Location and Structure
The ciliary muscle occupies a major volume of the ciliary body.
- Attachment points: Extends from the inner scleral spur (near the limbus) to the choroid.
- Structure: Composed of multidirectional smooth muscle fiber groups. The muscle contains smooth myocytes oriented in two primary directions:
- Meridional (longitudinal) fibers: Run from the limbus/sclera to the choroid.
- Circular fibers: Run parallel to the limbal contour.
Innervation and Reflex Arc
As an intrinsic eye muscle, it receives autonomic (parasympathetic) innervation. The ciliary muscle features muscarinic receptors.
The efferent pathway of the accommodation reflex arc:
- Descending fibers from Visual Area 19 of the visual cortex project to midbrain structures, reaching the pretectal area.
- From the pretectal area, signals are distributed to the oculomotor complex nuclei, including the parasympathetic Edinger-Westphal nucleus (nucleus of Perlia).
- Preganglionic parasympathetic fibers travel within the oculomotor nerve (n. oculomotorius, CN III) to the ciliary ganglion (ganglion ciliare).
- Postganglionic fibers from the ciliary ganglion innervate the ciliary muscle (m. ciliaris), driving accommodation.
Mechanism of Accommodation
The ciliary muscle is the central effector of accommodation: the eye's ability to adjust its refractive power for clear vision at varying distances. Accommodation occurs via changes in lens curvature.
- Near vision: The m. ciliaris contracts via parasympathetic stimulation. The muscular ring narrows and shifts anteriorly. Tension on the suspensory ligaments (zonules) decreases, the lens capsule relaxes, and the lens assumes a more convex (spherical) shape. Its refractive power increases.
- Distance vision: The ciliary muscle relaxes. The suspensory ligaments are under high tension. The lens capsule is stretched, flattening the lens, which minimizes the eye's refractive power.
Note: During near accommodation, the response does not occur in isolation but as part of a triad: simultaneous convergence (contraction of the medial recti muscles of both eyes to align visual axes) and miosis (pupillary constriction to increase depth of field for near viewing).
Pharmacology and Clinical Significance
The function of the ciliary muscle depends on cholinergic synaptic transmission within the eye.
- Drug Effects:
- Direct/Indirect Mimetics (Parasympathomimetics): Cause contraction of the ciliary muscle and relaxation of the zonular fibers. The lens becomes more convex, inducing spasm of accommodation (focused exclusively on near vision).
- M-cholinergic antagonists (Anticholinergics/Cycloplegics): Cause relaxation of the ciliary muscle and tightening of the zonular fibers. The lens flattens, inducing cycloplegia (paralysis of accommodation), leaving the eye focused strictly on distance with inability to focus up close.
- Cranial Nerve Involvement in Botulism and Severe Neurotoxin Poisoning:
Ophthalmoplegia can result from impairment of cranial nerves III, IV, and VI. Ocular clinical manifestations include anisocoria, impaired accommodation and convergence, mydriasis, strabismus, ptosis, and restricted extraocular movements.