Transcript Vascular coat of eye

Tashkent Medical Academy
 Vascular Layer
Vascular Layer
The 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 tract
 Iris
 Choroid
 Ciliary body
(includes zonular fibers, ciliary muscles, and ciliary processes)
The ciliary body is closely associated with the:
 Lens
 Trabecular meshwork
 Canal of Schlemm
Iris
 Towards 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 Iris
 The 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 Body
 Towards 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 processes
ciliary muscles
zonular fibers
An 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.
Choroid
 The 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, multilayered 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.