Anatomy of Iris
Iris is a circular pigmented diaphragm which is situated between the cornea and the lens
Parts : It has
Two margins
Peripheral margin attached with anterior surface of ciliary body :
Central circular margin known as pupil c
Two zones :
Ciliary zone contain ciliary crypts
Pupillary zone contain pupillary ruff the reflected area of posterior pigmented area
Two zone is separated by collaratte a smooth ridge of tissue
Shape: flatten cone with central opening, iris is push forward due to the anterior surface of the lens
Iris divide anterior segment of eyeball into anterior and posterior chamber , both the chamber filled by aqueous humor which circulate through the opening of pupil
Histology of iris
From before backwards , The iris is composed of three layers:
Anterior surface of the iris : an outer discontinuous layer of melanocytes and fibroblasts;
Stroma of the iris the intermediate fibrous layer (FL), housing pigment cells (Pc) and fibroblasts;
Muscle layer :
smooth muscle near the pupil ,
smooth muscle in the deeper part of stroma
Anterior Pigmented epithelium
Posterior pigmented epithelium
Anterior surface of the iris :
The anterior surface of the iris is not covered by a distinct endothelium. Instead, it is formed by layers of branched fibroblasts and melanocytes, which blend with the pectinate ligament derived from the Descemet’s membrane of the cornea.
Stroma of the iris
It contains collagen fibers, fibroblasts, melanocytes and blood vessels and nerves , sphincter and dilator pupillae muscles. Stromal spaces are in free communication with the fluid of the anterior chamber
Sphincter pupillae is a circular muscle, about 1 millimeter wide. It is located in the pupillary zone of stromal layer of the iris, attaching to and encircling the pupillary margin of iris. The muscle itself consists of six to eight circles of smooth muscle fibers, between of which are found the nerves and blood vessels that supply each fiber.
Development : ectoderm
Innervation
The sphincter pupillae receives parasympathetic innervation from the postganglionic fibers of ciliary ganglion, called short ciliary nerves, which are the branches of oculomotor nerve (CN III). Note that these particular nerve fibers originate from the accessory nucleus of oculomotor nerve (Edinger-Westphal) - a general visceral efferent nucleus - which provides autonomic (involuntary) control to this muscle.
Function
When contracting, the sphincter pupillae constricts the pupil, which is called miosis. This action happens during accommodation and pupillary light reflexes.
Accommodation
Accommodation is a reflexive event that adapts the eyes for observing close objects after being focused on distant ones. The reflex pathway includes optic nerve (CN II), visual and frontal cortex, oculomotor and accessory oculomotor nuclei and oculomotor nerve (CN III).
Once the eyes need to focus on a near object, the optic nerve sends signals to the visual cortex, which then stimulates the eye field within frontal cortex. The frontal neurons then send fibers to the ipsilateral oculomotor nucleus (unicortical innervation), and to both ipsi- and contralateral accessory oculomotor nuclei (bicortical innervation). The oculomotor nerve sends these signals to the ciliary ganglion, which then outsources the final stimuli via its two sets of branches;
Short ciliary nerves that cause the contraction of sphincter pupillae and miosis
Long ciliary nerves which stimulate the medial recti and ciliary muscles; the former causes the convergence of the eyeballs, while the latter thickens the lens increasing its refractive power
Pupillary light reflex
The pupillary light reflex happens when the eyes are exposed to bright light and the amount of light that falls onto the retina needs to be decreased in order to maintain clear vision. The reflex arc includes optic nerve (CN II), pretectal nucleus of midbrain, accessory oculomotor nucleus and oculomotor nerve (CN III).
Once the retinal photoreceptors detect light, the optic nerve sends a neural impulse to the pretectal nucleus. The pretectal nucleus stimulates the accessory oculomotor nucleus, which in turn innervates and constricts the sphincter pupillae via the short ciliary nerves. As pretectal nucleus connects to both accessory oculomotor nuclei, stimulation and miosis of one eye (direct pupillary light reflex) will always be followed by miosis on non-stimulated eye (indirect pupillary light reflex).
The dilator pupillae muscle is a ring of contractile cells within the iris. These cells are arranged radially, such that their contraction facilitates pupillary dilation (mydriasis). The dilator pupillae muscle receives innervation from the sympathetic nervous system.
Gross anatomy
The dilator pupillae muscle is a circumferential, spoke-like arrangement of contractile myoepithelial cells located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. Like the sphincter pupillae muscle, it is located posterior to the rich connective tissue stroma and neurovasculature of the anterior iris leaf
Innervation
The dilator pupillae muscle is innervated by the sympathetic nervous system. Postganglionic sympathetic fibers project from the superior cervical ganglion to join the carotid plexus. These fibers then course with the ophthalmic artery, forming a number of long ciliary nerves that supply the dilator pupillae muscle
Action
Contraction of the dilator pupillae muscle facilitates dilation of the pupil (mydriasis). This increases the amount of light impinging on the retina
Clinical importance
alongside endogenous sympathetic stimulation, sympathomimetic drugs (e.g. stimulants) may act on the dilator pupillae muscle, resulting in mydriasis
prolonged significant mydriasis narrows the iridocorneal angle for aqueous humor drainage, increasing the risk of closed angle glaucoma
the miosis seen in patients with Horner syndrome is mediated by inactivation of the dilator pupillae muscle due to a lesion interrupting sympathetic outflow
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