ciliary muscle

**Anatomy** - Smooth muscle within the ciliary body (between the iris root and choroid), with longitudinal (meridional), radial, and circular (Müller) fibers. - The ciliary body also contains the ciliary processes/epithelium, which secrete aqueous humor (the nonpigmented epithelium makes it; carbonic anhydrase is involved). - Suspensory ligaments (zonular fibers) connect the ciliary body to the lens capsule.

**Innervation (high-yield pathway)** - Parasympathetic: Edinger-Westphal nucleus → CN III (oculomotor) → ciliary ganglion (synapse) → short ciliary nerves → ciliary muscle (and the sphincter pupillae). Receptor: M3 muscarinic (Gq). - Sympathetic input (β-adrenergic) causes mild relaxation and has a minor role in accommodation (far vision).

**Physiology: accommodation** - Near vision: ciliary muscle contracts (sphincter-like) → ciliary ring diameter decreases → zonular fibers slacken → lens capsule relaxes and the lens becomes more spherical (thicker, with a greater anterior curvature) → refractive power increases. - Accommodation reflex ("near triad"): accommodation + convergence (medial rectus) + miosis (sphincter pupillae). - Distance vision: ciliary muscle relaxes → zonules taut → lens flattens.

**Clinical correlations** - Presbyopia: age-related loss of lens elasticity (and sclerosis) → decreased accommodative ability, typically after about age 40. Patients hold reading material at arm's length. Treated with convex (reading) lenses. - Cycloplegia: paralysis of the ciliary muscle (blurred near vision) from muscarinic antagonists (atropine, tropicamide, cyclopentolate; used for cycloplegic refraction and uveitis pain relief) or CN III palsy. Mydriasis occurs together with it, which can precipitate acute angle-closure glaucoma in predisposed patients. - Cholinergic agonists (pilocarpine, carbachol, physostigmine, echothiophate): ciliary muscle contraction and miosis. Pilocarpine pulls on the scleral spur, opening the trabecular meshwork and increasing aqueous outflow, so it is used in open-angle and acute angle-closure glaucoma. - Adie tonic pupil: damage to the ciliary ganglion/short ciliary nerves → poor pupillary light response and impaired accommodation, with a slowly reactive pupil. - CN III palsy: "down and out" eye, ptosis, dilated pupil, loss of accommodation. - Hyperopia: excessive accommodation, which can cause accommodative esotropia in children. - Aqueous humor outflow: the ciliary muscle contributes to the uveoscleral pathway and trabecular outflow tone. - Drugs that increase aqueous production are targeted differently: β-blockers (timolol) and carbonic anhydrase inhibitors (acetazolamide) act on the ciliary epithelium, not the muscle. α-agonists (brimonidine) decrease production and increase uveoscleral outflow. Prostaglandin analogs (latanoprost) increase uveoscleral outflow by relaxing the ciliary muscle and remodeling the extracellular matrix.

**Mnemonic**: "Contract to Close/Near": the ciliary muscle contracts → zonules relax → lens fat → near vision.