ciliary body

STRUCTURE: Part of the uveal tract (iris – ciliary body – choroid). A ring-shaped structure with two parts: (1) pars plicata – anterior, ridged ciliary processes that secrete aqueous humor; (2) pars plana – posterior, flat, continuous with the retina at the ora serrata. Components: ciliary muscle (smooth muscle), ciliary processes, and a two-layered ciliary epithelium (outer pigmented layer, inner non-pigmented layer). The non-pigmented epithelium forms the blood-aqueous barrier via tight junctions. Zonular fibers (suspensory ligament) connect the ciliary processes to the lens capsule.

FUNCTIONS: 1) Aqueous humor production: The non-pigmented ciliary epithelium secretes aqueous humor, with carbonic anhydrase and Na+/K+-ATPase involved. Flow: posterior chamber → through the pupil → anterior chamber → trabecular meshwork → canal of Schlemm → episcleral veins (conventional pathway). A minor uveoscleral outflow pathway also exists, and prostaglandin analogs enhance it. 2) Accommodation: Near vision → parasympathetic stimulation (CN III → ciliary ganglion → short ciliary nerves; M3 muscarinic receptors) → ciliary muscle contracts (sphincter-like, moves forward/inward) → zonular tension decreases → lens becomes more spherical, increasing refractive power. Distance vision → ciliary muscle relaxes → zonular fibers taut → flatter lens. 3) Vitreous/lens support: Zonules attach here; also contributes to hyaluronic acid secretion in the vitreous.

CLINICAL CORRELATES: - Glaucoma pharmacology (high-yield): Drugs that reduce aqueous production at the ciliary body: β-blockers (timolol; block β2 receptors), α2 agonists (brimonidine, apraclonidine; decrease production, brimonidine also increases uveoscleral outflow), carbonic anhydrase inhibitors (acetazolamide, dorzolamide). Increase outflow: prostaglandin analogs (latanoprost; uveoscleral; side effects: eyelash growth, iris hyperpigmentation), cholinomimetics (pilocarpine, carbachol; contract ciliary muscle and open the trabecular meshwork; miosis), and rho-kinase inhibitors (netarsudil). - Cycloplegics (atropine, tropicamide, cyclopentolate) paralyze the ciliary muscle → loss of accommodation (cycloplegia) and mydriasis; used for cycloplegic refraction and uveitis pain (ciliary spasm). - Presbyopia: age-related loss of lens elasticity (and ciliary muscle efficiency) → impaired near vision, usually after ~40 years of age; corrected with convex (reading) lenses. - Uveitis: Anterior uveitis includes iritis and iridocyclitis. Intermediate uveitis involves the pars plana (pars planitis; associated with multiple sclerosis and sarcoidosis). Posterior uveitis involves the choroid/retina. Associations of anterior uveitis: HLA-B27 diseases (ankylosing spondylitis, reactive arthritis, psoriatic arthritis, IBD), JIA, sarcoidosis. - Acute angle-closure glaucoma: Acute rise in intraocular pressure with a fixed mid-dilated pupil, painful red eye, halos, and headache/nausea; mydriasis (including anticholinergic/sympathomimetic drugs) can precipitate it. Treat with acetazolamide, topical β-blockers, α2 agonists, pilocarpine, and laser peripheral iridotomy. Avoid mydriatics. - Ciliary body melanoma is a uveal melanoma subtype (most common primary intraocular malignancy in adults; uveal melanoma is often GNAQ/GNA11 and BAP1 related, with liver metastases). - Horner syndrome (loss of sympathetic input) affects the dilator pupillae rather than the ciliary muscle, causing miosis, ptosis, and anhidrosis. - Adie tonic pupil: damage to the ciliary ganglion/short ciliary nerves → dilated, poorly reactive pupil with impaired accommodation. - Trauma can cause angle recession, cyclodialysis (separation of the ciliary body from the scleral spur) and hypotony.

EMBRYOLOGY: Ciliary epithelium derives from the neuroectoderm (optic cup, 2 layers: outer pigmented continuous with retinal pigment epithelium, inner non-pigmented continuous with neural retina). Ciliary muscle derives from neural crest.

INNERVATION/BLOOD SUPPLY: Parasympathetic (CN III; Edinger-Westphal nucleus → ciliary ganglion → short ciliary nerves) to the ciliary muscle; sympathetic (superior cervical ganglion) to the vessels and slight relaxation (β2). Blood supply from the long posterior ciliary arteries and anterior ciliary arteries (branches of the ophthalmic artery).