Week 6: Iris, Pupil and Ciliary Body Flashcards
Explain the general overview of the iris
- Suspended in the aqueous between cornea and lens
- Attached by its root to the ciliary margin where it is thinnest
- Thickest at collarette ridges
- Colour of the iris is produced by the pigment in the melanocyte
- The pupil constantly changes in size due to the action of the sphincter and dilator muscles
Explain the iris structure
Anterior surface
- Has centre pupillary zone & peripheral ciliary zone divided by collarette
- Velvety and has no epithelium (consists of melanocyes & little collagen fibers)
- Some areas have no melanocytes (aka Crypts of Fuch’s)
Posterior surface
- Connective tissue gives rise to radial streaks near free margin
- Circular contraction furrows found in ciliary portion of iris
What is the structure and function of the iris stroma?
- Made of highly vascular connective tissue
- Fibroblasts, melanocytes, type I, type III
Function:
- Allows fluid to move in and out of the stroma quickly during dilation and contraction
What is the structure, innervation and function of the iris sphincter pupillae muscle?
Structure:
- Smooth muscle that consists of muscle bundles
- Joined by gap junctions
- Innervated by parasympathetic neurons
- Function: action is miosis (pupil constriction) in response to light, convergence and sleep
What is the structure, innervation and function of the dilator pupillae muscle?
Structure:
- Composed of myoepithelial cells
- Joined by gap junctions
- Arranged radially
- Innervated by sympathetic neurons
Explain the anterior pigment epithelium
- Forms a ruff on the anterior surface
Explain the posterior pigment epithelium
- Heavily pigmented
- Cells are joined by gap junctions and desmosomes
- It forms radially arranged furrows near the margin
Describe the visual light reflexes: direct and consensual reflex (pupil constriction pathway)
- Light falls on the retina
- Afferent signal transmitted by RGC axons via optic nerve to chiasm, then optic tract to midbrain
- Synapse in pretectal area with interneurons which project to ipsilateral and contralateral Edinger-Westphal nuclei
- Efferent output from Edinger-Westphal nucleus via CNIII as pre-ganglionic fibres to ciliary ganglion.
- From Ciliary Ganglion, innervate sphincter via short posterior ciliary nerves
Describe the pupil dilator pathway
- Acetylcholine begins in the hypothalamus and descends through the midbrain to synapse with a pre-ganglionic neuron
- Pre-ganglionic neuron then to superior cervical ganglion (SCG)
- Poster-ganglionic neuron from SCG, innervates dilator via the ciliary ganglion (short ciliary nerves) and the long ciliary nerves via branches of CV1 (ophthalmic division of CV)
- Muscles action medicated by norepinephrine on both alpha and beta adrenoreceptors
What are the structures and functions for the ciliary body?
Structures:
- Middle structure of the uvea
- Base is towards the anterior chamber, apex blends with the choroid
Functions:
- Production of aqueous humour
- Production of lens zonules
- Holds the lens in place
- Facilitates accommodation
What are the portions/sections of the ciliary body?
- Pars plicata
- Ridged anterior region
- Ciliary processes providing a large surface area for aqueous production
- Processes are highly vascular - Pars plana
- Smooth posterior region of the ciliary body, continuous into the choroid and retina - Ciliary processes
- Arise from the pars plicata
- CilIary processes comprised of major and minor radial ridges
What are the layers of the ciliary body?
- Non-pigmented ciliary epithelium (outer layer)
- Pigmented ciliary epithelium (inner layer)
- Ciliary stroma
- Contains bundles of loose connective tissue & melanocytes - Ciliary muscle
- The bulk of the ciliary body
- Composed of smooth muscle fibers
- Innervated by autonomic nervous system
- Divided into 3 groups: a) Longitudinal fibres, b) Oblique fibres, c) Circular fibres
What is the role of the ciliary body on accommodation?
- Contraction of the ciliary muscle
- Pulls the ciliary body forward towards the lens
- Forward movement relieves tension in suspensory ligaments, making elastic lens more convex leading to an increase of refractive power of the lens
- Innervated by sympathetic and parasympathetic
Describe the process of aqueous formation/production?
- Ciliary body, particularly epithelium produce & secrete aqueous
- Provides nutrients to avascular cornea & lens
- Production follows a natural cycle
- Autonomic nerves located in ciliary body affect aqueous production
Ocular Anti-hypertensive medications:
- Increase Aqueous outflow
- Reduce Aqueous production
Explain the blood aqueous barrier and possible cause of breakdown of it
Blood Aqueous Barrier
- Ciliary body capillaries are fenestrated
- Blood aqueous barrier formed and maintained by tight junctions between the cells of the non-pigmented epithelium
- Designed to keep the aqueous plasma and protein free
- Allow the passage of small ions and H2O
Possible cause of breakdown:
- Trauma
- Infection
- Inflammation
- Vascular disease
