Ciliary muscle

Location and anatomy

The ciliary muscle occupies a significant part of the volume of the ciliary body.

• Attachment points: from the internal scleral spur (near the limbus) to the choroid.
• Structure: consists of multi-directional groups of smooth muscle cells. The ciliary muscle contains bundles of smooth myocytes oriented in two directions:
• Meridional: from the limbus/sclera to the choroid proper.
• Circular: parallel to the limbal contour.

Innervation and reflex arc

The muscle belongs to the intrinsic muscles of the eye and receives autonomic (parasympathetic) innervation. Muscarinic acetylcholine receptors are located on the ciliary muscle.

Efferent pathway of the accommodation reflex arc:

1. From Area 19 of the visual cortex, descending fibers pass to midbrain structures, reaching the pretectal area.
2. From the pretectal area, signals are distributed to the nuclei of the oculomotor complex, including the parasympathetic nucleus of Perlia.
3. Preganglionic parasympathetic fibers travel within the oculomotor nerve (n. oculomotorius, cranial nerve III) to the ciliary ganglion (ganglion ciliare).
4. Postganglionic fibers from the ciliary ganglion innervate the ciliary muscle (m. ciliaris), which drives the act of accommodation.

How does the accommodation mechanism work?

The ciliary muscle is a key element of accommodation: the ability of the eye to change its refractive power for clear vision of objects at varying distances. Accommodation is achieved by changing the curvature of the lens.

• Near vision: m. ciliaris contracts under parasympathetic innervation. The muscular ring narrows and shifts anteriorly. The tension of the zonule of Zinn weakens, the lens capsule relaxes, and the lens assumes a more convex (spherical) shape. Its refractive power increases.
• Distance vision: the ciliary muscle is relaxed. The suspensory ligaments holding the lens are under tension. The lens capsule is stretched, the lens flattens, and the refractive power of the eye is at a minimum.

Note: When viewing a near object, accommodation does not work in isolation, but as part of a triad: convergence (contraction of the medial rectus muscles of both eyes, bringing the visual axes together) and miosis (pupillary constriction to increase visual acuity when viewing near objects) occur simultaneously.

Pharmacology and clinical significance

The state of the ciliary muscle is linked to the activity of cholinergic synapses and the innervation of the eye.

• Drug effects:
• M-cholinomimetics: cause contraction of the ciliary muscle and relaxation of the zonule of Zinn. The lens assumes a more convex shape, leading to accommodation spasm: the eye is focused on the near point of vision.
• M-cholinoblockers: cause relaxation of the ciliary muscle and tension of the zonule of Zinn. The lens flattens, leading to cycloplegia (paralysis of accommodation): the eye is focused on the far point of vision, making near focus impossible.
• Cranial nerve palsy in botulism and acute intestinal infections:

Oculomotor disorders are caused by damage to cranial nerves III, IV, and VI. Clinical ocular manifestations include anisocoria, impaired accommodation and convergence, mydriasis, strabismus, ptosis, and impaired eye movements.

Class and exam questions

What else to study on this topic

• Ciliary body and aqueous humor production
• Oculomotor nerve (CN III)
• Lens and zonule of Zinn
• Accommodation and convergence reflex

Other topics in the "Anatomical Structures" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.