Vascular coat of eye

Tashkent Medical Academy

Vascular LayerVascular LayerThe middle coat of the eye is the vascular layer, also called

the uvea or uveal tract. The uveal tract makes up the vascular pigmented coat and has three critical structures:

Structures of the uveal tractIrisChoroidCiliary body (includes zonular fibers, ciliary muscles, and ciliary processes)The ciliary body is closely associated with the:LensTrabecular meshworkCanal of Schlemm

IrisTowards the front of the eye, the muscles of

the iris (colored portion of the eye) control the amount of light entering the eye. Pigments in the iris give it color. The iris represents the anterior portion of the uveal tract. The iris is made up of smooth muscle fibers that adjust pupil size.

The pupil (black aperture of the eye) is not a structure at all; rather, it is an opening created by the circular iris. Its blackness is due to the lack of reflection of light from within the eye. The pupil allows light into the eye.

Control of the IrisThe iris is composed of a pigmented epithelium, the

anterior sphincter muscle, and the dilator muscle. The sphincter and dilator are both smooth muscles.

The sphincter muscle contracts in response to parasympathetic stimulation, while the dilator contracts in response to sympathetic stimulation. Changes in the amount of light falling on the retina stimulate or inhibit each pathway:

In bright light, the sphincter muscle contracts while the dilator muscle relaxes, closing down the pupil and making the aperture smaller.

In dim light, the sphincter muscle relaxes while the dilator muscle contracts, opening up the pupil and making the aperture larger.

This opening and closing action controls the amount of light passing through the iris and falling on the retina to optimize illumination of the photoreceptors.

Dilation can also be caused by excess sympathetic stimulation of the dilator muscle. Numerous stimulants, including nicotine, caffeine, and amphetamines have this effect.

The neurotransmitter of the parasympathetic nervous system is acetylcholine, which binds to acetylcholine receptors on the smooth muscle it stimulates.

The pupil may be artificially dilated or constricted by drugs which block or promote either dilator or sphincter function. Drugs that cause:

• iris dilation are called mydriatics ; • iris constriction are called miotics.

Ciliary BodyTowards the front of the eye, the choroid merges into the ciliary body.

The ciliary body is a blood-rich tissue, whose structures work together to produce the aqueous humor. The ciliary body is made up of:

ciliary processesciliary muscleszonular fibersAn important part of the ciliary body is the ciliary muscle, whose

contractions control the shape of the lens during the process of focusing. The lens is suspended from the ciliary muscle by zonular fibres. Ciliary muscles are innervated by the parasympathetic and sympathetic nervous systems. Parasympathetic stimulation causes the muscles to contract, while sympathetic stimulation causes muscle relaxation. Changes in ciliary muscle tension affect both vision (by focusing light through the lens) and the outflow of aqueous humor in the eye.

The ciliary processes are the folds found on the surface of the ciliary body that produce aqueous humor. Although scientists do not understand exactly how aqueous humor is formed, they do know that it involves the secretion of salts (e.g., sodium chloride) by the ciliary epithelium.

The  canal of Schlemm provides a route for excess aqueous humor to drain out of the eye. Before it can leave the eye, the fluid must pass through the trabecular meshwork. Changes in the resistance of the trabecular meshwork to the outflow of aqueous humor maintain the appropriate fluid pressure within the eye. This function gives the trabecular meshwork and canal of Schlemm key roles in regulating intraocular pressure (IOP), which is the pressure exerted on the eye by the fluids it contains.

ChoroidThe posterior portion of the uveal tract is represented by

the choroid, a vascular membrane that separates the fibrous layer from the retina. The choroid receives its blood supply from the central retinal artery that emerges from the back of the eye.

The choroid provides all eye layers with 90% of their blood supply. The blood vessels, embedded in a meshwork of connective tissue, supply the fibrous layer and much of the retina with nutrients.

This blood supply meets the oxygen and metabolic needs of the eye; it provides the eye with the nourishment it needs to perform basic cellular functions and removes waste products from the eye. The circulation of blood within the choroid is closely related to the pressure of fluids within the eye.

- The ciliary body contains ciliary muscle that is composed of smooth muscle. Contraction and relaxation of the ciliary muscles change the tension of the zonular fibers, or suspensory ligaments, of the lens. This allows the lens to change shape, a process known as accommodation.- The ciliary processes are folds of connective tissue that are covered by two layers of epithelium. There is also a complex vasculature that cannot be seen easily. Fluid from these vessels is processed and transported by the epithelial cells to the posterior chamber as aqueous humor. The epithelial cells constitute the blood-aqueous barrier.- The aqueous humor enters the anterior chamber through the pupil as it flows between the lens and the iris.- Aqueous humor leaves the anterior chamber through the trabecular meshwork and into the canal of Schlemm. This is an endothelial lines, circumferentially arranged vessel that communicates with veins in the sclera and returns the aqueous humor back to the general circulation.- Obstruction of the trabecular meshwork and canals of Schlemm are thought to be the major cause of elevated intraocular pressure, which could then lead to glaucoma.

- The iris is detailed here in higher magnification. Note the anterior and posterior chambers to help orient yourself.- The anterior surface of the iris contains loose, variably pigmented stroma. It is open to the circulating aqueous humor within the anterior chamber.- Two layers of heavily pigmented epithelium cover the posterior surface of the iris.- Note that the sphincter pupillae muscle can be easily seen near the pupil margin. It is smooth muscle controlled by parasympathetics. The dilator pupillae muscle is more difficult to identify, but it dilates the pupil upon sympathetic innervation.

- Now we will look at the other portions of the eyeball. The eye can essentially be divided into 3 layers:

- 1) sclera – tough connective tissue that continues anteriorly as the cornea.

- 2) choroid or uveal layer – vascular, loose connective tissue that includes the ciliary body and the iris anteriorly.

- 3) retina – contains photosensory cells in the posterior portion and a non-sensory two-layered epithelium in the anterior portion. The boundary is marked by the ora serrata.

- Note how the photosensory retina is interrupted by nerve fibers and blood vessels leaving the eye at the optic nerve.- The fovea centralis in the macula is a focal region of thinner retinal epithelium important for visual acuity.

- The ora serrata is shown at higher magnification in the bottom panel. Note the transition from the tall, multi-layered photosensory epithelium into the two-layered pigmented epithelium.- The layer adjacent to the vitreous body and aqueous humor is unpigmented, but the bottom layer remains pigmented.- Note the other two layers of the eye – the choroid and the sclera – as labeled.