraction. The efferent pathway consists of several sequential steps:
- Central pathway: originates in the hypothalamus and descends through the brainstem.
- Spinal center: preganglionic fibers emerge from the spinal cord at the C8, T1, and T2 segments.
- Sympathetic trunk: preganglionic fibers ascend to the superior cervical ganglion (ganglion cervicale superius), where synapses are formed.
- Postganglionic fibers: travel within the plexus surrounding the internal carotid artery (a. carotis interna) toward target structures. The internal carotid plexus innervates structures of the eye, including the pupillary dilator muscle.
- Entry into the orbit: sympathetic fibers ascend, join the ophthalmic artery, and enter the orbit.
- Ciliary ganglion (ganglion ciliare): features a sympathetic root (radix sympathicus). Short ciliary nerves (nn. ciliares breves) branch from the ganglion to innervate the m. dilatator pupillae.
Note: In the clinical presentation of botulism and acute intestinal infections, intrinsic eye muscles including the m. dilatator pupillae are sometimes mentioned in the context of oculomotor nerve (n. oculomotorius, cranial nerve III) involvement.
Clinical Significance
Disruption of the sympathetic innervation to the pupillary dilator muscle leads to characteristic neurological and ophthalmological signs:
- Horner's syndrome: occurs due to damage to the sympathetic pathway at any level (central sympathetic fibers, ciliospinal center, superior cervical ganglion, or postganglionic fibers en route to the orbit). Paralysis of the pupillary dilator allows the parasympathetic sphincter to unopposedly constrict the pupil, causing miosis. The syndrome also classically presents with ptosis, narrowing of the palpebral fissure, apparent enophthalmos, anhidrosis, and vasodilation on the ipsilateral side of the face.
- Botulism and acute intestinal infections: lesions affecting cranial nerves III, IV, and VI lead to extraocular movement disorders. Ocular findings may include anisocoria, mydriasis, impaired accommodation and convergence, strabismus, ptosis, and restricted extraocular motility.