Anatomy of the Iris, Ciliary Body, Choroid & Pupillary Light Reflex

1. Iris

➤ Anatomy

• 

  muscles
  Part of the uveal tract (vascular tunic).
• Located between the cornea and lens.
• Forms the pupil in its center.

➤ Histology

• Stroma: contains melanocytes, vessels, and the sphincter pupillae.
• Posterior pigmented epithelium: dark layer preventing light scatter.

➤ Muscles

• Sphincter pupillae (circular fibers): constricts pupil (miosis).
• Dilator pupillae (radial fibers): dilates pupil (mydriasis).

➤ Innervation

• Sphincter pupillae:
  • Parasympathetic fibers from Edinger–Westphal nucleus → CN III (oculomotor) → Ciliary ganglion → short ciliary nerves.
• Dilator pupillae:
  • Sympathetic fibers from T1 spinal segment → superior cervical ganglion → long ciliary nerves.

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🔄 2. Ciliary Body

➤ Anatomy

• 

  eye
  Lies posterior to iris, encircling the lens.
• Contains:
  • Ciliary muscle (smooth muscle)
  • Ciliary processes (secrete aqueous humor)
  • Suspensory ligaments (zonular fibers)

➤ Function

• Accommodation: Changes shape of the lens.
• Aqueous humor production: Maintains intraocular pressure and provides nutrients.

➤ Innervation

• Ciliary muscle:
  • 
    Parasympathetic: Edinger–Westphal nucleus → CN III → Ciliary ganglion → short ciliary nerves.
  • Contraction → relaxation of zonular fibers → lens becomes more convex for near vision.

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🩸 3. Choroid (Choroidea)

➤ Anatomy

• Layer between the retina and sclera.
• Extends from optic disc to ora serrata.
• Layers: Bruch’s membrane, choriocapillaris, Sattler’s layer, Haller’s layer.

➤ Function

• Highly vascularized: Supplies outer retina with oxygen/nutrients.
• Pigmented: Absorbs stray light → improves visual acuity.

➤ Innervation

• Autonomic (sympathetic and parasympathetic) innervation via the long and short ciliary nerves.
• Also contains sensory fibers from CN V1 (nasociliary branch).

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💡 4. Pupillary Light Reflex

➤ Definition

An involuntary reflex causing pupil constriction in response to light, protecting the retina and adjusting for light intensity.

➤ Pathway

➤ Afferent Limb:

• Retina → Optic nerve (CN II) → optic chiasm → optic tract → pretectal nuclei in midbrain.

➤ Central Integration:

• Pretectal nucleus sends bilateral projections to Edinger–Westphal nuclei.

➤ Efferent Limb:

• Edinger–Westphal nucleus → Oculomotor nerve (CN III) → Ciliary ganglion → short ciliary nerves → sphincter pupillae muscle.

➤ Clinical Tests

• Direct response: Light in one eye → that eye constricts.
• Consensual response: Opposite eye also constricts.
• Loss of reflex can localize optic nerve, CN III, or brainstem lesions.

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📖 References

1. Sobotta Atlas of Human Anatomy, Vol. 2 – Head, Neck, Neuroanatomy. Elsevier.
2. Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 42nd Ed. Elsevier.
3. MEMORIX Anatomy (2019). Triton. [Czech edition used in Charles University]
4. Kenhub:
   • Ciliary body
   • Choroid
5. NCBI Bookshelf:
   • Pupillary Light Reflex
6. Osmosis.org: Visual pathway and pupillary reflex video
7. Charles University First Faculty: Course PDFs on “Neuroanatomy and Special Senses”.

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📷 Recommended Diagrams

• Uveal tract anatomy – Sobotta / Kenhub
• Pupillary Light Reflex Pathway – NCBI / Osmosis.org
• Innervation of the eye muscles and iris – Gray’s Clinical Neuroanatomy